Plastic Injection Molding
Terms and Glossary Guide

Air Burn: A patch or streak of brown or black material on the component caused by air or gases that have not been properly vented from the mold and have caused the material to overheat and burn.

Antioxidant: Additive used to help protect plastics from degradation through sources such as heat, age, chemicals, stress, etc.

Antistatic Agent: Additive used to help eliminate or lessen static electricity from the surface of the plastic part.

Aspect Ratio: Ratio of total flow length to average wall thickness.

Back Pressure: The pressure applied to the plastic during screw recovery. By increasing back pressure, mixing and plasticating are improved; however, screw recovery rates are reduced.

Backing Plate: A plate used as a support for the mold cavity block, guide pins, bushings, etc.

Blistering: Irregular area in the surface of a fused plastisol; usually caused by water, solvent, or air contamination.

Boss: Protuberance on a plastic part designed to add strength, facilitate alignment, provide fastening, etc.

Broken Mold Marks: Filled in areas not per drawing specification due to mold damage.

Bubbles: Air pockets that have formed in the material of the component. Bubbles may vary in size.

Cavity: The space inside a mold into which material is injected.

Charge: The measurement or weight of material necessary to fill a mold during one cycle.

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 Plate: A plate fitted to a mold and used to fasten the mold to a platen.

Clamping Pressure: The pressure applied to the mold to keep it closed during a cycle, usually expressed in tons.

Clarifiers: Additive used in polypropylene random copolymers to improve clarity.

Closed-loop Control: System for monitoring complete, Injection Molding- process conditions of temperature, pressure and time, and automatically making any changes required to keep part production within preset tolerances.

Cold Flow/Orange Peel/Lumps: Any material that has not cooled uniformly causing the appearance of either a speck of material lighter than what was used or a rippling effect on the surface of the component.

Cooling Channels: Channels located within the body of a mold through which a cooling medium is circulated to control the mold surface temperature.

Crack/Splits/Chips: A physical separation or tearing of the part.

Cushion: Extra material left in barrel during cycle to try and ensure that the part is packed out during the hold time.

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 in the fully open position.
Cycle Time: The time required by an Injection Molding system to mold a part.

Delamination: When the surface of a finished part separates or appears to be composed of layers. Strata or fish-scale-type appearance where the layers may be separated.

Diaphragm Gate: Used in symmetrical cavity filling to reduce weld-line formations and improve filling rates.

Dimensional Issues: Parts not made to drawing dimensional specifications due to internal part stress warping, mold damage, incorrect mold manufacturing, etc.

Direct Gate: The sprue that feeds directly into the mold cavity.
Discoloration: Any change from the designated color of the material or component. Incorrect color of the component.

Dispersion Aids: Perforated plates placed in the plasticator nozzle to aid in mixing or dispersing colorant as it flows through the perforations.

Draft: The degree of taper of a mold-cavity sidewall or the angle of clearance designed to facilitate removal of parts from a mold.

Drag Marks: A form of deep scratch or scratches on the surface of the component that have no visible signs of loose chips or material.

Drooling: The extrudation or leakage of molten resin from a plasticator nozzle or nozzle sprue bushing area while filling or shooting the mold.

Dwell: A pause in the applied pressure to a mold during the injection cycle just before the mold is completely closed. This dwell allows any gases formed or present to escape from the molding material.

Ejection Pin Marks: See Raised Ejector Site.

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.

Ejector Return Pins: Projections that push the ejector assembly back as the mold closes. Also called surface pins or return pins.

Ejector Rod: A bar that actuates the ejector assembly when the mold opens.

Embedded Particulate: Any completely embedded foreign particulate such as grease, metal, paint chips, burnt material, etc. that cannot be smeared or removed from the component.

Family Mold: A multi-cavity mold where each of the cavities forms one of the component parts of an assembled finished part.

Fan Gate: A gate used to help reduce stress concentrations in the gate area by spreading the opening over a wider area. Less warping of parts can usually be expected by the use of this type of gate.

Fill: The packing of the cavity or cavities of the mold as required to give a complete part or parts that are free of flash.

Fin: The web of material remaining in holes or openings in a molded part which must be removed for final assembly.

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 the direction of the flow of the melt into the mold.

Flow Marks: Wavy surface appearances on a molded part caused by improper flow of the melt into the mold.

