"Answers to Technical Problems in Injection Molding and Injection Molding Machine Use"
1. Reasons and solutions for flash of product when machine is just turned on, and lack of glue after a period of production.
When machine is just turned on, melt in material tube of injection molding machine has a long heating time, low melt viscosity, and good fluidity, so product is easy to flash. After a period of production, melt continuously takes away heat, resulting in insufficient melt, high viscosity, poor fluidity, and lack of glue in product. After a period of production, gradually increase temperature of material tube to solve problem.
2. During production process, product lacks glue, and sometimes increasing injection pressure and speed is ineffective. Why?
After a period of production, melt continuously takes away heat, resulting in insufficient melt, high viscosity, poor fluidity, and lack of glue in product. Increase temperature of material tube to solve problem.
3. Reasons and solutions for ellipse of product.
Ellipse of product is due to uneven glue injection, which causes uneven pressure around product, making product elliptical. Three-point glue injection is used to make product glue injection uniform.
4. Requirements for molds for precision products.
Mold material is required to have good rigidity, small elastic deformation, and small thermal expansion coefficient.
5. Purpose of product acid resistance test.
Product acid resistance test is to detect internal stress of product and position of internal stress point, so as to eliminate internal stress of product.
6. Reasons and solutions for easy cracking of metal inserts in product.
When inserts are placed in product, hot melt meets cold inserts during injection molding, which will form internal stress, reduce strength of product and make it easy to crack. During production, inserts are preheated.
7. Rationality and selection method of mold exhaust point.
If mold exhaust point is unreasonable, it will not only fail to achieve exhaust effect, but will cause product to deform or change in size, so mold exhaust point should be reasonable. When selecting mold exhaust point, exhaust should be opened at the place where product is finally full of glue and place where product is trapped and burned.
8. Reasons and solutions for easy brittle cracking of product.
Easy brittle cracking of product is caused by use of too much nozzle material and inferior material in product, or material stays in material pipe for too long, causing rubber material to age and product to be easy to crack. Increase proportion of new materials, reduce number of times nozzle material is recycled and used, generally not more than three times, and avoid rubber material staying in material pipe for a long time.
9. Reasons and solutions for fiber overflow in glass fiber products.
It is due to low melt temperature or low mold temperature, insufficient injection pressure, which causes glass fiber to not be well combined with plastic in glue, causing fiber overflow. Increase melt temperature, mold temperature, and increase injection pressure.
10. Effect of feed port temperature on product.
Too high or too low feed port temperature will cause machine to return material unstably, feeding amount will be unstable, size and appearance of product will be affected.
11. Reasons and solutions for white spots in transparent products.
White spots in transparent products are caused by cold glue entering product or dust in material. Increase nozzle temperature, add cold material well, and pay attention to preservation of raw materials to prevent dust from entering.
12. What is injection capacity of injection molding machine.
Injection capacity PW = injection pressure (kg/cm2) x injection volume (cm3) / 1000
13. What is injection horsepower of an injection molding machine?
Injection horsepower PW (KW) = injection pressure (kg/cm2) x injection rate (cm3/sec) x 9.8 x 100%
14. What is injection rate of an injection molding machine?
Injection rate V (cc/sec) = n/4 x d2 x r d2: material tube diameter r: material density
15. What is injection thrust of an injection molding machine?
Injection thrust F (kgf) = л/4 (D-D) x PX2 D1: cylinder inner diameter D2: piston rod outer diameter P: System pressure
16. What is injection pressure of injection molding machine?
Injection pressure

 

