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Form of reasons and improvement measures for injection molding process defects

Time：2021-08-10 11:36:17 / Popularity： / Source：

Causes of defects in injection molding process and improvement measures
Shortcoming Reason Improvements
1. Injection of unsaturated molds 1) Insufficient injection volume 1) Adjust injection capacity
2) Insufficient melt flow 1) Increase injection pressure
2) Increase temperature of nozzle of feed pipe
3) Increase mold temperature
4) Increase injection time & speed
3) Incorrect mold design 1) Enlarge inlet, runner and nozzle.
2) Improve design of inlet, runner and nozzle
3) Improve entrance location
4) Increase vent holes
2. Product embrittlement 1) Overheating 1) Reduce temperature of material pipe
2) Reduce molding cycle
2) Contains internal stress 1) Increase temperature of barrel
2) Reduce injection pressure
3) Increase molding cycle
4) Increase mold temperature
5) Enlarge inlet, runner and nozzle.
3) Weld Lines 1) Increase injection pressure
2) Increase injection time & speed
3) Increase temperature of barrel
4) Increase mold temperature
5) Add an overflow at junction line and then cut flow
6) Change location and design of entrance
7) Improve vent
(4) Finished product lacks moisture. 1) Water treatment makes finished product absorb moisture
5) Poor quality of recycled materials. 1) Increase ratio of new materials
3. Finished product has unmelted particles 1) Resin is not fully melted 1) Increase temperature of barrel
2) Increase molding cycle
3) Change to high plasticizing capacity injection machine
4) Increase back pressure
4. Surface of finished product is recessed or there is space inside. 1) Melt filling is not enough to shrink low-selling finished product after cooling. 1) Increase injection time & pressure.
2) Increase mold temperature. (but not suitable for surface depression)
3) Reduce temperature of material tube
4) Resin is fully dried or filled with a lubricant to reduce injection pressure loss
5) Enlarge inlet, runner and nozzle.
5.Linear air lines and wrinkles on the surface are like fish eyes. 1) Temperature is too high to vaporize. 1) Reduce temperature of material pipe
2) Reduce temperature of nozzle of feed pipe
3) Reduce molding cycle or use a small barrel.
2) Resin moisture absorption 1) Dry again.
2) Use unbroken resin.
3) Material pipe has dead ends. 1) Material pipe and nozzle are designed to be streamlined.
4) Poor entrance design. 1) Enlarge entrance size or change position.
6. Scorch 1) Poor mold exhaust air is compressed in mold cavity. 1) Reduce injection pressure. Reduce injection speed.
2) Improve mold exhaust.
3) Change entrance position, size and type
7. Finished product sticks to female mold or entrance part 1) Too much resin injected into cavity exceeds its shrinkage. 1) Reduce injection pressure, time, speed.
2) Reduce mold temperature.
3) Reduce temperature of material pipe
4) Extend cooling time.
2) Mold design. 1) Check inner groove of UNDERCUTS.
2) Check ejection system
3) Increase demolding angle. (R 3--5)
8. Rough edges. 1) Insufficient clamping pressure. 1) Reduce injection pressure and speed.
2) Reduce temperature of material pipe
3) Lower mold temperature.
4) Reduce injection volume.
5) Increase amount of clamping pressure.
  2) Improper mold fit or bent mold plate. 1) Polish mold surface again.
2) Use a thicker post mold plate.
3) Check whether there are foreign objects on mold surface.
9. Finished product is warped and deformed. 1) Two mold halves have different temperatures. 1) Determine average mold temperature.
2) Excessive shrinkage. 1) Reference 11.
3) Uneven thickness of finished product changes too much. 1) Redesign mold.
2) Make two mold halves have different temperatures.
3) Use cooling fixtures.
4) Extend cooling time.
4) Push out before fully curing. 1) Extend cooling time.
2) Lower mold temperature.
3) Increase ejection area.
5) Injection direction. (Specially contains glass fiber resin) 1) Change entrance position.
2) Change to contain glass resin.
6) Not tough enough. 1) Increase thickness of finished product.
