The Xometry Ultimate Injection Molding Glossary


A-Side: The half of the mold that creates the exterior of a cosmetic part.
Air Trap: Air Traps can happen when a bubble of air is trapped as plastic flow fronts coincide. The air bubble, or air trap, can cause defects in a plastic part such as blemishes on the surface or incomplete filling.
Alloy: Composite material made by blending polymers or copolymers with other polymers or elastomers under selected conditions.
Automatic Mold: A mold for injection, compression or transfer molding that repeatedly goes through the entire molding cycle, including ejection, without human assistance.
B-Side: The half of the mold where ejectors, side-action cams and other complex components are located. The B-side usually creates the inside of a cosmetic part. It is also called the “core”.
Barrel: The component of the injection-molding machine where the resin pellets are melted, compressed and injected into the mold’s runner system.
Binder: A resin or other material used to hold particles together. The binder is the continuous phase in a reinforced plastic, which provides mechanical strength or ensures uniform consistency, solidification, or adhesion to a surface coating. Typical binder materials include resin, glue, gum and casein.
Blister: A raised section on the surface of a molding caused by the pressure of gases on its incompletely hardened surface.
Blush: A cosmetic imperfection that is created where the resin is injected into the part, usually visible as a blotchy discoloration on the finished part at the site of the gate.
Boss: A raised stud feature that is used to engage fasteners or support features of other parts.
Bridge Tool: An injection mold tool used to supply parts while the production tool is being built.
Bulk Pack: When plastic parts are placed in a box without any form of stacking.
Cavity: A hollow part of a mold that forms the outer part of a plastic-molded object.
Clamp: The part of an injection molding machine incorporating the platens that provides the force necessary to hold the mold closed during injection of the molten resin and open the mold to eject the molded part.
Clamping Pressure: The pressure applied to the mold to keep it closed during the molding cycle.
Class A Surface: In automotive design, a class A surface is any of a set of freeform surfaces of high efficiency and quality.
Cold Slug: A defect characterized by a small non-uniform area on the part caused by an improperly heated piece of plastic becoming attached to the part.
Cooling Channels: Channels or passageways located within the body of a mold or inserted through which a cooling medium is circulated to control the temperature of the mold.
Core-Out: A core-out is the gutted portion of a completed injection molded part. This gutting is done to create uniform wall thickness (see “wall thickness” definition below), reduce the part weight, and/or reduce warping (see “warp” definition below).
Crazing: Fine cracks which may extend in a network on or under the surface, or through a layer of plastic material.
Cycle Time: The period, or elapsed time, between a certain point in one cycle and the same point in the next.
Degassing: Opening and closing of a mold to allow gas to escape. Trapped gas and/or air can cause parts defects such as blistering and bubbles.
Draft: Referring to a section of an injection molded part that has been designed with a taper, drafts make it easier to remove the part from the mold. With very few exceptions, injection molded parts are generally designed with drafts.
Draft: The angle or degree of taper in a side wall to help facilitate removal of the parts from the mold.
Durometer: The hardness of a material as measured by the Shore Durometer. It is measured on a numeric scale with numbers ranging from lower (i.e. softer) to higher (i.e. harder).
Ejector Pin Retainer Plate: The retainer into which ejector pins are assembled.
Ejector Pins: Pins installed in the B-side of the mold that push the part out of the mold when the part has cooled sufficiently.
Family Mold: A mold where more than one cavity is cut into the mold to allow for two or more different parts to be made simultaneously.
Filler: An additive to resins for the purpose of improving physical properties (impact resistance, hardness, dimensional stability, etc.), or to reduce cost of resin.
Fillet: A curved face where a rib meets a wall. This helps improve the flow of material to minimize any mechanical stress the finished part. It also can minimize molding stress on the mold and reduce cracks in the mold.
Flash: A thin fin of material which forms at the parting line of the mold.
Galling: A mold defect when the metal mold material creates friction upon itself from rubbing.
Gate: Molten resin flows through the gate in order to get from the runner to the cavity of the mold.
Gate: The area of the mold where resin enters the mold cavity.