Flow Rate: The volume of material passing a fixed point per unit time

Gas-Assisted Injection Molding: In the gas assisted process, an inert gas is injected into the center of the flow of plastic. This method provides parts which combine thick and thin walls, parts with hollow sections and elongated shapes, and more complex parts replacing multipart assemblies.

Gate: An orifice through which the melt 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.

Hob: A master model in hardened steel. The hob is used to sink the shape of a mold into a soft metal block.

Homopolymer: Plastic that results by the polymerization of a single monomer.

Hopper Dryers: Auxiliary equipment that removes moisture from resin pellets.

Hopper Loader: Auxiliary equipment for automatically loading resin pellets into machine hopper.

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.

Injection Pressure: The pressure on the face of the injection screw or ram when injecting material into the mold, usually expressed in PSI.

Inmold Decoration (IMD): A molding process where films, coatings or printing are placed in the mold prior to injection to become a more permanent part of the molded item.

Inmold Labeling (IML): A molding process where a label is placed in the mold prior to injection to become a more permanently attached to the part.

Insert Molding: Insert molding is the process of molding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials.

Insulated Runner: See hot-runner mold.
Izod Impact Test: Test to determine impact strength of a sample by holding a sample bar at one end and broken by striking. Sample specimen can be either notched or unnotched.

Jetting: A turbulent flow in the melt caused by an undersized gate or where a thin section rapidly becomes thicker.

Jig: A tool for holding parts of an assembly during the manufacturing process.

Knit Lines: Where melted material flows together to form a line or lines that may cause weakening or breaking of the component.
Knockout Pins: A rod or device for knocking a finished part out of a mold.
L/D Ratio: A term used to help define an injection screw. This is the screw length-to-diameter ratio.
Melt Flow Rate: A measure of the molten viscosity of a polymer determined by the weight of polymer extruded through an orifice under specified conditions of pressure and temperature. Particular conditions are dependent upon the type of polymer being tested.
MFR usually is reported in grams per 10 minutes. Melt flow rate defines the flow of a polypropylene resin. An extrusion weight of 2160 grams at 446°F (230°C) is used.

Melt Index: Term that defines the melt flow rate of a polyethylene resin. An extrusion weight of 2160 grams at 310°F (190°C) is used.

Mold: A series of machined steel plates containing cavities into which plastic resin is injected to form a part.

Mold Changer: An automated device for removing one mold from a machine and replacing it with another mold.

Mold Frame: A series of steel plates which contain mold components, including cavities, cores, runner system, cooling system, ejection system, etc.

Mold-Temperature-Control Unit: Auxiliary equipment used to control mold temperature. Some units can both heat and cool the mold. Others, called chillers, only cool the mold.

Moving Platen: The platen of an Injection Molding machine that is moved by a hydraulic ram or mechanical toggle.

Multi-Cavity Mold: A mold having two or more impressions for forming finished items in one machine cycle.

Multi-Material Molding: The injection of two-or-three materials, in sequence, into a single mold during a single molding cycle. The Injection Molding machine is equipped with two-or-three plasticators. (See also co-injection)

Nest Plate: A retainer plate in the mold with a depressed area for cavity blocks.

Non-Return Valve: Screw tip that allows for material to flow in one direction and closes to prevent back flow and inject material into the mold.
Nozzle: The hollow-cored, metal nose screwed into the injection end of a plasticator. The nozzle matches the depression in the mold. This nozzle allows transfer of the melt from the plasticator to the runner system and cavities.

Nucleating Agent: Additive used with polypropylene to increase crystallization rate by providing additional sites for crystal growth.

Orange Peel: A surface finish on a molded part that is rough and splotchy. Usually caused by moisture in the mold cavity.

Over 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- or under-packing results in less than optimum fill.

Part Picker: An auxiliary unit usually mounted on fixed platen, which reaches into the open mold to grab parts and remove them prior to next molding cycle. Also called a robot, the device is used when you do not want to drop parts from mold upon ejection.

Parting Line: On a finished part, this line shows where the two mold halves met when they were closed.
Peeling: An open blister.
PET: Polyethylene Terephthalate, a type of polyester and a leading recyclable plastic material.

Pin Marks: See Raised Ejector Site.

Pinpoint Gate: A restricted gate of 0.030 in or less in diameter, this gate is common on hot-runner molds.

Piston: See ram.

Plasticate: To soften by heating and mixing.

Plasticator: The complete melting and injection unit on an Injection Molding machine.

Platens: The mounting plates of a press on which the mold halves are attached.

Plate-out: The blooming of additives onto machinery during processing of plastics.

