Injection molding defect form, this is too comprehensive! Please forward to each other

Time:2023-12-04 20:14:11 / Popularity: / Source:

Problem Possible Cause Solution
Small black spots Inner wall of glue barrel is burnt and glue pieces fall off 1. Clean inner wall of barrel (remove screw and clean it)
2. Use harder glue to clean inner wall of barrel (for small dock machines, use cleaning agent and acrylic, etc.)
3. Prevent rubber material from being exposed to high temperatures for a long time (material temperature should not be too high, and material storage should not be too much)
Black spots Air brings dirt or there is air in mold cavity. 1. Capping hopper.
2. Glue should be sealed well and kept clean.
3. Increase or decrease temperature of melt barrel and mold to change speed of rubber entering mold cavity.
4. Reduce injection pressure or speed.
Black stripes Barrel or screw is not clean, and raw materials are not clean. Clean barrel, screw and handle rubber carefully to avoid contamination.
Barrel can partially overheat rubber material. Make each section of barrel evenly heated
Cold rubber materials are burned when they rub against each other or against wall of barrel. A. Add rubber material with external lubricant; B. Add a small amount of white mineral oil to rubber material; C. Increase temperature of rear section of barrel.
Nozzle is overheated and burns rubber material. Reduce temperature of nozzle.
Temperature of nozzle changes greatly Use a thermostatic controller to control electric heat
Yellow dot, yellow line, brown stripe discoloration Sol barrel is fully or partially overheated. A Reduce temperature; B Reduce screw speed; C Reduce return back pressure
Rubber sticks to inner wall of barrel or nozzle and even burns. Clean nozzle or barrel screw.
Rubber stays in barrel for too long Shorten injection molding cycle
There is a dead space in barrel. Replace screw.
Product matt white Raw materials are wet. Dry raw materials for reproduction.
Bubbles Insufficient filling of mold cavity. Reason:  
A Cut surface of product is thick and there are protruding lines on mold cavity wall. A Redesign mold.
B Injection pressure is too low. B Increase injection pressure.
C Injection time is too long. C Increase injection time.
D Insufficient material feeding D Increase injection speed and increase width of water inlet.
Rubber material is moist. Avoid excessive temperature changes and dry rubber particles before molding.
Mold temperature is uneven. Rearrange water transport channels to make mold temperature consistent.
Rupture or cracking Filling mold too firmly A Reduce speed; B Reduce pressure; C Reduce injection time
Mold temperature is too low. Increase mold temperature.
Inappropriate demoulding design such as angle and undercut position. Modify mold
Improper ejector pin or ring. Press and release ejector pin again so that product can be ejected smoothly from mold.
Too much material is used (graining nozzle). Use less or no graining nozzle or nozzle
Product dimensional changes Molding conditions are unstable. A Adjust operating conditions until maximum smooth operation is obtained.
B Length of plastic molding time depends on operating conditions of plastic molding workers.
Injection molding conditions are inconsistent with normal conditions. A Correct injection molding conditions that are not used in normal times.
B Check injection molding temperature, pressure, mold temperature, molding time, feeding material, voltage.
Ambient air temperature is not fixed when inspecting plastic parts. A Keep ambient air temperature constant.
B Measure and inspect products under same conditions.
Glue leakage Rubber material is too hot. A Reduce temperature of rubber material.
B Reduce screw speed and back pressure
Injection pressure is too high. A Reduce pressure.
B Reduce pillow pressure time.
Poor contact between convex and concave surfaces of mold or nozzle. Polish mold nozzle again.
Injection platform is not pressurized enough. Check injection platform injection pressurization system of machine.
Glue dripping from injection nozzle when opening mold Rubber material is overheated A Reduce temperature of furnace nozzle
B Reduce barrel temperature.
Injection plastic material is not dry enough. Dry rubber material.
Flow lines and corrugations on plastic surface Material is not hot enough. Increase temperature of rubber material
Mold temperature is not hot enough. Increase mold temperature.
Injection pressure is too low Increase injection phenomenon.
Injection speed is too low Increase injection speed
Gate is too small, causing rubber to spray in mold cavity. Expand gate and reduce injection pressure.
Thickness of cut surface of plastic product is uneven. A Design plastic part so that thickness of cut surface is uniform.
B Remove raised lines on product
Insufficient exhaust. Increase exhaust.
Plastic is layered at the gate. Mold temperature is too low. Increase electric heat and increase mold temperature.
Injection speed is too slow. Increase injection speed.
Insufficient injection pressure. Increase injection pressure.