17. What is plasticizing capacity of injection molding machine?
Plasticizing capacity W (g/sec) = 2.5x(d/2.54)2x(h/2.54)xNxSx1000/3600/2 h = screw front end tooth depth (cm) S = raw material density
18. What is system pressure? What is difference with injection pressure?
System pressure (kg/cm2) = the highest working pressure set in hydraulic circuit, injection pressure refers to actual pressure of injection molding machine, the two are not equal.
19. Requirements for hydraulic oil for injection molding machines:
(1) Appropriate viscosity and good viscosity performance
(2) Good lubricity and rust resistance
(3) Good chemical stability, not easy to gasify into colloid
(4) Good foaming properties
(5) Low corrosion to machine parts and sealing devices
(6) Ignition point (flash point) requirements, low freezing point
20. What is effect of hydraulic oil viscosity on injection molding machines?
When system working environment temperature is high, oil with higher viscosity should be used. Otherwise, oil with lower viscosity should be used. When system working pressure is high, oil with higher viscosity should be used. Because under high pressure, sealing is more difficult and leakage is main problem. Otherwise, when system working pressure is low, oil with lower viscosity should be used. When working parts of hydraulic system run at high speed, flow rate of oil is also high. At this time, pressure loss will also increase, while leakage will be relatively reduced. Oil with lower viscosity should be used. Otherwise, when working parts move at low speed, oil with higher viscosity should be used.
21. Loosening setting.
Correct loosening position = position of rubber ring + distance of screw offside.
22. Importance of setting release position
If release position is set too high, return material will absorb oxygen, oxidize rubber and generate bubbles. If position is set too low, pressure in barrel will be high, and shear force will be too high, causing rubber to decompose and nozzle to drool. Position error cannot exceed 0.4mm.
23. Setting melt position
Melting position = product weight/(maximum stroke/maximum melt volume)
24. Main advantages of gas-assisted injection molding (GAM)
It can evacuate core of thick materials to make hollow pipes, which can save materials and shorten cycle time.
Use of gas in injection molding can make pressure evenly distributed. When plastic cools and solidifies, gas can compensate for volume shrinkage of plastic by expansion, reduce internal stress of molded product, thereby improving shape stability, eliminating deformation and warping.
25. Is piston rod outer diameter small in the middle and large at both ends?
Due to uneven shrinkage of middle-hole needle overheating, piston rod outer diameter is small in the middle and large at both ends. Middle-hole needle can be made of phosphor copper material with fast heat dissipation, and mold is vented in the middle part of product.
26. Spherical cracking after silk screen printing
Due to stress on the surface of product, it cracks after silk screen printing. Increase mold temperature to reduce stress. Annealing can be used to eliminate stress.
27. Problem of easy breakage of nozzle edge of eyeglass frame
Injection pressure and holding pressure are large, and residual internal stress on nozzle edge causes product to break easily. Try to reduce injection pressure and holding pressure, and appropriately increase mold temperature to solve it.
28. Problem of easy bursting when screwing four columns of electrical housing
Due to existence of water clamping lines on columns, product assembly columns are easy to burst. Mold increases exhaust, appropriately increases mold temperature, and speeds up injection speed to reduce water clamping line.
29. Product deformation problem
Product deformation is mainly caused by imbalance during heat shrinkage, or by product's own internal stress.
30. Bubble problem of transparent PC shell
Insufficient drying of raw materials, uneven glue thickness in products, poor mold exhaust, and easy decomposition of raw materials may all cause product bubbles. Fully dry, increase mold exhaust, and minimize uneven glue thickness.
31. Injection molding problem of magnetic materials for copiers.
High mold temperature and fast injection method should be used.
32. Product glue nozzle shrinkage problem.
Poor mold exhaust, slow injection speed, insufficient holding pressure and time can all cause nozzle shrinkage. Increase mold exhaust, appropriately increase injection speed, and increase holding pressure and time.
33. Internal stress in product causes product to burst after being placed for a period of time.
Due to residual stress in product, product bursts due to effect of stress after being placed for a period of time. Increase mold temperature during injection molding and reduce injection pressure to eliminate product stress. Product can be annealed to eliminate stress.
34. When using black masterbatch for ABS material, product is prone to breakage and peeling.
It is because too much carbon powder is used in masterbatch pigment, which causes product to peel. Replace masterbatch pigment.
35. A 180-ton 14-ounce machine produces four CD boxes with a total weight of 120 grams. Appearance is good and there is no batching, but one of them weighs 2 grams. Why?
Mold product produces four. Due to imbalance of mold glue feeding, one of products is full and dense, and it weighs 2 grams.
36. A 100-ton hydraulic elbow machine has been used for three years, and mold often cannot be opened after locking.
It is because machine elbow is worn, causing mold opening to be unbalanced, so mold often cannot be opened after locking.
37. A 7-ounce machine has been used for two years, and injection is unstable. One mold has a batching and one mold lacks glue. Oil seal and glue distributor have been replaced, and system pressure is stable, but it does not work.
Due to wear or damage of screw, return material is uneven, so injection is unstable.
38. A new 150-ton machine has been producing PP nozzle materials for half a year. The fastest melting time was 3 seconds, but now it takes 6 seconds. Return speed is slow due to wear of screw.
39. A plastic molding master was debugging a product and found that there was a lack of glue. Speed and pressure were increased a little, but product did not respond. If speed and pressure were increased a little more, there would be a flash.
Machine's clamping mechanism was worn, causing a gap in clamping, so there would be a flash.
40. A machine was used for two years. Temperature between barrels was too high. Turning off power did not work.
Because screw rod was worn and became rough, friction generated heat when material was returned during plastic molding, which made temperature in the middle of barrel too high.
41. A factory has more than a dozen new and old machines. Oil seals often leak oil. After being replaced for a period of time, they leak again?
Oil temperature is too high, which makes oil seals easy to age, damage and leak oil. Oil cylinder core is worn, causing oil seals to be scratched and leak oil.
42. Explanation of starting circuit of oil pump motor
Starting of oil pump motor adopts star-delta starting.
43. Explanation of principle of power-saving pump
Power-saving pump is a variable flow pump. When machine is in motion, pressure oil pushes mechanical action through oil valve cylinder and then returns to oil tank. When machine is not in motion, pressure oil returns directly to oil tank.
44. Explanation of output and input of electronic circuit board.
Signals of various motion sensing switches and electronic rulers are input into electronic board. After computer calculates, it outputs to oil valve to execute action.
45. Omitted
46. Causes and prevention of stubborn oil leakage of oil seal
Wear and tear of oil cylinder core causes stubborn oil leakage. Keep oil cylinder core clean to avoid wear and tear to prevent oil seal from causing oil leakage.
47. Influence of pressure and flow linearity on injection molding process
Pressure and flow linearity are proportional, which is of great significance to accuracy and stability of parameters of injection molding process.
48. Reasons and improvement measures for slowdown of production at the same time
Main reasons for slowdown of production cycle are extension of cooling time and extension of return time due to wear of screw. Improve cooling effect of mold and shorten cooling time.
Replace worn screw to shorten return time and speed up production cycle.
49. Causes and treatment plans for squealing during melting
Squealing during melting is caused by friction between screw and material or barrel. Polish or electroplate screw to make surface smooth and reduce friction, and adjust center of screw so that it does not rub against barrel.
50. Detection and adjustment method of clamping parallelism
Use four dial indicators to measure stretching length of coring column when machine is clamped to see if it is within allowable tolerance. Do not detect parallelism of coring column clamping. Then adjust large nut of coring column to adjust parallelism of clamping.
51. Causes and preventive measures for breakage of coring column
Reason for breakage of coring column is due to imbalance of clamping. Adjust parallelism of clamping to prevent coring column from breaking.
52. Analysis of cause of elbow wear
Cause of elbow wear is poor lubrication of elbow.
53. Causes and prevention of screw and rubber head breakage
Reason for breakage of screw and rubber head is that plastic has not reached melting temperature or there is an iron block in barrel that blocks screw, and high pressure during return of material causes screw and rubber head to be twisted off.
54. Analysis of easy leakage of cooler
Due to excessive acidity or saltiness of cooling water, cooler pipes are corroded, causing cooler to leak easily.
55. Installation and application of nitrogen injection
Nitrogen injection is an additional device installed in injection oil circuit. During injection, nitrogen expands rapidly to squeeze hydraulic oil, increasing flow rate of hydraulic oil to achieve rapid injection.
56. Application of gas-assisted equipment
Gas-assisted is to fill product with air to make it empty when making injection-molded products, which can reduce plastic materials.
57. Comparison between open-loop and closed-loop oil circuits
Comparison between open-loop and closed-loop oil circuits is that closed-loop oil circuit adds a pressure sensor at nozzle. When set parameters deviate from actual data, pressure sensor will feedback signal to computer, and computer will correct deviation value to make set value equal to actual value.
58. Influence of levelness on injection molding machine
Levelness of injection molding machine is of great significance to balance of machine's opening and closing molds, and plays a role in ensuring smooth operation of machine
59. Causes and preventive measures for mold plate (head plate, second plate, tail plate) rupture
Main reason for template rupture is stress in mold plate. Under action of stress, mold plate ruptures. Mold plate should eliminate stress in time during processing to prevent mold plate from rupture.
60. When using engineering plastics, solution to insufficient melting torque and insufficient injection pressure
When using engineering plastics, insufficient melting torque may be solved by increasing first-level melting motor, and insufficient injection pressure can be solved by reducing screw diameter.
61. Reasons and solutions for high temperature in the middle section of material pipe
High temperature in the middle section of material pipe is mainly caused by insufficient smoothness of screw surface and friction between screw and material. Screw surface should be polished or electroplated to reduce back pressure of return material.
62. Reasons and solutions for decrease in clamping force and loose mold in production of hydraulic elbow injection molding machine
It is mainly caused by elbow wear and aging of clamping cylinder oil seal. Replace elbow copper sleeve and replace clamping cylinder oil seal.
63. Comparison of lubricating oil and grease (butter).
Lubricating oil and grease are both used to lubricate mechanical activities of machine. Lubricating oil is easier to penetrate into mechanical moving parts than grease. Grease can adhere to mechanical moving parts for a long time.
64. Comparison between electronic ruler and decoder
Electronic ruler and decoder are both machine motion rulers. Accuracy of electronic ruler is worse than that of decoder, but it is more stable than decoder and will not change origin. Decoder is more accurate than electronic ruler, but it is not as stable as electronic ruler and is easy to change origin.
65. Analysis of increase in oil pump noise
Mainly due to oil pump wear or oil pump bearing wear.
66. Analysis of reasons why heating coil burns frequently under normal production conditions
Mainly due to poor contact of heating coil, which causes wire to burn, or heating coil heating wire is not resistant to high temperature and is easily oxidized and burns out
67. Reasons and solutions for frequent breakage of fixing screws of ejector cylinder
Mainly due to insufficient strength of fixing screws or easy vibration loosening of fixing screws. Replace fixing screws with high strength, and add anti-slip rings to fixing screws during installation to prevent screws from vibrating loose.
68. Analysis of impact of installing energy-saving inverters on machine
Installation of energy-saving inverters has an impact on stability of machine, causing machine to start pressure slowly.
69. Analysis of whitening and deterioration of hydraulic oil
Whitening of hydraulic oil is caused by water mixed in hydraulic oil.
70. Reason why melt drive shaft is easy to break and bearing is easy to damage
It is mainly caused by loosening of drive shaft fixing nut.
71: Reasons and treatment plans for excessive product weight deviation
Increase in product weight is caused by machine clamping mold not being locked, or clamping mechanism being worn. Excessive injection pressure or back pressure can cause product weight to increase. Check whether machine clamping mechanism is worn, whether mold adjustment is in place, and reduce injection pressure or back pressure.
72. Reasons and preventive measures for molds that often cannot be opened
Mold often cannot be opened. Check whether oil circuit is stuck or blocked, check whether clamping mechanism is worn and unbalanced. If mold is locked too tightly and for too long, mold will not open.
73. Analysis of causes of unstable injection end point
Mainly due to wear of flow-stop ring of injection head, injection end point is unstable.
74. Calculation of thrust of product in mold cavity
Projected area of product x injection pressure per unit area = thrust in mold cavity
75. Calculation of injection pressure and selection of screw
Injection pressure

