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Plastic Industry Definitions
Additives are materials that are added to a polymer to produce a desired change in material properties or characteristics. A wide variety of additives are currently used in thermoplastics, to expand or extend material properties, enhance processability, modify aesthetics, or increase environmental resistance. Additives enhance properties like flame retardancy and UV light stability.
The applied hydraulic pressure used to restrict the shot size formation. Applied to the back of a melt accumulator or reciprocating screw. Used to control screw drift, mixing, and shot size adjustments. In molding, back pressure increases the temperature of the melt, and contributes to better mixing of colors and homogeneity of the material.
Cylinder that contains the screw and the heaters. Built to withstand pressure of 7,500-20,000 psi.
Heaters that raise the barrel temperature in order to transform the thermoplastic material into a melt.
Blow Needles, Extrusion Blow Molding
Device used to pierce and inflate the parison, usually actuated by an air cylinder.
Blow Pressure, Extrusion Blow Molding
Dimension of bottle divided by the parison diameter. Bottles usually have many ratios. However, only the maximum ratio usually needs to be considered.
The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling.
A mixture of resin and the ingredients necessary to modify the resin to a form suitable for processing into finished articles.
The process of selection of additives and their incorporation into a polymer. Compounding is done to obtain desirable properties for particular uses. Modification of the polymer properties is done by ingredients such as polymeric resins, plasticizers, fillers, reinforcing agents, various stabilizers, lubricants, coloring agents, flame retardants, etc.
A method of molding in which the molding material, generally preheated, is placed in an open heated mold cavity, the mold is closed with a top force, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. This process is most often used with thermoses.
The elapsed time required for the melt to reach its Vicat softening temperature.
This term usually, but not always, denotes a polymer of two chemically distinct monomers.
Complete, repeating sequence of operations for injection molding a part.
In a molding operation, cycle time is the time elapsing between a particular point in one cycle and the same point in the next cycle.
Density and specific gravity are used interchangeably, which is formally incorrect. The difference is the following: density is mass per unit volume of a material at 73°F (23°C); specific gravity is the mass of a given volume of material at 73°F (23°C) divided by an equal volume of water at the same temperature. The conversion is: density = specific gravity X 0.99756. The often used English term “relative density” has the same meaning as “specific gravity.
Diameter of the barrel described in inches. Term used to characterize the extruder.
Metal attachment to the end of the extruder that gives the polymer melt its particular shape.
Pressure required to force the melt through the die.
Occurs when one area of the part cools at a different rate or when the mold surfaces are at different temperatures. Warping results from differential cooling.
Equipment in the process following the extruder. Generally consists of a sizing fixture, used for sizing and cooling of the extrudate; a puller, used to remove the extrudate from the extruder at a continuous rate; and a cutter.
Dropping a Parison
Extruding a parison of sufficient length to position it between the two mold halves to produce a part.
Entrance to the part from the runner located on the parting line.
A rod, pin or sleeve which pushes a molding off of a core or out of a cavity of a mold.
The ability of a material to return to its original state after deformation: the yield point is not exceeded: elastic behavior. Plastics in general respond elastically. If a material’s yield point is exceeded when stressed, it does not return to its original state after removal of the stress: permanent deformation by plastic behavior. Plasticity is the inverse of elasticity. Another way of explaining this is the following. During the first part of the pulling process in the tensile test, both tensile stress and tensile strain continue to increase, and in proportion. When this takes place, the material acts like a spring, and is said to have elastic behavior. Some materials – such as methacrylates – will be broken when they have been strained only a small amount, and while still showing essentially elastic behavior. Other materials – such as polycarbonates – can be stretched many times their original length before they break. The latter have a yield point, and a corresponding yield stress.
A material which at room temperature can be stretched repeatedly to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.
End Of Flow
The melt is just touching the last part of the mold to be filled and the pressure at that point is zero.
A broad term covering all plastics, with or without fillers or reinforcements, which have mechanical, chemical and thermal properties suitable for use, in construction, machine components and chemical processing equipment.
