Plastic Injection Molding Glossary
Learn the meanings of common injection molding terms
Injection Molding Glossary: Key Terms Explained
Our customers span a wide range of industries and experience levels, from multinational appliance manufacturers to individual inventors and entrepreneurs. Many are developing brand-new products or improving existing designs and may be working with an injection molder for the first time.
Because our customers bring varying levels of knowledge, clear communication is essential throughout the manufacturing process. Having a shared understanding of common injection molding terminology helps ensure projects move forward smoothly.
This injection molding glossary defines the most commonly used terms in the industry to support that communication.
| Molding Term | Definition | |
|---|---|---|
| Aging | The change of a material with time under defined environmental conditions, leading to improvement or deterioration of properties. | |
| Barrel | The part of the molding press where resin is melted. | |
| Boss | The round protrusions on plastic parts, often designed for fasteners. | |
| Bridge Tool | A temporary low production tool mold. It is built for the purpose of making production parts until a high volume production mold is ready. | |
| Bubbles | Air pockets that have formed in the material of the component. Bubbles may vary in size. | |
| Cavity or Die Cavity | The negative of a part in a mold, which forms the outer surface of the molded part. Depending on the number of such depressions, molds are designated as a single cavity or multi-cavity. | |
| CMF | Color, Material, Finish | |
| Core | The B-side of the injection mold tool that creates the internal features of a part. | |
| Cycle | The complete sequence of operations in a process to complete one set of moldings. The cycle is taken at a point in the operation and ends when this point is again reached and moving platens of the clamp unit are in the fully open position. | |
| Design for Manufacturing (DFM) | This review evaluates all design changes that need to be made and reviews the proposed molding strategy on all parts. 2D drawings are reviewed for characteristics to be controlled in production. CMF (color, material, finish) is finalized along with the production schedule. | |
| Draft | The degree of taper of a mold-cavity sidewall or the angle of clearance designed to facilitate removal of parts from a mold. Generally, all plastic components should be designed with draft where possible. | |
| Durometer | The hardness of a material as measured by the Shore Durometer. | |
| EDM | Sometimes referred to as spark machining, spark eroding, burning, die sinking or wire erosion, Electric Discharge Machining is a manufacturing process whereby a desired shape is obtained using electrical discharges. Material is removed from the part by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. | |
| Ejector Pins | Pins that are pushed into a mold cavity from the rear as the mold opens to force the finished part out of the mold. Also called knockout pins. | |
| Engineering Change Order (ECO) | Document which records and approves any changes made to the design, cost, or timing. | |
| Family Mold | A multi-cavity mold where each of the cavities forms one of the component parts of an assembled finished part. | |
| Filled Resins | Filled resins combine materials such as glass and carbon with either thermoplastic or thermosetting polymers (most commonly nylon). These materials often have improved mechanical properties compared to unfilled. | |
| Flash | Any excess material that is formed with and attached to the component along a seam or mold parting line. | |
| Flow Line | Marks visible on the finished items that indicate where plastic flow occurs. | |
| Flow Marks | Wavy surface appearances on a molded part caused by improper flow of the melt into the mold. | |
| Gas-assisted Molding | A small amount of gas is blown into the mold during cooling to apply pressure. Typically used to mitigate the formation of sink marks. | |
| Gate | An orifice through which the molten plastic enters the mold cavity. | |
| Gate Trim | Remnant of plastic left over from cutting the component from the runner or sprue, usually to be cut flush with the edge of the component. | |
| Hand Load | For low production molds with undercuts, steel fixtures which create the undercuts are manually removed from the mold during the part ejection process. | |
| Heel | Refers to the portion of an automatic injection mold that keeps the slide in the forward position when the molding machine is closed on the mold. | |
| Hot-Runner Mold | A mold in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner molds make parts that have no scrap. | |
| Insert Molding | The process of molding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials. | |
| Jetting | Turbulent flow in the melt caused by an undersized gate or where a thin section rapidly becomes thicker. | |
| Knit Lines | Where melted material flows together to form a line or lines that may cause weakening or breaking of the component. | |
