Temperature: Measurement and control of temperature is 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 composed of two different wires connected at the end.
If one end is hotter than the other, a small electrical signal will be generated. The more heated, the stronger signal.
Temperature control: Thermocouples are also widely used as sensors in temperature control systems. On control instrument, required temperature is set, and sensor display is compared with temperature generated at set point. In this simplest system, when temperature reaches set point, it will turn off, and power will turn back on when temperature drops. This system is called on-off control because it is either on or off.
Melt temperature: Melt temperature is very important. Temperature of injection cylinder used is only a guide. Melt temperature can be measured at nozzle or using air injection method.
Temperature setting of shot cylinder depends on melt temperature, screw speed, back pressure, shot size and injection cycle.
If you have no experience with a particular grade of plastic, start with the lowest setting. For ease of processing, shot cylinder is divided into zones, but not all are set to same temperature.
If operation time is long or at high temperature, set temperature of the first zone to a lower value. This will prevent plastic from melting and diverting prematurely. 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 and can be measured by a sensor on nozzle or hydraulic line. It has no fixed value, and the more difficult mold is to fill, 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 shot pressure may be required to maintain injection speed at required level. High pressure is no longer required after mold is filled. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), structure will deteriorate due to sudden change in pressure, so sometimes it is not necessary to use secondary 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 injection 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 meganewtons per square meter. However, this is only a low value and should be used as a very rough empirical value, because once injection molded part has any depth, side wall 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 filling plastic, and increases stress of injection molding machine. Therefore, the lower back pressure, the better. In any case, it should not exceed 20% of injection pressure (maximum rating) of injection molding machine.
Nozzle pressure: Nozzle pressure is pressure inside nozzle. It is roughly 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. On a piston injection molding machine, pressure loss can reach about 10%. On a piston injection molding machine, pressure loss can reach 50%.
Injection speed: This refers to filling speed of mold when screw is used as a punch. When injecting thin-walled products, a high shooting speed must be used to completely fill mold before molten glue solidifies and produce a smoother surface. A series of program shooting speeds are used during filling to avoid defects such as spraying or trapped air. Injection molding can be carried out under an open-loop or closed-loop control system.
Regardless of injection speed used, speed value must be recorded on record sheet along with injection time, which refers to time required for mold to reach predetermined first-stage shot pressure, which is a portion of screw advancement time.
Mold Venting: Due to rapid filling of mold, mold must allow gas to vent, in most cases this gas is just air in mold cavity. If air cannot escape, it will be melted and compressed, causing temperature to rise and cause plastic to burn.
Venting points must be located near water clamp and final injection part. General venting point is a groove 6 to 13 mm wide and 0.01 to 0.03 mm deep, usually located at parting of one of mold halves.
Holding Pressure: During filling phase of injection molding cycle, it may be necessary to use a high shot pressure to maintain injection speed at required level. After mold is filled, it enters holding phase, when screw (acting as a punch) advances 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 second stage. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), crystal structure will deteriorate due to sudden change of pressure, so sometimes there is no need to use second stage pressure.
Use of recycled plastics: Many injection molding machines use a mixture of new materials and recycled plastics (commonly known as sprue materials). Surprisingly, use of recycled plastics can improve performance of injection molding machines, that is, its use produces more consistent injection molded parts, but it is worth noting that it is best to remove dust from recycled materials before use to avoid causing differences in plastic feed amount and resulting in color distribution deviations in injection molded parts.

 

Exact proportion of recycled plastics to be used must be determined based on experimental data. This data must be obtained without affecting physical properties of injection molded parts. General empirical 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 molded part is satisfactory before any measurements are actually made on injection molded part. At each injection, selected parameter is measured and compared to set or stored value.
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 rejected, or, if it exceeds it only slightly, it will be stopped 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 each molded part is compared with stored required image during injection molding.
Each injection molded part is compared with standard injection molded part for dimensions and visual defects.
Recording injection molding conditions: Never forget that purpose of injection molding is to produce injection molded parts of satisfactory quality 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, appropriate record sheets should be completed and injection samples should be retained for future reference.
Shutdowns: It is most important to adopt a reasonable shutdown process, which can save a lot of time and money. If you need to stop machine, for example to burn plastic, then there is no need to drain plastic. You may save cost of completely shutting down and cleaning machine.
Temporary stoppages: If machine is stopped, it is necessary to spray out remaining plastic several times or to run other plastic through machine to clean remaining plastic from shot cylinder. Number of sprayings should be increased if plastic fades. When performing minor repairs, shot cylinder heater should be adjusted to the lowest setting to minimize possibility of thermal decomposition. On more modern machines this process may start automatically.
Overnight stoppages: Before molding thermoplastics (such as PS), if machine is shut down overnight, it is only necessary to turn off bottom slide and shot cylinder heater and spray shot cylinder clean. Nozzle is completely cleaned, shot cylinder is cooled as much as possible, and after machine has cooled down, all equipment is turned off and machine is ready to heat again.
Heat-sensitive plastics: If plastic decomposes in injection molding machine, it can burn and eventually change color, making injection molded part a waste part. In this case, injection molding machine must be completely shut down and spray must be cleaned. Preventive method is to use a plastic with higher thermal stability to spray heat-sensitive plastic, so that it can withstand constant heating at any time. In order to deal with problem of plastic oxidation, operator can fill injection 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 injection pressure transmission, 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. Excessive pressure will damage machine, mold and waste electricity. Therefore, principle of appropriate clamping force is that finished product should not have burrs on parting surface when it is injected into mold.
Screw function: Screw has functions of conveying, mixing, exhausting, dehumidifying, melting and metering raw materials. Seventy percent of heat required for melting plastic raw materials comes from friction heat generated when screw rotates, and thirty percent comes from heat supplemented by electric heater. Low viscosity, small screw, and 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 excessive burrs, burning and mold sticking, too slow speed will easily cause short shot shrinkage, obvious joint line, and it must be adjusted in sections according to actual needs
Injection pressure: Injection pressure and injection speed have some common influence, both of which determine how raw materials in mold can be evenly distributed, thoroughly and appropriately filled in every corner. Too low pressure will cause short shot shrinkage, too high pressure will cause burrs, mold sticking, residual internal stress, deformation, cracking, easy damage to molds, machines, etc.
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 colors, and high internal stress in finished product. If temperature is too high or too long, finished product will easily produce burrs due to too 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 product
High mold temperature has high crystallinity and better surface properties
Low mold temperature causes rapid curing of material, high forming pressure, and residual stress. Uneven crystallinity can easily cause post-crystallization and unstable dimensions.
3. To prevent deformation of finished products
Insufficient cooling causes shrinkage and sinking.
Uneven cooling causes uneven shrinkage, which causes warping and distortion.
Different thickness, density and shrinkage will also be different.
4. Mold temperature control type
1. Refrigerator Cooling between 80℃-15℃, 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 indirectly cools with water.
4. Electric heating plate and rod within 200℃, be careful of leakage.