Gibbs: The Area of the injection mold that holds the slide down so the cam is able to actuate it.
Grooming: Machining of molded parts to acheive a better finish
Gusset: A triangular rib that strengthens areas such as a wall to a floor or a boss to a floor.
Hand Load: A hand load is a feature in a mold — usually made from steel or aluminum — that creates undercuts (see “undercuts” definition below) in a molded part. Hand loads must be manually removed from a completed part after ejection.
Hand Pull: The part of the injection mold that is used for making undercuts in plastic parts
Heel Block: Blocks used to keep the slide in the forward position as the molding machine is being closed on the mold
Hot Runner Mold: A thermoplastic injection mold in which the runners are insulated from the chilled cavities and remain hot so that the center of the runner never cools in normal cycle operation. Runners are not, as is the case usually, ejected with the molded pieces.
Injection molding: A manufacturing process in which melted plastic is injected into a mold to form a part.
Insert Molding: The process of forming an object by molding plastic around metal inserts.
Knit lines: Knit lines are natural occurrences in the part where separated flows of cooling material meet and rejoin, sometimes resulting in incomplete bonds and/or a visible line.
Laminar Flow: Laminar flow of thermoplastic resins in a mold is accompanied by solidification of the layer in contact with the mold surface that acts as an insulating tube through which material flows to fill the remainder of the cavity. This type of flow is essential to duplication of the mold surface.
Line of Draw: The direction in which the two mold halves will separate and allow the part to be ejected without any obstructions.
Living hinge: Very thin section of plastic used to connect two parts and keep them together while allowing them to open and close. They require careful design and gate placement. A typical application would be the top and bottom of a box.
Medical Grade: Resin that may be suitable for use in select medical applications.
Melt Flow: The viscosity of a polymer based on the polymer’s weight when extruded under certain pressure and temperature -- varying by specific polymer.
Melt Index: The amount, in grams, of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subjected to 2160 grams force in 10 minutes at 190°C.
MUD Set: (Master Unit Die) Inserts which are quickly changed over by using a standard frame.
Multi-cavity mold: A mold where more than one cavity is cut into the mold to allow for two or more of the same parts to be made simultaneously. The key difference between a family mold and multi-cavity mold is that the cavities of the family mold produce different parts, whereas the cavities of a multi-cavity mold produce the same part.
Notch Sensitivity: The extent to which the sensitivity of a material to fracture is increased by the presence of a surface in homogeneity such as a notch, a sudden change in section, a crack, or a scratch. Low notch sensitivity is usually associated with ductile materials, and high notch sensitivity with brittle materials.
Nozzle: The tapered fitting on the end of the barrel of the molding press where the resin enters the sprue.
Overmolding: overmolding lets you combine multiple materials into one part. One material, usually a thermoplastic elastomer (TPE/TPV), is molded onto a second material, which is often a rigid plastic. Think about your toothbrush handle where the single piece has both rigid and rubbery components.
Parting Line: The edge of a part where the mold halves separate.
Piece Price: How much the molded parts cost per piece. This amount is separate from the cost of creating the mold or tool.
Polymer: A substance that has a molecular structure consisting chiefly or entirely of a large number of similar units bonded together, e.g., many synthetic organic materials used as plastics and resins.
Prototype Mold: A simplified mold construction often made from a light metal casting alloy or from an epoxy resin in order to obtain information for the final mold and/or part design.
Purging: The process of cleaning the injection machine of remnant color or materials prior to running a new part.
Ram: A plunger-like part which pushes the melted material into the mold.
Release agent: A compound, which is sprayed on the mold, or as an additive, molded into the part to help facilitate the release of the part.
Retainer Plate: The plate on which demountable pieces, such as mold cavities, ejector pins, guide pins, and bushings are mounted during molding: usually drilled for water lines.
Rib: A reinforcing member of a molded part.
RIFT: Resin Infusion Under Flexible Tooling
Runner System: The term usually applied to all the material in the form of sprues, runners and gates which lead material from the nozzle of an injection machine or the pot of a transfer mold to the mold cavity.
Runner: The runner is a channel cut into an injection mold, connecting the sprue (see “sprue” definition below) with the gate and ultimately the cavity, through which molten plastic travels during injection.
