In the daily injection molding process, we often encounter various quality issues, including minor surface defects and more serious quality issues, which affect the performance and safety of products. They are usually caused by problems with molding processes, material use, mold design, or a combination of the three. The optimal process settings are crucial for the final productivity level, part quality, and final injection molding costs. Systematically answer common injection molding problems and solutions.

Question 1: Short shot

A short shot occurs when the molten material stream does not completely fill the cavity in the mold, resulting in incomplete molded parts after cooling. This effect is often far from the gate, especially if it involves long flow distances, thin walls (or combinations), or molded parts filled with polymers, resulting in defects in the functionality or appearance of the product

Common reasons may be:

1. Insufficient amount of material injected

2. Insufficient injection filling speed

3. Insufficient pressure

4. Insufficient injection time

5. Unbalanced multi-cavity mold

6. Foreign matter blocking the nozzle

7. The injection temperature is too low

8. The gate or vent is too small

9. The mold temperature is too low

10. Excessive wear of screw and barrel

Solution:

Redesign the mold with wider channels or gates to achieve better fluidity

Increase injection speed or pressure or select a thinner substrate to improve fluidity

3. Increase the mold temperature to prevent excessive cooling of the material

Add additional vents or expand existing vents in the mold to allow trapped air to escape

Question 2: Injection

Spray refers to a deformation in a molded component that begins to solidify before the cavity is filled when the initial "jet" of molten material is injected into the mold cavity. Spraying typically manifests as a wavy line on the surface of the finished part, typically starting from the initial gate of the injection. This visible flow pattern may lead to partial weakness.

The reason for this phenomenon is that when a molten polymer or other material is injected through a small gate under high pressure, it usually quickly ejects from the gate rather than gradually filling the mold cavity. As the first row of this material cools the mold wall and begins to harden, the remaining mold material pushes it, leaving an impression on the surface of the finished component.

Solution:

Reduce the injection pressure to prevent rapid injection of material into the mold cavity

Increase material and mold temperatures to prevent early solidification of initial material jets

Design the mold using an injection gate so that the material passes through the mold rather than longitudinally

Question 3: Flash

Flash (also known as "splash" or "burr") is excess molding material that appears as thin lips or protrusions at the edges of the component. The flash occurs because the material has flowed outside the predetermined flow path and into the space between the tool plates or injector pins. Flashes usually occur near seams, sealing surfaces, exhaust passages, or injectors.

Common reasons may be:

1. Injection pressure too high (filling or packaging)

Foreign matters (pollutants, dirt, etc.) on the mold surface

3. Injection molding and mold temperatures are too high

4. The viscosity of resin melt is too low

5. The surface of the closed mold does not match

Solution:

If the steel plates cannot fit together properly or allow material to flow out of the channel, the mold can be redesigned

Increase plate clamping force to limit material flow path

Adjust mold temperature, injection pressure, and ventilation to improve material flow

Question 4: Sink marks

The sink trace is an aesthetic flaw in the surface. They appear as depressions, usually appearing in the thicker parts of the molded part. The root cause is usually shrinkage of the material during crystallization.

Sedimentation marks are mainly caused by thermal shrinkage (shrinkage) during the cooling process. After the external material has cooled and solidified, the internal material begins to cool. Its contraction pulls the external material (surface) inward, causing sink marks.

Common reasons may be:

Insufficient injection pressure

2. Insufficient pressure holding time

3. Insufficient material quantity

4. Insufficient cooling or injection time

5. The injection speed is too high

6. Melt or mold temperature is too high

7. Poor part design, non-uniform walls, and/or excessive wall thickness

Solution:

Increase the holding pressure and time to cool the material near the surface of the part

Increase cooling time to limit shrinkage

Design the mold with a thinner component wall for faster cooling near the surface

Question 5: Voids (bubbles)

Voids appear as small bubbles or bubbles in molded products, usually caused by air, gas, or vacuum. If the item is highly transparent, the gaps become more apparent. One of the main reasons for cavities is that the molding pressure is not sufficient to force trapped air out of the mold cavity.

