Injection Molding Terminology
Complete Guide to Plastic Injection Molding Terms
Injection molding terminology is essential for engineers, product designers, and buyers working with plastic parts and molds.
This guide provides a structured explanation of key plastic injection molding terms, covering mold design, processing, defects, and surface finish.
How to Use This Terminology Guide
This page is organized into categories:
Mold structure terms
Injection molding process terms
Design terminology
Common defects
Surface finish terminology
Each section includes links to detailed guides for deeper understanding.
1. Mold Structure Terminology
Core
The core forms the internal geometry of a molded part.
Cavity
The cavity forms the external surface of the part.
Parting Line
The line where two halves of the mold meet.
Slider (Side Action)
Used to form undercuts or complex geometries.
Ejector Pin
Pushes the part out of the mold after cooling.
Gate
The entry point of molten plastic into the mold cavity.
See detailed explanation:
Injection Molding Surface Finish
Runner
Channels that guide molten plastic to the cavity.
Sprue
Main channel connecting machine nozzle to runner.
2. Injection Molding Process Terms
Injection Pressure
The force used to inject molten plastic into the mold.
Holding Pressure
Maintains pressure to compensate shrinkage during cooling.
Cooling Time
Time required for plastic to solidify before ejection.
Cycle Time
Total time for one injection molding cycle.
Short Shot
Incomplete filling of mold cavity.
Overpacking
Excess pressure causing stress or defects.
3. Design Terminology
Draft Angle
A slight angle added to facilitate part ejection.
Wall Thickness
Uniform thickness is critical for quality and cost.
Rib
Used to strengthen parts without increasing thickness.
Boss
Cylindrical feature for screws or assembly.
Undercut
Feature that prevents direct mold opening.
Fillet
Rounded internal corner to reduce stress.
4. Common Injection Molding Defects
Sink Mark
Depression caused by uneven cooling.
Warpage
Part deformation due to uneven shrinkage.
Flash
Excess material at mold edges.
Weld Line
Visible line where melt flows meet.
Flow Mark
Patterns caused by flow variation.
Burn Mark
Discoloration caused by trapped air overheating.
5. Surface Finish Terminology
SPI Surface Finish
Standard system defining mold surface quality.
See full guide:
SPI Surface Finish Chart
Mold Polishing
Process used to achieve required surface finish on mold.
Glossy Finish
Highly reflective surface achieved by polishing.
Related page:
Glossy Plastic Parts
Matte Finish
Non-reflective surface used to reduce glare.
Textured Finish
Patterned surface created by etching or blasting.
6. Material & Processing Terms
Shrinkage
Reduction in size during cooling.
Melt Flow Index (MFI)
Indicates material flow characteristics.
Shear Rate
Rate of deformation during flow.
Viscosity
Resistance to flow of molten plastic.
Hygroscopic Materials
Materials that absorb moisture (e.g., nylon).
7. Advanced & Industry Terms
Thin Wall Injection Molding
Process for very thin plastic parts.
Insert Molding
Embedding metal parts into plastic.
Overmolding
Molding one material over another.
Hot Runner System
Keeps plastic molten inside runner system.
Cold Runner System
Plastic solidifies in runner and must be removed.
Why Understanding Injection Molding Terminology Matters
Clear understanding of terminology helps:
Improve communication with manufacturers
Reduce design errors
Optimize cost and production efficiency
Ensure correct specification of mold and surface finish