The product or material delivered from an extruder, for example, film, pipe profiles.
A machine for producing more or less continuous lengths of plastics sections such as rods, sheets, tubes, and profiles.
The process of foaming continuous shapes by forcing a molten plastic material through a die.
A multiple cavity mold which produces non-identical parts simultaneously.
The section of the screw just under and slightly forward of the feed hopper.
The contour the melt takes sequentially as it fills the cavity. The mold should fill with a straight flow front with no changes in direction throughout filling. Mold-filling analysis software can be used to predict these filling patterns.
The pressure required to fill the cavity.
A relatively inert substance added to a plastic compound to reduce its cost and/or to improve physical properties, particularly hardness, stiffness and impact strength.
Fillers are generally inert materials characterized by a low aspect ratio (4D). They are commonly used as extenders; fillers do in fact improve certain material properties such as wear, resistance, electrical properties, modulus values, HDT and others.
Films are distinguished from sheets in the plastics industry only according to their thickness. In general, films have thicknesses no greater than .030″.
The surface texture of a finished article.
Describes a very small void which has not blended completely with the surrounding material. Usually caused by a minute quantity of silicone, oil or other surface contaminate on the surface of the substrate.
Deprecated term: not recommended, owing to risk of misunderstanding.
Having the ability to resist combustion (A flame retardant plastic is considered to be one that will not continue to burn or glow after the source of ignition has been removed.) Retardant, or retarder: a substance used in small proportion to reduce the reaction rate of a chemical system. Flame retardant: a substance added, or treatment applied, to a material in order to suppress, significantly reduce or delay the propagation of flame. Flame retardants may be added to plastics materials (external flame retardants) or incorporated as chemical groups in the base polymer during the polymerization process (internal flame retardants). Flame retardance is the property of a material, either inherent or by virtue of a substance added or a treatment applied, to suppress significantly, reduce or delay the propagation of flame. Flame retarded: treated with a flame retardant. fire retardant: a substance added, or treatment applied, to a material in order to suppress, significantly reduce or delay the combustion of the material.
Resin that is forced out of the mold cavity area and onto the parting line or into the vents of the mold. A very thin film of cured resin attached to the molded part.
Flash pockets provide relief areas outside the pinch-offs to limit the amount of compression for mold closing. The flash pockets are sometimes referred to as gutter or relief areas. Ideally, flash pockets will be deep enough to lessen the compression of the flash material, while providing enough surface contact to cool the flashed material.
The temperature at which solvents volatilize sufficiently to produce a flammable mixture.
The ability of the molten resin to move or travel during injection. Flow can be influenced by temperature, part design, process conditions and tool design.
Modifying flow paths, particularly runner sections, so that all flow paths within a mold fill in equal time with equal pressure.
A small area in the mold (usually no more than 0.200″ to 0.300″ wide) which locally thickens the wall to allow material to flow into outer areas more smoothly. A molders “trick” to get a few more inches of flow from the material. Usually not designed in the initial stage of tool construction. Use should be based on experience or qualified molder inputs. Also known as a flow rib.
The contour the melt takes sequentially as it fills the cavity. The mold should fill with a straight flow front with no changes in direction throughout filling. Mold-filling analysis software can be used to predict these flow patterns.
The volume of material passing a fixed point per unit time.
A small area in the mold which usually thins the wall to stall the flow of resin to select areas of the mold. Used to steer the material in another direction
Foaming is the process of producing a cellular plastic by using foaming agents. A foaming agent is a material mixed with or dissolved in a plastic to make it foam. It includes expanding agents that produce gas on heating – such as easily volatile solvents – or chemical blowing agents that produce gas by thermal decomposition. Weight reduction is the percent volume of a foamed part which contains gas instead of resin. To get 10% weight reduction, the mold is filled 90%, then allowed to “foam” and fill that last 10%. Since only 90% of the volume is resin, the part weighs 10% less than a solid part of the same dimensions would. High weight reductions result in reduced material properties when compared to low weight reductions.
Any substance which alone or in combination with other substances is capable or producing a cellular structure in a plastic mass.
