When purchasing injection molded parts, it is important to understand some of the common but scary problems and defects that impact product quality. Being familiar with these imperfections and their causes can help you work with injection molders to insure the highest quality part production is achieved.
Most defects in plastic parts can be traced back to three sources:
- The material being used to make the part
- The processing of the material in the mold
- The mold itself
We grouped the defects together by their most common cause; however one or more factors may contribute to a defect.
Common defects linked to the plastic resins or additives being used to manufacture a part include:
Color streaks – Just like the name implies, color streaks are random areas of color change that are often attributed to the non-uniform mixing of resins and colorants.
Delamination – This defect appears as a flaky surface layer on the part and is often caused by contamination or moisture in the resin pellets.
Discoloration – This can occur when the hopper and feed zone have not been flushed properly to remove any residual color.
Embedded contaminates – Particles or flecks of residual foreign material that can originate in the barrel of the press.
Splay marks or silver streaks – Circular marks appearing where the molten plastic enters the mold cavity. This is often caused by excessive moisture in the resin.
Common defects linked to the processing of the plastic resin in the mold include:
Blistering – Bubbles or raised imperfections that are generally caused by too much heat and/or inadequate cooling.
Burn marks – Black or brown blemishes (which are carbon deposits) that are caused by improper ventilation or prolonged heating in the mold.
Cold slugs – A small non-uniform area on the part caused by an improperly heated piece of plastic becoming attached to the part.
Flow marks – A wavy pattern or discoloration caused by a slow injection speed which allows the material to cool too quickly.
Sink marks or shrinkage voids – Depressions or hollows in a part that can be attributed to excessive press pressure, non-uniform heating, inadequate cooling time or part design.
Stress cracking or stress crazing – This defect usually occurs as a result of over exposure to a high temperature.
Stringing – A thin strand of material attached to a part generally caused by a nozzle that is too hot.
Common defects linked to improper mold design and/or maintenance include:
Drag marks – Scratches that occur when the part is ejected from the mold. This is usually due to an improperly designed ejector system or one that is out of alignment.
Flash or burrs – A thin lip or protrusion beyond the body of the part that is generally caused by poor clamping force, improper mold design and/or mold damage.
Jetting – A snake-like line of material that cools independently of the material around it. This defect is generally due to poor tool design often relating to incorrect gate size and length or placement.
Short shot – An incomplete part due to lack of a filled mold. This problem is often attributed to a blockage or improper injection pressure.
Warping – A part with a distorted shape can be due to a poor cooling system in the mold. When the plastic material is cooled unevenly, the result is a bowing effect.
Most of the defects listed here can be addressed by making changes to the processing, the material or the mold itself. The best way to avoid these part defects is to work with a plastic injection molder that has a great deal of experience with various resins and their applications. Using a turnkey manufacturer, who also builds and maintains the mold, can help avoid any costly machining charges or mold replacement.