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, too little can cause the material to freeze off causing short shot
Shear: Shear refers to speed- and pressure-induced friction inside of a mold that maintains the melt of the plastic while the cavity fills. Too little shear can lead to material freeze-off and subsequent short shot (see “short shot” definition below), while too much shear can burn the plastic.
Short Shot: Commonly caused by low shear, short shot refers to an injection that fails to completely and properly fill a mold cavity.
Shot Capacity: The maximum weight of material which a machine can produce from one forward motion of the plunger or screw.
Shrink Rate: Shrink rate is the measure of how much a plastic will shrink as it cools. Every plastic has a different shrink rate, which can be as little as 0.001 inches or as great as 0.060 inches, with an average between 0.004 inches and 0.021 inches. Shrink rate is generally accounted for during the design phase and added to the size of the mold so that completed parts are the proper size after shrinking.
Shutoff: The surfaces where the A-side and B-side of the mold contact.
Side-action A portion of the mold that is pushed into place as the mold closes. Typically, side-actions are used to resolve an undercut, or sometimes to allow an undrafted outside wall. As the mold opens, the side action pulls away from the part, allowing the part to be ejected.
Sink: Imperfections such as dimples that occur as plastic cools at different rates along the surface.
Slides: A portion of the mold which is made to travel at an angle to the normal movement of the molding machine, to produce recesses, undercuts and holes.
Soft Tooling: Usually a term meant for RTV Molding, but also used for aluminum tooling for plastic injection molding compared to steel. Although, aluminum has been proven as a reliable mold material for production injection molding.
Spider Gate: Multi-gating of a part through a system of radial runners from the sprue.
Splay: Streaks that occur as the result of moisture in resin.
Sprue: The sprue of an injection molding die is the passage that connects the nozzle of the molding machine with the runner of the die
Steel Safe: Refers to the amount of metal left on the mold in order to tweak in a dimension. For example, if you have an inside diameter that is supposed to be .500 you may leave the mold at .505 in case you get excessive shrink
Straight Pull Mold: A mold that does not require side actions.
Submarine Gate: A type of edge gate where the opening from the runner into the mold is located below the parting line or mold surface as opposed to conventional edge gating where the opening is machined into the surface of the mold. With submarine gates, the part is broken from the runner system on ejection from the mold.
Surface Finish: Finish of molded product.
T1 Shots: The first stage test shot samples produced based on your 3D files.
Tear Strip: A removable feature that creates a crisp end of a molded part.
Texture: A surface treatment applied to the mold to create texture on the parts. There are a variety of available textures, and draft requirements vary depending on the selected texture.
Tie Bars: Bars which provide structural rigidity to the clamping mechanism of a press often used to guide platen movement.
Tolerance: A specified allowance for deviations in weighing, measuring, etc., or for deviations from the standard dimensions or weight.
Tool: The mold used to form plastic parts in an injection machine.
Tunnel Gate: A gate that allows the material to flow into the plastic part. But when the part is ejected, the opening mold shears off the gate. (No second operations).
Undercut: An undercut is any part design feature — such as a clip, window, or hole — that requires a hand pull, slide, or hand load to create.
Vent: A very small opening in the mold cavity that allows air to escape from a mold while the resin is injected. Typically on the perimeter of the part at the parting line. They can also be made off ejector pins and sub-inserts.
Vertical Flash Ring: The clearance between the core and the vertical wall of the cavity in a positive or semi-positive mold; also the ring of excess material which escapes from the cavity into this clearance space.
Vestige: Vestige refers to any extra material extending from a molded part at the area of the gate after gate runner removal (such as by an automatically trimmed gate). Vestiges generally require manual removal.
Void: A bubble occurring in the center of a heavy thermoplastic part. The void is usually caused by excessive shrinkage.
Wall Thickness: Wall thickness is a measure of the thickness of any given cross section of a plastic part.
Warp: Warp describes any area of a completed injection molding part that has been distorted in some way, whether during injection or cooling. Warp is often, but not exclusively, caused by lack of uniformity in wall sections of a part design.