Injection molding defects and solutions

Definition: shrinkage dents on the surface of plastic parts, as shown in Figure 1.
Natural causes: During cooling process of plastic, plastic shrinks due to thermal effects. If these shrinkages are not compensated in time, shrinkage marks (commonly known as dents) will appear at certain locations of plastic parts. Due to insufficient cooling, stress generated when surface of injection molded part is cooled while it is still unstable pulls inwards. There are three reasons for this defect:
1. Solidification speed is too slow;
2. Effective holding time is too short;
3. Due to great resistance of melt flowing in mold cavity, or part of injection molded part and gate system are too narrow, there is not enough holding pressure to be transmitted to certain locations of mold cavity.

 

Definition: Silver-white stripes appearing on the surface of plastic part, as shown in Figure 2.
Natural causes: Charred marks are produced when melt is damaged by thermal effects. Thermal damage can shorten plastic molecular chain (forming silver marks) or change macroscopic molecular structure (forming brown marks).
Solutions:
1. Reduce injection speed
2. Avoid sharp edges
3. Avoid too small flow channel
4. Check main flow channel and gate system
5. Check temperature of nozzle cross section
6. Check function of stop valve
7. Check drying of shackles
8. Reduce amount of recycled shackles
9. Use rubber or colorant that can withstand high temperature.

 

Definition: Water wave-like stripes (wrinkles) appearing on the surface of rubber part, as shown in Figure 3.
Natural cause: Rubber particles absorb moisture from air during storage or injection molding. When rubber particles melt, moisture turns into water vapor bubbles. Due to difference in speed between flow crest surface and center of molten plastic, these bubbles are pushed to the surface of molten plastic. Compressed bubbles burst due to compensation pressure, are then deformed by flow crest of molten plastic and solidified on mold wall.
Solution:
1) Plastic material should be fully dried before use
2) Reduce molding temperature of plastic
3) Reduce back pressure and melt speed
4) Reduce injection speed and increase injection pressure
5) Wipe off moisture or volatile components on mold surface before molding
6) Increase mold temperature
7) Improve mold exhaust

 

Definition: Small grooves or points or lines that appear on the surface or side of plastic part or cut into surface of material, as shown in Figure 4.
Natural cause: When filling mold cavity, gas is not discharged in time, but is dragged and pressed on the surface of injection molded part along flow direction, especially in word mark, reinforcing ribs, hemispheres and concave parts. Gas will be trapped by melt in front to form air marks. If gas is pulled by front end of screw during pressure test, air marks of injection molded part will appear near gate. Gas is brought into mold cavity during injection and is pushed to and solidified on mold wall. Air will also be drawn into plastic during plasticizing process. If pushing pressure is insufficient or plasticizing amount is too large, and air cannot escape from hopper, air marks will be formed and will appear on the entire injection molded part.
Solution:
1) If there are air grooves a. Adjust injection speed b. Modify sharp edges of transition position c. Shallow depth of engraved mark.
2) If air marks are close to gate a. Reduce screw rotation speed when applying pressure b. Reduce pressure c. Use a stop valve.
3) Whether there are bubbles in plastic melt: a. Increase pushing pressure b. Check plastic material and its conveying conditions c. Increase plasticizing amount.
4) Provide sufficient exhaust;
5) Change gate position.

 

Definition: Color deviation of plastic part (darker or lighter), or surface gloss of product does not match, as shown in Figure 5.
Natural causes: During coloring process, color masterbatches form blocks, resulting in uneven mixing, which will cause color difference marks. Plastics, injection molding parameters and other additives will affect mixing of color masterbatch and plastic, resulting in undesirable color. Chromatic aberration marks are often caused by inability of dyeing material to completely blend with rubber compound. Similar to thermoplastic rubber materials, color masterbatch and dye powder are extremely sensitive to excessive processing temperature and excessive residence time, such as cause of color difference marks caused by thermal damage, and treatment methods are same as scorch marks. Excessive stress in injection molded parts, such as caused by excessive demoulding force or distortion, will also cause color differences, because light will be intercepted differently in deformed area than in other areas, causing visual differences. In terms of pre-coloring ingredients, problem of uniform composition also causes color difference.
Solution:
1 Improve mechanical fusion of plastic;
a Increase pushing pressure and speed;
b Increase injection speed;
c Use finer gates and main channels;
2 Coloring;
a Use smaller masterbatch;
b Use color paste or pellets;
c Use finer pellets;
3 Use smaller rubber particles;
4 Replace injection molding machine or plasticizing system;
a Increase L/D ratio;
b Use a screw with shearing and mixing effects;
c Use a dense mixer and stop valve.

