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Useful Data
Glossary  - Common terms used in the plastics industry
ADDITIVE - A substance added to a plastic compound to alter its characteristics. Examples are plasticizers, and flame retardants.

ALLOY - A combination of 2 or more plastics which form a new plastic. See BLEND.

AMORPHOUS - A plastic material in which the molecular structure is random and becomes mobile over a wide temperature range. See CRYSTALLINE.

ANNEAL - To heat a molded part up to a temperature just below its melting point and slowly cooling it back; down to room temperature. This relieves molded stresses. See CONDITIONING.

ANISOTROPIC SHRINKAGE - Shrinkage that occurs more in one direction (usually the direction of flow; reinforced materials shrink more across the direction of flow) than another.

AUTOMATIC OPERATION - The term used to define the mode in which a molding machine is operating when there is no need for an operator to start each cycle.

BARREL - A metallic cylinder in which the injection screw (or plunger) resides in the molding machine. Also called CYLINDER.

BLEND - A mixture of 2 or more plastics.

BOSS - A projection of the plastic part, normally round, which is used to: strengthen an area of a part; provide a source of fastening; or to provide an alignment mechanism during assembly.

CARTRIDGE HEATERS - Pencil-shaped electrical heater devices sometimes placed in molds to raise the temperature level of the mold. Especially beneficial when molding high-temperature crystalline materials.

CAVITY - A depression or female portion of the mold which creates the external plastic part surface.

CHECK RING - A ring shaped component that slides back and forth over the tip end of the screw. The check ring eliminates the flow of molten material backwards over the screw during the injection process.

CLAMP FORCE - The force, in tons, that the clamp unit of a molding machine exerts to keep the mold closed during the injection process.

CLAMP UNIT - That section of the molding machine containing the clamping mechanism. This is used to close the mold and keep it closed against injection pressure created by the injection process. The clamp unit also contains the ejection mechanism.

COLD SLUG WELL - A depression (normally circular) in the ejection half of an injection mold, opposite the sprue, designed to receive the first front, or "cold" portion, of molten plastic during the injection process.

COMPRESSION RATIO - A factor that determines the amount of shear that is imparted to plastic material as it travels through the barrel. It is determined by dividing the depth of the screw flight in the feed section by the depth of the screw flight in the metering section.

CONDITIONING - Exposing a molded part to a set of conditions (such as hot oil) which impart favorable characteristics to the product. See ANNEAL.

COOLING CHANNELS - Drilled holes or channels machined into various plates or components of an injection mold providing a flow path for cooling medium (such as water) in order to control the temperature of the mold.

CORE - a) An extended or male portion of the mold which creates the internal plastic part surface. b) A pin or protrusion designed to produce a hole or depression in the plastic part.

COUNTERBORE - A recessed circular area. Commonly used to fit the head of an ejector pin (return pin, sucker pin, etc.) in the ejector plate.

CRYSTALLINE - A plastic material in which the molecular structure becomes mobile only after being heated above its melting point. See AMORPHOUS.

CUSHION - A pad of material left in the barrel at the end of the injection stroke. It is excessive to the amount needed to fill the mold and acts as a focus point for holding pressure against the cooling melt.

CYCLE - The total amount of time required for the completion of all operations needed to produce a molded part. Sometimes referred to as the "gate-to-gate" time, meaning the time from when an operator first closes the gate until the time the operator closes the gate again for starting the next cycle.


DECOMPRESSION - A method of relieving pressure on the melt after preparing it for injection during the upcoming cycle. This minimizes the drooling that occurs when a shutoff nozzle is not utilized.

DEFECT - An imperfection in a molded part that results in the product not meeting original design specifications. These defects can be visual, physical, and/or hidden.

DRAFT - An angle (or taper) provided on the mold to facilitate ejection of the molded part.

EJECTOR HALF - That half of the mold which is mounted to the moving platen of the injection machine. Sometimes called the "live" half or the "moveable" half because it moves. This half of the mold usually contains the ejection system:

EJECTOR PIN - A pin, normally circular, placed in either half of the mold (usually the ejector half) which pushes the finished molded product, or runner system, out of a mold. Also referred to as a "knockout" pin, for obvious reasons.

FEED THROAT - The area at the rear end of the injection unit that allows fresh plastic to fall from the hopper into the heating barrel.

FEED ZONE - That area of the screw that is at the rear and receives fresh material from the feed throat.

