Causes and solutions of shortcomings in injection moulding
Time：2019-12-23 08:48:13 / Popularity： / Source：
To form a qualified product requires a perfect fit between injection moulding process, injection moulding machine and injection molding mold. If there is a problem on either side, it is impossible to produce a qualified product. Table below lists some common injection moulding defects and suggestions for your reference.
In order to reduce downtime and identify cause of problem as quickly as possible, operator should record all the best injection moulding conditions on “Injection Moulding Condition Record” for future reference.
Note: Information provided in this appendix is not completely accurate. We are not responsible for accuracy and completeness of this information. Users should decide availability of data at their discretion.
Finished product is incomplete
| Cause of problme |
Solution |
| Temperature of compound is too low |
Increase temperature of melt cylinder |
| Injection pressure is too low |
Increase injection pressure |
| Insufficient amount of injection |
Increase amount of injection |
| Sprue sleeve is not properly matched with nozzle R. Nozzle overflows. |
Replace the nozzle or sprue sleeve |
| Injection time is too short |
Increase injection time |
| Injection speed is too slow |
Speed up injection |
| Mold temperature is too low |
Increasing mold temperature |
| Uneven mold temperature |
Re-adjust mold cooling water distribution |
| Poor mold exhaust |
Open vent hole in proper position |
| Nozzle temperature is too low |
Increase temperature of nozzle |
| Uneven injection |
Reopen mold overflow position |
| Sprue or spill is too small |
Increase the sprue or spill |
| Insufficient lubricant in plastic |
Increase lubricant |
| Insufficient back pressure |
Slightly increased back pressure |
| Over head combination or screw wear |
Remove for inspection, repair or replacement |
| Maximum injection volume is not enough |
Replace larger size injection machine |
| Product is too thin |
Using nitrogen gas injection |
Product shrinkage
| Insufficient amount of injection |
Increase amount of injection |
| Injection pressure is too low |
Increase injection pressure |
| Back pressure is not enough |
Increase back pressure |
| Injection time is too short |
Increase injection time |
| Injection speed is too slow |
Speed up injection |
| Spill imbalance |
Adjust size or position of mold spill |
| Nozzle or sprue hole is too thin to reduce back pressure effect |
Adjust mold or replace nozzle |
| Material temperature is too high |
Lower material temperature |
| Improper mold temperature |
Adjust to appropriate temperature |
| Insufficient cooling time |
Prolong cooling time |
| Too much pressure retaining compound |
Compound is terminated as far as possible at the front end |
| Product itself or ribs are too thick |
Review product design |
| Over-head combination or screw wear |
Remove for repair or replace |
| Gate is too small, plastic solidifies and loses back pressure |
Increase gate size |
Finished sticking mold
| Injection pressure is too high |
Reduce injection pressure |
| Excessive amount of injection |
Reduce amount of injection |
| Injection time is too long |
Reduce injection time |
| Material temperature is too high |
Lower material temperature |
| Uneven feeding makes partial oversufficiency |
Change size or position of spill |
| Mold temperature is too high or too low |
Adjust mold temperature and relative temperature on both sides |
| Mold has no draft angle |
Increase draft angle |
| Mold surface is not smooth |
Increase mold surface finish |
| Demoulding creates vacuum |
Opening or ejector slowing down, or add a air poppet valve |
| Injection cycle is too short |
Enhanced cooling |
| Insufficient release agent |
Slightly increase amount of release agent |
Sprue (nozzle) sticking mold
| Injection pressure is too high |
Reduce injection pressure |
| Plastic temperature is too high |
Reduce plastic temperature |
| Sprue cooling is not enough |
Extend cooling time or reduce cooling temperature |
| Runner's draft angle is not enough |
Modify mold to increase draft angle |
| Sprue bushing and nozzle R are not properly matched |
Realignment |
| Inner surface of runner is not smooth |
Replacement bushing |
| No runner shoveling |
Add a shoveling |
| Filling too full |
Reduce amount of injection, time and speed |
| Insufficient release agent |
Slightly increase amount of release agent |
Burring, flash
| Plastic temperature is too high |
Reduce plastic and mold temperature |
| Injection speed is too high |
Reduce injection speed |
| Injection pressure is too high |
Reduce injection pressure |
| Filling too full |
Reduce amount of injection |
| Poor clamping line or matching surface |
Overhaul mold |
| Insufficient clamping force |
Increase clamping pressure or replace injection moulding machine with large clamping force |
Product is broken when mold is opened or when it is ejected.
