Analysis of defects and causes of plastic parts

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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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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