10.Spray marks (JETTNG) 1) Melt filling is too fast. 1) Reduce injection speed.
2) Change location and type of entrance (e.g. side entrance).
11.Excessive contraction. 1) Molding conditions. 1) Lower mold temperature.
2) Increase injection pressure
2) Entrance is too small. 1) Enlarge entrance.
12.Lamination separation. (LAMINATION) 1) Contains incompatible foreign matter. 1) Check resin. (especially recycled materials)
  2) Filling mold cavity leads to a high effect. 1) Increase mold temperature and barrel temperature
2) Reduce injection pressure and speed.
13.Surface cracks and bubbles. 1) Surface of cavity is too cold. 1) Increase mold temperature.
2) Reduce injection speed.
3) Insert is preheated.
14.Surface of glass fiber is like a blanket. 1) Improper injection speed. 1) Increase injection speed & pressure
2) Increase temperature of barrel
3) Increase mold temperature
15. Discoloration and embrittlement of non-flammable specifications 1) Overheating. 1) Reduce temperature of material pipe
2) Reduce molding cycle
Difficulty Reason Improvements
SURFACE IMPER PECTIONS (please refer to silver flow lines, shrinkage and flow lines) 1. Cold plastic 1) Increase plastic temperature
2. Mold temperature is too low 2) Increase mold temperature, or increase zone mold temperature control
3. Injection is too slow 3.A, Application of machine control system, boost and pump speed, increase injection speed
B. Use maximum force and quantitative injection or pre-plastic feeding
4. Flow material is unbalanced between feed port and plastic runner 4) Make feed inlet and plastic runner both have feed balance
5. Poor plastic flow of material in mold 5) Rearrange feed inlet or redesign product
UNMELED GRANULES 1. Plastic temperature is too low 1) Increase plastic temperature
2. Cycle is too fast or energy of barrel is insufficient 2) Extend cycle time.
3. Insufficient material flow restrictions 3) Use limited pressure injection nozzle
Product deformation (WARPING) 1. Finished product is too hot and not solidified when it is ejected 1.A Reduce plastic temperature
B Reduce mold temperature
C Extend mold opening time
2. Plastic temperature is too low 2.A Increase barrel temperature
B Increase mold temperature
3. Uneven thickness or special shape of product 3. A different temperature used on each mold side
B Make thickness of each part of product consistent
4. Too much feed 4.A Reduce feed
B Reduce injection pressure
5. There is more than one feed port, feed of each feed port is unbalanced 5) Restricting plastic flowing through feed port can only limit pressure close to main runner
6. Poor design or poor operation of ejector system 6) Redesign
7. Inconsistent mold temperature 7) Maintain mold surface at same temperature
8. Too much stuffing around feed inlet 8.A. Increase or decrease forward speed of pump
B. Reduce or increase size of feed inlet
SILVER STREAKS 1. Plastic temperature is too high 1) Reduce temperature of barrel, and temperature of self-injection nozzle begins to drop.
2. Plastics are mixed with recycled materials of uneven particle size 2) Avoid using materials with uneven particles, or make them same size first
3 Temperature of plastic is inconsistent, because: A. Temperature of nozzle or barrel is too high 3): A. Reduce temperature of nozzle and barrel
B. Exceeding plasticizing ability of machine B. Extend cycle or use a machine with large heat energy
C. Temperature of plastic in hopper is inconsistent. C. Preheat material, or install an oven-type hopper to make temperature of material consistent
4.At colder end of machine, air gathers in particles of material. 4) Reduce temperature of barrel section and avoid use of recycled materials.
5.Moisture is attached to the material. 5) Dry material before use.
6.Flow of material with voids in mold cavity. 6) A Make feed of each feed port consistent.
B Relocate feed inlet.
C Keep mold temperature consistent.
D Make thickness of product as consistent as possible.
7. Insufficient external release agent. 7) Add zinc stearate 15g/100 lbs (recycled material).
8. External release agent is inconsistent. 8) Increase mixing time, or add a little lubricant.
9.Shooting speed is too fast. 9) A Add vent holes.
