Tech Tips

C&H Plastics Blog… We’re Back with Some Common Injection Molding Terms

Common Injection Molding Terms

Like every industry, the thermoplastic injection molding business has its own technical language. But we sometimes forget that what’s familiar to us may not be so clear to our customers. So, now that we have returned from a brief hiatus from our blog posts, we thought it would be helpful to start things off with a short glossary of molding terms.

You’ll see below some common words and phrases we use during the plastic parts manufacturing process. The order of the terms listed is the approximate order of production. We hope this helps when you talk with us about your plastic part both in the design and prototyping phases and during manufacturing.

So, welcome back to our blog and, as always, call us with any questions about the plastic parts you’ll need for your business. You can call me at 315.841.4101 ext. 29.


Rollin Bateman,
VP Sales & Engineering

Common Molding Terms

Plastic: A synthetic or natural material made from a large group of polymers (such as PVC, Nylon, and polyethylene), that is molded into various shapes.
Thermoplastic: Plastic materials that soften under heat and are capable of being molded into shapes with pressure. They become rigid upon cooling. These materials can be softened again when reheated. This happens without undergoing a chemical change.
Plastic Color Concentrate: Plastic material in which the color pigment is heavily concentrated.
Color Concentrate Letdown: The amount of plastic color concentrate that is mixed into a batch of base plastic material that results in the desired part color.
Pre-Colored or Custom Compounded Material: A custom thermoplastic material whose properties or appearance have been altered and/or improved by adding colorants and additives to it while in a melted process state. These compounds are ready for use by the plastics molders.
Reground or Recycled Plastics: Scrap or waste plastics that can be reprocessed back into the molding process or used in a completely different application.
Runner or Sprue: Pieces of plastic formed in a channel that plastic flows through to get to part.  Sometimes ground up and re-used as “regrind.”
Gate: Area where plastic flows from runner or sprue into the molded part.
Reject: An unacceptable part – usually put in with sprues and material to be re-cycled as regrind unless part was contaminated.
Blush: A dull or cloudy spot on a shiny surface.
Contamination: Speck or streak, usually dark, on a part resulting from contaminated material.  Usually another type or color of material.
Crack: A fracture or break in the plastic material.
Drag Marks: Marks or scratches on the part running in the same direction as the part moves when mold opens up.
Flash: Usually a thin edge or burr of plastic not wanted on the part. Often found on parts molded on an old, worn mold.
Knit Line: Fine line formed on a part where two waves of plastic flow back together during molding.

Knock-out or

K.O. Pin Marks:

Blemish or distortion (often light-colored and circular) on the surface of a part, caused when the part is sticking in the mold upon ejection.
Shorts: Parts not complete – not enough plastic flowed into mold.
Silver Streaks: A splatter or streak of silver on the surface of a part.
Sink Marks: A dip or sunken spot on a normally flat surface.
Splay or Jetting: Silvery splash or streak on part, often near gate or sprue
Stress Mark: Light-colored spot on part caused by part sticking in the mold upon ejection.
Warping: Twisting or bowing of a part.