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Analysis of defects and causes of plastic parts

Time：2024-11-13 08:19:43 / Popularity： / Source：

 
Summary of plastic part defects
No Defect name No Defect name No Defect name
1 Flash 8 Cracks 15 Product clamping line
2 Deformation 9 Punch marks 16 Leather scratches
3 Air trap 10 Weld lines 17 White top, top cracks
4 Silver thread 11 Underfilling 18 Surface cold material
5 Burn 12 Bubble, air inclusions 19 Poor surface gloss, shadows
6 Shrinkage mark 13 Bright marks, dark marks    
7 Flow mark 14 Abrasions, strains    
 
Defect name Flash Defect picture plastic part defects 
Defect level C
Defect explanation Excess thin wings produced on the edge of workpiece
Mold type Cause Solution
Plastic mold 1. Excessive injection Reduce injection amount
2. Insufficient clamping force Increase clamping force
3. Excessive material fluidity (such as PA/PP/PE/POM) Change materials/lower mold temperature/lower material temperature/lower injection pressure
4. Parting surface is not tightly fitted, with gaps or debris Adjust mold and re-grind loose fitting parts/remove debris
5. Gap between sliding parts of cavity and core is too large Adjust mold to reduce gap between sliding parts of cavity and core
6. Overcutting exists in tight fitting part of mold Weld overcut parts and re-process and match them
7. Poor mold strength or rigidity Increase mold plate thickness/add mold plates or add support columns
8. Poor parallelism of various receiving surfaces of mold Fix mold so that receiving surfaces are parallel to each other
9. Mold is not pressed tightly during installation Reinstall and tighten mold
 
Defect name Deformation Defect picture plastic part defects 
Defect level C
Defect explanation Due to uneven molecular orientation and cooling shrinkage inside workpiece, shape and size of workpiece change, which is called deformation.
Mold type Cause Solution
Plastic mold 1. Insufficient cooling time and high mold temperature Increase cooling time and reduce mold temperature
2. Unreasonable design of shape of workpiece, uneven thickness If conditions permit, modify workpiece to meet molding process
3. Unreasonable position of feed port, molten material directly impacts core, and force on both sides is uneven Change feed port position
4. Unreasonable injection process (low material temperature and mold temperature, low injection pressure, fast injection speed, insufficient holding time) Adjust injection process
5. Large temperature difference in different parts of cavity, uneven cooling Reasonably control mold temperature to ensure that the overall temperature of mold cavity tends to be balanced
6. Insufficient or excessive feeding Quantitative feeding
7. Unreasonable position of mold ejection Adjust position of ejector mechanism
 
Defect name Air entrapment Defect picture plastic part defects 
Defect level B
Defect explanation During mold injection, end of molded part will be shiny due to cavitation.
Mold type Cause Solution
Plastic mold 1. Poor mold venting Increase exhaust or arrange ejector pins and inserts properly to fully exhaust air through gaps
2. Excessive moisture or volatiles in raw material Dry raw materials
3. Unreasonable gate setting (dead corners in flow) Change gate position
4. High injection pressure/fast injection speed Adjust injection pressure/injection speed
5. Part structure reasons (large wall thickness difference) Adjust part structure
 
Defect name Silver wire Defect picture plastic part defects 
Defect level A
Defect explanation Silvery-white texture (needle-shaped stripes or flaky mica patterns) produced on the surface of workpiece along flow direction of molten material is called silver wire.
Mold type Cause Solution
Plastic mold 1. Raw material contains too much water or volatiles Dry raw materials
2. There is water in mold cavity Remove moisture in mold cavity
3. Excessive use of lubricating oil and mold release agent in mold cavity Use lubricants and mold release agents reasonably
4. Wall thickness of workpiece is uneven. When molten material flows from thin wall to thick wall, it expands, volatiles vaporize and contact mold surface to liquefy into silver wire Improve design of plastic parts to make transition between thick and thin walls uniform, meet process performance
5. Improper ingredients, mixed with foreign matter or unmelted materials, stratification and detachment occur Pay attention to purity when mixing ingredients
6. Low mold temperature, low injection pressure, and low injection speed. Molten material fills slowly, cools quickly, and is easy to form a thin layer of silver-white or white reflective light Increase mold temperature, increase injection pressure and speed up injection speed
 
