20 common injection molding defects and their solutions

Injection molding, also known as injection molding, is a well-developed material forming method.

Advantages: fast production speed, high efficiency, automatic operation, unrestricted shape and size, accurate product size, easy product replacement, and the ability to form complex shaped parts.

Application: Injection molding is suitable for mass production and complex shaped products.

Defect type

1. Short Shot
2. Flash
3. Diesel Effect
4. Cold Slug
5. Trapped Air
6. Jetting
7. Record Effect
8. Tiger Lines/ Tiger Stripes
9. Welding Line
10. Delamination
11. Glossy Spots
12. Matted and Rough Surface
13. Burning Streaks/ Silver Streaks
14. Color Streaks
15. Grey and Dark Streaks
16. Moisture Streaks
17. Voids/Sink Mark
18. Microcracks, Stress Whitening
19. Warpage
20. Demolding Problem

1. Short Shot

Short Shot: Plastic does not completely fill the mold

The surface of short shot for injection molded parts is enlarged 7 times

The surface of short shot for injection molded parts is enlarged 50 times

Defects caused by

• Low melt flow
• Unreasonable design of mold exhaust structure
• Too narrow mold, high resistance to melt flow
• Insufficient injection pressure
• Unreasonable hot runner size

Solution

• Improve injection speed and melt flow
• Improve the exhaust system of the mold
• Increase the mold wall thickness and switch to a grade where the melt flows easily
• Increase the injection pressure
• Enlarge the cross section of the main flow channel and manifold

2. Flash

Flash: The flash is usually formed on the edge of the part or on the parting line of the mold.

Defects caused by

• Large mold fit tolerance
• Mold parting line damage
• Insufficient mold clamping force
• High melt temperature
• Too fast injection speed
• High internal pressure of the mold

Solution

• Avoid over-injection causing material buildup
• Open a gate at the thickest wall thickness for injection
• Position the fusion line out of sight
• Increase the cross section of gates and runners

3. Diesel Effect

Diesel Effect: Black marks in the end zone or cutoff area, due to burning by overheated compressed air.

Lack of mold ventilation, partial charring of ribs

Poor venting at the end of the runner

Defects caused by

• No mold exhaust port, or poor mold exhaust
• Injection speed is too high
• Air cannot escape from the mold parting surface

Solution

• Install or optimize the mold ventilation system and clean the flow path of the ventilation system
• Reduce the injection speed or injection pressure when the mold is about to be filled
• Rationalize gate location and mold wall thickness
• Temporarily reduce the closing pressure

4. Cold Slug

Cold Slug: The mark produced by the injection of the molten plastic into the cavity after it has begun to cool.

Defects caused by

• Nozzle is not heated or the heating on the nozzle is too small
• Machine nozzle or hot runner tip leakage
• Machine nozzle has no draft angle

Solution

• Install a heater band on the machine nozzle
• Reduce back pressure
• Install a pin closure device for the machine nozzle and hot runner
• Optimize the geometry of the machine nozzle runner
• Optimize gate design
• Avoid the use of tunnel gates when forming fragile materials

5. Trapped Air

Trapped air: The air bubbles formed during the injection molding process cannot be discharged.

Defects caused by

• Air is trapped during mold filling due to improper mold shape
• Insufficient mold venting

Solution

• Optimize the design of mold shape with the help of mold flow calculation
• Improve the design of the mold venting system
• Check the condition of the vent (for the presence of mold deposits)

6. Jetting

Jetting: The melt front is ejected directly from the gate in the cavity without any wall contact (no laminar flow). The defect is visible on the surface of the molded part and shows a serpentine melt jet or fusion line.

Defects caused by

• Improper gate location
• Small gate cross section
• High injection speed

Solution

• Change the position of the gate so that the melt flows down the wall of the mold
• Install an auxiliary core or pin across the gate
• Increase the cross section of the gate
• Reduce the injection speed to normal value

7. Record Effect

Record Effect: A ripple-like groove appears on the surface, mainly at the end of the runner.

Defects caused by

• Melt and mold temperature and injection speed are too low
• Too small flow cross section
• Too long flow path
• d. Insufficient pressure at the end of mold filling

Solution

• a. Increase melt temperature and mold temperature, increase injection speed
• b. Expand the flow cross section
• c. Use machines with appropriate hydraulic systems

8. Tiger Lines/ Tiger Stripes

Tiger Lines/Tiger Stripes: Concentric rings radiating outward from the sprue, light and dark. This molding problem occurs mainly in styrene copolymers and blends

Defects caused by

• The cross section of the gate and runner is too small
• Mold wall thickness is too low
• High pressure loss in the machine nozzle and runner system
• Unsuitable processing conditions

Solution

• Enlarge the cross section of the runner system and gates
• Increase mold wall thickness
• Increase melt temperature and inject at low to medium speed

9. Welding Line

Welding Line: due to the low temperature of the multi-strand melt convergence generated by the gap

Defects caused by

• Convergence of two or more melt strands of different temperatures
• Injection speed is too fast or too slow
• Low melt flow

Solution

• Position the melt marks in a position where they are not visible or do not negatively affect the mechanical properties by designing the gate position properly.
• Optimize mold/melt temperature and injection speed
• Improve the design of the mold venting system
• Check the condition of vent hole mold deposits

10. Delamination

Delamination: The wall of the part is delaminated. If the surface of the molded part is scratched with a blade, it will delaminate.

