polymer processing

Weld Line - The point on a part where two plastic flow fronts meet during injection. Witness Line - A visible line on part due to two molding surfaces coming together. Witness lines always occur at parting lines.

Variation - A change in the value of a measured characteristic. Vent - A shallow channel in the mold to allow air to escape from the cavity during injection. Virgin Material - Plastic material that has not been subject to use or processing other than that required for its initial manufacture. Viscosity - Internal friction or resistance to flow of a melt or liquid. The constant ratio of shearing stress to shearing rate. It is a measure of the materials resistance to flow.

Ultimate strength - Term used to describe the maximum unit stress a material can withstand before breaking when subjected to a load in compression, tension or shear. Undercut - An indentation or protuberance on a part that impedes the removal of core and/or cavity.

Tensile strength - The resistance of a material to longitudinal tension stress. Thermal conductivity - Ability of a material to conduct heat. Thermal degradation - Deterioration of material by heat. Thermal diffusivity - The ability of a material to transfer heat per unit time and area. Thermoplastic - Capable of being repeatedly softened by heat and hardened by cooling. Thermoset - A material that will undergo or has undergone a chemical reaction by the action of heat, catalysts, ultraviolet light,etc. Three-plate mold - Mold with two parting lines, one for part and the other to free trapped cold runner. Two-plate mold - Mold with one parting line used to free both part and runner.

Shear - An action resulting from applied forces which cause or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their place of contact. Shear heating - Frictional heat generated within a plastic due to shear forces either in the extruder or in the runner system during injection. Shot - The yield of parts from a mold after one complete molding cycles, including scrap. Shot capacity - The maximum amount of material that the injection units ram or plunger can push out in one forward stroke. Shot size - The amount of plastic required to fill the mold. Shrinkage - The decrease in dimensions a plastic undergoes from molten state to a solid state. Shrinkage is compensated for by packing pressure, but some material shrinkage occurs after gate is frozen and for as much as 24 hours after it is molded. Sink Mark - An imperfection or depression in the surface of a part, generally cause by non-uniform shrinkage. SPC - Statistical Proce...

Recovery - The injection unit building up (plasticizing) for a next shot. Regrind - A thermoplastic from one process or cycle (scrap, runners, etc.) ground up to be used for a future molding cycle. Resin - Any of a class of solid or semi-solid organic products of natural or synthetic origin. Most resins are polymers. Runner - Channel in mold that connects sprue to gate. Runnerless molds - Molds which are designed so that the runner is heated and stays molten. Runner is not ejected with part each shot.

Parting line - Line, visible on the mold part, where mold separates to release the molded part from the cavity. Pearlescent pigments - A class of pigments consisting of particles that are essentially transparent crystals of a high refractive index. The optical effect is one on partial reflection from the two sides of each flake. When reflections from parallel plates reinforce each other, the result is a silvery luster, iridescent effects and metallic sheen resembling natural pearl. PET - Polyethylene Terephthalate, known as a thermoplastic polyester. Has the unusual ability to exist in either an amorphous or highly crystalline state. Plasticize - To soften a material and make it plastic or moldable, either by means of an extruder or by heat or both. Platen - The plates on which the mold assembly is mounted. Large metal blocks on clamping ends of molding machine used to support mold. Polycarbonate resins - Polymers derived from the direct reaction between aromatic and aliphatic di...

Olefins - A group of unsaturated hydrocarbons and named after the corresponding paraffins by the addition of ene or ylene to the stem. Examples are polyethylene, polypropylene, etc. Orientation - The alignment of crystalline structure in polymeric materials so as to produce a highly aligned molecular structure.

Nozzle - The hollowed metal cone nose screwed into the extrusion end of the extruder screw that mates with the mold via the mold's nozzle bushing. The nozzle allows transfer of the molten plastic from the extruder to the mold runner system and cavities.

Matte finish - A surface which displays no gloss when observed at any angle. Mean - See Average Melt Flow Index - The amount, in grams, of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subject to 2,160 grams force for ten minutes at 230°C, per ATSM D1238 Metallizing - Applying a thin coating of metal to a non-metallic surface. Metering Zone - The final zone of an injection barrel or screw, at which the melt is advanced at a uniform rate to the screw tip. Mold - The cavity or matrix of cavities into which plastic melt is placed and takes form. Mold Components - Parts that make up a mold and determine how a mold will be operated. Mold Frame - Series of metal plates which support mold components. Multi-cavity Mold - A mold with two or more cavities.

L/D ratio - Length to diameter ratio of an extruder screw. LDPE - Low Density Polyethylene Lead In - Initial clearance through an initial angle or chamfer that helps align two close mating parts. Linear Molecule - A long chain molecule of two-dimensional structure which may contain side chains or branches. lLDPE - Linear Low Density Polyethylene Lubricants - Prevent materials form sticking or improve processability.

Knockout - Any part or mechanism of a mold used to eject the molded article. Knockout bar - A bar that ejects the molded article from the mold. It is usually activated automatically while the mold is opening.

Jetting - A turbulent flow in the resin melt caused by an undersized gate or where a thin section becomes thicker. Can also occur when gating into and along a thick section.

Impact strength - The ability of a material to withstand shock loading. Inert additive - A material added to a plastic compound, such as a filler, which may alter the properties of the finished article but which does not react chemically with any other constituents of the composition. Injection pressure - Pressure exerted on plastic to fill, pack, and hold mold. Comprised of Boost and Hold pressure. Injection ram - The ram which applies pressure to the plunger in the process of injection molding. In-mold decorating - The process of applying labels or decorations to plastic articles simultaneously with the molding operation. Interference fit - The mating of two points, lines, or surfaces, the dimensions of which cause negative clearance between them.

