Crown Plastic’s glossary


ABS: Acrylonitrile Butadiene Styrene (Thermoplastic Resin).
Abrasion: Wearing away by friction. Glass is highly resistant to abrasion from other materials, but can be damaged through contact with itself. Lubrication during processing and fabrication helps prevent abrasion.
Acrylic: Thermoplastic polymer made by the polymerization of esters of acrylic acid.
Additive: A material used to modify the properties of polymer resins. Examples include plasticizers, initiators, light stabilizers and flame-retardants.
Adhesion: The state in which two surfaces are held together by an interlocking action or force.
Adhesive: A film, liquid or paste capable of holding the surfaces of two materials together.
Air-Bubble: A localized, non-interconnected, spherically shaped entrapment of air within and between the plies of reinforcement.
Air Vent: Small outlet to prevent entrapment of gases in a molding or tooling fixture.
Alloy: In plastics, a blend of polymers or copolymers blended with other polymers or elastomers under select conditions.
Ambient: Surrounding environmental conditions, such as pressure, temperature, or relative humidity.
Amine Resins: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.
An Isotropic: Exhibiting different properties when tested along axes in different directions. see An Isotropic laminate 
An Isotropic Laminate: A laminate whose properties are different in different directions.
Angle-Ply Laminate : A laminate with fibers of adjacent plies oriented at alternating angles.
Antioxidant: Substance that, when added in small quantities to resin, prevents oxidation and degradation while maintaining the resin’s properties.
Antistatic Agents: Agents added to a molding material or applied to the surface of a molded object to make it more conductive and prevent the fixation of dust or buildup of electrical charge.
Aramid: A highly oriented organic material derived from polyamide with an aromatic ring structure that is used primarily as a high-strength, high-modulus fiber. Examples include Kevlar and Nomex.
Aspect Ratio: Ratio of length to diameter of a fiber.
ASTM: American Society of Test Methods.
Axial Winding: Filaments that are wound parallel or at a small angle to the axis (0° helix angle).


Balanced Construction: Equal parts of warp and fill in fiber fabric.
Balanced Laminate: Composite laminate whose 0° and 90° angles and are symmetrical around the centerline.
Bare Glass: Glass fiber  before a binder or sizing is applied.
Batch Oven: Large temperature-controlled oven used to heat-clean rolls of glass fiber fabric.
Batt: Felted fabrics or structures built by the interlocking action of compressing fibers, without spinning, weaving, or knitting.
Bearing Strength: The maximum amount of load stress that can be sustained.
Bearing Stress: Applied load in pounds divided by the bearing area. Maximum bearing stress is the number of pounds that can be sustained, divided by the original bearing area.
Bed: The mat of chopped glass fibers deposited over a layer of resin mix on carrier film following a chopping operation.
Bias Fabric: Warp and fill fibers placed at an angle to the length of the fabric.
Biaxial Load: Loading condition in which a laminate is stressed in two different directions in its plane.
Biaxial Winding: Filament winding in which the helical band is laid in sequence, side by side, without any fibers crossing over each other.
Bi-directional: Reinforcing fibers arranged in two directions, usually at right angles.
Bi-directional Laminate (Cross Laminate): A reinforced plastic laminate whose fibers are oriented in two directions in its plane.
Binder: Coating which is applied to the surface of materials to hold them together.
Blanket: Fiber or fabric plies that have been laid up in a complete assembly and placed on or in the mold all at one time
Blister: Flaw either between layers of laminate or between the gel coat film and laminate.
Bond Strength: Amount of adhesion between bonded surfaces. The stress required to separate a layer of material from the base to which it is bonded, as measured by load/bond area.
Boron Fiber: Fiber produced by vapor deposition of elemental boron, usually onto a tungsten filament core, to impart strength and stiffness.
Breathing: Opening and closing a mold so that gas can escape early in the molding cycle.
Bridging: Condition in which fibers do not move into or conform to radii and corners during molding, resulting in voids and dimensional control problems.
Buckling (Composite): Failure mode generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.
Buildup: Glass bundles or chopper fuzz that collect on the chopper, cot, static bars or machine frame.
Bundle: A discrete collection of many parallel glass filaments.


