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Plastic Injection Molding Terminology & Definitions
This plastic injection molding terminology and definitions table serves as a quick reference for engineers, designers, and manufacturers. For deeper technical standards, consult ISO 294 (Plastics—Injection Molding Test Specimens) or SPI Mold Standards.
Plastic Injection Molding Terminology & Definitions |
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Category |
Term |
Definition |
Related Terms |
| Process Terms | Injection Molding | Manufacturing process where molten plastic is injected into a mold cavity under high pressure. | Clamping, Injection, Cooling |
| Clamping Force | Pressure required to keep the mold closed during injection (measured in tons). | Tonnage, Mold Lock | |
| Cycle Time | Total time to complete one molding cycle (injection → cooling → ejection). | Cooling Time, Residence Time | |
| Shot Size | Maximum volume of plastic the machine can inject per cycle. | Barrel Capacity, Melt Volume | |
| Mold Terms | Cavity | Hollow space in the mold that forms the external shape of the part. | Core, Impression |
| Core | Mold component that forms internal features (e.g., holes) of the part. | Cavity, Insert | |
| Runner | Channel that delivers molten plastic from the sprue to the gates. | Cold Runner, Hot Runner | |
| Gate | Small opening where molten plastic enters the cavity. | Edge Gate, Pin Gate, Submarine Gate | |
| Ejector Pins | Pins that push the finished part out of the mold. | Ejection System, Stripper Plate | |
| Material Terms | Resin | Raw plastic material (pellets or granules) used for molding. | Polymer, Compound |
| Additive | Substances mixed with resin to enhance properties (e.g., colorants, flame retardants). | Filler, Masterbatch | |
| Melt Flow Index (MFI) | Measure of a plastic’s viscosity under standardized conditions (g/10 min). | Flow Rate, Viscosity | |
| Defects | Sink Mark | Surface depression caused by uneven cooling or thick sections. | Shrinkage, Warping |
| Flash | Excess plastic leaking out of the mold parting line. | Burrs, Overpacking | |
| Short Shot | Incomplete filling of the mold cavity due to insufficient material or pressure. | Flow Lines, Air Traps | |
| Weld Line | Visible seam where molten plastic flows meet but don’t fully bond. | Knit Line, Meld Line | |
| Tooling Terms | Tool Steel | High-hardness steel (e.g., P20, H13) used for mold fabrication. | Mold Base, Heat Treatment |
| Lifter | Mechanism to form undercuts by sliding during mold opening. | Side Action, Cam Pin | |
| Vent | Small gap in the mold to allow air escape during injection. | Gas Traps, Porosity | |
| Advanced Terms | Overmolding | Process of molding a second material over a pre-molded part. | Two-Shot Molding, Insert Molding |
| Gas-Assisted Molding | Technique using nitrogen gas to create hollow sections in thick parts. | Foam Molding, MuCell® | |
| Insert Molding | Embedding metal/plastic inserts into the molded part. | Outsert Molding, Hybrid Molding | |
| Quality Terms | Parting Line | Line where mold halves meet, often visible on the finished part. | Flash Line, Split Line |
| Draft Angle | Taper applied to mold walls for easier part ejection (typically 1°–3°). | Undercut, Release Angle | |
| Tolerance | Allowable dimensional deviation from the design (e.g., ±0.1 mm). | GD&T, Critical Dimensions | |
Key Categories Explained:
- Process Terms: Core steps in injection molding (e.g., clamping, injection).
- Mold Terms: Components that shape the plastic (e.g., cavity, runner).
- Defects: Common flaws and their causes (e.g., sink marks from thick walls).
- Tooling: Mold design and materials (e.g., tool steel types).
- Advanced Techniques: Specialized methods (e.g., overmolding for multi-material parts).
Example Application:
- Problem: “Flash” occurs → Check clamping force and mold alignment.
- Solution: Increase tonnage or repair worn mold parting lines.
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