A general term encompassing processes in which the shape of plastic pieces such as sheets, rods or tubes is changed to a desired configuration.
In injection molding, the channel through which the molten resin flows from the runner into the cavity.
A blemish or disturbance in the gate area of an injection molded article.
The hardness of a material can be measured by its resistance to scratching or to indentation. Mostly used hardness tests involve the determination of the material resistance to indentation under standardized conditions. A hard indenter of standard shape is pressed into the surface of the material under a specified load. The resulting area of indentation or the depth of indentation is measured and assigned a numerical value. Various methods can be used. For plastics, the most widely used methods are Ball hardness, Rockwell and Shore methods.
The applied hydraulic injection pressure maintained after the completion of mold filling. Also referred to as secondary pressure.
Homopolymers are polymers which are made up of one single repeated basic unit or (mono)mer.
The device above the machine barrel used to store the resin pellets.
Injection Or Fill Time
Time required to fill the cavity or mold.
Injection Compression Molding
A method of molding where the plastic is injected into a partially open cavity with injection pressure typically 50-75% lower than standard injection molding depending on wall thickness and mold open distance. The 2nd stage clamp action closes the telescoping core and subsequently compresses and distributes the melt into the far extremities of the cavity, including ribs and bosses.
The method of forming objects from granular or powdered plastics, most often of the thermoplastic type, in which the materials is fed from a hopper to a heated chamber in which it is softened, after which a ram or screw forces the material into a mold. Pressure is maintained until the mass has hardened sufficiently for removal from the mold.
The applied hydraulic pressure used to push the resin into the mold cavity.
The elapsed time required to fill the mold cavity.
A removable part of the mold imparting increased resistance to wear or heat transferability to that area of the mold.
An article of metal or other material which is incorporated into a plastic molded part either by pressing the insert into the finished molded part or by placing the insert in the cavity so that it becomes an integral part of the molding.
Insert Pull-Out Strength
Force required to pull an insert straight out of a material at least 0.020″ thick. Insert performance criterion maintained for the most part in molded parts.
A pin that ejects a molded article from the mold. Land Area The area of surfaces of a mold which contact each other when the mold is closed.
Land (Gate Area)
Gate dimension parallel to the direction of melt flow.
L/D Ratio Extrusion
Barrel length divided by the diameter of the barrel
Any of a number of processes, such as drilling, turning, sanding, etc., which may be performed on a piece of plastic.
The resistance of glossy plastic surfaces to abrasive action.
A masterbatch is a concentrate of colorants or additives properly dispersed into a carrier polymer, which is then blended into the natural polymer to be coloured or modified.
The file of the information on each material acceptably tested for use in MOLDFLOW analyses.
The molten material which will fill the mold cavity to form the part.
Also referred to as a plunger. A large capacity holding area for molten resin. The area of a structural foam molding machine that determines the shot size.
The amount of a thermoplastic resin, measured in grams, which can be forced through a specified orifice within ten minutes when subjected to a specified force. (ASTM D-1238)
The strength of the plastic while in the molten state. This is a pertinent factor in extrusion, blow molding and drawing of molten resin from a die.
The melt temperature or Tm is the temperature, measured under specified conditions, at which crystallinity disappears in a semi-crystalline polymer. Semi-crystalline materials have a clearly defined melt temperature. Amorphous materials soften over a wide temperature range above their glass transition temperature. They do not have a specific Tm, but a melting range. Melt temperature should be measured from a purge shot having the same residence time as in the production process.
Phenomena accompanying the softening of a material under the influence of heat: can be shrinking, dripping and burning of molten material. Melt drip: falling droplets of molten material, either burning or not.
In injection molding, a lubricant used to coat the surface of the mold to enhance ejection of the molded article or prevent it from sticking to the tool. Many resins are available with an internal mold release.
The difference in dimensions between the mold and the molded part.
The temperature at which the mold is maintained. Often the most important benefit of raising mold temperature is that it allows a slower injection rate without the plastic getting too cold.