Carbon: Element that provides the backbone for all organic polymers.
Carbon-Carbon: Composite material consisting of carbon or graphite fibers in a carbon or graphite matrix.
Carbon Fiber: Fiber produced by the pyrolysis of organic precursor fibers, such as rayon, polyacrylonitrile (PAN), and pitch, in an inert environment.
Casting: Process of pouring resin, fillers and/or fibers into a mold vs. building up layers through lamination.
Catastrophic Failures: Totally unpredictable failures of a mechanical, thermal, or electrical nature.
Caul Plates: Smooth metal plates free of surface defects with the same size and shape as a composite lay-up that contacts the lay-up during curing..
Cavity: Space inside a mold in which a resin or molding compound is poured or injected.
C-Glass: Glass with a soda-lime-borosilicate composition that maintains chemical stability in corrosive environments.
Chalking: Surface phenomenon indicating degradation of a cosmetic surface.
Chemical Size: A surface finish applied to the fiber that contains some chemical constituents other than water.
Clamping Pressure: In injection molding and transfer molding, the pressure that is applied to the mold to keep it closed in opposition to the fluid pressure of the compressed molding material.
Cloth: Fiberglass reinforcement made by weaving strands of glass fiber yarns.
Coefficient of Friction (COE): The ratio between the force needed to move one surface horizontally over another and the pressure between the two surfaces.
Coefficient of Thermal Expansion (CTE): How much a material’s shape will change per each degree of temperature fluctuation.
Cold Flow: Distortion that occurs in a material under continuous load within its working temperature range and without a phase or chemical change.
Compaction: Applying a temporary vacuum bag and vacuum to remove trapped air and compact the lay-up; also removing air in SMC machines prior to roll-up.
Compatibility: The ability of two or more substances to be combined in order to form a homogeneous composition of useful plastic properties; for example, the suitability of a sizing or finish for use with certain general resin types.
Composite: Chemical or mechanical bonding of dissimilar materials such as glass fiber and polyester resin, whose cumulative properties are superior to the individual materials.
Composite Material: A combination of two or more materials (reinforcing elements, fillers, and composite matrix binder).
Compression Molding: An open molding process in which material is introduced and shaped by the pressure of closing and by heat.
Compressive Modulus: Ratio of compressive stress to compressive strain below the proportional limit. Theoretically equal to Young’s modulus determined from tensile experiments.
Compressive Strength: The amount of nonmoving load that a bar can take before it is crushed.
Condensation Polymerization: A chemical reaction in which two or more molecules combine, with the separation of water or some other simple substance.
Conductivity:  The electrical or thermal conductance of any material.
Conformability: A materials ability to conform to difficult shapes without wrinkling.
Continuous Filaments: Filaments that extend substantially throughout the length of the yarn.
Continuous Heat Resistance: Maximum temperature to which material should be subjected in a continuous application. Below this temperature, the material is acceptable. At temperatures above the maximum, the material may decompose, melt, or otherwise fail in an application.
Continuous Laminating: Process for forming panels and sheeting in which fabric or mat is introduced to a resin and passed through a heating and cooling zones for cure. Paralell belts control thickness as the various plies are brought together.
Continuous Roving: Roving supplied in a package that allows for continuous processing.
Continuous Strand: Fiberglass mat of very long individual fibers that have a regular crossed pattern and are loosely held together with a binder.
Copolymer: A resin produced by copolymerization. A process in which unlike molecules are arranged in alternate sequence in a chain.
Corona Treatment: Surface treatment for the promotion of bonding from the generation of plasma by electrical excitement of ambient air.
Corrosion Resistance: A material’s ability to withstand ambient natural factors or those of a particular artificially created atmosphere, without degrading or changing in properties. For metals, this could be pitting or rusting; for or organic materials, it could be crazing.
Coupling Agent: Any chemical substance designed to react with both the reinforcement and matrix phases of a composite material to form or promote a stronger bond at the interface.
Crazing: Cracking of resin due to stress.
Creep: The slow movement of a plastic material with time.
Creep, Rate of: Rate of the slope of the creep-time curve at a given time. Deflection with time under a given static load.
Cross-linking: Chemical links between molecule chains in thermosetting resins. Styrene monomer is a cross-linking agent in polyester resins.
CTE: see Coefficient of Thermal Expansion
Cure: Cross-linking or total polymerization of a resin’s molecules that alters a material’s properties, changing it from a liquid to a solid.
Cure Cycle: The time/temperature/pressure cycle used to cure a thermosetting resin system or prepreg
Cure Temperature: Temperature at which a cast, molded, or extruded product, resin-impregnated reinforcement, adhesive or other material is subjected to curing.
Cure Time: The time required for liquid resin to reach a cured or fully polymerized state after catalyst has been added.
Cycle: The complete, repeating sequence of operations in a process or part of a process. In molding, cycle time is the period (or elapsed time) between a certain point in one cycle and the same point in the next.