 

Definition: There are hemp or strip-like glass fiber patterns on the surface of plastic part, as shown in Figure 6.
Natural cause: Because shape of glass fibers is slender, their arrangement direction during injection molding process will be affected by injection flow direction and is consistent with flow direction of rubber. If molten material suddenly solidifies when it contacts mold wall, part of glass fibers will not be sealed by rubber. In addition, surface of plastic part will become rough due to huge difference in shrinkage between glass fiber and plastic (shrinkage ratio of fiber to plastic is 1:200). Because glass fiber will hinder shrinkage of plastic during cooling, especially in warp direction of fiber, this will cause unevenness on plastic part.
Solution:
1) Optimize injection molding process and change filling material
a Increase injection speed
b Increase mold wall temperature
c Increase melt temperature
d Optimize holding time
2 Whether glass fiber mark appears near fusion line
a. Use shorter glass fiber
b. Use glass beads
c. Use compatible materials of same series
3 Check possibility of using a dual-material injection molding process (and sandwich injection molding process) to produce injection molded part.

 

Definition: Small bubbles formed by gas entering plastic part, as shown in Figure 7.
Natural cause: During injection molding process, gas is trapped in melt and causes air (bubbles) in injection molding or causes injection molded part to bend and deform.
Solutions:
1) Solutions to vacuum bubbles
a. According to wall thickness, determine reasonable gate and runner size. Generally, gate height should be 1/2 to 1/3 of product wall thickness.
b. Keep residual amount of barrel injection glue at 3 to 8 mm.
c. Injection time should be slightly longer than gate sealing time.
d. Reduce injection speed and increase injection pressure.
e. Use materials with high melt viscosity grade.
f. Use gas injection molding method.
2) Solution to bubbles caused by generation of volatile gases is mainly to use
a. Fully pre-dry.
b. Reduce resin temperature to avoid generation of decomposition gas.
c. Bubbles caused by poor fluidity of rubber can be solved by increasing temperature of resin and mold, increasing injection speed, etc.
3) Bubbles caused by exhaust problem of injection mold can be solved by increasing exhaust of mold.

 

Definition: Vacuum bubbles (not bubbles) generated inside thick wall of plastic part, as shown in Figure 8
Natural cause: When shrinkage (shrinkage amount) of material due to thermal effect cannot be compensated during cooling process of plastic, voids will be formed in corresponding parts.
If part becomes solid due to rapid cooling or suitable shape, they cannot be pulled inward by stress generated by cooling process of injection molded part, but cooling stress may tear uncondensed part of injection molded part, resulting in a vacuum cavity.

 

Solution:
To properly transmit holding pressure, gate of injection molded part must be designed at position with the largest cross-sectional area. If this is not possible, a flow acceleration groove can be used between gate and storage position to improve transmission of holding pressure. To prevent gate and main channel from solidifying prematurely, there must be enough flow area to remove accumulated molten material. Use ratio of current glue position thickness and rib thickness.

Definition: Line generated by plastic part at joint of molten material flow as shown in Figure 9
Natural cause: When two or more melt flows meet, a fusion line will be formed. When encountering other melt flows, arc-shaped flow peaks will be flattened and bonded with other melt flows. In this process, high-viscosity flow peaks will be stretched. If temperature and pressure at melt flow combination position are not sufficient, corners of front end of melt flow will be difficult to fill. On a smooth surface, gap along fusion line can be clearly seen, while on a structural surface, there will be a difference in gloss on the surface of fusion line. In addition, because combination position of melt flow is not a single-phase fusion, there is a chance that a weak point will be formed. For example, for plastics using additives, additives will be neatly arranged near fusion line due to flow direction, resulting in more obvious color deviation near fusion line.
Solutions are as follows:
1) Improve molding by improving fluidity, such as increasing barrel temperature, mold temperature, injection pressure and speed;
2) Add venting grooves;
3) Minimize use of mold release agents;
4) Set process overflow at location where fusion line is generated and remove it after molding;
5) If it only affects appearance, gate position can be changed to change location of fusion line. Or location where fusion line is generated can be treated as a dark glossy surface for modification;
6) If it is a hole shape, method of beer and then drilling can be used to solve problem of fusion line strength.