Common reasons may be:

1. Failure to completely fill the mold

Poor mold exhaust, especially around protrusions

3. The filling rate is too fast (trapped air produces short shots)

4. Mold temperature mismatch

Excessive component thickness

6. Wrapped with water

Porous or very fine additive powders that absorb air wrap the air

Solution:

1. Increase injection pressure to forcibly trap bubbles

Select materials with low viscosity to limit the risk of bubble formation

Place the gate near the thickest part of the mold to prevent the material from being most susceptible to premature cooling of voids

Question 6: Warpage

Warpage is a possible deformation of an injection molded product when different parts of the component contract unevenly. Just as wood can warp when it dries unevenly, plastics and other materials can warp during the cooling process because uneven shrinkage can exert unnecessary pressure on different areas of the molded component. This improper pressure can cause the finished part to bend or twist when cooled. One of the main reasons for warpage of injection molded plastics and similar materials is that cooling occurs too quickly. High temperatures or low thermal conductivity of molten materials can worsen the problem.

Solution:

1. Ensure that the cooling process is gradual and long enough to prevent uneven stress on the material

2. Reduce the temperature of the material or mold

3. Try switching to a material that shrinks less during the cooling process (for example, the shrinkage rate of thermoplastic filled with particles is much lower than that of semi crystalline or unfilled materials)

4. Redesign a mold with uniform wall thickness and partial symmetry to ensure greater stability of the parts during cooling

Question 7: Burn marks

Burn marks are defects caused by polymer degradation during injection molding, resulting in black or rust discoloration on the edges or surfaces of molded plastic components.

Common reasons may be:

1. The injection pressure is too high

2. The injection speed is too high

3. The back pressure is too high

4. The temperature is too high

5. Insufficient exhaust

6. Improper flow path location or design

7. Other material pollution

Solution:

Reduce melt and mold temperatures to prevent overheating

Reduce the injection speed to limit the risk of air retention within the mold

Enlarge the gas vent and gate to allow trapped air to escape from the mold

Shorten the mold cycle time so that any air and resin will not overheat

Question 8: Discoloration

Discoloration or "discoloration" occurs when the molded component is different from the expected color. Discoloration is usually limited to localized areas or several abnormal colors on the molded part. This defect usually affects the appearance of the part without reducing its strength.

Common causes of discoloration are residual particles in the hopper or residual resin in nozzles or molds during previous production processes. Other possible reasons are poor thermal stability of the colorants or improper mixing of the masterbatch.

Solution:

1. Ensure that workers properly clean the hopper, nozzle, and mold between production runs to eliminate any residual particles or substrate

2. Consider using a cleaning agent to remove excess color from the machine

Ensure that you or your supplier use colorants with sufficient thermal stability

4. Ensure that the color masterbatch is evenly mixed to achieve a consistent color output

Question 9: Welding line

Welding lines represent optical and mechanical defects in molded parts. Welding lines typically occur in areas where polymers flow together during injection molding. Flow marks occur due to turbulent flow during injection. Welding lines can appear on the surface of molded parts where molten material splits into two or more directions in the mold and converges. Hairy welding lines are the result of weak adhesion of the material, which reduces the strength of the component.

Common reasons may be:

1. The injection temperature is too low

2. The mold temperature is uneven or too low

3. Mold filling too fast or too slow

Too much mold release agent or lubricant

5. The mold surface is scratched or not clean

6. Filling speed and/or time is too low

7. Insufficient exhaust

Solution:

Increase material temperature to prevent local solidification

Increase injection speed and pressure to limit cooling before filling the mold with material

Redesign the mold to eliminate zoning

4. Switch to a material with a lower melting point or viscosity to allow faster flow and prevent early cooling

Question 10: Tiering

Delamination, sometimes referred to as lamination or delamination, is a defect on the surface of a molded component that can be peeled off layer by layer. Similar to schistose mica. This is often considered a fairly serious defect because it reduces the strength of the component.

The most common cause of delamination is contamination of resin particles or other substrates with foreign materials. Flaky separation occurs when the two materials cannot be bonded correctly. For example, common base plastics such as acrylonitrile butadiene styrene (ABS) are used in combination with incompatible plastics such as polypropylene (PP).

Possible causes:

1. High shear stress

2. Incompatible materials mixed together

3. Excessive use of release agent

4. Excessive moisture heats up and forms steam, resulting in surface delamination.

5. Material degradation

6. Caused by excessively high injection speed, residence time, or melt temperature.

Incorrect screw or runner system design can also lead to material degradation.

Solution:

1. Eliminate degradation and excessive shear stress

2. Reduce shear stress

3. Eliminate excessive moisture

4. Material suppliers can provide the best drying conditions for specific materials.

5. Reduce recycled materials

6. Avoid excessive use of release agents

Repair drainage systems or other issues to eliminate mold release difficulties, rather than overusing mold release agents.