Hardness - The resistance of a material to compression or indentation. Among the testing methods Brinell, Rockwell, and Shore HDPE - High density polyethylene Heat deflection or distortion point - The temperature at which a standard test bar deflects 0.010 inches under a stated load of either 66 or 264 psi. Indication of maximum temperature that you can eject a part from a mold. Hold (2nd Stage Pressure) - Sufficient pressure applied to cavity that doesn't allow plastic to flow into or out of the cavity until the gate is frozen. Hopper - Feed reservoir into which material is loaded from which it falls into a mold machine's injection unit. Hot runner mold - Type of runnerless mold. Runner is not ejected with part each cycle

Gate - An orifice through which the molten resin enters the cavity. Gate blush - A blemish or disturbance in the gate area of an injection molded article. It occurs when the melt fractures in leaving the gate due to relaxation of elastic forces. Gel permeation chromatography - A device used to determine the molecular weight distribution of a polymer. GPC - An abbreviation for Gel Permeation Chromatography. Guide pins - Devices to maintain proper alignment for molds.

Feed zone - Zone on injection screw directly below hopper. Plastic is pre-heated in this zone, not melted. Filler - A substance added to the plastic to make it less costly. Some fillers also improve mechanical properties. Finish - The ultimate surface of a plastic part. Flash - The thin, surplus web of material which is forced into crevices between mating surfaces during a molding operation, and which remains attached to the molded article. Flow lines - Wavy surface appearance caused by improper flow of the resin into the mold. Fluorescent - Organic dyes or pigments which may be blended into inks to provide a luminescent or fluorescent appearance typical of the "neon" colors. Frictional heating - See Shear Heating

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Ejection - That part of the molding cycle in which the finished part is removed from the mold mechanically. Ejection system - System used to eject the molded part from the mold after it cures. Ejector plate - A plate which backs up the ejector pins and holds the ejector assembly together. Elastic deformation - The part of the deformation of as part under load which is recoverable when the load is removed. Elasticity - The property of a substance which enables it to return to its original shape and size after removal of a deforming force. Elongation - Lengthwise stretching of a material usually denoted as a % of original length. Environmental stress cracking (ESC) - The susceptibility of a plastic to crack or craze under the influence of certain chemicals, stresses, or other agents. Ethylene plastics - Plastics based on polymers or co-polymers of ethylene. EVA (Ethylene-Vinyl Acetate) - Co-polymers from these two monomers retain many of the properties of ethylene, but have consi...

Daylight opening - Clearance between the two platens of the press in the open position. Decompression - See Suckback Degradation - A deleterious change in the chemical structure of a plastic. Degating - The removal of material left on a plastic part formed by the opening through which material was injected into the mold cavity. Degree of polymerization - The average number of monomer units per polymer molecule, a measure of molecular weight. In most plastics, the molecular weight must reach several thousand to attain worthwhile physical properties. Density - Mass per unit volume of a substance, exposed in units such as grams per cubic centimeter, pounds per cubic foot or pounds per gallon. Desiccant - A substance capable of absorbing water vapor from air or other gaseous material, used to maintain low humidity in a storage or test vessel. Draft - The degree of taper of a side wall or the angle of clearance designed to facilitate ejection of parts from the mold. Dry coloring - ...

CAD - Computer Aided Design. Use of computer to draw/design parts. Cap - Closure Capacity - The rated amount of volume (# parts) produced by a mold/molding machine per year. Capper - Machine that applies a closure to a bottle on a packing line. Cavitation - The number of molding stations within a mold. Cavity - Mold component(s) which form the exterior or external surfaces of a part. Centerline - A line drawn down on part drawing that represents the geometrical center of the part in that axis. Charge - The amount of material volume or weight required for the injection unit to plasticize for each molding cycle. Chemical resistance - Ability of a material to retain utility and appearance following contact with chemical agents. Child resistant closure (CRC) - A closure that requires dissimilar motions which may make removal by a child difficult. Subject to government regulations. Clamp speed - Speed at which the mold platens will open and close; controlled by operator settings....

Back draft - A slight undercut or tapered area in a mold tending to prevent removal of the molded part. Back plate - A plate used as a support for the cavity blocks, guide pins, bushing, etc. Sometimes called support plate. Back pressure - Pressure exerted by the injection unit's cylinder against the screw as it recovers or plasticates. Used to increase the melt homogeneity. Increases melt temperature. Baffle - A plug or other device inserted in a flow channel to divert cooling medium to a desired path. Balanced runner - A runner system designed to place all cavities at the same distance from the sprue. Ball check valve - An anti-back flow valve which prevents the plastic melt from flowing back over the screw during injection. Band heaters - Electrical heaters surrounding the barrel and nozzle of the injection unit. Bar chart - Histogram Barrel (extruder) - Hollow tube in which plastic is gradually heated and melted. Inside the barrel is a helical screw which compresses an...

ABS (Acrylonitrile-butadiene-styrene) - Blends of copolymers or styrene-acrylonitrile copolymer with butadiene-acrylonitrile rubber. Accumulator - Auxiliary cylinder and piston (plunger) mounted on injection molding machine. It is used to provide extremely fast molding cycles through accumulation of molten plastic from the main extruder (reducing plastication time) and forces the plastic into the mold during filling at high speed and pressure (reducing fill time). Acetal resins - The molecular structure of the polymer is that of linear acetal, consisting of unbranched polyoxmethylene chains. Achromatic color - A neutral color (white, gray or black) that has no hue. Acrylic - Polymethylmethacralate Additives - Used to enhance processing, performance, appearance, and/or economics of the basic plastic formulation. Aesthetics - The sum total of the visual response (color, shape, feel, etc.) to the appearance of a part. Amorphous phase - Devoid of crystallinity; no definite order. Th...