Definitions of Technical Terms

  • Accelerator

    A material which, when mixed with a material, will speed up the chemical reaction between the 2 components.

  • Addition Cure

    A silicone-curing system that uses platinum as the catalyst.

  • Air-Bubble Void

    Air entrapment that has occurred either on the surface or within mold rubber or casting.

  • Air Vent

    A small outlet designed as part of the mold, to prevent entrapment of gases.

  • Ambient

    The surrounding environmental work conditions such as pressure or temperature.

  • Barcol Hardness

    A hardness scale used for plastics, resins, and very soft metals. A Barcol Impressor is the device used to take measurements.

  • Bond Strength

    The amount of force required to separate an adhesive from the attached substrate.

  • Catalyst

    A substance which markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts of primary reactants.

  • Cavity

    Depression, space inside a mold wherein a casting medium is poured or injected.

  • Cavity-Pour Mold

    A mold created by pouring rubber between a model and a support shell. Also called a matrix or blanket mold.

  • Closed-Cell Foam

    Foam made mostly of enclosed, nonconnected cells.

  • Compression Mold

    Compression molding is a process in which material is placed into an open mold and shaped by heat and the pressure created when the mold closes.

  • Compression Set

    Measures how well a rubber material returns to its original shape after being compressed. A sample is held at 75% of its original height for a set time, then measured for permanent deformation.

  • Compressive Strength

    The force required to crush a material. It may be reported in PSI in the United States or in other units.

  • Curing Agent

    A catalytic or reactive agent which when added to a resin causes polymerization; synonymous with hardener.

  • Curing Temperature

    Temperature at which a cast, molded, or extruded product, a resin-impregnated reinforcement, an adhesive, etc., is subjected to curing.

  • Curing Time

    The period of time for a material to fully cure. Also called demold time.

  • Delaminate / Delamination

    Separation of layers in a material, often a composite, due to failure of the bond between those layers.

  • Demolding

    The process of removing a mold from a model or a casting from a mold; by mechanical means, by hand or by the use of compressed air.

  • Die B/C Tear Strength

    The force required to tear a specified ASTM test specimen.

  • Dilatant Flow

    The viscosity increases as shear rate increases, causing the material to appear thicker as it is mixed.

  • Draft

    The intentional slope or taper given to the vertical surfaces of shapes designed to aid in their removal from a mold.

  • Durometer

    The system used to measure hardness. A durometer gauge takes the measurement.

  • Dwell

    Letting latex sit in a stone mold to build a skin before pouring out the excess.

  • Elongation At Break

    Elongation recorded at the moment a material breaks when being pulled apart, often expressed as a percentage of the original length.

  • Epoxy Plastics

    Plastics based on resins made by the reaction of epoxides or oxiranes with other materials such as amines, alcohols, phenols, carboxylic acids, acid anhydrides and unsaturated compounds.

  • Exotherm

    The creation of heat during the curing of a plastic resin.

  • Expansion Ratio

    A foam’s expanded volume compared with its liquid volume.

  • Extenders

    Lower-cost materials used to extend higher-cost resins with minimal effect on physical properties.

  • Filler

    An inert material that is added to a plastic resin mixture to reduce cost, modify mechanical properties, serve as a base for color effects, or to improve the surface texture.

  • Flash / Flashing

    Flash or flashing is excess material attached to a casting along the parting line.

  • Flexural Modulus

    A mechanical property that measures a material’s stiffness or resistance to bending when force is applied.

  • Flexural Strength

    Measures the force, in PSI, needed to bend a material until it breaks. A flexural modulus can also be determined which measures the stiffness when bending the material.

  • Flow

    The movement of a liquid material as it fills a mold or surface.

  • F.R.P.

    Fiber-reinforced plastic plastic reinforced with cloth, mat, strands, or another form of fiber.

  • Full Cure

    The point when a material is no longer reacting and has reached its full properties.

  • Gel

    The initial semi-cured phase that develops as a material changes from a liquid to a solid.