 

D1: Cylinder inner diameter (CM) D2: Piston rod outer diameter (cm) P: System pressure
76. What are the biggest characteristics of thermoplastic and thermosetting plastics?
Thermoplastic is a material that becomes molten glue when heated to softening temperature of material, solidifies and forms after cooling, and can be reversed repeatedly.
Thermosetting is a material that solidifies and forms after heating, remains unchanged after cooling, and is irreversible.
77. Why are additives mixed in plastics?
Plastics are mixed with additives because of needs of product performance. Additives can improve many properties and functions of plastics.
78. How to set clamping force.
Clamping force is set based on projected area of product multiplied by pressure coefficient of plastic. (Refer to "Principles of Injection Molding Machine")
79. How to correctly set pressure holding switch point
Correct pressure holding switch point is when the product is filled to 98%.
80. What changes will the screw cause to plastic in screw if it rotates at high speed or low speed?
High-speed screw rotation can increase plasticization degree of molten glue. Since high-speed rotation increases shear force of molten glue, viscosity of molten glue decreases, which is beneficial to molding, but increased friction heat can easily cause molten glue to decompose and temperature to get out of control. Low-speed rotation reduces plasticization degree of molten glue, but reduces friction heat, which is beneficial to performance of plastic.
81. What changes will correct and incorrect use of back pressure cause to plastic?
Resistance of screw when rotating backward is back pressure. Purpose of setting this resistance is to make raw materials conveyed by screw. During compression process, air in raw material can be more tightly removed, raw material density will be higher, and ejected product will be more stable. Because raw material is stirred more times in material tube, melting heat will increase. For poor color mixing of finished product, it is necessary to quickly change color or plastic and finished product have air marks, and back pressure has a good effect. Therefore, if back pressure is too low, finished product is prone to internal bubbles or surface silver streaks. If back pressure is too high, raw material is prone to overheating, agglomeration at hopper discharge, screw does not retreat, cycle is extended, and nozzle overflows. There are two types of back pressure control, which use throttle valves or pressure regulating valves.
82. Reasons and solutions for formation of burrs on injection molded parts.

 