Injection time is too long. Shorten injection time.
There are impurities in glue. A Clean .glue barrel.
B Avoid mixing with other plastics.
Too much lubricant in mold Clean mold surface and use a small amount of lubricant (release agent).
Small nozzle. Adjust size of nozzle to achieve good pressure control.
There are streaks around nozzle. Material temperature is too high and flow rate is too fast. Reduce temperature and injection speed.
Molding cycle too long. Materials with high temperature are prone to shrinkage. A Lower temperature
B Reduce screw rotation speed or back pressure.
Mold temperature is too high Reduce mold temperature.
Unstable molding time (worker problem). Use timers and strengthen control of workers’ operating speed.
Temperature and time required for plastic melting are not enough. Increase temperature and extend time.
Insufficient cooling in thicker parts of plastic parts. A Change position of cooling water channel in mold.
B Use a chiller
Product deformation Injection pressure is too high Reduce injection pressure
Pressure holding time is too long. Reduce pressure holding time.
Difference between front and rear molds is too large. Make front and mold temperatures consistent
Water entry level is not designed regularly. Re-design water entry position.
Thickness of cut surface of plastic part is uneven. Keep thickness of cut surface as uniform as possible.
Mold temperature is too low. Increase mold temperature.
Material temperature is too low. Increase material temperature.
Screw back pressure is too high. Reduce screw back pressure.
Plastic parts stick to mold Injection pressure or melt barrel temperature is too high. A Reduce injection pressure or barrel temperature.
B Reduce screw rotation speed or screw back pressure.
Feed too much. Reduce feed.
Injection time is too long. Reduce injection time.
Insufficient cooling of mold (thermal expansion of plastic parts). Extend cooling time of mold.
Plastic parts stick to male mold due to shrinkage. Reduce cooling time so that front and rear molds have different temperatures.
Plastic parts in mold are not hardened. Extend holding pressure and strengthen cooling of mold.
There is an undercut in mold. Remove undercut and polish it to increase slope of demoulding part.
Air pressure in deep part of mold cavity is small (vacuum state). Set up a suitable exhaust channel.
Inner wall of mold is not smooth enough. Mold cavity wall is polished again.
Cracks Material temperature is too low. Increase material temperature.
There are oil stains such as release agent on mold surface. Wipe clean mold wall and spray no release agent or less release agent.
Melting level is too far from water inlet level. Reset water inlet level or add several symmetrical water inlets.
Poor exhaust Add sufficient exhaust
Thickness of cut surface of plastic product changes greatly. Redesign plastic part and set appropriate water entry position.
Mold temperature is too low. Increase mold temperature.
Low injection pressure and slow speed. Increase injection pressure or speed.
Rough surface of plastic parts Low mold temperature Increase mold temperature
Low injection pressure Increase injection pressure
Injection speed is slow Increase injection speed.
There is moisture in mold wall. Clean and repair water leakage cracks or prevent water vapor from condensing on mold wall.
Use too much release agent Use no or less release agent
Use too much internal or external lubricant. Check material is suitable and reduce amount of mineral oil when mixing.
Mold surface is rough. Polish mold wall again.
Insufficient injection Each section of melt tube is not enough. Add each section of melt tube.
Mold temperature is not enough. Temperature increases mold temperature.
Insufficient pressure. Increased pressure.
Nozzle is too cold when you first turn it on. Injection more or use a flame to heat nozzle.
Uneven mold temperature. Use no or less release agent.
Air cannot be discharged from mold cavity (trapped air). Increase number and size of exhaust channels.
Insufficient injection time. Increase injection time.
Melt flow pattern flowing into multi-cavity mold cannot be appropriately parallel. Correct parallel situation.
Small gate Expand gate
Amount of melt in mold cavity is larger than that of injection molding machine. Use a larger injection molding machine.
Bright stripes on the surface of plastic parts Insufficient material temperature. Increase temperature of melt barrel.
Low mold temperature Increase mold temperature.
There is moisture in mold. Heating and drying mold.
There are stains on mold. Clean mold.
Improper rotation speed position. Adjust appropriate rotation speed position.
There is moisture in material. Dry rubber material.
Oil temperature machine Using principle of heat conduction, heat transfer oil is heated to an appropriate temperature, transported to pipe of mold of molding machine, mold is heated to required temperature, and oil temperature is controlled within required temperature range. Suitable for molding with high mold temperature requirements. Improving quality of plastic products can also increase production efficiency.