Definition: Gloss of a local position or area on the surface of plastic part is dull (dim) or shiny, color of plastic part is deviated (darker or lighter), or gloss of product surface does not match, as shown in Figure 10.
Natural causes: Reflection degree of surface of injection molded part to light directly reflects its brightness. After light is projected onto the surface of injection molded part, it will change direction (reflection). The smoother surface of injection molded part, the smaller scattering angle of reflected light will be, and the rougher surface, the greater scattering angle will be. The smoother surface, the higher brightness. To achieve this effect, polished mold wall must have a clear projection, while etched mold wall does not need it. Uneven brightness is caused by molten material contacting uneven mold wall of cooling system and different shrinkage of injection mold.
If injection molded part is extended due to distortion after cooling, uneven brightness will also occur.

 

Solution:
1) Increase mold surface temperature;
2) Increase mold temperature;
3) Increase holding pressure;
4) Check holding time;
5) Optimize holding position to ensure that holding can be switched after injection reaches 98%;
6) Optimize injection rate;
7) Improve plasticization effect of rubber by adjusting back pressure and melt speed.

Definition: Snake-like jetting on the surface and inside of rubber part, as shown in Figure 11.
Natural cause: Jetting occurs because flow crest fails to form completely in mold cavity. From gate, molten material is injected into mold cavity uncontrollably. At the same time, molten material cannot be completely melted with plastic that enters mold cavity after cooling and solidification, thus forming jetting. This defect is common in injection molded parts with rapidly increasing cross-sectional area filled by high-speed injection.

 

Solution:
1) Forming conditions
a Increase resin temperature and reduce resin viscosity;
b When using non-crystalline resin, it is better to set mold temperature to about 20-30℃ lower than thermal deformation temperature of resin.
C Reduce injection speed.
2) Mold
a In order to reduce flow rate of resin through gate, cross-sectional area of gate can also be increased.

Definition: Wavy lines on the surface of plastic part are shown in Figure 12.
Natural cause: When molten material is injected into colder mold cavity, layer of plastic immediately following flow crest will solidify due to rapid cooling. This lower temperature solidified outer layer will cool molten material near flow crest of mold wall. If this situation occurs in a short period of time (especially when injection speed is slow), high viscosity or solidified flow crest will hinder subsequent flow of molten material along mold wall. Therefore, hot melt plastic flowing from back to the front will not be pushed against mold wall.
Solution:
1) Speed up injection speed;
2) Increase injection pressure and maintain pressure;
3) Increase temperature of resin and mold.

 

Definition: Shadows appearing on the surface near gate or step position of plastic part, as shown in Figure 13.
Natural Causes: Main causes of dull spots near gate:
1. Gate is too thin 2. Injection speed is too high.
Solution:
1) Reduce injection speed or use injection curve;
2) Make transition position from gate to cavity into an arc;
3) Increase gate diameter;
4) Change gate position;
5) Increase sol temperature;
6) Change temperature of mold wall.

 

Definition: Periphery or rib position of plastic part is not full of glue (incomplete structure), as shown in Figure 14.
Natural causes: Following natural causes cause under-injection defects in injection molded parts:
1 Amount of plastic injected is too small (such as: shot volume)
2 Flow of material is hindered by exhaust problems
3 Injection pressure and injection speed of injection molding machine are not enough
4 Molten material in flow channel section solidifies prematurely (such as: injection speed is too low, mold temperature is improperly controlled or gate position is wrong).
Solution:
1) Extend injection time to prevent backflow of tree before gate solidifies and difficulty in filling cavity due to short molding cycle;
2) Increase injection speed (must be coordinated with injection position);
3) Increase mold temperature;
4) Increase resin temperature;
5) Increase injection pressure;
6) Increase gate size. Generally, height of gate should be equal to 1/2 to 1/3 of wall thickness of product.
7) Reset gate position (generally set at the thickest position of glue);
8) Reasonably set exhaust groove or exhaust needle;
9) Keep about 3 to 8mm of injection cushion;
10) Choose low viscosity grade materials.

 

Definition: Partial burning (yellow, black) of plastic parts due to poor mold arrangement (air entrapment), as shown in Figure 15.
Natural causes: Burning effect is purely a venting problem, which is common in blind holes, fillets, flow ends, and at confluence of multiple flow waves. Because ejector gap is too small and design is poor, gas in mold cavity cannot be completely or timely discharged through parting surface, exhaust position or ejector gap during injection process. In the later stage of injection molding, these gases will be compressed and generate high temperature, causing plastic to burn.
Solutions:
1) Introduce a feasible exhaust system and check whether exhaust duct is contaminated
2) Reduce injection molding speed
3) Injection molding process can be changed to avoid air entrapment.