83. Why are there tiny pinholes on the surface of high-gloss products and insufficient gloss? How to solve it?
Molding conditions: Uneven melting of raw materials, partial overheating, too low nozzle temperature, overheating and decomposition of raw materials, insufficient buffer, too fast or too slow injection speed.
Plastic raw materials: Volatile gases such as water in raw materials, mixed with foreign raw materials, and volatile components in lubricants.
Mold design: Too much release agent, poor exhaust, water or oil attached to mold surface, too high or too low mold temperature, gate or runner is too small.
Injection machine: Nozzle is too small.
84. How to adjust injection molding process parameters (temperature, pressure, speed, position)?
Temperature: Temperature measurement and control are very important in injection molding. Although it is relatively simple to make these measurements, most injection molding machines do not have enough temperature sampling points or lines.
On most injection molding machines, temperature is sensed by thermocouples. A thermocouple is basically two different wires connected at the ends. If one end is hotter than the other, a small electrical signal is generated, and the hotter it is, the stronger signal will be.
Temperature Control: Thermocouples are also widely used as sensors in temperature control systems. On control instrument, desired temperature is set, and sensor display is compared with temperature generated at set point. In the simplest system, when temperature reaches set point, it will turn off, and when temperature drops, power will turn back on. This system is called on-off control because it is either on or off.
Melt Temperature: Melt temperature is very important, and shot cylinder temperature used is only a guide. Melt temperature can be measured at nozzle or using air injection method. Shot cylinder temperature setting depends on melt temperature, screw speed, back pressure, shot size and injection cycle.
If you have no experience in processing a specific grade of plastic, start with the lowest setting. For ease of control, shot cylinder is divided into zones, but not all are set to same temperature. If operation time is long or at high temperature, please set temperature of the first zone to a lower value. This will prevent plastic from melting and diverting too early. Before injection molding begins, make sure that hydraulic oil, hopper closer, mold and shot cylinder are at correct temperature.
Injection pressure: This is pressure that causes plastic to flow. It can be measured by a sensor on nozzle or hydraulic line. It has no fixed value, and the more difficult mold filling is, the higher injection pressure is. There is a direct relationship between injection line pressure and injection pressure.
First stage pressure and second stage pressure: During filling stage of injection cycle, high injection pressure may be required to maintain injection speed at required level. After mold is filled, high pressure is no longer required. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), pressure changes suddenly, which will deteriorate structure, so sometimes it is not necessary to use second stage pressure.
Clamping pressure: In order to counter injection pressure, clamping pressure must be used. Do not automatically select maximum value available, but consider projected area and calculate a suitable value. Projected area of molded part is maximum area seen from direction of application of clamping force. For most injection molding situations it is about 2 tons per square inch, or 31 trillion newtons per square meter, but this is only a low value and should be used as a very rough empirical value, because once molded part has any depth, side walls must be considered.
Back pressure: This is pressure that needs to be generated and exceeded before screw retreats. Although use of high back pressure is conducive to uniform color distribution and plastic melting, it also prolongs screw return time, reduces length of fiber contained in filled plastic, and increases stress of injection molding machine. Therefore, the lower back pressure, the better. In any case, it cannot exceed 20% of injection pressure (maximum rating) of injection molding machine.
Nozzle pressure: Nozzle pressure is pressure inside nozzle. It is approximately pressure that causes plastic to flow. It does not have a fixed value, but increases as difficulty of mold filling increases. There is a direct relationship between nozzle pressure, line pressure and injection pressure. On a screw injection molding machine, nozzle pressure is about 10% less than injection pressure. In piston injection molding machines, pressure loss can reach 50 percent.
Injection speed: This refers to speed at which mold is filled when screw is used as a punch. When injecting thin-walled products, high injection speeds must be used to completely fill mold before molten plastic solidifies and produce a smoother surface. A series of program injection speeds are used during filling to avoid defects such as spraying or trapped air. Injection molding can be carried out under open-loop or closed-loop control systems.
Regardless of injection speed used, speed value must be recorded on record sheet together with injection time. Injection time refers to the time required for mold to reach predetermined first-stage injection pressure, which is a part of screw advancement time.
Mold venting: Due to rapid filling of mold, mold must allow gas to be exhausted. In most cases, this gas is just air in mold cavity. If air cannot be exhausted, it will be melted and compressed, causing temperature to rise and cause plastic to burn. Exhaust position must be located near water stripe and final injection molding part. Exhaust position is generally a groove 6 to 13 mm wide and 0.01 to 0.03 mm deep, usually located at parting surface of one of half molds.
Holding pressure: During filling phase of injection molding cycle, high shot pressure may be required to maintain injection speed at required level. After mold is filled, it enters holding phase, when screw (acting as a punch) pushes additional plastic to compensate for plastic shrinkage. This can be done at lower or equally high pressures. Usually if high pressure is used in the first stage, lower pressure is used in the second stage. However, when injecting some semi-hard thermoplastics (such as PA and POM), crystal structure will deteriorate due to sudden change in pressure, so sometimes there is no need to use a second stage pressure.
Use of recycled plastics: Many injection molding machines use a mixture of new material and recycled plastic (commonly known as sprue material). Surprisingly, use of recycled plastic can improve performance of injection molding machine, that is, its use produces more consistent injection molded parts, but it is worth noting that recycled material is best to be dusted before use to avoid causing differences in amount of plastic fed and resulting in deviations in color distribution of injection molded parts. Exact proportion of recycled plastics to be used is determined by experimental data. This data must be obtained without affecting physical properties of injection molded parts. General experience value is between 15% and 25%.
Quality Control: Final characteristics (weight and size) of injection molded parts are closely related to production conditions: such as pad size, injection pressure and flow rate. This means that in many cases, it is possible to check whether injection molded parts are satisfactory before any measurement is actually made on injection molded parts. In each injection, selected parameters are measured and compared with set or stored values. As long as measured value is within pre-selected range, control system determines that injection molded part is acceptable. If measurement exceeds set limit, injection molded part will be discarded, or, if it exceeds it by a little, it will stop and wait for a second inspection by a qualified person. Today's injection molding machines are equipped with video recorders and computer systems, so that during injection molding, each plastic part is compared with stored required image. Each injection molded part must be compared with size and visual defects of standard injection molded part.
Recording Injection Molding Conditions: Never forget that goal of injection molding is to produce a molded part that meets quality requirements within a specified time and at a specified cost. To achieve this, it is basically to keep accurate records. On many injection molding machines, a button can do this. If there is no button, an appropriate record sheet should be completed and a sample of injection molding should be retained for future reference.
Shutdown: The most important thing is to adopt a reasonable shutdown process, which can save a lot of time and money. If you need to shut down machine, such as burning plastic, then there is no need to drain plastic. You may save cost of completely shutting down and cleaning injection molding machine.
Temporary Pause: If injection molding machine is suspended, it is necessary to spray out remaining glue or let other plastics pass through injection molding machine to clean remaining plastic in injection cylinder several times. In the case of discoloration of expected material, number of spray cleanings will increase. When making minor repairs, heater of injection cylinder must be adjusted to the lowest value to minimize possibility of thermal decomposition. On more modern injection molding machines, this process may be started automatically.
Overnight stop: Before injecting thermoplastics (such as PS), if machine has been shut down overnight, just turn off bottom slide and shot cylinder heater and spray shot cylinder clean. Clean nozzle completely and cool shot cylinder as much as possible. After injection molding machine cools down, turn off all equipment and injection molding machine is ready to heat again.
Heat-sensitive plastics: If plastic decomposes and burns in injection molding machine, it will eventually change color and turn injection molded parts into waste. In this case, injection molding machine must be completely shut down and sprayed clean. Prevention method is to use a plastic with higher thermal stability to spray heat-sensitive plastic, so that it can withstand long-term reheating at any time. In order to deal with problem of plastic oxidation, operator can fill shot cylinder with plastic, such as PE.
Important reasons for deformation of plastic products during molding:
(1) Thickness of finished product is different, and difference is too large, resulting in different shrinkage rates.
(2) Uneven transmission of injection pressure, caused by high and low density (gate position and type).
(3) Uneven mold temperature distribution. Cooling system should be cooler near gate, and vice versa.
(4) Molecular orientation difference is too large.
(5) Post-crystallization (crystalline plastic).
(6) Excessive internal stress.
Clamping pressure: Clamping pressure must be greater than the total pressure of plastic injected into mold. If it is too low, plastic may overflow from parting surface. Too high pressure will damage machine mold and waste electricity. Therefore, appropriate clamping force is based on principle that finished product is injected into mold without burrs on parting surface.
Screw function: Screw has functions of conveying, mixing, exhausting, dehumidifying, melting and metering for raw materials. 70% of heat required for melting plastic raw materials comes from friction heat generated when screw rotates, and 30% comes from heat supplemented by electric heater. Low viscosity, small screw, melt speed should be increased. High viscosity, large screw melt speed should be slowed down. Composite materials need to slow down speed.
Injection speed: Speed of injection mainly determines flow of raw materials in runner and cavity of mold. Too fast speed will cause overfilling, burning and sticking to mold, too slow speed will easily cause short shot shrinkage, obvious joint line, and must be adjusted in sections according to actual needs.
Injection pressure: Injection pressure and injection speed have some common influences, both of which determine how raw materials in mold can flow evenly, thoroughly and appropriately. Too low pressure will cause short shot shrinkage, too high pressure will cause burrs, sticking to mold, residual internal stress, deformation, cracking, easy damage to mold, machine, etc. in the future.
Raw material temperature: Heat and temperature required to properly melt raw materials during molding are different due to different melting temperatures and specific heats of each raw material. If temperature is too low, raw materials will melt unevenly, resulting in short shots, uneven color, and high internal stress in finished product. If temperature is too high or too long, finished product will easily produce hair due to good fluidity, and finished product will shrink due to difference in cooling temperature. In severe cases, raw materials will decompose, deteriorate or even burn.
Mold temperature: Raw material will transfer a large amount of heat into mold, and finished product will dissipate part of heat into air. Therefore, in order to keep mold at a certain constant temperature, chilled water, cold water, hot water, hot oil or electric heating rods are passed through mold to balance heat in and out of mold and maintain a certain constant temperature. If mold temperature is too low, finished product is prone to short shots, rough surfaces, high internal stress, and sticking to mold. If mold temperature is too high, finished product is prone to shrinkage and sagging, and cycle is extended. Therefore, cooling time and mold temperature can be set based on experience.
Necessity of temperature control:
1. For formability and forming efficiency
High mold temperature has good fluidity and needs to extend cooling time of finished product.
Low mold temperature shortens curing time and improves efficiency.
2. For physical properties of molded products
High mold temperature, high crystallinity, better surface properties
Low mold temperature, rapid solidification of materials, high molding pressure, causing residual stress. Uneven crystallization, easy to cause post-product formation, unstable size,
3. For preventing deformation of finished products
Insufficient cooling, shrinkage and sinking.
Uneven cooling, uneven shrinkage, causing warping and distortion
Different thickness, density and shrinkage.
4. Mold temperature control type
1. Cooling between 8℃-15℃ in freezer, pay attention to sweating and rusting.
2. Water temperature machine within 96℃, directly add water source.
3. Oil temperature machine within 150℃, oil temperature circulation and indirect water cooling,
4. Electric heating plate and rod within 200℃, be careful of leakage.
Influence of mold temperature on injection molding:
Mold temperature is the most important variable in injection molding----no matter what kind of plastic is injected, it must be guaranteed that mold surface is basically wet. A hot mold surface keeps plastic surface liquid long enough to build pressure in cavity. If cavity fills and cavity pressure can press soft plastic against metal before frozen skin hardens, cavity surface replication is high. On the other hand, if plastic entering cavity under low pressure pauses, no matter how short, its slight contact with metal can cause stains, sometimes called gate stains.
For each plastic and plastic part, there is a mold surface temperature limit beyond which one or more undesirable effects may occur (for example, component can flash). Higher mold temperatures mean less flow resistance. On many injection molding machines, this naturally means faster flow through gate and cavity, and because injection flow control valves used do not correct for this change, faster filling causes higher effective pressures in runner and cavity, which can cause flash. Because hotter model does not freeze plastic that enters flash area before high pressure is built, melt can flash around ejector pin and overflow into parting line gap. This suggests need for good injection rate control, which some modern flow control programmers do provide.
In general, increasing mold temperature reduces condensation layer of plastic in cavity, allowing molten material to flow more easily in cavity, resulting in greater part weight and better surface quality. At the same time, increasing mold temperature also increases part tensile strength.
Mold insulation methods: Many molds, especially engineering thermoplastics, operate at relatively high temperatures, such as 80 degrees Celsius or 176 degrees Fahrenheit. If mold is not insulated, heat lost to the air and injection molding machine can easily be as much as shot cylinder. So insulate mold-aircraft frame plate and, if possible, surface of mold. If considering a hot runner mold, try to reduce heat exchange between hot runner section and cooled injection molded part. This method can reduce energy loss and preheating time.
85. Generation of internal stress and solutions
Generally, before injection molding of finished products, pressure inside finished products is about 300kg/cm2-500kg/cm2. If injection pressure is too high due to improper adjustment, injection pressure will gradually decrease due to resistance between runner, gate, and finished products, and finished products will gradually cool down. However, there are different pressures at injection port and far end of finished products. After a period of heat contact, finished products will gradually release internal stress and cause deformation or rupture. When internal stress is too high, annealing can be implemented to solve it.
Generation of internal stress:
(1) Overfilling. (2) Uneven thickness, gaet is opened in thin part. (3) Too high density makes demolding difficult. (4) Strain around embedded parts, which is easy to cause cracking and large difference between hot and cold, resulting in different shrinkage. In order to fill mold around embedded parts, a large pressure must be applied, resulting in excessive residual stress. (5) Direct gates with thin and shallow gates are very likely to have residual stress. (6) Crystalline plastics, if cooled too quickly, internal stress is not easy to release.
Solutions and countermeasures:
(1) Increase material temperature and mold temperature, and set them within standard conditions of each raw material. (2) Shorten holding time. (3) For non-crystalline plastics, holding pressure does not need to be too high, because it is less likely to shrink. (4) Wall thickness design should be uniform, and gate should be opened at thick part. (5) Ejection should be uniform. (6) Implants should be preheated (inserted with clamps or gloves). (7) Avoid mixing with new materials. For example, PC is easy to decompose with water. If mixing is required, it must be thoroughly dried. (8) Enlarge vertical gate, horizontal runner, gate, etc. to reduce flow resistance, so that molded product can be easily conveyed from a distance. (9) Products that have been formed can be annealed according to conditions of 2-2-1. (10) Increase nozzle diameter. Long nozzles need to be controlled by heating plates. (11) Engineering plastics and those with glass fiber need to be molded at a mold temperature of more than 60℃.
86. Causes and countermeasures of warping of injection molded parts:

 

87. How to set screw advance time?
Injection cycle: Injection cycle refers to the time required for injection molding machine to complete a specific set of actions. Therefore, action time of each part may affect the entire cycle time. To achieve purpose of shortening cycle time and improving production efficiency, each part of action should be considered separately to identify part that may shorten time, so that each part can often save a little time. Although this saving may be small, when these times are added together, shortened time is very significant from perspective of the overall shortened percentage.
Idle time of injection molding machine: Idle time is time required for injection molding machine to complete a complete cycle when it is idle, that is, there is no plastic in injection molding machine. Regardless of size and type of injection molding machine, you should first understand idle operation of injection molding machine when you try to change operation, because it helps injection molder determine whether a particular injection molding machine is capable of producing or maintaining the output at a high output. So before trying to reduce operation time, consider whether operation time can be reduced in terms of injection molding machine status, age and idle time.
Table: General injection molding machine idle time

 

Cooling time factor is the longest part of injection molding cycle, but it is part where significant savings can be made. Although it can be calculated, it is usually determined empirically, such as gradually reducing cooling time until non-deformed injection molded parts are continuously produced. During cooling stage, sufficient time is required to retract screw (sometimes called screw reset or metering time) to refill plastic in injection cylinder (place injection molded object in mold again). Otherwise, injection molding process will not be able to proceed.
Calculating cooling time: Two main influences on controlling cooling time are: 1. Solidification time of thermoplastic being processed. 2. Design of cooling channels in mold.
Many injection molders rely on mold designers to determine type and amount of cooling required for a specific mold at any time, but cooling system used is not enough. Cooling energy required for mold must be calculated to obtain specified and operating time.
By calculating solidification time of a specific injection molded part and plastic combination, value obtained may become basic cooling requirement for a given mold.
Table: Cooling time (seconds) of different material thicknesses of common plastics

 