Damping: The decay with time of the amplitude of free vibrations of a specimen.
Daylight: The distance, in the open position, between the moving and fixed tables or the platens of a hydraulic press. In a multi-platen press, daylight is the distance between adjacent platens. Daylight provides space so a molded part can be removed from the mold.
Decibel: A unit used to measure the intensity of a sound.
Deflashing: A finishing technique used to remove excess, unwanted material (flashing) on a plastic molding.
Deflection Temperature Under Load: Temperature at which a simple beam has deflected a given amount under load (formerly called heat distortion temperature).
Deformation Under Load: Dimensional change of a material under load for a specific time following the instantaneous elastic deformation caused by the initial application of the load. (Also, ‘cold flow’ or ‘creep’.)
Delamination: Separation of composite layers.
Density: A material’s weight per unit volume.
Dielectric: A material’s ability to resist the flow of electrical current.
Dielectric Strength:  How much of an electrical voltage can be built up on one side of the material before it is communicated to the other side.
Dimensional Stability: A plastic part’s ability to retain the precise shape to which it was molded, cast, or otherwise fabricated.
Direct-Sized Yarn: Specially formulated sizing on textile yarns that allows them to be resin compatible.
Dispersion: Degree to which reinforcement is uniformly distributed in a material.
Distortion: Change in shape from that which is intended.
Doctor Blade or Bar: A straight piece of material used to spread resin, inks, or other liquids as applying a thin film of ink for printing or as an adhesive film.
Draft: The taper or slope of a mold’s vertical surface allowing molded parts to be removed.
Drape: The ability of pre-impregnated broad goods to conform to an irregular shape; textile conformity.
Dry Loft: Height of the bed of chopped fibers.
Dry Spot: Area of incomplete surface resin on laminated plastics.
Dwell: A pause in the application of pressure or temperature to a material allowing gas to escape from the molding material or heat to conduct.


E Glass: A family of calcia-alumina-silicate glasses with a certified chemical composition used for general purpose and most electrical applications. (ASTM D578-90.)
Elasticity: A material’s ability to recover its original size and shape after the force deforming it has been removed.
Ejection (Demolding): Removing a molded part from the mold by hand, mechanical means or use of compressed air.
Ejection Plate: A metal plate used to operate ejector pins; designed to apply a uniform pressure to them in the process of ejection.
Elastic Limit: The greatest stress a material can sustain without permanent strain after the stress has been completely released with non-recoverable deformation.
Elastomer: A material that substantially recovers its original shape and size at room temperature after a deforming force is removed.
Elongation: When a bar is pulled, how much longer it gets before it breaks.
Encapsulating: Completely surrounding an object with resin or a fiber resin composite.
Environment: The aggregate of all conditions (such as contamination, temperature, humidity, radiation, magnetic and electric fields, shock, and vibration) that externally influence the performance of an item.
Epoxy: A polymerizable thermoset polymer containing one or more epoxide groups cured by its reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans.
Extenders: Low-cost materials used to dilute or extend high-cost resins without extensive lessening of properties.
Extruder: Machine that pushes molten plastic through a die for the formation of plastic sheet.


Fabrication: The process of  producing a product from raw materials.
Fabricator: Manufacturer of  plastic products.
Fatigue: Failure or decay of mechanical properties after repeated applications of stress. Fatigue tests indicate a material’s ability to resist cracking, which eventually causes failure due to a large number of cycles.
Fatigue Life: How many cycles of deformation it takes before a test specimen will fail under a given set of oscillating conditions (stresses and strains).
Fatigue Limit: The maximum level under which a material can be stressed cyclically for an infinite number of times before it fails.
Fatigue Strength: The maximum cyclical stress a material can withstand for a given number of cycles before failure occurs. The residual strength after being subjected to fatigue.
Fiber: The major reinforcement material component in a composite matrix. Often, fiber is used synonymously with filament.
Fiber Content: The amount of fiber present in a composite. This is usually expressed as a percentage volume fraction or weight fraction of the composite.
Fiber Diameter Letter Designation: Fibers are generally classified in hundred thousandths, i.e., a “K” fiber has a mean average diameter of 50+ to 55 height. See chart below,
Fiber Direction: The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.
Fiber Glass: Primarily means glass in fiber form. Also used to describe composite processing and applications, for example, fiber glass molding plant, fiber glass car.
Fiber Glass Reinforcement: Major material used to reinforce plastic. Available in mat, roving, fabric and other forms, it is incorporated into both thermosets and thermoplastics.
Fiber Orientation: Fiber alignment in a non-woven or a mat laminate where the majority of fibers are in the same direction, resulting in greater strength in that direction.
Fiber Pattern: Visible fibers on the surface laminate or molding. The thread size and weave of glass cloth.
Fiber Reinforced Plastic (FRP): A general term for a composite that is reinforced with cloth, mat, strands, or any other fiber form.
Filament: A single, threadlike fiber of glass.
Filament Yarn: A yarn composed of continuous filaments assembled with or without twist.
Fill: The system of yarns running crosswise in a fabric (short for filling). Also known as weft. see Warp
Filler: An inexpensive substance that is added to plastic resins to extend volume, improve properties, and lower cost.
Fillet: A rounded filing of the internal angle between two surfaces of a plastic molding.
Fines: Excessively small particles.
Finish: see Size
Fire Retardants: Chemicals that reduce a resin’s tendency to burn.
Flame Treatment: Surface treatment for the promotion of bonding by the generation of plasma from the burning of natural gas.
Flammability: How fast a plastic material will burn when subjected to a particular ASTM test. In this test, a flame is applied to one end of a strip of material. When the material starts burning the flame is removed and the time to consume a given amount of material is measured. Units are measured in inches per minute (in/min.). Higher numbers indicate that the material will burn faster under this particular test’s conditions. S.E. means self extinguishing; S.E. classified materials stop burning when the flame is taken away.
Flash: Portion of the charge that flows or is extruded from the mold cavity during the molding. Also refers to extra plastic attached to a molding along the parting line that must be removed before the part is considered finished.
Flash Point: Lowest temperature at which a substance gives off enough vapors to form a flammable mixture.
Flexible Molds: Rubber or elastomeric plastic molds used for casting plastics. They can be stretched to remove cured pieces with undercuts.
Flexural Modulus: A number referring to a material’s stiffness. It is used to calculate how far a bar will bend when a bending load is applied. Units are normally millions of pounds per square inch. (106 psi) – Giga Pascals (gPa). In two materials of equal thickness, the one with a higher number is more resistant to deflection.
Flexural Strength: Also known as bending strength. Describes how much nonmoving load can be applied to a bar before it yields or breaks. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascals (mPa). Higher numbers indicate stronger materials that can withstand a heavier load.
Flow: The movement of resin under pressure, allowing it to fill all parts of a mold or fiber. The gradual but continuous distortion of a material under continued load, usually at high temperatures; also called creep.
Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding.
Flow Marks: Wavy surface appearance of an object molded from thermoplastic resins, cased by improper flow of resin into the mold.
Fly: Fibers that fly out into the atmosphere during handling and processing.
Foam: Lightweight, cellular plastic material containing glass-filled voids. Typical foams include urethane, PVC, and polyester.
Force: The amount of pressure exerted onto an object.
Forming: Process of glass fiber production during which fibers are drawn, attenuated from molten glass and collected in forming cakes.
Fracture: When a surface ruptures without the laminate completely separating, or where there is complete separation of a body because of external or internal forces.
Fracture Stress: The true, normal stress on the minimum cross-sectional area at the beginning of fracture.
Fracture Toughness: The damage tolerance of a material containing initial flaws or cracks. Used in aircraft structural design and analysis.
FRP: Acronym for fiber glass-reinforced or fiber-reinforced plastic, polymer or polyester.