The purpose of mold venting is to exhaust air from mold cavities to enable the inflation of the part. Drilled holes, vent bushings and continuous venting along mold seams are typical methods. If venting is needed on appearance surfaces, the vents are sometimes textured to match the part finish.
The temperature of the mold and melt and the time required to fill the mold.
The period of time occupied by the complete sequence of operations on a molding press requisite for the production of one set of molded articles.
The pressure applied to the ram of an injection machine or press to force the softened plastic completely to fill the mold cavities.
The variability of the pressure to fill the cavity and temperature of the melt at the part as influenced by changes in injection time and barrel melt temperature.
A monomer is the molecular unit from which polymers are prepared. A polymer is a molecular chain formed by combining many smaller molecules. Polymers are the product of a reaction called polymerization, the process of connecting many (poly) single units (mers or mono-mers) to form long chain molecules of higher molecular weight. Polymerization reactions may be controlled to produce molecules of a specific length or molecular weight. All plastic resins or materials are polymeric in nature.
Multiple Cavity Flow
Produces more that one identical part with each cycle.
Hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine designed to form a seal under pressure between the cylinder and the mold
A round, hollow tube of molten plastic that is extruded from the head of the blow molding machine.
The tendency for a parison to lose its tubular shape because of conditions in the extrusion process. Generally, the bottom of the parison tends to drape irregularly, causing the entire parison to lose its original shape, creating difficulties in molding the part.
Parison Melt Strength
Parison melt strength depends directly on the melt characteristics of the resin being extruded. The ability to extrude a parison of sufficient dimensions to produce the desired part depends on the melt strength of the parison. The larger and heavier the parison, the greater its melt strength requirements.
Parison Pinch Bars
Various methods are used to close or seal the bottom of the parison before mold closing or part inflation. The devices are called pinch bars and are usually spring loaded, or hydraulically or pneumatically actuated. They may be attached to the bottom of the mold or positioned directly under it.
A pinch-off is needed when the parison falls outside the cavity of the mold. It is the protruding edge separating the cavity from the flash pocket, and it compresses the flash to the point of severance. Inserted beryllium copper is preferred because the alloy has thermal conductivity equal to that of the aluminum alloy used in the mold. Steel pinches are used when pinch wear is critical – for example, when molding materials such as polycarbonates are used.
Introducing air pressure into the parison before closing the mold halves. This provides better distribution of wall thickness and prevents the parison wall from coming in contact before the inflation of the part.
Varying the wall thicknesses in a parison to conform to the wall thickness requirements of a given part.
The bottom portion of a parison that is severed by the lower pinch-offs and falls outside the mold.
The ability of a material to withstand continuous and permanent deformation by stresses exceeding the yield value of the material without rupture.
To render a material softer, more flexible and/or more moldable by the addition of a plasticizer.
A substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability or extensibility. Some plasticizers have been known to have detrimental effects on certain types of plastic, end use testing is recommended.
An objectionable coating gradually formed on metal surfaces of molds during processing of plastics due to extraction and deposition of some ingredient such as pigment, lubricant, stabilizer or plasticizer.
The large metal plates the mold attaches to on a plastic molding machine.
PLC (Performance Level Categories)
As defined by UL: “In order to avoid an excessive level of implied precision and bias, material performances for several tests are recorded as PLC, based on the mean test results (rather than recording the exact numerical results)”. PLC levels are assigned to electric properties, tested according to UL 746A
Polycarbonate Resin (PC)
A family of special types of polyesters in which groups of dihydric phenols are linked through carbonate linkages.
The product of a polymerization reaction. The product of polymerization of one monomer is called a homopolymer, monopolymer or simply a polymer. When two different monomers are polymerized simultaneously the product is called a copolymer. The term terpolymer is sometimes used for polymerization products of three monomers.