B Reduce injection pressure & speed & reduce barrel temperature
10.Too much external release agent. 10) Increase proportion of recycled materials or materials without release agent.
11.Mold temperature is too low. 11) Increase mold temperature
12.Injection pressure is too high. 12) Reduce injection pressure
Weld line is poor or obvious. 1.Temperature of plastic is too low. 1) A increase barrel temperature & mold temperature
B Increase injection pressure
C Extend cycle time.
2. Bonding line is too far away from feed inlet. 2) Repositioning of feed inlet.
3. Air is not removed quickly enough. 3) Improved exhaust system.
4. Thickness of product is not enough. 4) Redesign product, or improve position of material port.
5. Mold temperature is too low 5) Increase mold temperature
6. Insufficient pressure. 6) Increase pressure.
8.Injection speed is too slow. 7) Increase injection speed.
9.Gas accumulation. 9) An escape hole is added at the top.
Scaling (SINKMARKS) 1. Plastic in mold is not enough to withstand shrinkage during cooling. Because:
A. Part of product is thick or has protruding parts.		 A. Repair mold or increase injection pressure
B. Insufficient feed. B. Increase feed & increase pressure
C. Advancing time of pump is too short. C. Increase advance time of pump
D. Injection pressure is too low. D. Increase injection pressure.
2.There is moisture in material. 2) Dry material before use.
3.Mold temperature is uneven. 3) Rearrange temperature control waterway to make mold temperature consistent.
CRACKING OR CRAZING 1. Excessive cushioning. 1.A: Reduce amount of shot material.
B: Reduce injection pressure
2.Mold temperature is too low. 2) Increase mold temperature
3. Improper demolding angle or depression. 3) Modify mold.
4. Ejector pin position is wrong. 4) Ejector pin should have a balanced ejection force. It is better to eject product than to pull it out.
Size changes. 1. Operating conditions are unstable. 1). Consistent operating conditions.
2. Molding conditions are unstable. 2). Consistency of molding conditions. (material temperature, pressure, mold temperature cycle feeding voltage...)
3. Surrounding air humidity and temperature changes 3) Consistent conditions.
Surface of product is rough. (DIMCNSIONAL VARIATION) 1.Mold temperature is too low. 1) Increase mold temperature
2.Injection pressure is too low. 2) Increase injection pressure
3.There is moisture on mold surface. 3) Clear or repair water leaks, or avoid condensation of water vapor.
4. Excessive release agent on mold surface. 4) Reduce or remove mold release agent on mold surface.
5.Plastic is not enough to make plastic completely contact mold cavity. 5) A Increase injection pressure
B Increase shot volume
C Increase mold temperature
6. Forward speed of pump is too low. 6) A Increase pump temperature.
B Increase temperature of plastic.
SHORT SHOTS 1.Plastic temperature is insufficient. 1) Increase temperature of plastic.
2.Mold temperature is too low. 2) Increase mold temperature
3.Insufficient injection pressure. 3) Increase injection pressure
4. Inconsistent mold temperature 4) Reinstall waterway.
5.Insufficient injection volume. 5) Increase injection volume
6.Air gathers. 6) Increase vent hole or enlarge vent hole.
7. Insufficient release agent. (additional). 7) Increase mold release agent.
8. Advancing time of pump is too short. 8) Increase pump forward time
9.Injection speed is too slow. 9) Increase injection speed.
10.Feed port is too small. 10) Increase feed port.
BROWNSTREAKS 1. The whole or part of barrel is overheated. 1) Lower temperature.
2. Barrel or nozzle is blocked. 2) Cleaning nozzle of barrel.
Internal bubble (BUBBLUS) 1. Material in mold cannot withstand excessive shrinkage due to following reasons
A thicker part, protrusion.		 A) Modify mold.