| Living Hinge | A thin flexible hinge made from the same material as the two rigid pieces it connects. It is typically thinned to allow the rigid pieces to bend along the line of the hinge to allow them to open and close. They require careful design and gate placement. A typical application would be the top and bottom of a box. | |
| Mold Frame | A series of steel plates which contain mold components, including cavities, cores, runner system, cooling system, ejection system, etc. | |
| Moldflow or Moldflow Analysis | Simulation software for plastic injection molding used in computer-aided engineering. | |
| Multi-Cavity Mold | A mold having two or more impressions for forming finished items in one machine cycle. | |
| Over Molding or Multi-Shot Molding | A process in which a mold cavity is first partially filled with one plastic and then a second shot is injected to encapsulate the first shot. | |
| Packing | The filling of the mold cavity or cavities as full as possible without causing undue stress on the molds or causing flash to appear on the finished parts. Over-packing or under-packing results in less than optimum fill. | |
| Parting Line | On a finished part, this line shows where the two mold halves met when they were closed. | |
| Platen | The base plate on an injection molding machine onto which the mold plates are attached. | |
| Polymer | The molecular designation of long repeated linked chains, which includes resins used in injection molding. | |
| Product Requirements Document (PRD) | The PRD is an initial project document that defines the purpose and scope of the project and breaks it down into features. It also determines a timeline and identifies stakeholders. | |
| Production Approval | Production approval is necessary to move from first articles to production parts. | |
| Purging | The forcing of one molding material out of the nozzle tip with another material prior to molding a new material for cleansing purposes. Special purging compounds are used. | |
| Resin | Resin is the material being injected into the mold, and is the thermoplastic material that the final part will be made from. Examples of available resins can be found here: https://icomold.com/material-list/ | |
| Ribs | Thin support features (usually triangle-shaped) on a part used for strengthening wall sections and bosses. | |
| Runner | The channel that connects the sprue with the gate for transferring the molten plastic to the cavities. | |
| Shear | Refers to when plastic enters into the mold and the melt is maintained by friction produced by speed and pressure. Too much shear can cause the plastic material to burn, while too little can cause the material to freeze off, causing a short shot. | |
| Short Shot | Failure to completely fill the mold or cavities of the mold. Edges may appear melted. | |
| Shrink Rate | The dimensional differences between a molded part and the actual mold dimensions. | |
| Single-Cavity Mold | A mold having only one cavity and producing only one finished part per cycle. | |
| Sink Marks | A shallow depression or dimple on the surface of a finished part created by shrinkage or low fill of the cavity. | |
| Slide | Part of the plastic injection mold used for creating undercuts, which are required for automatic injection molds. | |
| Slide Action | A sliding mechanism in the mold designed for the molding of parts with undercuts. The undercut-steel fixture is held in place during the injection process and then slides out of the way prior to ejection. | |
| Sprue | The feed opening provided in injection molding between the nozzle and cavity or runner system. | |
| Steel (Metal) Safe | A part design change will only require removing metal from the mold instead of adding metal to the mold. | |
| Straight Pull | No side actions. | |
| T0, T1, T2… Shots | Production shoots the first parts and samples are sent to the client for review and checked for key dimensions. Any necessary changes are recorded and corrected. | |
| Thermoplastic | Thermoplastics are plastics that become moldable at elevated temperatures, and re-solidify upon cooling. | |
| Undercuts | A feature or indentation that impedes withdrawal from a two-piece rigid mold. | |
| Vent | A shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity. | |
| Vestige | A mark created by trimming the injection molded part gate. | |
| Wall Thickness | The thickness of the cross section of the plastic part. | |
| Warp | Dimensional distortion in a molded object caused by internal stresses from uneven material flow, cooling, and compression. | |
| Weld Line | Where melted material flows together during molding to form a visible line or lines on a finished part that may cause weakening or breaking of the component. |
Launch Your Injection Molding Project with Confidence
Partner with Experts Who Get It Right the First Time
Every project is reviewed by a seasoned manufacturing engineer to optimize part design, material selection, and tooling strategy — ensuring smarter decisions before steel is cut and production begins.