Plunger: See ram.

Preform: A plastic test tube shaped part produced by Injection Molding systems in the first step of a two-stage Injection Molding and blow molding process used to produce PET bottles or containers. The perform is subsequently re-heated and stretch blown through a blow molding process into the final container shape.

Pressure Pads: Reinforcements of hardened steel distributed around the dead areas in the faces of a mold to help the land absorb the final pressure of closing without collapsing.

Pulled Gate: Area where the part was connected to the sprue or runner that has been drawn out or stretched from the surface.

Purging: The forcing one molding material out of the plasticator with another material prior to molding a new material. Special purging compounds are used.

Raised Ejector Site / Ejection Pin Marks / Pin Marks: Where the ejector site is either heightened or raised above the surface of the component.

Ram: The forward motion of the screw in the plasticator barrel that forces the melt into the mold cavity.

Recovery Time: The length of time for the screw to rotate and create a shot.

Re-grind Problems: See Silver/Splay. Use of re-ground material increases susceptibility for moisture problems as well as polymeric chain length degradation.

Restricted Gate: A very small orifice between runner and cavity in an Injection Mold. When the part is ejected, this gate readily breaks free of the runner system. Generally, the part drops through one chute and the runner system through another leading to a granulator and scrap reclaim system.

Retainer Plate: The plate on which demountable pieces, such as mold cavities, ejector pins, guide pins and bushings are mounted during molding.

Retractable Cores: Used when molding parts in cavities not perpendicular to the direction in which the part is ejected from the mold. The cores are automatically pulled from the mold prior to the mold opening and reinserted when the mold closes again and prior to injection.

Rib: A reinforcing member of a molded part.

Ring Gate: Used on some cylindrical shapes. This gate encircles the core to permit the melt to first move around the core before filling the cavity.

Robot: Automated devices for removing parts upon ejection from an open mold rather than letting the parts drop. Also see parts picker. Robots also can perform secondary functions, such as inspection, degating, precise placement of parts on a conveyor, etc.

RMS Roughness: A measure of the surface roughness/smoothness of a material. The root mean square (RMS) average of the "peaks and valleys" of a surface is determined using a Profilometer. The lower the number, the smoother the surface: a reading of one or two would be a very polished and smooth surface.

Rockwell Hardness: A measure of the surface hardness of a material. A value derived from the increase in depth of an impression as the load of a steel indenter is increased from a fixed minimum value to a higher value and then returned to the minimum value. The values are quoted with a letter prefix corresponding to a scale relating to a given combination of load and indenter.

Runner: The channel that connects the sprue with the gate for transferring the melt to the cavities.

Runnerless molding: See hot-runner mold.

Scratch: Mark made via abrasion, not as specified in visual or cosmetic specification criteria.

Screw Travel: The distance the screw travels forward when filling the mold cavity.

Short Shot: Failure to completely fill the mold or cavities of the mold. Edges may appear melted.

Shot: The complete amount of melt injected during a molding cycle, including that which fills the runner system.

Shot Capacity: Generally based on polystyrene, this is the maximum weight of plastic that can be displaced or injected by a single injection stroke.

Shrinkage: The dimensional differences between a molded part and the actual mold dimensions.

Side Bars: Loose pieces used to carry one or more molding pins and operated from outside the mold.

Side-Draw Pins: Projections used to core a hole in a direction other than the line of closing of a mold and which must be withdrawn before the part is ejected from the mold. See also Retractable Cores.

Silver: See Splay.

Sink/Shrink: A depression or valley on a component surface that would not normally have a depression.

Silver Streaks: See splay marks.

Single-Cavity Mold: A mold having only one cavity and producing only one finished part per cycle.

Sink Mark: A shallow depression or dimple on the surface of a finished part created by shrinkage or low fill of the cavity.

Slip Agent: Additive used to provide lubrication during and immediately following processing of plastics.

Slip Plane: Marks evident in or on finished parts due to poor welding or shrinking upon cooling.

Spiral Flow: Test performed by Injection Molding a sample into a spiral mold and used to compare the processability of different resins.
Splash Marks: See splay marks.

Splay: A shiny area on the surface that is usually small in size and may have the appearance of a lightly tinted or silver streaks.

Splay Marks: Marks or droplet type imperfections on the surface of the finished parts that may be caused by the splaying of the melt through the gates and into the cool cavity where they set up.

Split-Ring Mold: A mold in which a split cavity block is assembled in a channel to permit the forming of undercuts in a molded piece. These parts are ejected from the mold and then separated from the piece.