 

Definition: Excess glue (overflow) generated around plastic parts, ejector pins, and buckles, as shown in Figure 16.
Natural causes: Mold is not strong enough, tolerance is too large during manufacturing, or sealing position is damaged; mold opening force is greater than clamping force, so that mold cannot remain closed or clamping pressure causes mold to deform; injection molding pressure is too high, so that hot melt glue is pushed into a thin gap; mold cavity pressure is too high and flow resistance of melt is thin, which is easy to produce flashes.
Solution:
1) In order to solve problem of insufficient clamping force, calculate clamping pressure according to following formula and select injection molding machine: clamping pressure projection area x effective injection pressure in mold, effective injection pressure in ABS mold is about 25 tons/in2, and PC is about 27 tons/in2.
2) Increase clamping force;
3) Increase mold strength;
4) If mold is aged, clamping surface accuracy is not enough, and there are more flashes, you can consider overhauling or re-opening mold;
5) In terms of molding conditions, following measures can be taken to improve:
a. Reduce injection pressure and holding pressure
b. Reduce injection time
c. Adjust injection position to avoid excessive injection
d. Reduce barrel temperature and mold temperature
6) Choose plastic with higher viscosity.

 

Definition: Turtle-like cracking marks appearing inside and on the surface of plastic part, as shown in Figure 17.
Natural causes: When deformation of injection molded part is too large (such as affected by external stress or torque), stress reflection or stress cracking will occur. Maximum deformation of injection molded part is affected by type of material used, molecular structure, injection molded part conditions and surrounding climate environment. Material is affected by different temperatures and time during processing process, which makes ability of injection molded part to resist internal and external stresses suddenly decrease. In these processes, chemical strength between molecules will be weakened by processes such as moisture, diffusion and expansion, increasing risk of cracking. In addition to being generated when melt is cooled or in flow process, internal stress can also be caused by expansion. Internal expansion stress is caused by pressure retained in mold cavity. During demolding process. Injection molded part that has been under pressure and residual pressure will suddenly become compressed under atmospheric pressure, which will cause inner layer of injection molded part to pressurize outer layer. Therefore, when residual stress is still large, demolding will cause stress reaction and stress cracking. Insufficient cavity size or excessive pressure in cavity are main reasons for demolding of injection molded parts under high residual pressure.

 

Solutions:
1) Reduce external stress;
2) Switch to holding stage earlier (optimize holding transition point)
3) Reduce holding pressure
4) Shorten holding time (extend cooling time)
5) Change mold wall temperature
6) Change sol temperature
7) Change injection speed
8) Modify shape of injection molded part

Definition: White marks or cracks appear on the surface of ejector pin of plastic part, as shown in Figure 18.
Natural causes: Causes of visible ejector marks can be roughly divided into 4 main groups:
1 Process influences, such as improper adjustment of injection molding machine causing plastic part to be separated from mold prematurely or demolding force is too large;
2 Geometry influences, such as inappropriate ejector length or wrong installation;
3 Mechanical strength influences, such as errors in design and size of mold, injection molded parts and demolding system;
4 Thermal effects, such as too large temperature differences in mold or ejector wall;

 

Solutions:
1) Reduce injection pressure
2) Increase demolding slope;
3) Increase number or area of push rods;
4) Reduce surface roughness of mold;
5) Spraying demolding agent is also a method, but care should be taken not to adversely affect subsequent processes such as hot stamping, coating, etc.

Definition: Bending, sinking, and deformation of structure or horizontal surface of plastic part, as shown in Figure 19.

 