Gate: Point at which molten thermoplastic enters the injection molding tool cavity.
Gel: A semisolid, similar to gelatin in consistency.
Gel Point: When a material begins to exhibit pseudo-elastic properties.
Gel Time: Time required to change a flowable liquid resin into a non-flowing gel.
GFRP: Glass fiber-reinforced plastic, polymer or polyester.
Glass Blends: When several different fiber types, i.e. different lengths and diameters, are blended in the fiber slurry.
Glass Content: Percentage of glass in the compound.
Glass Fiber Wet-Process: Process of forming a glass mat on modified papermaking equipment.
Glass Transition: Reversible change in an amorphous polymer or amorphous regions of a partially crystalline polymer from or to a viscous, rubbery, or hard and relatively brittle condition.
Glass Transition Temperature (Tg): The appropriate midpoint of the temperature range over which glass transition takes place.
Good Side: Side of molding in contact with a mold surface.
Graphite Fiber: Fiber made from a precursor by oxidation, carbonization and graphitization process (which provides a graphitic structure).
Green: Resin, which has not completely cured and is still rather soft and rubbery.
Green Strength: That ability of the material, while not completely cured, to undergo removal from the mold and handling without tearing or permanent distortion.
GRP: A derivation commonly used in Europe referring to glass-reinforced plastic, polymer or polyester.


Hand: Fabric softness as determined by touch (individual judgment).
Hardener: A substance or mixture added to a plastic composition to promote or control curing by taking part in it.
Hardness: Resistance to surface indentation usually measured by the depth of penetration (or arbitrary units related to the depth of penetration) of a blunt point under a given load using a particular instrument according to a prescribed procedure.
HDPE: High density polyethylene polymer.
Heat Distortion Temperature: The temperature at which a material will bend under a given load.
Heat Distortion Point: Temperature at which a material’s strength begins to degrade. Now called deflection temperature.
Heat Resistance: The ability of plastics and elastomers to resist deterioration due to elevated temperatures.
Heat Sink: A contrivance for the absorption or transfer of heat away from a critical element or part.
High-Pressure Laminates: Laminates molded at pressures not lower than 6.9 MP (1.0 ksi), and more commonly between 8.3 to 13.9 Mpa (1.2 to 2.0 ksi).
Homogeneous: Describes a material with a uniform composition.
Homopolymer: A compound produced by polymerization.
Honeycomb: Manufactured product of resin-impregnated sheet material (paper, glass, fabric) or metal foil, formed into hexagonal-shaped cells. Used as a core material in sandwich construction.
Hoop Stress: The circumferential stress in a material of cylindrical form subjected to internal or external pressure.
Hybrid: A composite laminate consisting of two or more composite material systems. Two or more different fibers, such as carbon and glass or carbon and aramid, combined into a structure.
Hydraulic Press: A press in which molding force is created from pressure exerted by a fluid.
Hygroscopic: Material that absorbs moisture from the air.