| Filling too full |
Reduce injection pressure, time, speed and amount of injection |
| Mold temperature is too low |
Increased mold temperature |
| Part of draft angle is not enough |
Overhaul mold |
| Product cannot be balanced when demolded |
Overhaul mold |
| Insufficient thimble or improper position |
Overhaul mold |
| Partial vacuum during demolding |
Open mold to make a slow speed, add a air poppet valve |
| Insufficient release agent |
Increase amount of release agent |
| Poor mold design, excessive stress in finished product |
Improved finished product design |
Bonding line
| Poor plastic melting |
Increase plastic temperature, back pressure, and screw speed |
| Mold temperature is too low |
Increase mold temperature |
| Nozzle temperature is too low |
Increase temperature of nozzle |
| Injection speed is too slow |
Speed up injection speed |
| Injection pressure is too low |
Increase injection pressure |
| Plastic is dirty or infiltrated with other materials |
Check plastic |
| Too much release agent |
Use less mold release or try not to use |
| Sprue or overflow is too large or too small |
Adjusting mold |
| Place where glue is bonded is too far from sprue |
Adjusting mold |
| In-mold air is not removed |
Increase vent or check if original vent is blocked |
| Insufficient amount of injection |
Use a larger injection moulding machine |
Flow texture
| Poor plastic melting |
Increase material temperature, back pressure and speed up screw speed |
| Mold temperature is too low |
Increase mold temperature |
| Improper mold cooling |
Re-adjust mold water pipe |
| Injection is too fast or too slow |
Adjust proper injection speed |
| Injection pressure is too high or too low |
Adjust proper shot pressure |
| Plastic is dirty or infiltrated with other materials |
Check plastic |
| Spill is too small to produce texture |
Increase overflow |
| Thickness of finished product is too different |
Change finished design or location of spill |
Splay, bubbles
| Plastic contains moisture |
Plastic drying thoroughly to improve backup pressure |
| Plastic temperature is too high or too long, residence time in the barrel is too long |
Reduce material temperature, or replace small injection volume injection machine. |
| Decomposition of other additives in plastics |
Reduce additive usage or replace high temperature alternatives |
| Uneven mixing of additives in plastics |
Well mixed |
| Injection speed is too fast |
Slow down injection speed |
| Injection pressure is too high |
Reduce injection pressure |
| Melt speed is too low |
Increase melt speed |
| Mold temperature is too low |
Increase mold speed |
| Uneven plastic grain thickness |
Use granular homogeneous raw materials |
| Air is trapped in the barrel |
Reduce temperature of rear section of barrel, increase back pressure, and reduce length of compression section |
| Improper flow of plastic in mold |
Adjust size and position of overflow, keep mold temperature balanced, thickness of finished product is even. |
Finished surface is not shiny
| Mold temperature is too low |
Increase mold temperature |
| Insufficient plastic dose |
Increase speed, time and amount of injection pressure |
| Too much mold release oil in mold |
Wipe clean |
| Improper drying of plastics |
Improve drying treatment |
| Water on inner surface of mold |
Wipe and check for water leaks |
| Inner surface of mold is not smooth |
Grinding mold |
Finished product deformation
| Product has not been cooled when ejected |
Reduce mold temperature, extend cooling time |
| Plastic temperature is too low |
Increase mold and plastic temperature |
| Product shape and thickness asymmetry |
Mold temperature zone control, fixed by a fixed frame after demolding, changing mold design. |
| Filling too full |
Reduce speed, time and amount of injection pressure |
| Several overflow feeds are uneven |
Change spill |
| Thimble system imbalance |
Improved thimble system |
| Uneven mold temperature |
Adjust mold temperature |
| Poor holding pressure |
Increase pressure holding time |
There are air holes in finished product
| Insufficient amount of filler to prevent excessive shrinkage of finished product |
|
| Finished section, rib or column too thick |
Change finished design or location of spill |
| Injection pressure is too low |
Increase injection pressure |
| Insufficient amount of shot and time |
Increase amount and time of injection |
| Sprue spill is too small |
Increase sprue and spill |
| Injection speed is too fast |
Slow down injection speed |
| Plastic moisture |
Plastic is completely dry |
| Plastic temperature is too high to decompose |
Reduce plastic temperature |
| Uneven mold temperature |
Adjust mold temperature |
| Cooling time is too long |
Reduce in-mold cooling time |
| Back pressure is not enough |
Increase back pressure |
Black spots
| Overheated part of plastic is attached to inner wall of melt cylinder |
Thoroughly empty injection, remove melt cylinder to clean, reduce plastic temperature and heating time, strengthen plastic drying treatment. |
| Plastic mixed with impurities |
Check plastic and injection it completely. |
| A focal spot is produced when it is injected into mold |
Reduce pressure and speed of injection, reduce plastic temperature, strengthen venting holes of mold, and change position of spill. |
| A dead corner in melt cylinder that causes mold to overheat. |
Check contact surface between nozzle and melt cylinder for clearance or corrosion. |
Black lines
| Plastic temperature is too high |
Reduce plastic temperature |
| Melt speed is too fast |
Reduce melting speed |
| Screw and melt cylinder are eccentric to generate abnormal friction heat |
Overhaul machine |
| Nozzle hole is too small or temperature is too high |
Readjust aperture or temperature |
| Amount of injection is too large |
Replace smaller injection machine |
| A dead corner in melt cylinder that causes material to overheat. |
Check contact surface between nozzle and melt cylinder for clearance or corrosion. |
Unstable cycle
Various molding defects listed above, causes and countermeasures are mostly related to stability of cycle. Proper plasticization of plastic in melt cylinder, or temperature control of mold, is result of heat transfer balance. That is to say, during entire injection cycle, plastic in melt cylinder receives frictional heat from rotation of screw and heat of electric heating coil. Thermal energy is injected into mold with plastic. Heat of mold comes from thermostat of plastic and mold, which is lost in the release of finished product, lost in the air or carried away by cooling water. Therefore, if temperature of melt cylinder or mold is to be maintained, it is necessary to maintain heat transfer balance of inlet and outlet. Maintaining a balanced heat transfer requires maintaining a stable injection cycle. If injection cycle time is getting shorter and shorter, heat energy in melt cylinder will not be enough, so that it is not enough to melt plastic, heat energy of mold is more than enough, so mold temperature is rising. Otherwise, there is opposite result. Therefore, in any injection moulding operation, especially manual operation, it is necessary to control stable cycle time and try to avoid different speeds. If other conditions remain same, acceleration of cycle will result in short shots, shrinkage and deformation of finished product, and sticking.
Delay of cycle will result in: flash, hair, sticking, deformation of finished product, overheating of plastic, or even charring. Compound remaining in mold may cause damage to mold. Overheated plastic in melt cylinder may corrode black lines of melt cartridge and finished product.