1 Analysis of causes of shrinkage and dents in injection molded parts

Injection molding defect 
During injection molding process, it is a common phenomenon for products to shrink and dent. Main reasons for this situation are:

Machine aspect

(1) If nozzle hole is too large, melt will flow back and cause shrinkage; if it is too small, resistance will be large and amount of material will be insufficient, causing shrinkage.
(2) Insufficient clamping force will cause flash to shrink, so check whether there is any problem with clamping system.
(3) If amount of plasticizing is insufficient, a machine with a large amount of plasticizing should be used to check whether screw and barrel are worn.

Mold aspect

(1) Design of parts should make wall thickness uniform and ensure consistent shrinkage.
(2) Cooling and heating system of mold should ensure that temperature of each part is consistent.
(3) Pouring system must be smooth and resistance must not be too large. For example, size of main runner, runner, and gate must be appropriate, smoothness must be sufficient, and transition area must be an arc transition.
(4) For thin parts, temperature should be increased to ensure smooth material flow, and for thick-walled parts, mold temperature should be lowered.
(5) Gate should be opened symmetrically, try to open it in thick-walled part of part, and cold slug well volume should be increased.


Crystalline plastics shrink more than amorphous plastics. During processing, amount of material should be appropriately increased, or a replacement agent should be added to plastic to speed up crystallization and reduce shrinkage dents.
(1) Temperature of barrel is too high and volume changes greatly, especially temperature of front furnace. For plastics with poor fluidity, temperature should be appropriately increased to ensure smoothness.
(2) Injection pressure, speed, and back pressure are too low, and injection time is too short, resulting in insufficient material quantity or density, resulting in shrinkage caused by excessive pressure, speed, and back pressure, or too long time, causing flash and shrinkage.
(3) When amount of material added is too large, injection pressure will be consumed; if it is too small, amount of material will be insufficient.
(4) For parts that do not require precision, after injection and pressure-holding is completed, outer layer is basically condensed and hardened but sandwich part is still soft and can be ejected. Eject mold as soon as possible and let it cool slowly in air or hot water, which can make shrinkage dents gentle and less conspicuous without affecting use.

2 Analysis of causes of vibration marks on injection molded parts

Rigid plastic parts such as PS form dense ripples centered on gate on their surfaces near gate, sometimes called vibration lines. Reason is that when melt viscosity is too high and mold is filled in a stagnant flow, front material quickly condenses and shrinks as soon as it contacts surface of mold cavity, subsequent melt expands shrunk cold material and continues process. Continuous alternation causes material flow to form surface vibration patterns as it advances.


(1) Increase temperature of barrel, especially nozzle, and mold temperature.
(2) Increase injection pressure and speed to quickly fill mold cavity.
(3) Improve size of flow channel and gate to prevent excessive resistance.
(4) Mold must be well vented and a large enough cold slug well must be installed.
(5) Do not design parts too thin.

3 Analysis of causes of swelling and bubbling of injection molded parts

Some plastic parts will soon develop swelling or bubbling on the back of metal insert or in particularly thick areas after being molded and demoulded. This is caused by expansion of gas released by plastic that is not completely cooled and hardened under action of internal pressure.


1. Effective cooling. Reduce mold temperature, extend mold opening time, reduce drying and processing temperature of material.
2. Reduce mold filling speed, shorten molding cycle, and reduce flow resistance.
3. Increase holding pressure and time.
4. Improve situation where wall surface of part is too thick or thickness changes greatly.
Injection molding defect 

4 Analysis of causes of cracking of injection molded parts


(1) Too much processing pressure, too fast speed, more filling, injection, too long pressure holding time will cause excessive internal stress and cracking.
(2) Adjust mold opening speed and pressure to prevent demolding and cracking caused by rapid forced drawing of parts.
(3) Increase mold temperature appropriately to make parts easy to demould, and lower material temperature appropriately to prevent decomposition.
(4) Prevent cracking due to lower mechanical strength due to welding marks and plastic degradation.
(5) Use release agents appropriately and pay attention to frequently removing aerosol and other substances attached to mold surface.
(6) Residual stress of part can be eliminated by annealing heat treatment immediately after molding to reduce generation of cracks.