Impact Strength-Izod: A moving load is one that is moving when it strikes a bar. Units are measured in foot pounds per inch of width; sometimes given as foot pounds per inch of notch.
Impact Test: Measure of the energy necessary to fracture a standard sample by an impulse load.
Impregnate: In reinforced plastics, to saturate a reinforcement, especially fiberglass, with a resin.
Injection Molding: Method of forming plastic to the desired shape by forcing a heat-softened thermoplastic polymer into a relatively cool cavity under pressure or thermosetting polymer into a heated mold.
Inorganic Pigments: Natural or synthetic metallic oxides, sulfides, and other salts that impart heat and light stability, weathering resistance, color, or migration resistance to plastics.
Insert: An integral part of plastic molding consisting of metal or other material that may be molded into the part or pressed into position after the molding is completed.
In-Situ: In the place of use.
Instron: Instrument for determining the tensile and compressive properties of materials.
Interface: A surface that lies between two different materials.
Interlaminar: Descriptive term pertaining to an object existing or occurring between two or more adjacent layers.
Interlaminar Shear: Shearing force that produces a relative displacement between two layers in a laminate along the plane of the interface.
Intumescent: Fire-retardant technology causing an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.
Irreversible: Not capable of re-dissolving or re-melting.
Isobaric: Constant pressure process.
Isochoric: Constant volume process.
Isotropic: Having uniform properties in all directions.
Izod Impact Test: A test for shock loading in which a notched specimen bar is held at one end and broken by striking, and the energy absorbed is measured.


Jackstrawing: Visual effect of glass fiber turning white in a cured laminate.


Kevlar: An organic polymer composed of aromatic polyamides having a para-type orientation (parallel chain extending bonds from each aromatic nucleus).
Knitted Fabric: A textile structure produced by inter looping one or more ends of yarn or comparable material.


Laminate: Primarily means a composite material system made with layers of fiber reinforcement in a resin. Sometimes used as a general reference for composites, regardless of how made. Examples of usage: laminate consumption by market, compression-molded laminate.
Land: Portion of a mold which provides the separation or cutoff of the flash from the molded article; in the screw of an extruder, the bearing surface along the top of the flights; in an extrusion die, the surface parallel to the flow of material; in a semi-positive or flash mold, the horizontal bearing surface; in a two-piece mold, a platform build up to the split line.
Lay: In glass fiber, the spacing of the roving bands on the roving package expressed in the number of bands per inch; in filament winding, the orientation of the ribbon with some reference, usually the axis of rotation.
Laydown: The degree of uniformity in thickness that a bed of chopped glass fibers exhibits across the width of the chopped pattern.
Layer: The horizontal plane of doffs on a pallet. A pallet usually has four layers of twelve doffs each. also see Cell 
Lay-up: Act of building up successive layers of polymer and reinforcement. Layers of catalyzed resin and fiberglass or other reinforcements are applied to a mold in order to make a part. Also refers to the reinforcing material placed in position in the mold, the process of placing reinforcing material in position in the mold, or the resin-impregnated reinforcement.
L/D Ratio: A term used to define an extrusion screw, which denotes the ratio of the screw length to the screw diameter.
Lengthwise Direction: Refers to the cutting of specimens and application of loads. For rods and tubes, lengthwise is the direction of the long axis. For other shapes of materials that are stronger in one direction than in the other, lengthwise is the direction that is stronger. For materials that are equally strong in both directions, lengthwise is an arbitrarily designated direction that may be with the grain, direction of flow in manufacture, longer direction, etc.
Liquid-Crystal Polymer: A newer thermoplastic polymer that is melt process capable and develops high orientation in molding, resulting in tensile strength and high-temperature capability.
Load-Deflection Curve: A curve in which the increasing tension or compression of flexural load are plotted on the ordinate axis and the deflections caused by those loads are plotted on an abscissa axis.
Loss on Ignition: Weight loss, usually expressed as a percent of the total, after burning off an organic sizing from glass fibers, or an organic resin from a glass fiber laminate.
Low-Pressure Laminates: Laminated, molded, and cured using pressures from 400 psi down to and including the pressure obtained by mere contact of the plies.
Lubricant: A material added to materials to improve the handling and processing properties of molded polymer products.