  • Gel Coat

    A resin applied to the surface of a mold and allowed to gel prior to laying up subsequent layers. Usually used to improve the surface appearance and protect the underlying laminate from the environment.

  • Gel Time

    The duration it takes for a material to transition from a viscous liquid to a soft, elastic gel, marking the point where it begins to lose its ability to flow.

  • GFRC / GFRG

    Glass Fiber Reinforced Concrete / Glass Fiber Reinforced Gypsum.

  • Glove Mold

    A seamless flexible mold that can be stretched off a casting.

  • Guide Pin

    A pin or key used to bring mold halves or sections into alignment on closing or joining together so they will not shift. Usually consists of a depression in one part and a matching protrusion in another part.

  • Hand Lay-up

    The process of placing and working successive layers of reinforcing material or resin-impregnated reinforcement in position on a mold by hand.

  • Hardener

    The component in a two-part system that causes the chemical reaction to occur. Some customers may refer to Part B of a 101 silicone as the hardener.

  • Hardness

    Shore A Durometer; Shore D Durometer; Barcol 935 Impressor. Hardness measures the resistance of a material to indentation.

  • Heat Deflection Temperature

    Heat Deflection Temperature is the temperature at which a material deforms by a specified amount under a specified load.

  • Heat Distortion Temperature

    Heat Deflection Temperature is the temperature at which a material deforms by a specified amount under a specified load.

  • Hydrostone

    A high-strength gypsum used for functional finished gypsum products.

  • Impact Strength

    The ability of a material to resist cracking, fracturing, or permanent deformation under sudden impact or shock loads.

  • Inhibition

    The restriction or prevention of a process, most commonly a material not curing properly.

  • Injection Molding

    The forcing of liquid or molten material into a mold cavity. It is commonly used with thermoplastics for mass production.

  • Keys

    Features used in mold making to align mold or support-shell halves and prevent distortion.

  • Laminate

    To unite layers of material, such as fiberglass or carbon fiber, using a bonding material.

  • Lay-Up

    As used in reinforcing; process of brushing or hand laying material into a mold.

  • Life Casting

    The process of making a mold of a living person's face, head or other body part.

  • Load-Deflection Curve

    A curve in which the increasing flexural loads are plotted on the ordinate axis and the deflections caused by those loads are plotted on the abscissa axis.

  • Master Model

    An exact duplicate of a model, used to make numerous molds. Saves the actual model from the damage during moldmaking.

  • Mat

    A fibrous material for reinforced plastic or gypsum consisting of randomly chopped filaments or swirled filaments; available in blankets of various widths, weights, and lengths.

  • Micron

    One micron = .001 millimeter = .00003937 inch.

  • Mixed Viscosity

    A material’s viscosity after its components are mixed.

  • Mix Ratio

    The correct proportion, by weight or volume, of the components in a multi-part system.

  • Model

    The original object or pattern used to create a mold. It may be any shape, material, or texture that you want to reproduce.

  • Modulus

    Key physical property that indicates hardness of a material. More specifically, the modulus is the amount of force needed to deform a material a set amount.

  • Modulus Of Elasticity

    The ratio of linear stress to elastic, nonpermanent strain. Also called Young’s Modulus.

  • Mold

    The cavity / form that carries a negative or reverse impression of an original model. Molds can be made of a rigid material, such as plaster or plastic resin or more commonly, a flexible material such as rubber.

  • Mold-Release Agent

    A spray, liquid or powder used to prevent sticking of molded articles in the cavity.

  • Gang Mold

    A mold with two or more mold impressions; that is, a mold which produces more than one casting per casting cycle.

  • Parting Line

    The path on a model where a multi-part mold will separate.

  • Pinhole

    A tiny hole in the surface of a material, usually occurring in multiples.

  • Pit

    A small crater in the surface of a material or cast part.

  • Plaster

    A white powder consisting mostly of gypsum. You mix plaster powder with water to make a thick liquid.

  • PLI

    Pounds per linear inch.

