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Defects in Plastics and How to Prevent Them

When acquiring injection molded parts, it is critical to recognize some of the frequent but concerning faults and errors that affect product quality.  Understanding these flaws and their causes will help you collaborate with injection molders to ensure the best quality part production.

Defects in plastic parts are typically caused by three factors: the material used, the manufacturing process, and the product itself.

We have organized the defects by the most common cause; however, one or more sources may contribute to a defect. And it’s no secret that getting good products requires careful selection of a reliable injection molding service.

Material-Related Defects

Common faults associated with the plastic resins or additives used to make a product are:

  1. Color streaks are random areas of color shift caused by non-uniform resin and colorant mixing.
  2. Often caused by contaminants or moisture in the resin pellets, delamination appears as a flaky surface layer on the part.
  3. Discoloration can occur when the hopper and feed zone have not been thoroughly cleansed to remove any remaining color.
  4. Embedded contaminants are particles or flecks of residual foreign material that can form in the barrel of the press.
  5. Splay markings, also known as silver streaks, are circular patterns that emerge where molten plastic enters the mold chamber. This is frequently caused by excessive moisture in the resin.

Manufacturing Defects

Common faults associated with the manufacturing of the plastic resin in the mold include:

  1. Blistering refers to bubbles or elevated defects generated by excessive heat and/or inadequate cooling.
  2. Burn marks – Black or brown blemishes (carbon deposits) due by poor ventilation or extended heating in the mold.
  3. Cold slugs are small, non-uniform areas on a part created by an incorrectly heated piece of plastic being adhered to it.
  4. Flow marks are wavy patterns or discolorations generated by a sluggish injection speed, allowing the material to cool too quickly.
  5. Sink marks or shrinkage voids are depressions or hollows in a part that can be caused by high press pressure, uneven heating, insufficient cooling time, or improper part design.
  6. Stress cracking or stress crazing – This fault is mainly caused by excessive exposure to high temperatures.
  7. Stringing – A thin string of material adhered to a part, usually caused by a hot nozzle.
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Mold Design and Maintenance Issues

Common faults associated with incorrect mold design and/or maintenance include:

  1. Drag marks are scratches that occur when a part is ejected from the mold.  This is frequently caused by an ejector system that was not properly constructed or is out of alignment.
  2. Flash or burrs are a thin lip or protrusion beyond the body of the item that is typically caused by insufficient clamping force, incorrect mold design, and/or mold degradation.
  3. Jetting is a snake-like line of material that cools independently of the material surrounding it.  This problem is typically caused by improper tool design, often resulting in erroneous gate size, length, or placement.
  4. Short shot: An incomplete portion due to a lack of a filled mold.  This issue is frequently related to a blockage or incorrect injection pressure.

  5. Warping – A deformed shape on a part might be caused by a faulty cooling system in the mold. When plastic material is cooled unevenly, it causes a bending effect.

Partnering for Quality

Most of the flaws described here can be fixed by altering the processing, the material, or the mold itself. Choose an injection molding service that is a guarantee of quality. The easiest method to avoid these part flaws is to work with a plastic injection molder who has extensive experience with various resins and their applications. Using a turnkey manufacturer, who also produces and maintains the mold, can help you avoid expensive machining fees or mold replacement.