Mat: A fibrous material for reinforced plastic consisting of randomly oriented chopped filaments, short fibers (with or without a carrier fabric) or swirled filaments loosely held together with a binder.
Mat Binder: Resin applied to glass fiber and cured during the manufacture of mat that holds fibers in place and maintains the mat’s shape.
Matched Metal Molding: A reinforced plastics manufacturing process in which matching male and female metal molds are used (also called compression molding) to form the part with time, pressure and heat.
Matrix: The resin component of a polymer composite. Both thermoplastic and thermoset resins may be used, as well as metals, ceramic and glasses. (Plural: matrices.)
Mat Strength: The mat’s ability to resist being pulled apart under tension during impregnation and molding.
Matchsticks: Strand-to-strand adhesion. A matchstick is a wide bundle that has 3 to 4 times as many filaments than the majority of bundles in the bed.
Mechanical Properties: A material’s properties, such as compressive and tensile strength and modulus, that are associated with elastic and inelastic reaction when force is applied. The individual relationship between stress and strain.
Metallic Fiber: Manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic or core completely covered by metal.
Micro balloons: Microscopic bubbles of glass, ceramic or Phenolic, used as a filler or to create syntactic foam or putty mixtures.
Micro cracking: Crack formed in composites when thermal stresses locally exceed the strength of the matrix.
Micron: One micron = .001 millimeter = .00003937 inch.
Mil: Unit used to measure the diameter of glass fiber strands (1 mil = 0.001 in.).
Milled Fiber: Continuous glass strands hammer milled into very short glass fibers. Modulus, Initial: The slope of the initial straight portion of a stress strain or load-elongation curve.
Modulus of Elasticity: How much a material can bend without losing its ability to return to its original physical properties.
Moisture: The amount of volatiles on the glass expressed as a percentage of the total weight.
Moisture Content: The amount of moisture in a material determined under prescribed conditions, expressed as a percentage of the mass of the moist specimen; that is, the mass of the dry substance plus the moisture present.
Mold: The cavity or matrix into or on which the plastic composition is placed and from which it takes form. The tool used to fabricate the desired part shape.
Molded Edge: An edge that is not physically altered after molding for use in final form, and particularly one that does not have fiber ends along its length.
Molding: The forming of a polymer or composite into a solid mass of prescribed shape and size.
Molding Compounds: Plastics in a wide range of forms meeting specific processing requirements. Granules or pellets are popular forms.
Molding Cycle: The period of time required for the complete sequence of operations on a molding press to produce one set of moldings.
Molding Pressure: Pressure applied to the ram of an injection machine or compression or transfer press forcing softened plastic to completely fill mold cavities.
Molding Pressure, Compression: Unit pressure applied to material in the mold. The area is calculated from the projected area taken at right angles to the direction of applied force and includes all areas under pressure during complete closing of the mold. Unit pressure is calculated by dividing the total force applied by the projected area, expressed in pounds per square inch.
Mold-Release Agent: Lubricant, liquid or powder (often silicone oils and waxes) that prevents molded articles from sticking in the cavity.
Mold Shrinkage: Immediate shrinkage that occurs when a part is removed from a mold and cooled to room temperature; the difference in dimensions, expressed in inches between a molding and the mold cavity in which it was molded (at normal temperature measurement); the incremental difference between the dimensions of the molding and the mold from which it was made, expressed as a percentage of the dimensions of the mold.
Monomer: A simple molecule capable of reacting with like molecules to form a polymer;


Non-Woven Fabric: A textile structure produced by bonding or interlocking fibers, or both, accomplished by mechanical, chemical, thermal and/or solvent means.
Notch Sensitivity: The extent to which a material’s sensitivity to fracture is increased by 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.


Orange Peel: Gel coated or painted finish that is not smooth and is patterned similar to an orange’s skin.
Organic: Matter originating in plant or animal life, or composed of chemicals of hydrocarbon origin, either natural or synthetic.
Orientation: Position with relation to flow of polymer in the mold.
Overlay Sheet: Non-woven fibrous mat (of glass, synthetic fiber or other material) used as the top layer in a cloth or mat lay-up to provide a smoother finish, minimize the appearance of a fibrous pattern, or permit machining or grinding to a precise dimension. Also called surfacing mat.
Overspray: A specially formulated binder applied to texturized yarn that helps retain the yarn bulk after texturizing.