FILLER - Specific material added to the basic plastic resin to obtain particular chemical, electrical, physical, or thermal properties.

FLASH - A thin film of plastic that tends to form at parting line areas of a mold. May also be found in vent areas and around ejector pins. Flash is caused by too great a clearance between mating metal surfaces, which allows plastic material to enter.

FLIGHT - The helical metal thread structure of the injection screw.

GATE - An opening found at the entrance of a cavity (end of the runner system) which allows material to enter.

GRANULATOR - A machine designed to grind up rejected pre-molded plastic (products or runners). The material generated by this process is called REGRIND.

GUIDE PINS - A pin (usually circular) which normally travels in a bushing in order to provide alignment of two unattached components, such as the two halves of an injection mold. Also called LEADER PINS.

HEATING CYLINDER - That section of the injection molding machine in which the plastic resin is heated to the proper molding temperature prior to injection into the mold.

HEATING ZONE - An area of the heating barrel that is controlled by a temperature controller attached to a set of heater bands. There are four major zones: rear, center, front, and nozzle.

HEATER BANDS - Bracelet-shaped electrical heaters that are placed around the outside circumference of the heating barrel.

HOPPER - A funnel-shaped container mounted over the feed throat of a molding machine. It holds fresh material to be gravity fed into the feed zone of the heating barrel. Hoppers are normally designed to hold an average of 2 hours worth of material for a given machine size.

HYDRAULIC CLAMP - A term used to describe the use a large hydraulic cylinder to open and close the clamp unit of a molding machine.

HYGROSCOPIC - A term applied to those plastics (such as ABS and NYLON) which absorb moisture from the atmosphere.

INJECTION CAPACITY - A rating of the maximum amount of plastic material, in ounces, a machine is capable of injecting in a single stroke of the injection screw or plunger. It is based on the specific gravity of polystyrene as a standard.

INJECTION MOLDING - The process of pushing a molten plastic material into a relatively cooled mold in order to produce a finished product.

INJECTION PRESSURE - That pressure which performs the initial filling of the mold. It is supplied by the injection screw or plunger as it pushes material out of the heating barrel and into the mold.

INJECTION UNIT - That section of the molding machine which contains the injection components, including the hopper, heating cylinder, screw (or plunger), nozzle, and heater bands.

ISOTROPIC SHRINKAGE - Shrinkage that occurs equally in all directions. See ANISOTROPIC SHRINKAGE.

KNOCKOUT PIN - See Ejector Pin

LAND - A term used to describe the area in which the gate, or vent, resides. It can also be thought of as the "length" dimension in the "L, W, H" terminology used for describing the dimensions of the gate or vent. See also SHUTOFF LAND.

L/D RATIO - The result of a calculation which divides the entire length of flighted area on a screw by its nominal diameter.

LEADER PIN - See Guide Pin

MANUAL OPERATION - The term used to define the mode in which a molding machine is operating when there is a need for an operator to start and finish each phase of the total cycle.


MELT - A term given to describe the condition of molten plastic prior to injection into a mold. A proper melt has the consistency of warm honey.

METERING ZONE - That area of the screw at the front end which contains properly melted plastic that is ready to inject.

MOLD - The term given to the entire tool (cavity, core, ejectors, etc.) needed to produce molded parts from molten plastic material.

MONOMER - A molecular unit of an organic substance, usually in the form of a liquid or gas. See POLYMER.

MOVING PLATEN - The platen of a molding machine that travels (opens and closes). It is connected to the clamp unit and is the mounting location for the "B", or traveling, half of the mold.

NON-RETURN VALVE - A mechanism mounted in (or at) the nozzle of the injection machine, which operates to shut off injection flow at the end of the injection cycle. This eliminates material from the upcoming shot from drooling out of the nozzle when the mold opens to eject parts from the previous shot.

NOZZLE - A device mounted at the end of the heating barrel which focuses plastic material to flow from the machine into the mold.


PARTING LINE - A plane at which two halves of a mold meet. Also applies to any other plane where two moving sections come together and form a surface of a molded part.

PLASTIC - A complex organic compound (usually polymerized) that is capable of being shaped or formed.

PLATENS - The flat surfaces of a molding machine onto which the two halves of the mold are mounted. One is stationary and the other travels. There is a third platen (stationary) at the clamp end of the machine which serves as an anchoring point for the clamp unit.

PLUNGER - The injecting member of a non-screw design molding machine. Plungers do not rotate, (auger) to bring material forward in preparation for the next cycle. Nor do they blend the material as a screw does.