Mold aspect

(1) Ejection must be balanced. For example, number of ejector pins and cross-sectional area must be sufficient, draft slope must be sufficient, and cavity surface must be smooth enough to prevent cracking due to concentration of residual stress in ejection caused by external forces.
(2) Structure of parts should not be too thin, arc transitions should be used as much as possible in transition parts to avoid stress concentration caused by sharp corners and chamfers.
(3) Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between insert and finished product.
(4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent formation of vacuum negative pressure.
(5) Sprue is large enough to allow gate material to be demoulded before it has time to solidify, making it easy to demould.
(6) Connection between main flow bushing and nozzle should prevent cold hard material from being dragged and causing part to stick to fixed mold.


(1) Content of recycled materials is too high, resulting in low strength of parts.
(2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing their strength and causing ejection and cracking.
(3) Material itself is not suitable for environment in which it is being processed or is of poor quality. Contamination will cause cracking.
Machine aspect
Plasticizing capacity of injection molding machine must be appropriate. If it is too small, plasticization will be insufficient and it will not be fully mixed and it will become brittle. If it is too large, it will degrade.

5 Analysis of causes of bubbles in injection molded parts

Gas in bubble (vacuum bubble) is very thin and belongs to vacuum bubble. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. Vacuum bubbles are formed due to insufficient filling of plastic or low pressure. Under rapid cooling of mold, fuel in contact with cavity is pulled, resulting in volume loss.


(1) Increase injection energy: pressure, speed, time and material volume, and increase back pressure to make mold full.
(2) Increase material temperature and smooth flow. Lower material temperature to reduce shrinkage, and increase mold temperature appropriately, especially local mold temperature where vacuum bubble is formed.
(3) Set gate in thick part of part to improve flow conditions of nozzle, runner and gate, and reduce consumption of pressure.
(4) Improve mold exhaust condition.

6 Analysis of causes of warpage deformation of injection molded parts

Occurrence of deformation, bending, and twisting of injection molded products is mainly due to fact that shrinkage rate in flow direction is greater than that in vertical direction during plastic molding, causing parts to warp due to different shrinkage rates in each direction. Also because there is inevitably a large internal stress remaining inside part during injection molding, causing warping. These are all manifestations of deformation caused by high stress orientation. So fundamentally speaking, mold design determines warping tendency of parts. It is very difficult to suppress this tendency by changing molding conditions. Final solution to problem must start with mold design and improvement. This phenomenon is mainly caused by following aspects:

Mold aspect

(1) Thickness and quality of parts should be uniform.
(2) Cooling system should be designed to make temperature of each part of mold cavity uniform. Pouring system should make material flow symmetrical to avoid warpage due to different flow directions and shrinkage rates. Appropriately thicken branch channels and main channels in difficult-to-form parts, and try to eliminate density differences, pressure differences, and temperature differences in mold cavity.
(3) Transition area and corners of thickness of part must be smooth enough and have good demoulding properties, such as increasing demoulding margin, improving polishing of mold surface, and ejection system must be balanced.
(4) Exhaust should be good.
(5) Increase wall thickness of part or increase anti-warping direction, and use reinforcing ribs to enhance anti-warping ability of part.
(6) Material used in mold has insufficient strength.


Crystalline plastics have more chances of warping than amorphous plastics. In addition, crystalline plastics can correct warping deformation by using crystallization process, which decreases as cooling rate increases and shrinkage rate decreases.


(1) Injection pressure is too high, holding time is too long, melt temperature is too low and speed is too fast, which will cause internal stress to increase and warpage deformation will occur.
(2) Mold temperature is too high and cooling time is too short, causing parts to be overheated during demolding and cause ejection deformation.
(3) Reduce screw speed and back pressure to reduce density while maintaining minimum charging amount to limit generation of internal stress.
(4) If necessary, parts that are prone to warping and deformation can be soft-shaped or de-moulded and then de-metered.

7 Analysis of color bars, lines, and scratches of injection molded parts

Occurrence of this defect is mainly a common problem in plastic parts colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye paste in terms of color stability, color purity, and color migration, distribution properties, that is, uniformity of mixing of color particles in diluted plastics, is relatively poor, and finished products naturally have regional color differences.
Injection molding defect 


(1) Increase temperature of feeding section, especially temperature at rear end of feeding section, so that temperature is close to or slightly higher than temperature of melting section, so that color masterbatch can melt as soon as possible when entering melting section, promote uniform mixing with dilution, and increase opportunity for liquid mixing.
(2) When screw speed is constant, increasing back pressure increases melt temperature and shearing effect in barrel.
(3) Modify mold, especially pouring system. If gate is too wide, turbulence effect is poor when melt passes through, and temperature rise is not high, so it is uneven and ribbon mold cavity should be narrowed.