8. Avoid material contamination

11. Streamlin

Streamlines appear as wavy patterns, usually slightly different in color from the surrounding area, and typically on the narrower portion of the molded component. They may also appear as annular bands on the product surface near the mold entry point, or as "gates" through which molten material flows. Flow marks generally do not affect the integrity of the component. However, if they are found in certain consumer products, such as high-end sunglasses, they may be ugly and may not be acceptable.

Streamlines are typically the result of changes in the cooling rate of the material as it flows in different directions throughout the mold. Different wall thicknesses can also cause materials to cool at different rates, leaving streamlines behind. For example, molten plastic cools very quickly during the injection process, and when the injection speed is too slow, flow traces are evident. While still filling the mold, the plastic becomes partially solid and sticky, resulting in ripples.

Solution:

Increase injection molding speed, pressure, and material temperature to ensure that the material fills the mold before cooling

Increase the wall thickness of the mold around the corners to help maintain consistent flow and prevent flow lines

Reposition mold gates to create more distance between them and the mold coolant to help prevent premature cooling of the material during the flow process

4. Increase nozzle diameter to increase flow rate and prevent early cooling

12. Groove

A groove is a surface defect in which a "ring" appears on the surface of a molded part, mainly near the needle point gate and concentrically diffuses onto the molded part.

Possible causes:

Insufficient inventory temperature

2. Insufficient injection speed

3. The mold temperature is too low

Improper injection port position or design

13. Air stripe

Air streaks in molded parts exhibit matt, silver, or white streaks (stripes) on the surface of the molded part. They can usually be found where the wall thickness of domes, ribs, and molded components may vary. They can also appear near the gate or near engraving and recesses.

Possible causes:

1. Insufficient exhaust

2. The injection speed is too fast

3. Air suction

4. Excessive moisture in the material

5. The mold temperature is too low

14. Moist stripe

Moist streaks may appear on the surface of the molded part because the U-shaped profile opens against the direction of flow. They typically exhibit silver stripes with rough or porous surfaces. The moisture streaks caused by moisture on the mold surface appear as large and matt layered structures.

The main reason is the condensation of water in the resin and the mold surface.

Solution;

Reduce the moisture content in the resin

15. Color Stripes

Due to the uneven distribution of color pigments in the product or the different orientations of isotropic pigments in the molded product, color streaks may appear on colored components. Thermal effects (pigment degradation) can also cause different color shadows to appear on items.

Possible causes:

1. Color dispersion or insufficient distribution

2. Incorrect masterbatch

3. Improper design

4. Lack of dispersed lubricants

5. The temperature is too high

6. Injection pressure too high

16. Stress whitening

Stress whitening is caused by stress cracking in polymer materials caused by internal and external stresses. In PP, stress whitening typically occurs with microcracks between the copolymer as a matrix and the rubber phase. A typical appearance location is the release point. Stress cracks caused by internal stress can typically occur a few days or even weeks after the production of the relaxation process for molded parts.

Possible causes:

1. Characteristics of plastic itself (copolymer, impact resistance, etc.)

Residual stress caused by molding conditions (e.g., melt temperature, part design, resin flow behavior, etc.)

3. Chemical resistance (ESCR)

17. Gloss difference

Gloss differences typically occur due to differences in wall thickness of molded parts and subsequent different cooling rates in different areas.

Possible causes:

1. The fluidity of the mold surface

2. Processing temperature

3. Variable cooling conditions

4. Residual stress in the mold

18. Sticking in the mold

The molded part may stick to the mold and not eject correctly. This usually occurs when the shrinkage level is too low or too high to be easily sprayed, or when the shape and surface finish of the molded product create a "sticking" effect in the mold itself.

Possible causes:

1. Outer packaging - injection pressure too high

2. Underlayer packaging - excessive shrinkage

Insufficient cooling

4. Highly polished core surfaces>wire drawing and polishing

5. Irregular mold surface

6. Insufficient center and wall taper

7. Unreasonable design of cutter retraction groove

It should be noted that the problems and solutions encountered during the injection molding process are not unique, but are also affected by machine usage, machine settings, mold type and complexity, materials, and the production environment.

Generally, injection molding requires a significant upfront investment in molds. Therefore, it is particularly important to design the mold for the first time, and try to avoid redesigning it after finding serious defects. In contrast, defects related to molding processes or materials are often easier to resolve and cost less. However, regardless of the cause, defects in molded products can greatly limit the production of molded devices. The above 18 common injection molding defects and solutions will help ensure smooth and qualified production of injection molding products.