B injection pressure is insufficient. B)Increase injection pressure
C. Advancing time of pump is too short. C) Increase advance time of pump
D. Injection speed is too slow. D) Increase injection speed.
E. Feed port is too small. E) Increase feed inlet.
2.Temperature of plastic is too high. 2) Reduce temperature of plastic. (Barrel & Nozzle)
Main runner cannot be separated. (SURUE STICKING) 1.Injection pressure is too high. 1) Reduce injection pressure
2.Temperature of plastic is too high. 2) Reduce temperature of plastic. (Barrel & Nozzle)
3. Main runner is too long. 3) Shrink main runner.
4. Too much feed. 4) Reduce amount of feed
5. Insufficient draft angle 5) Increase draft angle
6. Improper matching of sprue nozzle 6) Injection nozzle hole should be smaller than vertical gate (sprue)
  1. Depression or rough surface
 7) Eliminate dents and touch polished surface
8. Pump stays too long 8) Decrease advancing time of column
LAMINATION AT GATE 1. Material temperature is not enough 1) Increase temperature of plastic.
2.Mold temperature is too low. 2) Increase mold temperature
3. Injection speed is slow 3) Increase injection speed
4. Insufficient pressure 4) Increase pressure
5. Pump stays too long 5) Reduce injection time
6. Material is contaminated 6.A. Cleaning material tube
B. Avoid mixing with other plastics
7. Too much release agent 7) Clean surface of mold and use it occasionally
8. Plastic runner and feed port are too large or too small 8) Trim plastic runner to achieve better pressure control
9. Mold temperature is too high, especially in main runner or central feed port 9) Use auxiliary cooling equipment on hot mold surface
PARTS STICKING IN MOLD 1. Injection pressure or barrel temperature is too high 1). Reduce injection pressure or barrel temperature
2. Too much feed 2) Reduce feed
3. Advancing time of pump is too long 3) Reduce time to advance column
4. Feed inlets of multiple writing molds are unbalanced, or feed inlets of single writing molds are unbalanced 4) Restrict plastic process, as close as possible to main runner.
5. There are recesses in mold. 5) Eliminate dents, polish or increase demoulding angle
6. Deep tube part demoulding vacuum problem 6) Provide sufficient escape airway
7. Change of mold opening time 7) Maintain a fixed mold opening time, if necessary, use a timer
8. Improper finishing of mold surface 8) Polishing mold surface
9. Die core misalignment 9) Adjust die core and use withdrawal angle to lock it
FLASH 1. Material is too hot 1.A. Reduce material temperature
B. Reduce mold temperature
2. Pressure is too high 2.A. Reduce stress
B. Reduce boost timing
3. Overshoot 3) Reduce feeding
4. Unstable shooting 4) Maintain front end of pump column with same amount of material
5. Parting line or parting surface is poor 5) Re-modify parting line or parting surface
6. Unstable cycle time 6) Use a timer to maintain consistency throughout cycle
7. Insufficient mold opening pressure 7.A. Increase mold opening pressure
B. Adjust to machine production with higher mold opening pressure
FLOW LINES 1. Plastic temperature is too low 1) Increase plastic temperature.
2. Mold temperature is too low 2) Increase mold temperature
3. Feed port is too small to cause overflow 3) Enlarge feed port and reduce injection speed
4. Thickness of each part of product is inconsistent 4.A. Redesign product, but thickness is same
B. Eliminate protrusions or fuss
Cycle instability (CYCLES) 1. Length of time to keep mold open is variable 1) Use mold opening timer to maintain a fixed cycle time
2. Unstable pressure 2.A. Keep enough pressure to make shots consistent
B. Check whether pressure system is leaking
3. Barrel temperature is inconsistent 3.A. Check temperature control system to maintain normal operation
B. Use the best temperature control system
C. Check voltage and make it stable
D. Make sure heater is working properly
E. Before material is added to hopper, temperature of each barrel of material should be kept consistent
F. Amount of material at the front end of pump column should be consistent
G. Machine should be maintained in a balanced condition
4. Inconsistent mold temperature 4.A Use mold temperature controller
B. There should be proper water channels in  mold
C. Appropriate venting should be provided in  mold
D. Usually water pipe in mold should have proper joints
5. Unstable feeding 5) Check feeding mechanism
 
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