Defect name Burn Defect picture plastic part defects 
Defect level Product first level surface: Class A;
Product second level surface: Class B
Product third level surface: Class C:
Defect explanation Black spots, black stripes or carbonization along surface of workpiece are called burns
Mold type Cause Solution
Plastic mold 1. Material is not cleaned properly or there are impurities Carefully clean material tube to make it clean, check whether plastic has impurities and remove them in time
2. Mold is poorly vented or clamping force is too large Reasonably repair mold exhaust system to reduce clamping force
3. There are flammable volatiles in melt or on the surface of cavity Clean cavity surface, there should be no debris and moisture
4. Plastic is damp and hydrolyzed to black Dry plastic before use to remove moisture
5. Uneven dyeing, with dark objects Or pigment deterioration Reasonable ingredients
6. Plastic components decompose and deteriorate Use new materials
7. Melt temperature is too high, causing melt to decompose and carbonize Control melt temperature
8. Melt is heated for too long, causing melt to decompose and carbonize Control melt heating time
 
Defect name Shrinkage marks Defect picture plastic part defects 
Defect level Product first level surface: Class A;
Product second level surface: Class B
Product third level surface: Class C;
Defect explanation Depression on the surface of workpiece caused by material shrinkage is called shrinkage marks.
Mold type Cause Solution
Plastic mold 1. Low injection pressure and slow speed (failure to compress material to appropriate density) Increase injection pressure and injection speed
  2. High material temperature, high mold temperature, and short cooling time Reduce material temperature and mold temperature, increase cooling time
  3. Improper injection volume adjustment and short holding time Adjust injection volume and increase holding time
  4. Large shrinkage rate of raw materials Add glass fiber/asbestos when allowed
  5. Uneven mold cooling Adjust mold cooling water layout and cooling water flow
  6. Uneven thickness of workpiece or too thick wall thickness Improve plastic part design or add process holes at wall thickness
  7. Feed port is too small or number is insufficient Increase feed port, or increase feed quantity
  8. Feed port is improperly located, which is not conducive to feeding Improve feed port location
 
Defect name Flow marks Defect picture plastic part defects 
Defect level Product first level surface: Class A;
Product second level surface: Class B:
Product third level surface: Class C;
Defect explanation After molten material is injected into cavity, fluidity is reduced, resulting in ripples on the surface of workpiece
Mold type Cause Solution
Plastic mold 1. Low material temperature/mold temperature/nozzle temperature Increase mold temperature/material temperature/nozzle temperature
2. Low injection pressure and slow injection speed Increase injection pressure and injection speed
3. Unreasonable design of cold material hole, cold material inside is not removed Improve cold material hole and remove cold material
4. Poor fluidity of raw materials Adjust injection process/replace raw materials with good fluidity
5. Pouring system has a long process and a small cross-sectional area; size, shape and position of feed port are incorrect, which hinders flow of molten material and cools quickly Improve pouring system and increase its cross-sectional area
6. Runner is tortuous, narrow and has a rough surface Fix runner and polish it to make it smooth
 
Defect name Cracking Defect picture plastic part defects 
Defect level A
Defect explanation Cracks on the surface of workpiece
Mold type Cause Solution
Plastic mold 1. Poor demoulding Re-polish mold and make polishing direction consistent with demoulding direction/increase demoulding slope
2. Insufficient cooling, workpiece is ejected before it is completely cooled, causing damage Extend cooling time or change cooling method
3. Unreasonable ejection during demoulding, uneven ejection force distribution Adjust mold ejection mechanism to make it evenly stressed and reliable
4. Workpiece has sharp corners or notches, which is prone to stress concentration Improve design of plastic parts
5. Cooling time is too long or too fast Reasonably control cooling time
6. Raw materials are mixed with impurities Use clean raw materials and remove impurities
7. There are more secondary materials in melt Reasonably configure proportion of secondary materials
8. Cracks on mold are copied to workpiece Check corresponding position of mold cavity and repair mold
 
