Ji-Horng Plastic Co., Ltd. Injection Molding: Defect and Solution
Plastic injection molding may cause various kinds of defect, the reasons are not only the malfunction of the machine, but also performance of the machine, design of the mold, and feature of the material, etc.
Molding conditions (such as temperature, pressure, additive, venting, etc.) are key factors for injection molding as well.
The different molding conditions may spend different time tending towards stability, and the results can be taken as references upon steady.
e.g. If you just adjusted the temperature, it is necessary to keep observing for at least ten minutes until it has become stable.
Therefore, once adjusting the molding condition, here are few things which should consider:
1. Do not adjust more than one molding condition at one time.
2. After adjusting the molding condition, it is necessary to observe until it becomes steady.
See below form for the reasons and solutions of injection defect:
Ⅰ . Short Shot
| Reason | Solution |
| Temperature of the material is too low | Raise the temperature |
| Insufficient injection pressure | Raise the injection pressure |
| Insufficient plastic material | Add in extra plastic material |
| Insufficient injection time | Extend the time period |
| Speed of inject is too slow | Faster the speed of injection |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Uneven temperature of mold | Adjust the position of the cooling water jiffy plug |
| The ventilation of the mold is poor | Applied additional vent hole on appropriated position |
| Clogged injection nozzle | Remove and clean |
| Filling uneven | Redo the position of the gate |
| The gate is too small | Enlarge the gate |
| Lubricants not enough | Add in extra lubricants |
| Poor fluidity of plastic material | Add in extra lubricants or use another material |
| Check ring was worn out | Remove and repair |
| Performance of the machine is poor | Use another suitable machine |
Ⅱ. Shrink
| Reason | Solution |
| Insufficient plastic material | Add up extra plastic material |
| Insufficient injection pressure | Raise the injection pressure |
| Insufficient injection time | Extend the time period |
| Injection speed is too fast | Slower the speed of injection |
| Filling uneven | Redo the position and the size of the gate |
| Clogged injection nozzle | Remove and clean |
| Temperature is too high | Reduce the temperature |
| Insufficient cooling time | Extend the cooling time |
| The ventilation of the mold is poor | Applied additional vent hole according to the position which shrinked |
| Undersize runner | Replace with larger runner |
| Under packing | Extend packing time |
| Check ring was worn out | Remove and repair the mold |
| Matrial is too soft | Use another material |
| Excessive shrinkage rate | Use another material |
Ⅲ. Sticking on Mold
| Reason | Solution |
| Injection pressure is too high | Reduce the injection pressure |
| Material injected too much (Overpack) | Using adequate mold release agent |
| Close time is too long | Reduce the close time |
| Injection is too fast | Slower the speed of injection |
| Temperature of the material is too high | Reduce the temperature |
| Some parts are overpack due to the uneven material feeding | Redo the position and the size of the gate |
| Insufficient cooling time | Extend the cooling time |
| Temperature of the mold is too high or too low | Adjust the temperature of male part and female part |
| Under-cut | Modify the mold and remove the undercut |
| Mold interior walls is not glossy | Polish the mold |
Ⅳ. Sprue Sticking
| Reason | Solution |
| Injection pressure is too high | Reduce the injection pressure |
| Temperature of material is too high | Reduce the temperature of the meterial |
| Oversize runner | Modify the mold |
| Insufficient cooling time of runner | 1. Extend the cooling time 2. Reduce the temperature of runner |
| The angle of the runner is too low | Modify the mold to increase the angle |
| Sprue bushing is not aligned with injection nozzle | Redo or adjust the mold |
| Rough runner interior walls | Maintenance and repair the mold |
| Under-cut | Maintenance and repair the mold |
| Broken runner external hole | Maintenance and repair the mold |
| No snatch pin | Add in extra snatch pin |
| Too much material injected (Overpack) | 1. Reduce usage of material 2. Reduce injection time 3. Slow down speed of injection |
Ⅴ. Flash / Burr
| Reason | Solution |
| Temperature of the material is too high | 1. Reduce the temperature of material 2. Reduce the temperature of mold |
| Injection pressure is too high | Reduce the injection pressure |
| Too much material injected (Overpack) | 1. Reduce usage of material 2. Reduce injection time 3. Slow down speed of injection |
| Parting line of mold is untight | Maintenance and repair the mold |
| Insufficient locking pressure | Raise locking pressure |
| Close time is too long | Reduce the close time |
| Injection speed is too fast | Slower the speed of injection |
| Excess fluidity of plastic material | Use another material |
Ⅵ. Broken / Cracked
| Reason | Solution |
| Too much material injected (Overpack) | 1. Reduce usage of material 2. Reduce injection time 3. Slow down speed of injection 4. Decrease injection pressure |
| Close time is too long | Reduce the close time |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Draft angle is too small | Maintenance and repair the mold |
| The gate is too small | Maintenance and repair the mold |
| Ejector pin doesn't work because of position or quantity | Maintenance and repair the mold |
| Under-cut | Maintenance and repair the mold |
| Material has excessive hygroscopicity | 1. Dry the material 2. Use another kind of material |