Note: Above calculated value is time required for material to cool to mold temperature, but in many cases it is the time it takes for material to cool to deformation resistance temperature. This time determines whether injection molded part can be ejected without deformation. Therefore, above value can be understood as maximum value.
Setting of screw forward time (SFT): Calculate mold filling time, add 0.5 seconds to it, and produce about 5 injection molded parts at this setting. Each injection molded part should be weighed and/or measured, then value should be marked. Average value should be calculated and then process should be repeated as SFT time increases (for example, 0.5, 1.0, 1.5, 2.0 seconds, etc.). Time is increased until average weight or measurement of injection molded parts remains unchanged, which gives correct SFT time.
Effect of gate size on SFT: For above process to be efficient, appropriate size gate should be used for each injection. Gate hole should not be too small to prevent gate from freezing before mold cavity is filled with molten plastic and closing gate. On the other hand, gate size should not be too large. To prevent cold or semi-solid plastic from being pushed through gate and into mold, which causes pressure and cracks in gate area. For these reasons, wall thickness (depth) should be between 0.61 and 1.0t (t is wall thickness of specified part)
88. How to use traditional equipment to achieve thin-wall injection molding
In plastic injection molding, wall thickness of part is a very critical parameter. Thin-walled injection molded parts have many benefits. It reduces weight of part, production scale, material expenses and molding cycle. However, expensive high-speed injection molding machines must be used to manufacture thin-walled products, which is not cost-effective. Whether traditional injection molding machines can do it, let's analyze this issue below.
First, we need to understand what thin-wall injection molding is. Generally speaking, thin-wall injection molding refers to an injection molded part with a surface of 50 square centimeters, and its wall thickness is 1m. This level of injection molding can be called thin-wall injection molding.
However, traditional injection molding machines often cannot meet requirements of thin-wall injection molding. Take a traditional injection molding machine for making 3mm thick parts as an example: when leading edge of molten thermoplastic flows through mold cavity, it will contact core or inner wall of cavity with a lower temperature and form a solidified thin skin. This pre-solidified skin accounts for about 20% of the total wall thickness. Inside this skin, injected molten material is still flowing forward. Obviously, if wall thickness of part is reduced and reaches level of thin wall, its cooling rate will also be accelerated, resulting in an increase in proportion of above solidified skin to the total wall thickness, that is, molten core that subsequently flows into cavity will be reduced. On the contrary, interval between condensation time of part is shortening. This makes it more difficult for material to continue to flow, making it more difficult to achieve requirement of filling part before condensation.
In order to overcome filling difficulties of thin-wall injection molding, injection molding machine is usually specially designed or modified, such as using a multi-channel injection port, applying an injection pressure of up to 241Pa and an injection speed of 1000MMS. However, these practices will cost considerable funds.
So, can we control certain process parameters on a traditional standard injection molding machine without modification to achieve requirements of thin-wall injection molding?
Answer is yes. According to reports, someone has done experiments in this regard on a traditional injection molding machine with a maximum clamping force of 90 hectares and a maximum injection volume of 170g: a mold with a fan-shaped injection insert and a sprue and a cavity is placed on this machine. Length-to-thickness ratio of insert is 140:1, and cavity thickness is 1mm. Plastics used are LexanSP7602 and Magnum9015.
Weight of product part is the only variable output value. Under same mold cavity conditions, change in part weight is obviously closely related to degree of filling of molten material in cavity during injection molding process. It is said that reliability of results of analysis of part weight changes can be as high as 95%. Therefore, experiment started with relationship between relevant process parameters and part weight. For this purpose, five pressure and temperature converters were specially installed in cavity. A data control system tracks pressure and temperature curves in cavity.
Experiment used a half fractional factorial design to study nozzle temperature, mold temperature, cooling time, injection speed and whitening holding pressure. All five parameters are said to affect part weight. In order to establish these parameters to determine their effect on part weight, injection molding was performed with different combinations of high and low values.
Experiments were conducted on two materials, PC and ABS. Experimental conditions were: respective melting temperatures, standard mold temperature and part weight, standard part tensile strength and the highest allowable injection speed. In addition, relative viscosity of two materials can also be established at different shear rates.
Experimental results are as follows:
E. When melting temperature of ABS material is increased from 260℃ to 280℃, its part weight will increase from 6.6 grams to 7.4 grams, that is, a 12% increase.
F. For PC material, when its melting temperature is increased from 290℃ to 300℃, part weight increases from 7.3 grams to 8.9 grams, that is, an increase of 22%.
G. When mold temperature is increased from 80℃ to 90℃, part weight of both PC and ABS materials increases, but PC is more sensitive, and the latter's part weight can increase from 8.4 grams to 8.8 grams, an increase of 4.8%.
H. Changes in melting temperature and mold temperature will lead to changes in part tensile strength. However, increase in melting temperature will reduce strength, while increase in mold temperature will increase strength.
1. Shortening cooling time and increasing injection speed will increase weight of PC material parts, while ABS material is not affected by these two parameters:
●Result analysis:
1. For PC material, melting temperature, mold temperature, cooling time and injection speed are all key parameters affecting part weight; while for ABS, parameters affecting its part weight are only melting temperature and mold temperature.
Ⅱ. Increase in melting temperature will give material higher thermal energy and reduce viscosity of material, making it easier for molten material to flow. It forms a longer flow length and fills cavity more smoothly. However, if melting temperature is too high, it will cause material to degenerate and downgrade. Therefore, this parameter can only be used to ensure filling of cavity within upper limit allowed by material.
Ⅲ. Increase in mold temperature will reduce condensation layer of material in cavity, making it easier for molten material to flow in cavity, thereby obtaining a larger part weight and better surface quality.
IV. Shorter cooling time allows molten material to stay in container for a shorter time and reduces possibility of degradation. It is believed that reducing wall thickness by 50% will lead to a 4-fold reduction in cooling time. In addition, cooling time constitutes about 70% of molding cycle, and its reduction means improved production efficiency.
V. Machine injection should be as large as possible. Because this also helps to reduce residence time of molten material in container.
VI. Increasing injection speed will also reduce viscosity of molten material relatively. This is result of pseudoplastic effect when shear becomes thinner. At the same time, this shear heating only occurs in a fraction of a second, which is insignificant for causing significant degradation.
Ⅶ. Although increase in injection speed will reduce viscosity of PC materials and increase weight of parts, increase in part weight is much less than when melting temperature increases. However, since it can also make material less prone to degradation, increasing injection speed still has its advantages.
Ⅷ. Change in injection speed will hardly have any effect on ABS materials, because its relative viscosity has not decreased significantly at this time.
By changing some process parameters under conditions of traditional injection molding machines, effect of increasing part weight has been achieved. This result actually reflects increase in ability of plastic to fill a 1mm cavity in a molten state, that is, ability to form thin walls has been improved.
Based on experimental results, it is also possible to process thin-walled parts on traditional injection molding machines. During operation, injection speed can be adjusted to maximum upper limit allowed. On this basis, two parameters can be increased as much as possible according to maximum melting temperature limit and maximum temperature standard of mold recommended by material. This is main countermeasure to achieve high-quality thin-wall injection molding with low cost on traditional injection molding machine.
89. Precision injection molding and ordinary injection molding
Characteristics of precision injection molding are: size of injection molded parts requires high precision and small tolerance. To achieve precision requirements, conditions that must be met are:
J. Mold material should be good, rigidity should be sufficient, size accuracy of cavity, finish and positioning accuracy between mold plates should be high.
K. Use a precision injection molding machine.
L. Use a precision injection molding process.
M. Choose materials suitable for precision injection molding process.
Most commonly used precision injection molding materials are as follows: POM\POM+CF (carbon fiber), POM+GF (glass fiber), PA\FRPA66 (reinforced PA), PC, etc.
An important method to ensure dimensional accuracy of injection molded parts during precision injection molding is to control shrinkage of injection molded parts, including: thermal shrinkage, phase shrinkage, orientation shrinkage, compression shrinkage and elastic recovery of injection molded parts.
Characteristics of ordinary injection molding are: dimensional accuracy requirements of injection molded parts are not high, generally based on standard of assembly, and appearance requirements of injection molded parts are relatively high. If necessary, secondary processing (such as oil injection) may be used to improve appearance defects.
Ordinary injection molding does not require a particularly precise injection molding machine or specially specified materials. Generally, commonly used thermoplastics can be used for production. Therefore, ordinary injection molding technology is also widely used in modern plastic industry.
90. Injection molding machine safety operation guidelines
Injection molding machine is a high-pressure, fast-moving, and high-temperature machine that often causes operator to be careless for a while, causing irreparable personal injury and lifelong regret. Injection molding machine is dangerous in every operation, especially when opening and locking mold. To avoid danger, operator must pay attention to following safety operation issues during operation:
N. Keep injection molding machine and its surroundings clean.
O. Keep space around injection molding machine as unobstructed as possible. After adding lubricating oil or pressure oil, wipe off leaked oil as soon as possible.
P. Clean debris (such as glue particles) on melt cylinder before turning on electric heating to avoid fire. Unless it is necessary to repair machine, heat insulation protective cover on melt cylinder must not be removed at will.
Q. Check whether pressing emergency button or opening safety door can terminate mold clamping during operation.
R. When shooting platform moves forward, do not use your hands to remove melt leaking from nozzle to avoid getting your hands caught between shooting platform and mold.
S. When cleaning barrel, nozzle temperature should be adjusted to the most appropriate higher temperature to keep nozzle unobstructed, then use a lower injection pressure and speed to remove remaining glue in barrel. When cleaning, do not directly touch freshly injected glue with your hands to avoid burns.
T. Avoid leaving heat-sensitive and corrosive plastics in barrel for too long. Follow shutdown and cleaning methods provided by plastic supplier. When replacing plastics, ensure that mixing of new and old plastics will not produce chemical reactions (for example, mixing POM and PVC in sequence will produce toxic gases), otherwise other plastics must be used to remove old materials in barrel.
U. Before operating injection molding machine, check whether mold is firmly installed on dynamic platen and head plate of injection molding machine.
V. Pay attention to whether ground wire and other wiring of injection molding machine are connected securely.
W. Do not cancel safety door or safety door switch in order to increase production speed.
X. When installing mold, lifting ring must be fully screwed into mold lifting hole before it can be lifted. After mold is installed, length of injection molding machine safety rod should be adjusted according to size of mold, so that when safety door is opened, machine safety block (mechanical lock) falls down to block injection molding machine from locking mold.
Y. During normal injection molding production process, it is strictly forbidden for operator to remove injection molded parts from top or bottom of injection molding machine without opening safety door. When inspecting mold or temporarily stopping production, oil pump motor of injection molding machine should be turned off in time.
Z. When operating injection molding machine, if it can be operated by one person, multiple people are not allowed to operate it. It is forbidden for one person to operate control panel while another person adjusts mold or performs other operations.
91. What are maintenance points for electrical system, hydraulic system, clamping part, and injection part of injection molding machine?
Injection molding machine production is generally 24-hour operation (shift system). Except for reduction of orders or holidays, it will not stop. For machines that are in working state for a long time, we must do a good job of maintenance and strive to find and solve problems before machine fails. Otherwise, once machine fails, it must be stopped and repaired, which seriously affects production and delays delivery. Therefore, it is particularly important to do a good job of maintenance of injection molding machine.
To do a good job of maintenance of injection molding machine, maintenance content must be classified according to frequency of possible failures. Content with the highest frequency is included in daily maintenance, and content with a slightly lower frequency is included in weekly maintenance. And so on, work content of monthly maintenance and annual maintenance is classified.
After maintenance content is determined, it is best to arrange a dedicated person to be responsible for it to ensure that determined maintenance work is carried out effectively according to planned arrangement. When each maintenance work is completed, necessary records must be made so that there is a basis for evaluation of machine in future work.
For hydraulic injection molding machines, maintenance work is mainly carried out from following aspects:
A. Maintenance of electrical systems
(1) When checking high-voltage components in the machine, main power should not be turned on unless necessary.
(2) When replacing mold, it is impossible to let cooling water flow into control box.
(3) Check whether temperature in control box is too high, which will affect normal operation of electronic board.
(4) When replacing relay, specified voltage relay should be used.
B. Maintenance of hydraulic system
Pressure temperature of injection molding machine should be maintained between 30-50 degrees Celsius. If oil temperature exceeds 60 degrees Celsius, following problems will occur:
(1) Pressure oil deteriorates due to oxidation.
(2) Viscosity of pressure oil decreases, causing damage to oil pump, oil leakage, pressure drop and working efficiency of pressure relief valve.
(3) Accelerate aging of oil seal.
(4) Prevent cooling water from leaking into oil cylinder, and pay special attention to check whether there is water leakage inside oil cooler. Oil cooler should be removed and cleaned once every 6 months.
(5) Pressure oil of injection molding machine must be replaced every 3000-4000 hours of operation. When replacing pressure oil, do not mix new and old oils. At the same time, unscrew oil screen in oil cylinder for cleaning.
(6) If oil valve fails due to valve core being blocked by foreign objects, valve core should be removed from oil valve and cleaned with diesel or kerosene (or immersed in clean pressure oil for cleaning), then foreign objects should be removed with compressed air. Unless it is determined that oil valve is blocked by foreign objects and causes injection molding machine to fail, oil valve should not be removed casually.
C. Maintenance of clamping part
(1) Hinge of clamping part has a long working life, but each moving part should be properly lubricated, otherwise hinge will wear and reduce its life.
(2) Keep four coring columns clean.
(3) Keep sliding feet and rails of moving mold plate clean and lubricated.
(4) Avoid using clamping pressure close to or exceeding working pressure.
(5) When adjusting mold, do not use a very fast clamping speed.
(6) Control clamping stroke position at the most appropriate position to reduce impact on machine during clamping.
D. Maintenance of injection molding part
(1) Keep guide rod of injection molding platform lubricated and clean.
(2) Keep surface of injection molding platform clean and dry.
(3) Do not put anything into hopper except plastic, pigment and additives. If you use return material, you must add a hopper magnetic frame to hopper to prevent metal fragments from entering melt cylinder.
(4) When melt cylinder has not reached preset temperature, do not start melt motor, and do not use reverse (loose) action to avoid damaging rotating system components.
(5) Before using special plastics, consult plastics manufacturer to find out which injection screw is more suitable for this type of plastic.
(6) Use correct methods for replacing plastics and cleaning melt cylinder provided by plastics manufacturer.
(7) Periodically check various parts of injection molding platform, tighten loose parts, and ensure that two injection cylinders are installed in a balanced manner to avoid damage to injection cylinder oil seal, resulting in oil leakage and wear of cylinder pump core.
(8) Regularly drain grease of the hydraulic motor bell assembly and replace it with new grease.
(9) When melt temperature is normal but black spots or discoloration continue to appear on melt, check whether injection screw rubber ring and rubber medium are damaged.
(I) Repair starts with maintenance
(II) Mechanical maintenance
1. Grease
(III) Composition of machine:
2. Add lubricating oil
3. Check safety door switch
1. Mechanical part:
(1) Mechanical parallelism adjustment (2) Mechanical assembly and disassembly (3) Use and protection of mold plate teeth
2. Oil circuit:
(1) Oil circuit diagram analysis
(2) Oil system application
(3) Oil pump assembly and disassembly
(4) Oil seal replacement, how to use the simplest method
3. Circuit:
(1) Wiring method and working principle of each electrical component
High current contactor, Timer, Relay, Thermometer, Approach starter
4. Additional devices, core pullers and pick teeth
5. Machine modification and machine operation
6. Common mechanical failure analysis
92. Causes and solutions for black spots and dark brown stripes on injection molded parts
(I) Dark brown spots
Description: Injection molded parts have correct color but spots or stripes are occasionally visible.