Defect name Punching marks Defect picture plastic part defects 
Defect level Product first level surface: Class A
Product second level surface: Class B:
Product third level surface: Class C;
Defect explanation When high-speed melt passes through narrow areas such as nozzles, runners, and gates, it enters a relatively open cavity, contacts cavity surface and solidifies, and is then pushed by subsequent melt, leaving serpentine marks.
Mold type Cause Solution
Plastic mold 1. Small gate size Increase gate size
2. Too fast injection speed Appropriately reduce injection speed
3. Low mold temperature Increase mold temperature
4. Excessive injection pressure Reasonably control injection pressure
5. Improper gate position Change gate position
 
Defect name Weld line Defect picture plastic part defects 
Defect level Product first level surface: Class A
Product second level surface: Class B:
Product third level surface: Class C:
Defect explanation During injection molding, thin line produced at confluence of molten materials is called weld line.
Mold type Cause Solution
Plastic mold 1. Low injection pressure and slow injection speed Increase injection pressure and injection speed
2. Low mold temperature Increase mold temperature to make molten material naturally fuse well
3. Unreasonable use of release agent Control amount and number of times of release agent
4. Poor mold exhaust Increase exhaust or arrange ejector pins and inserts reasonably to fully exhaust gas through gaps
5. Low melt temperature Increase material temperature
6. Poor raw material fluidity Adjust injection process/use raw materials with good fluidity
7. Unreasonable gate setting Reasonable gate setting
8. Diversion caused by penetration Change gate position and reasonably set welding angle
 
Defect name Incomplete filling Defect picture plastic part defects 
Defect level B
Defect explanation After molten material enters cavity, it is not completely filled, resulting in incomplete parts, which is called incomplete filling.
Mold type Cause Solution
Plastic mold 1. Insufficient injection volume Increase injection volume
2. Low injection pressure/short injection time/short holding time Adjust injection process
3. Injection speed is too fast or too slow Reasonably control injection speed
4. Poor fluidity of raw materials Adjust injection process/select plastic materials with suitable fluidity
5. Too many flashes Eliminate/reduce flashes
6. Shape and size of gate/main channel or branch channel are unreasonable, and flow resistance of molten material is large Change shape and size of runner, and place gate position reasonably
7. Mold temperature is low and molten material cools quickly Increase mold temperature
8. Molten material contains cold material/molten material temperature is low Add (trim) cold material hole/increase molten material temperature
9. Mold exhaust is poor Increase exhaust or arrange ejector and insert reasonably, and use gap to fully exhaust
 
Defect name Bubbles, air inclusions Defect picture plastic part defects 
Defect level B
Defect explanation During filling process, molten material is disturbed by gas and presents a hollow protrusion on the surface of product, or a hollow bubble is generated inside product.
Mold type Cause Solution
Plastic mold 1. Raw material contains too much water and there are volatile substances Replace new raw materials or dry before use
2. Material temperature is high, heating time is long, and raw material decomposes Lower temperature and reduce heating time
3. Mold exhaust is poor Increase exhaust or arrange ejector and inserts reasonably, and use gap to fully exhaust
4. Injection molding machine plunger (screw) retreats early and holding time is insufficient Increase holding time
5. Mold temperature is low and molten material cools quickly Increase mold temperature
6. Injection speed is too fast/injection pressure is low Reasonably control injection speed/injection pressure
7. There is water, oil in mold cavity or release agent is used improperly Remove moisture and oil from mold cavity/reasonably use release agent
 
Defect name Bright print, dark print Defect picture  
Defect level Product first level surface: Class A
Product second level surface: Class B:
Product third level surface: Class C:
Defect explanation There are bright or dark areas on product surface
Mold type Cause   Solution
Plastic mold 1. There is moisture, oil or too much release agent on cavity surface (bright print) Clear moisture and oil in cavity/use release agent reasonably
2. High injection pressure (bright print) Adjust injection pressure
3. Edge stress of part is large, and material density is greater than other parts (bright print) Adjust gate position and injection pressure
4. Gap between lifter, internal extraction, ejector block, ejector pin and insert is large, and injection pressure is high (dark print) Fix mold and adjust gap between mold mating parts
5. Ejector plate is deformed (dark print) Increase mold plate thickness/level ejector plate
 