  • Post-Cure

    Additional elevated temperature cure, usually without pressure, to improve final properties and/or complete the cure. In certain resins, complete cure and ultimate mechanical properties are attained only by post curing.

  • Pot Life

    The amount of time that a catalyzed material remains workable (pourable for a liquid or trowelable for paste) after components have been mixed together.

  • PPE

    Personal Protective Equipment, such as gloves, eyewear, respirators, sleeves, or suits.

  • Premix

    The shaking or stirring of individual parts prior to dispensing. Done to ensure even distribution of the components in each part.

  • Pressure Casting

    Curing material under pressure to minimize visible bubbles.

  • Pressure Pot

    A chamber used to compress any entrapped air bubbles out of casting.

  • PSI

    Pounds per square inch.

  • Pseudoplastic Flow

    Viscosity decreases as the Shear Rate increases making the material flow more easily the harder it is mixed.

  • Reinforcement

    An inert material such as glass fiber, cotton, or burlap added to increase finished-part strength. It does not have to be strong on its own.

  • Release Agent

    A material applied to the surface of an original model or mold surface to prevent adhesion between two materials that would otherwise stick together.

  • Resin

    A natural or synthetic material that can form polymers through a curing process.

  • Room Temperature Vulcanization

    RTV Certain rubbers need heat treatment to cure, or vulcanize. RTV rubbers cure with no special treatment.

  • Rotational Cast

    Rotational casting, also called spin casting or slush casting, uses a rotating mold to coat the interior and create a hollow casting.

  • SDS

    A Safety Data Sheet describing product composition, possible hazards, and safety precautions.

  • Sealing Agent

    A material that is used to coat your original model so that the rubber mold does not mechanically adhere to it. Models that are porous need to be sealed.

  • Set-Up

    To harden, as in curing.

  • Shear Rate

    Measures how vigorously a material is being mixed. Low-Shear Rate is gentle mixing.

  • Shelf Life

    The period during which a properly stored material remains suitable for use.

  • Shore Hardness

    The measurement of how firm or soft a particular material is on a specified durometer scale.

  • Shrinkage

    The rate at which a material reduces in size after curing. Mold materials and casting materials may both shrink.

  • Skin Safe

    A designation based on a product’s specific testing, intended use, and instructions.

  • Specific Gravity

    A comparison of how dense a material is relative to water. It is expressed as a number without a unit.

  • Spray-Up

    A technique in which spray equipment applies any suitable sprayable material, including rubber, resin, foam, or concrete systems.

  • Sprue

    A delivery channel for casting material into a mold.

  • Support Shell / Mother Mold

    A rigid structure that supports a flexible mold.

  • TB

    A Technical Bulletin containing product instructions and technical information.

  • Tear Resistance

    Tear Resistance is a material’s resistance to tearing, commonly measured in PLI. Results depend on the test method used.

  • Tensile Properties

    Include Ultimate Tensile Strength, Elongation at Break and 100% Modulus. Ultimate Tensile Strength is the force, measured in PSI, needed to stretch a material until it breaks.

  • Tensile Shear Strength

    Measures a materials adhesive strength by determining the force needed to pull two pieces of metal apart that have been bonded together.

  • Thixotropic Flow

    Similar to pseudoplastic flow in that the viscosity decreases as the Shear Rate increases. However, when the mixing stops thixotropic materials do not immediately return to their original viscosity. 

  • Tooling

    What molds are called in the composite industry. Resin/plastic molds are called Plastic Tooling and rubber molds are called Flexible Tooling.

  • Undercut

    Any indentation or protrusion in a shape that will prevent its withdrawal from a one-piece mold.

  • Vacuum Degassing

    Using a vacuum to remove trapped air from a liquid.

  • Viscosity

    The resistance of the material flow measured in Centipoise (CPS). A material with a low viscosity will flow easily.

  • VOC

    A volatile organic compound that evaporates readily and may affect air quality or health.

  • Volumetric Yield

    The amount of space a material can fill.
Disclaimer
This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.