Sprue Bushing: A hardened-steel insert in the mold that accepts the plasticator nozzle and provides an opening for transferring the melt.

Sprue Gate: A passageway through which melt flows from the nozzle to the mold cavity.

Sprue Lock: The portion of resin retained in the cold-slug well by an undercut. This lock is used to pull the sprue out of the bushing as the mold opens. The sprue lock itself is pushed out of the mold by an ejector pin.

Sprue: The feed opening provided in Injection Molding between the nozzle and cavity or runner system.

Stack Molds: Two or more molds of a similar type that are positioned one behind the other to allow for additional parts to be manufactured during a cycle.

Stationary Platen: The large front plate of an Injection Molding press to which the front plate of the mold is secured. This platen does not move during normal operation.

Stress Cracking: There are three types of stress cracking: 1. Thermal stress cracking is caused by prolonged exposure of the part to elevated temperatures or sunlight. 2. Physical stress cracking occurs between crystalline and amorphous portions of the part when the part is under an internally or externally induced strain. 3. Chemical stress cracking occurs when a liquid or gas permeates the partís surface. All of these types of stress cracking have the same end result: the splitting or fracturing of the molding.

Striations: Marks evident on the molded-part surfaces that indicate melt flow directions or impingement.

Stringing: Occurs between the finished part and the sprue when the mold opens and the melt in this area has not cooled sufficiently.

Strings: Strings of material due to poor gate cut off. See pulled gate.

Stripper Plate: A plate that strips a molded piece from core pins or force plugs. The stripper plate is set into operation by the opening of the mold.

Structural Foam Molding: Process for making parts that have solid outer skin and foamed core. An inert gas foaming agent (e.g., nitrogen is injected and mix with the plastic material under high pressure inside the extruder barrel. The mold is filled with a short shot and the gas expands the material forming a solid skin with a cellular structure inside. This process is often used for large structural plastic parts with high strength and low weight.

Submarine Gate: A gate where the opening from the runner into the mold cavity is located below the parting line. Also called a tunnel gate.
Suck-back: When the pressure on the sprue is not held long enough for the melt to cool before the screw returns. Some of the melt in the cavities or runner system may expand back into the nozzle and cause sinks marks on the finished part.

Tab Gate: A small removable tab about the same thickness as the molded item, but usually perpendicular to the part for easy removal.

Thermoplastic: A polymer which melts or flows when heated. Thermoplastic polymers are usually not highly cross-linked, and act much like molecular solids: low melting and boiling points, high ductile strength.

Thermoset: A polymer that doesn't melt when heated. Thermoset polymers "set" into a given shape when first made and afterwards do not flow or melt, but rather decompose upon heating. They are often highly cross-linked polymers, with properties similar to those of network covalent solids, i.e., hard and strong.

Tie-Bar Spacing: The space between the horizontal tie-bars on an Injection Molding machine. Basically, this measurement limits the size of molds that can be placed between the tie-bars and into the molding machine.

Toggle: A type of clamping mechanism that exerts pressure by applying force on a knee joint. A toggle is used to close and exert pressure on a mold in a press.

Tonnage: The measure by which Injection Molding machines are typically categorized, representing the clamping force of the Injection Molding machine.

Tunnel Gate: See submarine gate.

Undercut: A protuberance or indentation that impedes withdrawal from a two-piece rigid mold.

Valve Gating: A type of gate where a pin is held in the gate or channel by spring tension. As the injection stroke moves forward, this gate compresses the plastic in the runner. When this pressure build-up is sufficient to overcome the spring tension, the pin is then pushed back (pulled) and the fast decompression of the melt fills the cavity at extremely high speed.

Vent: A shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity.

Vented Barrel: Special plasticator unit with a vent port over the compression section of the screw to permit escape of gases prior to injecting melt into mold. Often used when molding moisture-sensitive resins.

Vertical Flash Ring: The clearance between the force plug and the vertical wall of the cavity in a positive or semi-positive mold. Also the ring of excess melt which escapes from the cavity into this clearance space.

Voids: Air pockets in the part which have opened or were not filed with material, leaving an opening or hole.  

Warpage: Dimensional distortion in a molded object. Caused by internal stresses via un-even 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.

Wisps: Similar to stringing but smaller in size. These also may occur as slight flashing when the mold is over packed or forced open slightly. Mold-parting-line wear or misalignment can also cause wisps.

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