Defect name Scratches, strains Defect picture plastic part defects 
Defect level Product first level surface: Class A:
Product second level surface: Class B;
Product third level surface: Class C;
Defect explanation Workpiece rubs against mold during demolding, leaving traces along demolding direction
Mold type Cause Solution
Plastic mold 1. Cavity surface is not smooth Clean cavity surface
2. Demolding slope is small Increase demolding slope
3. Demolding direction is inconsistent with processing texture Trim mold texture/polish
4. Holding pressure/holding time is too large Adjust holding pressure/holding time
5. Insufficient cooling time Increase cooling time
6. Unreasonable mold ejection distribution Adjust ejection mechanism position
7. Unbalanced ejection Adjust ejection mechanism to balance ejection
 
Defect name Product clamping line Defect picture  
Defect level Product first level surface: Class A
Product second level surface: Class B
Product third level surface: Class C
Defect explanation Fine lines generated by workpiece at mold parting position
Mold type Cause Solution
Plastic mold 1. Large processing error Improve processing accuracy
2. Poor mold positioning accuracy Improve positioning accuracy
 
Defect name Leather grain injury Defect picture  
Defect level Product first level surface: Class A;
Product second level surface: Class B;
Product third level surface: Class C;
Defect explanation Leather grain area of workpiece rubs against mold during demolding process, leaving traces along direction of demolding
Mold type Cause Solution
Plastic mold 1. Small leather grain demolding angle Trim leather grain
2. Deep leather grain depth Trim leather grain
3. High injection pressure Adjust injection pressure
 
Defect name White top, top crack Defect picture plastic part defects 
Defect level Product first level surface: Class A
Product second level surface: Class B
Product third level surface: Class C;
Defect explanation Whitening, bulging or cracking caused by part when it is ejected from mold.
Mold type Cause Solution
Plastic mold 1. Unbalanced ejection Adjust ejection mechanism to balance ejection
2. Temperature difference between melt and mold is too large, which increases internal residual stress and is prone to white top when subjected to local force Control temperature difference
3. Poor demoulding Increase demoulding slope/polishing
4. High molding pressure (high injection pressure, high holding pressure, and long holding time) Adjust injection process
5. Unreasonable mold ejection distribution Adjust position of ejection mechanism
6. Small number of mold ejections Add ejector pins and ejector blocks
7. Insufficient cooling time Adjust cooling time
8. Too fast ejection speed Use a reasonable ejection speed
9. Low yield strength of raw materials Select raw materials with suitable mechanical properties
 
Defect name Surface cold material Defect picture plastic part defects 
Defect level Product first level surface: Class A;
Product second level surface: Class B
Product third level surface: Class C;
Defect explanation Material that is not fully plasticized or rapidly cooled when passing through runner and nozzle enters mold cavity, is poorly fused with surrounding materials, forming defects on the surface of product
Mold type Cause Solution
Plastic mold 1. Uneven plasticization of melt Uniform plasticization
2. Mold temperature is too low Increase mold temperature
3. Refractory miscellaneous materials are mixed into melt Use clean raw materials
4. Temperature of hot nozzle is too low Increase temperature of hot nozzle
5. There is no cold material hole or size of cold material hole is unreasonable Add cold material hole/change size of cold material hole
6. Injection speed is too fast or too slow Adjust injection speed
 
Defect name Poor surface gloss, shadow Defect picture plastic part defects 
Defect level Product first level surface: Class A;
Product second level surface: Class B;
Product third level surface: Class C;
Defect explanation Melt flow is blocked and direction is changed, resulting in inconsistent gloss on the surface of workpiece.
Mold type Causes Solutions
Plastic mold 1. Poor fluidity of raw materials Adjust injection process/select raw materials with suitable fluidity
2. Low melt temperature Increase melt temperature
3. Low mold temperature Increase mold temperature
4. Excessive injection pressure/injection speed Adjust injection pressure/injection speed
5. Poor mold exhaust Increase exhaust or arrange ejector and inserts reasonably to fully exhaust air through gap
6. Low crystallinity of raw materials or uneven wall thickness of product, resulting in uneven color Improve processability of product
7. Poor surface finish of cavity Cleaning mold/polishing
8. Raw materials are not dried/volatile substances exist Replacing new raw materials or drying before use
9. There is water, oil or improper use of release agent in mold cavity Removing moisture and oil from mold cavity/reasonable use of release agent
 
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