PA: Polyamide thermoplastic polymer.
PAI: Polyamide-imide  polymer.
Parting Line: A mark on a molded piece where mold sections have met in closing.
PBT: Polybuthlene Therephthalate (Thermoplastic Polyester Resin).
PEEK: Polyether Ether Ketone thermoplastic polymer.
PEI: Polyetherimide thermoplastic polymer.
PEKK: Polyetherketoneketone thermoplastic polymer.
PET: Polyethylene Terephthalate (Thermoplastic Polyester Resin).
Pigment: Colorant added to gel coat or resin.
Pinholes: Small holes on the exposed gel coated surface that are about the diameter of common pins and may be easily counted.
Pit: Small regular or irregular crater in the surface of a plastic, usually of equal width and depth.
Plastic: Material of which an essential ingredient is an organic polymer of large molecular weight and also contains hardeners, fillers and reinforcements; is solid in its finished state; and has been shaped by flow during some stage of its manufacture or processing. Made of plastic. A plastic may be either thermoplastic or thermoset.
Plastic Deformation: Change in dimensions of an object under load that is not recovered when the load is removed; opposed to elastic deformation.
Plasticizers: Material added to increase a plastic’s workability and flexibility. Normally used in thermoplastics. Also a lower molecular weight material added to epoxy to reduce stiffness and brittleness, thereby resulting in a lower glass transition temperature for the polymer.
Platens: Mounting plates of a press to which the entire mold assembly is bolted.
Plug: A composite industry term for a pattern or model.
Ply: The number of single yarns twisted together to form a plied yarn; also the number of plied yarns twisted together to form a cord. The individual yarn in a plied yarn or in a cord. One of several layers of fabric.
Poisson’s Ratio: The ratio of transverse strain to axial strain during axial load.
Polyamide: A polymer in which the structural units are linked by amide or thioamide groupings.
Polyester Combination Yarn: A polyester/fiber glass hybrid yarn.
Polyester (Unsaturated): Product of an acid-glycol reaction commonly blended with a monomer to create a polymer resin.
Polymer: Chain molecule composed of many identical groups, commonly found in plastics.
Polymerization: Chemical bonding of polymer molecules during the curing reaction.
Porosity: Entrapped gas bubbles or voids in a gel coat film.
Post Bake: see Post Cure
Post Cure: Heat cycle that a polymer goes through after fabrication for completion of cross-linking.
Pot Life: The length of time that catalyzed resin retains a viscosity low enough to be used in processing.
PP: Polypropylene thermoplastic resin.
PPO: Polyphenylene Oxide (Thermoplastic Resin).
PPS: Polyphenylene Sulfide (Thermoplastic Resin).
Preform: Preshaped fibrous reinforcement for molding.
Pregel: An unintentional extra layer of cured resin on part of a reinforced plastic’s surface.
Premix: A compound prepared prior to the molding operation containing all components required for molding.
Prepreg: A ready-to-mold material in sheet form. Cloth, mat, or unidirectional fiber impregnated with resin.
Pressure Bag: A membrane conforming to the inside of a laminate laid up on a mold. The membrane or bag is then inflated, applying pressure that consolidates and densifies the laminate.
Print Through: Distortion in a part’s surface through which the pattern of the carbon or fiberglass reinforcement is visible. Also known as print out, telegraphing or read through.


Reinforced Molding Compound: Compound consisting of a polymer and reinforcement fiber or filler, supplied by a raw material producer as ready-to-use materials.
Reinforced Plastics: Molded, formed, filament-wound, tape-wrapped or shaped plastic parts consisting of resins to which reinforcing fibers, mats and fabrics have been added before the forming operation to provide strength properties greatly superior to those of the base resin.
Reinforcement: Strength enhancing materials bonded into a matrix of a sheet to improve its mechanical properties. Reinforcements are usually long fibers, chopped fibers, whiskers and particulates. The term should not be used synonymously with filler.
Release Agent: Compound used to reduce surface tension or adhesion between a mold and a part.
Resin: The polymer material used in reinforced plastics, used to bind together the reinforcement material.
Resin Content: The amount of resin in a laminate as a percent of total weight or total volume.
Resin-Rich Area: Localized area filled with resin and lacking reinforcing material.
Resin-Starved Area: Localized area of insufficient resin, usually identified by low gloss, dry spots or fiber showing on the surface.
Resin Tearing: Separation of resin affecting cosmetic appearance.
Rib: Reinforcing member of a fabricated part.
Ribbon: A strip of material with a long length and narrow width.
Room Temperature Curing Adhesives: Adhesives that set to full strength without heating.
RP: Reinforced plastic or polymer.
RTP: Reinforced thermoplastic.
Runner: The channel through which thermoplastic material moves through a mold.