8 Analysis of causes of transparent defects in injection molded parts

Melting spots, silver streaks, cracked polystyrene, transparent parts of plexiglass, sometimes through light you can see some sparkling filament-like silver streaks. These silver streaks are also called flash spots or cracks. This is due to stress generated in vertical direction of tensile stress. Polymer molecules undergo heavy flow orientation and folding rate difference between non-oriented parts is reflected.


(1) Eliminate interference of gas and other impurities and fully dry plastic.
(2) Reduce material temperature, adjust barrel temperature step by step, and increase mold temperature appropriately.
(3) Increase injection pressure and reduce injection speed.
(4) Increase or decrease pre-molding back pressure and reduce screw speed.
(5) Improve exhaust conditions of runner and cavity.
(6) Clean possible blockages in nozzle, runner and gate.
(7) Shorten molding cycle. After demoulding, annealing can be used to eliminate silver streaks: for polystyrene, hold at 78℃ for 15 minutes, or at 50℃ for 1 hour. For polycarbonate, heat to above 160℃ for several minutes. .

9 Analysis of causes of uneven color of injection molded parts

Main reasons and solutions for uneven color of injection molded products are as follows:
(1) Poor colorant diffusion, which often causes patterns to appear near gate.
(2) Plastics or colorants have poor thermal stability. To stabilize color tone of parts, production conditions must be strictly fixed, especially material temperature, material quantity and production cycle.
(3) For crystalline plastics, try to keep cooling rate of all parts of part consistent. For parts with large wall thickness differences, colorants can be used to mask color difference. For parts with more uniform wall thicknesses, material temperature and mold temperature must be fixed. .
(4) Shape and gate form and position of part have an impact on plastic filling situation, causing color differences in some parts of part, which must be modified if necessary.

10 Analysis of causes of color and gloss defects

Under normal circumstances, gloss of surface of injection molded parts is mainly determined by type of plastic, colorant and smoothness of mold surface. However, defects such as surface color and gloss defects, surface darkness, etc. are often caused by some other reasons. Reasons and solutions for this are analyzed as follows:
(1) Mold finish is poor, there are rust stains on the surface of cavity, etc., and mold exhaust is poor.
(2) Pouring system of mold is defective. Cold slug well should be enlarged, runner should be enlarged, main channel should be polished, runner and gate should be polished.
(3) Material temperature and mold temperature are low. If necessary, local heating of gate can be used.
(4) Processing pressure is too low, speed is too slow, injection time is insufficient, and back pressure is insufficient, resulting in poor compactness and a dark surface.
(5) Plastic must be fully plasticized, but degradation of material must be prevented, heating must be stable, and cooling must be sufficient, especially for thick walls.
(6) Prevent cold material from entering part. If necessary, use a self-locking spring or reduce nozzle temperature.
(7) Too much recycled material is used, plastic or colorant is of poor quality, mixed with water vapor or other impurities, and lubricant used is of poor quality.
(8) Clamping force must be sufficient.

11 Analysis of causes of silver streaks in injection molded parts

Silver streaks on injection molded products include surface bubbles and internal pores. Main cause of defects is interference of gases (mainly water vapor, decomposition gas, solvent gas, and air). Specific reasons are analyzed as follows:
Injection molding defect 

Machine aspect

(1) Barrel and screw are worn or there is a dead corner in material flow in rubber head and rubber ring, which will decompose due to long-term heating.
(2) Heating system is out of control, causing temperature to be too high and decomposing. Check whether there are any problems with heating elements such as thermocouples and heating coils. Improper screw design may cause loosening or easily introduce air.

Mold aspect

(1) Poor exhaust.
(2) Friction resistance of runners, gates, and cavities in mold is large, causing local overheating and decomposition.
(3) Unbalanced distribution of gates and mold cavities and unreasonable cooling systems will cause unbalanced heating, resulting in local overheating or blocking of air channels.
(4) Water leaks from cooling passage and enters mold cavity.


(1) Plastic has high humidity, too much recycled material is added, or it contains harmful debris (debris is easily decomposed). Plastic should be fully dried and debris should be eliminated.
(2) To absorb moisture from atmosphere or from colorant, colorant should also be dried. It is best to install a dryer on machine.
(3) Amount of lubricants, stabilizers, etc. added to plastic is excessive or unevenly mixed, or plastic itself contains volatile solvents. Decomposition can also occur when mixed plastics are heated to a difficult balance.
(4) Plastic is contaminated and mixed with other plastics.

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