POLYMER - A group of long chains of monomers, bonded together in a chemical reaction to form a solid. This term is often used interchangeably with PLASTIC, but there can be a difference.

PURGING - A process of injecting unwanted plastic material from the injection cylinder into the atmosphere for the purpose of changing materials, changing colors, or removing degraded material. Also, the name given to the mass of material that is purged.

RECIPROCATING SCREW - A helical flighted, metal shaft which rotates within the heating cylinder of a molding machine, shearing, blending, and advancing the plastic material. After rotating, the screw is pushed forward which injects the plastic into the mold. Also, simply referred to as "the Screw".

REGRIND - Plastic material formed by granulating pre-molded material. Regrind is virgin material that has been exposed to at least one heating cycle.

RUNNER - Grooves or channels cut into either or both halves of the injection mold to provide a path for the molten plastic material to be carried from the sprue to the gate(s) of the cavity.


SCREW SPEED - The rotating speed of the screw as it augers new material towards the metering zone. It is expressed in RPM (revolutions per minute).

SECONDARY OPERATION - Any activity performed after the molding process required to produce a finished product suitable for its designed purpose.

SEMI-AUTOMATIC OPERATION - The term used to define the mode in which a molding machine is operating when there is a need for an operator to start each cycle.

SHOT - A term given to the total amount of plastic material that is injected (or shot) into a mold in a single cycle.


SHUTOFF LAND - A raised area of the mold surface surrounding the cavity image. This area is usually between 0.002 and 0.003 inch high, approximately 1/2 inch wide and is used to focus clamping pressure on the mold. The use of a shutoff land reduces the amount of tonnage required to keep a mold closed against injection pressure.

SLIDE - A section of the mold which is made to travel at an angle to the normal movement of the mold. Used for providing undercuts, recesses, etc.

SPRUE - The plastic material that connects the runner system to the nozzle of the heating cylinder of the molding machine. It is formed by the internal surface of a bushing that joins the mold to the machine's nozzle.

SPRUE BUSHING - A hardened bushing that connects the mold to the molding machine nozzle and allows molten plastic to enter the runner system.

STATIONARY PLATEN ("A") - The platen at the injection end of the molding machine that does not travel. It contains the "A" half of the mold and locates the mold to the nozzle of the injection unit. The moving platen travels between this platen and stationary platen "B".

STATIONARY PLATEN ("B") - The platen at the clamp end of the molding machine that does not travel. The moving platen travels between this platen and stationary platen "A".

STRESS - A resistance to deformation from an applied force. Molded plastic products tend to contain stresses molded in as a result of forces applied during the injection process. These stresses may result in fractures, cracks, and breakage if they are released during use of the product.


SUPPORT PILLAR - A circular rod mold component used to support the ejector half of the mold. It is required because of the tremendous amount of pressure exerted against the "B" plate by the injection phase of the molding process.

THERMOCOUPLE - A device made of two dissimilar metals which is used to measure the temperature of a heated area such as a barrel or nozzle. It sends a signal to a controller which then turns off or on to control the temperature of that area.

THERMOPLASTIC -A plastic material which, when heated, undergoes a physical change. It can be reheated, thus reformed, over and over again. See THERMOSET.

THERMOSET - A plastic material which, when heated, undergoes a chemical change and "cures". It cannot be reformed, and reheating only degrades it. See THERMOPLASTIC.

TIE BARS - Large diameter rods that connect stationary platen "A" to stationary platen "B". The moving platen contains bushings which are used for sliding over the tie bars, allowing the moving platen to travel between the 2 stationary platens.

TOGGLE CLAMP - A term used to describe the use of a mechanical "scissors action" system to open and close the clamp unit of a molding machine. It is operated by a relatively small hydraulic cylinder.

TRANSITION ZONE - That area in the center of the screw (between the feed zone and metering zone) This section has a tapering flight depth condition which compresses the plastic material in preparation for injection.

UNDERCUT - A recess or extension on the molded part, located in such a way as to prevent or impede ejection of the part by normal molding machine operation.

VENT - A shallow groove machined into the parting line surface of a mold in order to allow air and gases to escape from the cavity, or runner, as the molten plastic is filling the mold. Sometimes also located on ejector and core pins.

VENTED BARREL - A heating barrel designed with an automatic venting port which allows moisture and gases to escape from molten plastic prior to being injected into a mold.

Glossary  - Common terms used in the plastics industry