Defects are caused by

• Contamination by incompatible materials or impurities
• High shear stress can lead to layer formation even in compatible systems

Solution

• Increase the melt temperature
• Reduce the injection speed
• Increase the cross-section of the flow channel or the wall thickness of the mold
• Try to avoid contamination by impurities

11. Glossy Spots

Glossy Spots: Uneven luster or color of the product

Defect generation causes

• Material buildup, such as sudden changes in wall thickness, ribs, and fixed tabs can create dents, which can lead to glossy spots or differences in gloss
• Gates and gate cross-sections are too small
• Mold surface temperature, melt temperature and injection speed are too low
• Insufficient holding time
• Due to the quality of the inner wall of the mold

Solution

• Avoid material build-up
• Cast molded parts in the area with the thickest walls
• Increase cross section of gates and runners
• Increase mold temperature, melt temperature and injection speed
• Optimize holding pressure and pressure retention time

12. Matted and Rough Surface

Matted and Rough Surface: A surface that is rough, lusterless or coarse. This problem is most common in glass fiber reinforced materials

Defects caused by

• Cold mold temperature
• Low injection speed
• Low melt temperature

Solution

• Increase the mold surface temperature
• Fill the cavity with a higher injection rate
• Increase melt temperature

13. Burning Streaks/ Silver Streaks

Burning Streaks / Silver Streaks: the surface of the parts have silver or black stripes

Defects caused by

• Damage to the surface of the product due to high temperature or long residence time, resulting in gaseous products
• Excessive shear heat at the gate and hot runner
• Too fast injection speed will produce degradation gas, too slow will produce silver wire
• Unreasonable gate position

Solution

• Check the temperature control
• Install a small plasticizing device
• Reduce the injection speed
• Reduce back pressure and screw speed
• Lower the melt temperature
• Optimize the cross section of the gate

14. Color Streaks

Color Streaks: The appearance of color streaks

Defects caused by

• Uneven color distribution
• Contamination of the material
• “Dead spots” in the plasticizing device or hot runner that can overheat the material

Solution

• Master batch is not suitable for the material and actual melting temperature
• Use a screw with better mixing capability or a nozzle with mixing parts
• Clean the screw, check valves, nozzles, cylinders and hot runners to reduce the presence of impurities
• Redesign the plasticizing device and hot runner system

15. Grey and Dark Streaks

Grey and Dark Streaks: Grey to black streaks on the surface of the part

Defects caused by

• The screw feed system is too long or too deep, and the molten material is already molten so that air is trapped in the melt
• Dead spots in the plasticizing device or hot runner system
• Screw retraction stroke is too long (air intake)
• Wear of the plasticizing device

Solution

• Use a suitable screw
• Reduce the temperature of the feed system
• Increase the back pressure
• Use corrosion and wear resistant plasticizers

16. Moisture Streaks

Moisture Streaks: Oval-shaped silver streaks on the surface of the part

Defects caused by

• High moisture content of the material

Solution

• Pre-dry the material to the moisture content recommended by the material supplier
• Increase back pressure (helps with styrene polymers, but not recommended for PBT and PA)

17. Voids/Sink Mark

Voids/Sink Mark: Small cavities inside the molded part, usually invisible from the outside unless it is a transparent material. Internal cavities are formed when the outer skin is rigid, and surface depressions are produced when the outer layer is normally rigid.

Defects caused by

• Insufficient holding pressure
• Material buildup
• Improper gate position
• Too small runner cross section
• Difference in mold surface temperature

Solution

• Check melt buffer, increase holding time, optimize melt and mold temperature
• Avoid material build-up due to excessive wall thickness difference, optimize rib design: 0.5 times the basic wall thickness is recommended
• Gating should be set in the thick wall area
• Design reasonable gate and gate cross section
• Check mold surface temperature, cool thick-walled parts with cold water, optimize cooling/heating system

18. Microcracks, Stress Whitening

Microcracks, Stress Whitening: White dents or cracks on the plastic surface due to stress.

Defects caused by

• The material holding time is too long and the material is soft
• The pressure on the surface of the product is too high when the ejector rod is released from the mold, resulting in indentation

Solution

• Optimize holding pressure
• Select the top bar with larger cross section
• Optimize mold design to avoid edge gnawing

19. Warpage

Warpage: Failure of a molded part to retain the desired shape. Straight edges twist to the inside or outside

Defect causes

• Difference in wall thickness of mold part
• Difference in mold surface temperature
• Different shrinkage difference due to loss of injection pressure caused by long flow path
• Difference in orientation of glass fiber in the flow direction and perpendicular to the flow direction

Solution

• Make the mold wall thickness as uniform as possible to avoid material build-up
• Optimize the mold heating system by using multiple heating circuits to equalize the temperature difference at each location
• Reduce the length of the runner by increasing the number of gates
• Perform mold filling studies by partially filling the mold and reconstructing the fiber orientation, which can be done by adding flow aids or flow restrictors
• Use of materials with low warpage tendency

20. Demolding Problem

Demolding Problem: Parts are damaged or have obvious ejector marks on the surface

Defects caused by

• Low number of ejector bars or unreasonable position
• Scratches and edge gnawing in the gate system or mold, roughness of the inner wall causing difficulties in mold release
• Insufficient ejector bar release angle
• The part is not sufficiently cooled before demoulding
• Excessive ejector release speed and pressure
• Overfilling

Solution

• Increase the number of ejector bars and increase the release angle appropriately
• Reduce the internal roughness of the mold
• Fully cool down before demoulding
• Properly control the holding pressure to avoid too high
• Slow down the demoulding speed and reduce the ejector bar demoulding force.

Finally, the 20 most common problems in injection molding described above are related to the choice of materials, in addition to unreasonable design. Therefore, in order to better avoid these problems, we have compiled a list of 20 materials commonly used in the injection molding industry, with detailed information on the application areas, process conditions, chemical and physical properties of each material. You can find out more about them by clicking here.

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