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Common Problems and Solutions of Injection Molding - Part 2

Injection Molding Defect Problems

In last part, we introduced the causes and solutions of some problems in injection molding. Today we will continue to introduce causes and solutions of problems like delamination, streamlines, grooves, various streaks, stresses and whitening.

Problem 10: Delamination

Delamination, sometimes called layering, with the surface of a molded part peeling off layer by layer. It’s generally considered a fairly serious defect as it reduces the strength of the assembly.

Common Causes

The most common cause of delamination is contamination of resin particles or other substrates with foreign matter. Sheet separation occurs when two materials are not properly bonded. For example, combine common base plastics such as acrylonitrile butadiene styrene (ABS) with incompatible plastics such as polypropylene (PP).

  • High shear stress
  • Mixing of incompatible materials
  • Excessive use of release agents
  • Excessive moisture heats and forms steam, causing surface delamination.
  • Material degradation
  • Caused by excessive injection speed, dwell time or melt temperature.
  • Incorrect screw or sprue system design can also cause material degradation.
Delamination

Solutions

  • Eliminates degradation and excessive shear stress.
  • Reduce shear stress.
  • Eliminate excess moisture.
  • Material suppliers can provide optimum drying conditions for specific materials.
  • Reduce recycled material.
  • Avoid excessive use of release agents.
  • Repair drain systems or other problems to eliminate mold release difficulties, rather than overusing mold release agents.
  • Avoid material contamination.

Problem 11: Streamline

Streamlines appear as a wavy pattern, usually slightly different from the color of surrounding area. Besides, it’s usually on a narrower portion of the molded component.

They may also appear as annular bands on the product surface near the mold entry point, or “gates” through which molten material flows. Flow marks generally do not affect the integrity of the assembly. However, if they are found in certain products, such as high-end sunglasses, they can be ugly and unacceptable.

Common Causes

Streamlines are generally the result of changes in the cooling rate of the material. Because it flows in different directions throughout the mold.

Differences in wall thickness can also cause the material to cool at different rates, leaving streamlines. For example, molten plastic cools very quickly during injection, and flow marks are evident when the injection speed is too slow. While still filling the mold, the plastic becomes partially solid and sticky, causing the ripples to appear.

Solutions

  • Increase injection speed, pressure and material temperature to ensure material fills the mold before cooling.
  • Increase the wall thickness of the mold around the corners to help keep flow consistent and prevent flow lines.
  • Reposition mold gates to create more distance between them and mold coolant to help prevent material from cooling prematurely during flow.
  • Increase nozzle diameter to increase flow rate and prevent premature cooling.

Problem 12: Groove

A groove is a surface defect in which “rings” appear on the surface of the molded part, mainly near the pin point gate and spread concentrically onto the molded part.

Groove

Common Causes

  • Insufficient stock temperature
  • Insufficient injection speed
  • Mold temperature is too low
  • Improper injection gate location or design

Problem 13: Air streaks

Air streaks in molded parts appear as matt, silver or white streaks (stripes) on the surface of the molded part. They can often be found in domes, ribs, and where the wall thickness of molded parts may vary. What’s more, they can also appear near gates or near engravings and depressions.

Air streaks

Common Causes

  • Insufficient exhaust
  • Injection speed is too fast
  • Air suck back
  • Material is too wet
  • Mold temperature is too low

Problem 14: Wet streaks

Wet streaks may appear on the surface of the molded part because the U-shaped profile opens against the flow direction. They usually appear as silver streaks with a rough or porous surface. Wet streaks caused by moisture on the mold surface appear as large and matt lamellar structures.

Wet streaks

Main Cause

The main reason is the condensation of moisture in the resin and that on the mold surface.

Solutions

Reduce moisture content in resin.

Problem 15: Color streaks

Color streaks may appear on colored parts due to uneven distribution of color pigments in the product or different orientations of isotropic pigments in molded products.

Thermal effects (pigment degradation) can also cause different shades of color to appear on items.

Color streaks

Common Causes

  • Color dispersion or insufficient distribution
  • Incorrect masterbatch
  • Improper design
  • Lack of dispersed lubricant
  • Temperature is too high
  • Injection pressure is too high

Problem 16: Stress whitening

Stress whitening is caused by stress cracking in polymeric materials due to internal and external stress. In PP, stress whitening usually occurs with the copolymer as the microcracks between matrix and rubber phase. A typical appearance location is the demold point. Stress cracks caused by internal stresses can often appear days or even weeks after the production of the relaxation process of the molded part.

Stress whitening

Possible Causes

  • Properties of the plastic itself (copolymer, impact resistance, etc.)
  • Residual stresses caused by molding conditions (eg: melt temperature, part design, resin flow behavior, etc.)
  • Chemical Resistance (ESCR)

Problem 17: Gloss difference

Gloss differences typically arise due to wall thickness differences in the molded part and subsequent different cooling rates in different areas.

Gloss difference

Possible Causes

  • Fluidity of the mold surface
  • Processing temperature
  • Variable cooling conditions
  • Residual stress in the mold

18. Glue in the mold

Molded parts may stick in the mold and not pop out properly. This usually occurs when the level of shrinkage is too low or too high to spray easily. Or when the shape and surface finish of the molded article creates a “stick” effect in the mold itself.

Possible Causes

  • Outer packaging – injection pressure is too high
  • Underpack – Excessive Shrinkage
  • Insufficient cooling
  • Highly polished core surface > brushed finish
  • Mold surface irregularities
  • Insufficient center and wall taper
  • Unreasonable design of undercut

In Conclusion

It should be noted that the problems in the injection molding process and their solutions are not unique. They are also affected by machine usage, machine setup, mold type and complexity, materials, and manufacturing environment.

Typically, injection molding manufacturing requires a large up-front investment in molds. Therefore, the first mold design is especially important. And we should try to avoid redesigning after serious defects are found. In contrast, defects related to the molding process or materials are often easier and less expensive to resolve. But whatever the cause, defects in molded products can greatly limit the production of molded devices. The above 18 common injection molding defect problems and solutions will be helpful to the smooth and qualified production of injection molding products.

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