SAN: Styrene Acrylonitrile (Thermoplastic Resin).
S Glass: Magnesium-alumina-silicate glass with a high mechanical strength.
Sandwich Constructions: Panels composed of a lightweight core material to which two  thin, dense, high-strength or high-stiffness faces or skins are adhered.
Scrim: Light woven or non-woven fabric with relatively large openings between the yarns, used to reinforce materials.
Semi Preg: Reinforcement coated with polymer without fiber wet out.
Set-Up: To harden, as in curing. Preparation prior to commencing production.
Shear:  The movement from applied force between plies of a laminate.
Shelf Life: Allowable storage time before a product must be used.
Shore Hardness: A material’s resistance to indentation from a spring-loaded indenter utilizing a Shore scale.
Shrinkage: The relative change in dimensions of a material between hot molded temperature and room temperature.
Single Yarn: The simplest strand of textile material suitable for weaving, knitting and other operations.
Sink Mark: A shallow depression or dimple on the surface of a molded part due to its surface collapsing from local internal shrinkage.
Size: Treatment applied to glass fiber that allows resin and glass to adhere to one another. Also allows glass fiber to be conveniently handled.
Skin Coat: First layer of laminate next to the surface layer.
SMC: Sheet Molding Compound
Solid: The amount of sizing on glass expressed as a percentage of the total weight.
Solvent Resistance: A material’s ability to resist being dissolved by a particular solvent.
Specific Gravity: A material’s weight in relation to the weight of an equal volume of water. volume of water. Because specific gravity is a ratio of values for two materials, there are no units. Higher numbers indicate heavier materials.
Specific Heat (Thermal Capacity): The amount of heat needed to raise the temperature of one pound of material one degree Fahrenheit (F). Units are measured in BTUs per pound per degree (BTA/LB/°F) – Joules/Kilogram Kelvin (J/KgK). Higher numbers indicate more input heat energy is needed to raise the temperature of a material.
Specimen: An individual piece or portion of a sample used for a specific test of specific shape and dimensions.
Sprue: Connector between the runner and material source in a molding or a casting.
Starved Area: Portion of a plastic part without sufficient resin to completely wet out the reinforcement.
Static: Buildup of an electrical charge in a material.
Stiffness: The relationship of load to deformation.
Strength, Flexural: Maximum stress that can be borne by surface fibers in a beam in bending. Flexural strength is the unit resistance to maximum load prior to failure by bending, usually expressed in pounds per square inch.
Stress-Strain: Stiffness, expressed in pounds per square inch or kilograms per square centimeter, at a given strain.
Stress-Strain Curve: Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissa to obtain a stress-strain diagram.
Surfacing Mat: Very thin mat used  to produce a smooth, resin-rich surface on a reinforced plastic laminate.
Surfactant: Chemicals used to modify or change the surface of a layer of resin or polymer.


Tack: Surface stickiness.
Tack Free: Surface which is not sticky after cure.
Tangent Modulus: Slope of the line at a predefined point on a static stress-strain curve, expressed in force per unit area per unit strain. This is the tangent modulus at that point in shear, tension or compression, as the case may be.
Tape: A narrow fabric whose mass per unit area is less than 0.5 kg/m 2 (0.1 LB/ft2) for each 25.4 mm (1 in.) of width.
Tensile Elongation: The amount of stretch in a sample during tensile strain.
Tensile Load: Load applied away from the opposite ends of a given sample.
Tensile Modulus: When a bar is pulled in tension, it gets longer. Tensile modulus calculates how much longer it will get when a certain load is applied.
Tensile Strength: The amount of nonmoving load a bar can withstand before it breaks due to elongation.
Tensile Stress: Normal stress caused by forces directed away from the plane on which they act.
Tension Device: A mechanical or magnetic device that controls tension.
Thermal Coefficient of Expansion: How much the length of a material will change when it is heated or cooled.
Thermal Conductivity (K factor): The amount of heat transferred by conduction.
Thermoplastic: A polymer capable of being repeatedly softened by an increase of temperature and hardened by a decrease in temperature.
Thermoset: A material that undergoes a chemical reaction caused by heat and forms a solid that cannot be reformed.
TPU: Thermoplastic polyurethane polymer.
Translucent: Permits a percentage of light to pass but not optically clear.


UHMW-PE: Ultra high molecular weight polyethylene.
Ultimate Tensile Strength: The ultimate or final stress sustained by a specimen in tension at moment of rupture.
Unidirectional: Strength lying mainly in one direction. A glass reinforcement in which the fiber is oriented in one direction.
Untreated: A material that has not been corona or flame treated.
UV Stabilizer: Chemical compound which improves resistance to degradation from ultraviolet radiation.


Vacuum Bag Molding: Process in which a sheet of flexible transparent film, bleeder cloth and release film are placed over the lay-up in the mold and sealed at the edges and vacuum applied between film and work piece. Also called bag molding.
Veil: Ultra thin reinforcing mat.
Void Content: The percentage of voids in a laminate.


Warp: Yarn running lengthwise in a woven fabric.
Warp Size: Chemicals applied to the warp yarn to improve strand integrity, strength and smoothness in order to withstand rigors of weaving.
Weave: Particular manner in which a fabric is formed by interlacing yarns; usually assigned a style number.
Weft: The system of yarns running crosswise in a fabric. Also known as fill.
Wet Flexural Strength (WFS): Flexural strength after water immersion, usually after boiling the test specimen for two hours in water.
Wet Lay-Up: Reinforced plastic with liquid resin applied after the reinforcement is laid up.
Woven Roving Fabric: Heavy fabrics woven from continuous filament in roving form.
Wrinkle: Surface imperfection pressed into laminated plastics similar to a crease or fold in paper, fabric or other base.


Yarn: Generic term for a continuous strand of textile fibers, filaments or material in a form suitable for knitting, weaving or intertwining to form a textile fabric.
Yield: Linear density of a roving or yarn, measured by the number of yards per pound.
Yield Point:  The point at which permanent deformation of a stressed specimen begins to take place.
Yield Strength: Stress at the yield point.
Young’s Modulus: Ratio of normal stress to corresponding strain for tensile or compressive stresses less than the proportional limit of the material.

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