A linear polymer is a polymer in which the monomers are bound to each other in a straight chain without any branches. Branched polymers have branched connections of molecules.Copolymers are polymers with repeating molecular units from at least two different monomers. Two kinds of arrangements are possible: random and alternating, resulting in random copolymers and alternating copolymers.Such polymers are called block-copolymers, characterized by both monomers A and B forming the backbone chain of the polymer. They have repeating monomers in linearly connected blocks. Another possibility is the formation of a graft-copolymer, which is essentially a branched-chain structure. It has side chains composed of one type of monomer unit attached to the backbone or main chain from another monomer unit.
The process of converting a monomer or a mixture of monomers into a polymer. Addition polymerization is the stepwise addition of a simple repeated unit. Or, the reaction that yields a product that is an exact multiple of the original monomeric molecule. Condensation polymerization is the combination of functional molecules, leading to the formation of a polymer with the liberation of simple by-products, usually water.
A general term referring to the relative positions, arrangement in space, and freedom of motion of atoms in a polymer molecule.
The class of polymers made by polymerising relatively simple olefins.
Time, usually in hours, during which a two or three component product can be used after it is mixed. Sometimes measured in terms of time to gel and/or double in viscosity.
The deviation caused by the material when a light beam passes through a transparent specimen.
Resin that has been previously molded and ground into small pieces for reprocessing. Regrind is usually comprised of sprues, runners and unfilled parts.
An inert fibrous or non-fibrous material incorporated in a plastic to improve or modify mechanical or physical properties.
A plastic with high-strength fibers embedded in the polymer matrix, resulting in increased strength properties superior to those of the base resin.
Reinforced Plastic A plastic composition in which fibrous reinforcements are imbedded, with strength properties greatly superior to those of the base resin.
The term is use to designate any polymer that is a basic material for plastics.
Rockwell hardness is the resistance of a material to indentation of a defined steel ball. Three scales exist (R, L and M) with different ball diameters and different levels of loading for testing materials of different hardness.
In an injection mold, the feed channel, usually of circular cross section, that connects the sprue with the cavity gate. The term is also used for the plastic piece formed in this channel.
Developing a runner system which delivers the required amount of melt to each cavity with the correct pressure to finish filling all the cavities simultaneously at the correct temperature for the part.
Using the runner as a flow control device (positioning the gate and using the size of the runner to control the filling pattern within the cavity) in addition to getting the melt into the cavity.
This term is sometimes used for the entire resin feeding system, including sprues, runners and gates, in injection molding.
Any output of a mold that is not usable as the primary product.
A rotating auger that uses a helical thread and a varying channel depth to convey, melt, mix, and pump resin. Consists of a feed section, transition zone, metering section and in some cased a mixing zone/zones.
Shore hardness values are measured by using calibrated durometers: Shore A for softest and Shore D for harder materials. The material is penetrated with a steel rod of predefined dimensions – different for Shore A and for Shore D. The values show no correlation with other hardness measurement values.
The amount of resin injected into a mold cavity at a volume less than required to fill the mold.
One complete cycle of a molding machine.
The percentage of reduction in overall part dimensions. Shrinkage occurs during the cooling phase of the process.
The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.
A device attached to the front of the molding machine barrel or hot manifold system. It opens to allow resin flow into the mold cavity. It closes after injection to help prevent drooling.
Single Cavity Mold
Produces one part with each cycle.
A depression on the part surface over a thick wall feature i.e., a rib, boss, thick sprue gate or change in wall section. Caused by the delayed cooling effects of the center of the thick area. When the hot center cools, it shrinks and pulls down the skin on the surface.
An indentation on the surface of the part as a result of significant local change in wall section. The mark will occur in the thicker area.
The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73F).
The amount of heat needed to raise one gram of material one ÞC in temperature. Equals “heat capacity” or “Cp.”
The volume of a unit of weight of a material; the reciprocal of density.
A substance used in the formulation of plastics to help maintaining the properties of the material during processing and service life.
Entrance to the part from the runner located below the parting line. On ejection the part breaks away from the subgate.
Break up of a full molding into component areas.
Distressed melt flow due to welding, melding, splay, etc. will appear as surface imperfections.