| Mold became vacuum during ejecting | 1. Slower the speed of opening or ejecting 2. Use air-intake equipment |
Ⅶ. Welding Line (Knit Line, Parting Line)
| Reason | Solution |
| Poor fluidity of plastic material | 1. Raise temperature 2. Raise back pressure to faster the rotating speed of the screw 3. Use another kind of material |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Injection speed is too slow | Faster the speed of injection |
| Injection pressure is too low | Raise the pressure of injecting |
| Residual impurities in plastic material | Check the material |
| Too much mold release oil | Reduce or prevent using mold release oil |
| Undersize runner or gate | Using adequate mold release agent |
| Not able to vent the air | Check the vents to see if they are clogged, or add extra vents |
| Lack of overflow well | Add extra overflow well under the welding line |
Ⅷ. Flow Lines
| Reason | Solution |
| Material is not melted appropriately | 1. Raise temperature 2. Raise back pressure to faster the rotating speed of the screw |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Injection speed is too slow or too fast | Adjust the speed of injection |
| Pressure of injection is too high or too low | Adjust pressure of injection |
| Accidentally mixed with other materials | Check the material |
| Undersized runner | Enlarge the runner |
| Thickness difference of cutting surface is substantial | Change the design of the finished product or the gate position |
Ⅸ. Silver Streaks (Slivery Line)
| Reason | Solution |
| The material is moist | 1. Dry the material completely 2. Raise the back pressure |
| The temperature of the material or the mold is too high, and result in generation of gas | Reduce the temperature of the material or the mold |
| Material is mixed with volatile addictives such as mold release agent, dye, etc. | Reduce the usage of addictives or replace with alternatives which is heat resistance |
| The material and the didn't mixed well | Mixed completely |
| Injection speed is too fast | Slower the injection speed |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Uneven mixed material grain | Use material which has even grained size |
| Inappropriate mold flow | 1. Adjust the position of gate 2. Maintain suitable mold temperature |
Ⅹ. Lack of Gloss
| Reason | Solution |
| Temperature of the mold is too low | Raise the temperature of the mold |
| Insufficient material | 1. Incrase usage of material 2. Extend injecting time 3. Increase injecting pressure |
| Too much mold release oil | Wipe the mold clean |
| Water in mold interior | Wipe and check if there's water leakage |
| Mold interior walls is not glossy | Polish the mold |
XXI. Warpage, Part Distortion
| Reason | Solution |
| The finished product has not cooled down when it is ejected | 1. Reduce the temperature of the mold or material 2. Extend the cooling time |
| The shape and the thickness of the molded plastic part is asymmetric | 1. Fix the shape with styling frame 2. Modify the design of finished product |
| Injection pressure or speed is not suitable (Overpack) | 1. Reduce usage of material 2. Reduce injection time 3. Slow down speed of injection 4. Decrease injection pressure |
| Filling uneven | Redo the gate |
| The ecjector pin is not balance | Improve the ejector system |
| Temperature of the material is too high | Reduce the temperature |
| Structure of the plastic article is too friable or too tight | Increase or decrease the close time |
| Uneven temperaure between male mold and female mold | Balance the temperature of male mold and female mold |
| Pressure of the mold is not suitable | Balance the pressure of the mold |
XXIX. Air Trap (Air Bubbles/Voids)
| Reason | Solution |
| Cutting surface of finish product is too thick | Modify the design of finish product or position of gate |
| Injection pressure is too low | Raise the injection pressure |
| Insufficient injection time | Extend the injection time |
| Runner or gate is too small | Enlarge size of runner or gate |
| Injection speed is too fast | Slower the injection speed |
| The material is moist | Dry the material completely, or raise the back pressure |
| Temperature of material is too high and result in decomposition | Reduce temperature of material |
| Unven mold temperature | Adjust mold temperature |
| Cooling time is not suitable | Reduce or extend cooling time |
| Insufficient back pressure | Raise back pressure |
| Inappropriate temperature of feed tube | Reduce temperature of injection nozzle and forepart, and raise temperature of rear part |
XXX. Black Streaks
| Reason | Solution |
| Temperature of the material is too high | Reduce the temperature of the runner |
| Rotating speed of screw is too fast | Slow down rotating speed of screw |
| Residual impurities in plastic material | Check the material and clean feed tube |
| Undersized hole size of injection nozzle | Modify the hole size |
| Scorched material in feed tube | Clean the feed tube |
XLV. Brittleness
| Reason | Solution |
| Temperature of feed tube is too high | Reduce the temperature of feed tube |
| Back pressure is too high | Reduce the back pressure |
| Rotating speed of screw is too fast(Result in fracture of resin molecular chain) | Slow down the rotating speed of screw |
| Resin is not dried completely | Dry with suitable time and temperature |
| Using too much re-grinding material | Reduce the usage of the re-grinding material |
| Different kinds of re-grinding material was used | Use the same kind of re-grinding material |
**Ji-Horng Plastic is a leading plastic injection molding manufacturer since 1986, for more information, please contact us!**
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