(II) Black-brown streaks (For causes and remedies, please refer to (I) "Black-brown spots")
Description: Injection molded parts have a certain color but spots or streaks are occasionally visible.
93. What are causes and solutions for brittle cracking of injection molded parts?
Description: Injection molded parts break during ejection, or are prone to breaking or cracking during handling.

94. Causes and solutions for bubbles (trapped air) in injection molded parts?
Description: If there is gas (volatile substances) in melt, there will also be bubbles in injection molded part when holding pressure disappears.

95. Causes and solutions for burn marks on injection molded parts?
Description: Discolored plastic (from yellow to black), usually appears at the end of runner and/or where air is compressed.

96. Causes and solutions for injection molded parts sticking in mold
Description: Injection molded part sticks in mold, making it very difficult to remove injection molded part

97. Causes and solutions of injection molded part cracks
Description: Small cracks or fissures on the surface of injection molded part, which form a white/silver appearance on transparent injection molded parts.

98. Causes and solutions for plastic degradation in injection molded parts
Description: Molded part or some part of molded part has changed color: color usually darkens at location of degradation, from yellow to orange to black.

99. Causes and solutions for flashing of injection molded parts
Description: There is excess material on molded part, such as corners or peripheral fins, which usually appear on parting line of mold part or seam or hole of mold.

100. Causes and solutions for incorrect shrinkage of injection molded parts
Description: Injection molded part does not show normal shrinkage value associated with plastic.

101. Causes and solutions for rough surface of injection molded parts
Description: Inconsistent surface finish on molded part, some parts glossier than others.