The amount of force required to elongate the plastic by a defined amount. The higher the value, the stronger the material. Or the maximum stress sustained by a material before failure in tension. When the maximum stress occurs at the yield point, it is called tensile strength at yield. When maximum stress at break: tensile strength at break.
The measure of the ability of a material to conduct heat along its length or through its thickness. Or the rate of heat flow under steady conditions through unit area, per unit temperature gradient in the direction perpendicular to the area. The higher the value, the more conductive: a material with a low value for thermal conductivity acts like an insulator
Rubber-like elasticity exhibited by a rigid plastic resulting from an increase in temperature.
The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to tender it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.
Materials that become soft when heated and solid when cooled to room temperature. This softening and setting may be repeated many times.
The family of polymers that resemble elastomers in that they can be repeatedly stretched without distortion of the unstressed part shape, but are true thermoplastics and thus do not require curing.
Thermoplastics vs. Thermosets
A thermoplastic is a polymeric material or plastic which becomes soft and formable when heated and rigid when cooled. This process may be repeated a number of times without chemically altering the material. A thermoset is a polymeric material which undergoes irreversible chemical changes when cured through heat, catalysts or ultraviolet light: cross-linking prevents movement of molecular chains after curing. Once cured, the structure cannot be changed.
Materials that may not be reheated and softened again. Once the structural framework is set, these plastics cannot be reformed.
Depth of the material and contributory to pressure requirements; thickening reduces the pressure required to fill the part.
A white pigment available in two crystalline forms, rutile and anatase, the former being the most widely used white and opacifying pigment in thermoplastics, printing inks and paints
A protuberance or indentation that impedes withdrawal from a mold.
The dominant flow of two confronting flows, over the other. The lesser flow reverses direction giving poor surface appearance and structural strength. Underflow should be avoided by positioning gates so that the flow fronts meet at the end of filling.
Uniform Cooling Time
Cooling time the same throughout the part to avoid warping.
An additive which protects materials by absorbing UV radiation.
Additive which stabilises organic materials against UV radiation.
A method of forming plastic sheets or films into three-dimensional shapes, in which the plastic sheet is clamped in a frame suspended above a mold, heated until it becomes softened, drawn down into contact with the mold by means of a vacuum, and cooled while in contact with the mold. Often used interchangeably with thermoforming.
Vicat Softening Temperature Undercut
Vicat Softening Temperature, is a measure of the temperature at which a plastic starts to soften at specified test conditions according to ISO 306. It is determined with a standard indenter (a flat-ended needle of 1 mm 2 circular cross section) penetrating into the surface of a test specimen under a predefined load. The temperature at 1 mm penetration is quoted as the VST in Co. VST gives an indication of a material’s ability to withstand limited short-term contact with a heated object. For material comparisons only.
Any plastic compound or resin that has not been subjected to use or processing other than that required for its original manufacture.
Viscosity, MFR, MVI, and MV
Viscosity is the resistance to steady flow shown within the body of a material. It is internal friction or the measure of a polymer melt’s resistance to flow. In testing: the ratio of the shearing stress to the rate of shear of a fluid. Which ‘Newtonian viscosity’, the ratio of shearing stress to rate of shearing strain is constant. In non-Newtonian behavior – which is the usual case with plastics – the ratio is not constant but varies with the shearing stress. Such a ratio is often called the apparent viscosity at the corresponding shearing stress. It represents one point on the flow curve. MFR, or melt flow rate, is the mass of thermoplastic material extruded in a given time through a defined orifice under specified conditions. Also called “flow rate.” The expression MVI, or melt volume index, equals MVR. MV, or melt viscosity, is a measure of a polymer at a given temperature at which the molecular chains can move relative to each other. It is expressed as the quotient of the real shear stress t and the real rate of shear y. Melt viscosity is considerably dependent on the molecular weight: the higher the molecular weight the greater the entanglements and the greater the melt viscosity.
To twist or deform from a desired shape. Often caused by molded in stress or shrinkage.
Dimensional distortion of a plastic part due to strain resulting from injection molding or other conversion methods.
Sections of the barrel which are controlled individually by temperature controllers.