Reasons for mold release deformation can be divided into following categories:
1 Demolding force required for injection molded part is also sufficient to destroy injection molded part;
2 Demolding action is hindered (for example: when injection molded part slides in mold).
Solutions:
1 Demolding under residual pressure;
a. Switch to holding stage early
b. Reduce holding pressure
c. Shorten holding time
d. Extend cooling time
e. Strengthen mold rigidity
2 Ejector pin is stuck in injection mold;
a. Extend cooling time
b. Check ejector pin size
3 It is caused by backlash problem or appears at backlash position;
a. Shorten cooling time
b. Increase ejector pin ejection speed
c. Check demolding system or process
d. Check allowable ductility of material
4 A larger demolding force is required because injection molded part shrinks toward core;
a Shorten cooling time
b Increase holding pressure
c Optimize holding time
d Use different ejection speeds
e Use a coating that reduces friction to reduce demolding force
5 Shrinkage of reinforcing ribs is insufficient and a larger demoulding force is required
a Reduce holding time (use a segmented holding curve)
b Shorten holding time
c Extend cooling time
d Use different ejection speeds
e Increase mold wall temperature along reinforcing rib position
6 A larger demolding force is required due to the surface quality of injection molded part
a Use different mold wall temperatures
b Use different ejection speeds
c Check exhaust of mold core
d Check demolding system
e Check demolding angle
f Use a friction-reducing coating to reduce demolding force
g Use a demolding agent
7 A larger demolding force is required due to material adhering to mold surface
a Polishing direction should match demolding direction
b Change surface roughness
c Use a friction-reducing coating to reduce demolding force

Definition: Straight strip-shaped drag marks appearing on the side of plastic part, as shown in Figure 20.
Natural causes: Surface structure, mold wall thickness and mold release angle are interrelated, and will also affect demolding of structural surfaces.

 

Solutions:
1 Check turning point
2 Shorten holding time
3 Reduce holding pressure
4 Increase cooling time
5 Increase wall thickness
6 Increase mold release angle
7 Use a lower surface finish
8 Strengthen mold rigidity

Definition: Surface layer and inner layer of plastic part are separated and delaminated, as shown in Figure 21.
Natural causes: Surface peeling is caused by insufficient adhesion between solidified plastic layers. Different plastic layers are formed under different plastic flow and cooling conditions. Shear stress, effect damage and material inconsistency can affect adhesion between layers. When it reaches a certain level, skin begins to peel off.

 

Solution:
1 Check whether granules are impure or mixed with other materials
2 Check compatibility of colorant
3 Check moisture content of granules
4 Check consistency and plasticizing ability of melt
5 Reduce injection speed
6 Change sol temperature
7 Increase mold wall temperature

Definition: Cold slugs appear on the surface or inside plastic part, as shown in Figure 22.
Natural cause: Before molten material is injected into mold cavity, it solidifies into cold glue in gate or nozzle, then is injected into mold cavity with next injection cycle. If these cold glues cannot be dissolved again, they will cause marks like comet tails and can spread to any part of injection molded part.

 

Solution:
1 Appropriate decompression
2 Early plasticizing system return procedure
3 Check nozzle temperature
4 Increase nozzle temperature
5 Increase nozzle cross-sectional area
6 Extend gate runner
7 Use stop valve

Definition: Product runner or gate appears like hair-like plastic, as shown in Figure 23.
Natural cause: Phenomenon of filamentation is caused by insufficient cooling of gate or main flow channel. Flowing molten material in cross section of injection molded part or gate is not properly cooled and solidified, and is pulled out by the solidified part, forming filamentation.

 

Solution:
1 Is there a wire drawing phenomenon at gate or injection nozzle position
a. Change nozzle temperature
b. Eject accumulation immediately or later after storing material
c. Extend holding time
d. Reduce melt temperature
e. Change injection pressure and screw speed
f. Optimize cooling process near main channel
g. Extend cooling time
2 Is there a wire drawing phenomenon at hot runner position
a. Reduce hot runner nozzle temperature
b. Extend holding time
c. Check position of nozzle end and gate sleeve
d. Check cross section of gate
e. Optimize cooling time at gate position
f. Check temperature sensor
g. Check whether nozzle section is blocked
h. Extend cooling time

Definition: Black or dark spots produced by white or transparent plastic parts, as shown in Figure 24.
Natural causes:
1 Process impact: Sol temperature is too high or time it stays in plasticizing system is too long, or heating program is wrong when using hot runner;
2 Mold impact: Gate system is contaminated or hot runner system has dead corners;
3 Injection molding machine impact: Plasticizing system is contaminated or screw and barrel are corroded;
4 Plastic or fuel impact: Pellets are mixed with impurities, proportion of recycled materials is too large or inappropriate colorants or masterbatches are used.

 

Solutions:
1 Avoid insufficient pellet purity
2 Reduce Sol temperature
3 Change barrel temperature
4 Change screw speed
5 Reduce push pressure
6 Shorten injection cycle
7 Increase storage time delay
8 Use a plasticizing system with a smaller capacity
9 Check hot runner temperature
10 Reduce amount of recycled materials
11 Check compatibility of dyes
12 Check whether plasticizing system, gate system and hot runner contain impurities, corrosion and dead corners, etc.