102. Causes and solutions of silver streaks in injection molded parts
Description: This is also called (mica mark), bubbles. Surface finish of injection molded part is inconsistent in some places, and a silver surface appears.

103. Causes and solutions for wrinkles on the surface of injection molded parts
Description: Flow marks often appear in the area around gate or surface wrinkles appear in the end area of plastic flow.

104. Causes and solutions for color distribution deviation of injection molded parts
Description: Color of injection molded parts is not uniform, that is, there are differences in shades, color stripes or patches.

105. Causes and solutions for decrease in strength at fusion line of injection molded parts
Description: Injection molded parts contain fusion lines, and injection molded parts are damaged or broken at these locations during ejection or use.

106. Causes and solutions for insufficient filling of injection molded parts
Description: Injection process is not complete because mold cavity is not filled with plastic or some details are missing during injection process.

107. Causes and solutions for injection molded part size differences
Description: Changes in weight and size during injection molding process exceed production capacity of mold/injection molding machine/plastic combination.

108. Causes and solutions for shrinkage marks on injection molded parts
Description: Usually related to surface marks (see also 21 "cavitation"), are formed when plastic shrinks and detaches from mold surface.

109. Causes and solutions for stains on molded parts
Description: Usually related to gate area: its surface is dull and sometimes streaks can be seen.

110. Causes and solutions for sticking of injection molded parts
Description: Gate is held by gate sleeve.

111. Causes and solutions for voids in injection molded parts
Description: May be easily seen in "air traps" in transparent injection molded parts, but can also occur in opaque plastics. This is thickness dependent and is often caused by plastic shrinking away from center of injection molded part.

112. Causes and solutions for bending of injection molded parts
Description: Shape of injection molded part is similar to mold cavity but is a distorted version of mold cavity shape.

113. Countermeasures for defects of injection molded parts
Description of defects: So-called defects of injection molded parts refer to failure of injection molded parts to meet quality requirements of original design and use of injection molded parts during injection molding process. When describing defects, a possible cause may be included. For example, undermolding, that is, incomplete injection molding, may be described as insufficient filling pressure of mold or insufficient plastic filling mold. It is a useful method to try to describe it in the simplest terms without involving any possible causes. With this method, we will not have any prejudice when we look for cause of defect in next step.
Finding cause of defect: This can be a long process because it requires considering processing of plastics, injection molding machines, and molds. Following is a recommended guide:
A. Plastics: Check grade or type, check for impurities, and confirm whether it meets manufacturer's specifications. If defect is obvious on several batches of plastics from same manufacturer or same plastic from another supplier, it means that there is no problem with plastic. Observe effect of recycled materials. Pay special attention to its different processing characteristics from same new material.
B. Injection molding machine: Check all functions of injection molding machine and consider any factors that may affect pressure, temperature, ratio and time. If defect occurs intermittently, this usually indicates an error in operation of injection molding machine, such as temperature fluctuations caused by a bad thermocouple. If defect occurs in same position of a single-cavity mold, this suggests that cause of problem lies in device of injection cylinder (such as a return valve) or in control adjustment of injection molding machine.
C. Mold: Make sure that mold is properly installed, at correct temperature, and that all parts are running smoothly. If defect always occurs in same or several cavities of a multi-cavity mold, defect is usually in feed system (that is, runners or gates of these cavities).
D. Processing process: Check whether pressure, temperature and time are set according to recommendations of plastic supplier. If defect disappears when mold is used on another injection molding machine, defect is probably due to consistency of processing conditions used with original machine. If defect disappears when another person operates injection molding machine, defect may be caused by human error. Speed, management of injection molding operation and dwell time of safety door opening and closing should be checked.
●Methods to solve defects in injection molded parts
Appearance of defects in injection molded parts will undoubtedly bring quality risks to injection molding production. To solve these defects, we can start from following aspects:
A. Determine impact of defect: If defect makes injection molded part unusable or unsaleable, this defect must be eliminated. If it is only a weak impact, it may not be necessary to completely eliminate it.
B. Determine where responsibility lies: This may only be an academic research interest, but if defect reappears, operator, plastic, injection molding machine, mold and processing process need to be reviewed.
C. Take action to avoid defects: Failure to take appropriate action will produce inferior injection molded parts and thus have a devastating impact on profitability of project.
D. Take steps to prevent defects from recurring: Record all conditions when confirming elimination of defects. Indicate repair or change of mold or injection molding machine, as well as change in type, grade and quality of plastic. If recycled materials are used, indicate quality of proportion used.
These steps to deal with defects may seem redundant, but unless all methods are taken into account, no method of finding defects is perfect. Although defective products can be recycled, it is uneconomical to manufacture defective products.
114. Injection Molding Defects and Remedies
People desire a logical, systematic approach to dealing with defects, and many actual injection molders have their operating strategies. The most important thing in compiling these strategies is to ensure that semantics of all terms are clear and can be understood by all concerned. To clearly describe defects of injection molded parts, all possible causes should be examined. Its effects should also be considered, and when cause is determined, necessary steps should be taken to reduce and prevent its recurrence. There are six strategies.
1. Name defect. Because defects have different names, once you decide on a name, don't change it.
2. Describe defect. When describing defect, you may include a possible cause. For example: "Low injection", that is, incomplete injection, may be described as "insufficient filling pressure of mold" or "insufficient plastic to fill mold". It is very useful to try to describe it in the simplest terms without involving any possible causes. In this way, we will not have any prejudices when we look for cause of defect in next step.
3. Finding cause of the defect can be a long process because it requires considering plastic, injection molding machine, and processing of mold.
Following is a recommended guide.
A: Plastic: Check grade or type, check for impurities, and confirm whether it meets manufacturer's specifications. If defect is similar in several batches of plastic from same manufacturer or same plastic from another supplier, it means that there is no problem with plastic.
B. Injection molding machine: Check function of all parts of injection molding machine and consider any factors that may affect pressure, temperature, ratio and time. If defect occurs intermittently, it usually indicates that injection molding machine is operating incorrectly, such as temperature fluctuations caused by a bad temperature sensor. If defect occurs in same position of a single-cavity mold, it suggests that cause of problem lies in device of shot cylinder or control adjustment of injection molding.
C. Mold: Make sure mold is properly installed, at correct temperature, and all parts are running smoothly. If defect disappears and defect always occurs in same or several cavities of multi-cavity molds, then defect usually occurs in feed system (i.e., runners or gates serving these cavities).
D. Processing: Check whether pressure, temperature and time are set according to plastic supplier's recommendations. If defect disappears when mold is used on another injection molding machine, defect is likely due to consistency of processing conditions used with original machine. If defect disappears when another person operates injection molding machine, defect may be caused by human error. Check speed, management of injection molding action and dwell time of safety door opening and closing.
4. Determine impact of defect. If defect makes injection molded part unusable or unsaleable, defect must be eliminated.
5. Determine where responsibility lies. This may only be of academic research interest, but if defect recurs, operator, raw materials, injection molding machine, mold and processing process must be reviewed.
6. Take action to avoid defect. Failure to take appropriate action will produce inferior injection molded parts and therefore affect profitability of project.
In each example of an injection molding defect, defect of injection molded part is usually caused by following factors: operation of injection molding machine, function of mold or processing performance of plastic, etc. Problem or defect usually has more than one cause. Each of following defects is provided with a suggested remedy under headings of injection molding machine, mold and plastic.
115. Causes and countermeasures of deformation of injection molded parts
Description: Injection molded part does not accurately replicate dimensions of mold cavity. Some parts are broken, bent or deformed.