Definition: Material that cannot be fused to the surface of plastic parts or molds and venting grooves, as shown in Figure 25.
Natural Cause: In processing of thermoplastic materials, fouling is a common defect. It often occurs when using materials such as POM PP PET ABS PC PSU PBT and PE. Materials with additives such as fire retardants, lubricants and colorants are prone to fouling.

 

Solution:
1) Lower melt temperature
2) Shorten cycle time
3) Reduce moisture content of plastic
4) Ensure sufficient venting
5) Use suitable lubricants
6) Check compatibility of materials, colorants and additives.

Definition: Defect of whitening or obvious damage on sheet product or the bent part, as shown in Figure 26.
Natural Cause: Flake hinges are mainly caused by excessive stress in plastic. They may partially or completely break, and excessive stress may also lead to stress whitening.

 

Solution:
1 Insufficient filling:
a Increase injection speed
b Increase melt temperature
c Increase mold wall temperature
d Increase wall thickness of flap hinge
e Use materials with better fluidity
f Move gate away from flap hinge
2 It takes a greater force to bend flap hinge
a Reduce wall thickness of flap hinge
b Use materials with a lower rigidity coefficient
c Check design of flap hinge
3 Flap hinge breaks immediately or after bending several times:
a Move fusion line away from flap hinge
b Increase injection speed
c Change mold wall temperature
d Pull flap hinge back immediately after injection molding
e Use materials with high viscosity
f Ensure that flow wave peak flows evenly through flap hinge
g Check design of flap hinge
4 Stress reflection appears on flap hinge: Bend under mild conditions Folding hinge.

Definition: Bending, sinking, or deformation of a plastic part structure or horizontal surface, as shown in Figure 27.
Natural cause: Usually caused by different shrinkage ratios on injection molded part.

 

Solution:

Natural cause: Defects from manufacturing process of injection molded part (bubbles, bubbles, and insufficient adhesion of fusion layer) are very difficult to distinguish on electroplated injection molded parts, especially when there are uneven distribution of bubbles and bubbles on the entire injection molded part. Usually all defects on injection molded part can be seen on electroplated part. In many cases, electroplating not only fails to hide these defects, but makes them more likely to emerge. It should be noted that some defects are not easy to see on injection molded part before electroplating, but are easier to see after electroplating. Therefore, to avoid these defects, all edges and transition positions should be rounded as much as possible, and large changes in wall thickness should be avoided. Injection molded parts must be produced under low stress conditions, special attention must be paid to material drying method, and supplier's instructions must be followed.
Solutions:
1) Solutions to cracking:
a. Basic solution is to remove resin flash for electroplating;
b. Cracks that occur in high temperature environments: plating solution should be adjusted, ductility of precipitation film should be increased, and electrodeposition stress should be reduced;
c. Cracks that occur in low temperature environments: thickness of metal layer should be thinned to reduce binding force;
d. In order to reduce expansion difference with metal layer, it is necessary to select a grade product with a low linear expansion coefficient;
e. Attention should be paid to residual plating solution of resin parts (especially raised parts, etc.), and design should be changed according to situation to avoid chemical erosion. In addition, reducing residual strain that occurs during molding is also a relatively effective method.
2) Do bubbles and air bubbles appear on any part of molded part?
a. Check drying process of material (moisture marks)
b. Check for air marks
c. Use other materials
d. Check electroplating process
e. Use recycled materials
3) Do bubbles and air bubbles only appear in same location
a. Check mold for structural damage and repair if necessary
b. Where possible, use mold steel remelted under vacuum
4) Rubber bubbles and air bubbles do not appear in same position
a. Check mechanical damage of parts
b. See Section 7 Bubbles
c. No release agent is used
5) There are adhesion problems at locations with high internal stress or directional alignment (such as corners, edges, parting surfaces)
a. Change injection speed
b. When using non-crystalline materials, mold wall temperature can be increased
c. Increase melt temperature
d. Optimize pressure transfer point
e. Shorten holding time
f. Reduce holding pressure (use segmented holding curve)
g. Avoid different wall thicknesses
6) Adhesion problems appear in same location
a. Check processing method
b. Check for scale buildup
c. Check exhaust system
7) There is an adhesion problem near gate: See Matting Spots Near Gate
8) Cracks occur
a. Avoid deformation during transportation or assembly
b. Avoid internal stress, as above
9) Check for contamination of accumulations or materials
10) Check for heat damage to plastic, see scorch marks.