Appendix A:

A Glossary of Terms

Accelerator: Any chemical additive which is mixed with plasters or plastic resins to quicken the cure or setting of the material.
Activator: see curing agent, catalyst.
Addition Reaction Cure: A chemical reaction in which all of the reactants are incorporated into the final product with no resulting by-products. The addition-reaction cure is to be distinguished from the evaporative cure.
Alginate: A powdered derivative of algae and kelp, which when mixed with water sets to a gelatin-like elastomer. Developed for use in dentistry. Is a short term molding material which will shrink or expand over time, depending on the ambient moisture.
Apron: see flange.
Bead: A small semi-spherical protruding feature on the surface of a cast, resulting from a bubble in the mold. A bead can be removed from the cast, but its scar will hide the morphology of the original specimen at that location.
Block: Usually, an irregularly shaped piece of plaster or rubber crafted to fill an undercut on the mold left after the last coat of rubber has been applied and has cured. Blocks lie between the rubber portion of the mold and the plaster jacket.
Boxing: The act of erecting containment walls around a set-up block or mold in order to contain both the molding material and the jacketing material.
Bubble: A void left in a cast when the casting material does not come into full contact with portions of the specimen impression in the mold. Results in a smooth, semi-spherical impression left in cast, usually around teeth and areas of minute detail. See Microbubbles.
Cast: A replica produced by filling a mold with a casting compound, such as plaster or plastic, and letting it harden inside the impression, taking on the shape and surface details of the original specimen.
Catalyst: A substance, usually added in small amounts, which precipitates the curing process of an elastomer. Synonymous with activator, curing agent.
Circle Shrink: see shrinkage.
Clay Fence: see containment walls or walls.
Coecal®: A brand of dental stone manufactured by GC Lab Technologies Inc., and favored by the author for making high-detail, very accurately painted casts. More information about this high-quality, high-detail, and very paintable buff-colored plaster can be found at GC Lab's Coecal Product Information Page.
CMC: An anacronym for Cold Molding Compound. Generally synonymous with RTV.
Cold Molding Compound: A term for any number of rubber molding materials that can cure without the use of an external heat source, as in vulcanization. Generally synonymous with Room Temperature Vulcanizing (or RTV) rubbers.
Containment Walls: A containment system for rubber and plaster during molding, usually made of acrylic sheets, dental wax or plastilina. Synonymous with clay fence and walls.
Contaminant Layer: see separator.
Contaminants
Creep: A measure of the deformation remaining in a material after a force resulting in elastic deformation has been removed. In other words, creep is the opposite of elastic memory; if a material has perfect elastic memory, there will be no creep.
Cross Link: A bond which forms between molecules in a polymeric compound during its curing which results in its hardening and/or solidification.

Cure: In the case of an elastomer, the chemical reaction which results in its vulcanization. A chemical cure begins with the addition of a catalyst, and ends at the total cure time.
Cure Time: The time which is necessary for the elastomer to finish cross-linking and become a stable compound. Related to demolding time
Curing Agent: A secondary compound, usually liquid, which when added to the primary compound brings about hardening, solidifying, or stabilization of the entire mixture. Synonymous with activator, catalyst.
Cyanoacrylate

Demold: 1. (verb) To remove a specimen or a cast from a mold. Synonymous with pull.; 2. (noun) The process of removing a specimen or cast from a mold.
Demold Time: The amount of time necessary to cure the elastomer to a point at which it can be removed from the specimen without permanent distortion. Often comes well before the total cure time. Roughly synonymous with cure time.
Density: A quantitative material property, or measure, consisting of the the amount of mass that a material has within any given unit volume. Materials which are typically heavy, even in small quantities (or volumes), such as lead or gold, have higher densities than those which are less massive per unit volume, such as aluminum or gaseous hydrogen. In the physical metric (SI) unit system, density is expressed with the standard dimensions of kilograms per cubic meter (kg/m^3 --or-- kg * m^-3).
Dental Stone: A type of casting plaster designed specifically for dentists and their need for fine detail on dental casts. Similar in most respects to other plasters, except in being ground to a finer grain size, being baked longer to be dried more fully, and including accelerants.
Dental Tools

Dental Wax

Detail Coat: The first coat of the molding elastomer, which is applied directly to the specimen. This coat is responsible for capturing the surface details on the specimen. Synonymous with print coat, gel coat.
Dike: A vertically oriented barrier used in the molding process which delineates the margins of the portion currently being made of a multi-piece mold. Usually made of plastilina or cardboard.
Durometer Hardness: A measure of the hardness of a material. Low durometers indicate soft materials. Can be measured by any of three scales: Shore 00 (for very soft materials, such as flesh), Shore A (for average hardness, such as many cured elastomers), or Shore D (for very hard materials, such as plaster).
Dymo® label maker: A hand-held device for embossing letters and numbers in relief on adhesive plastic strips.
Efflorescences: Soft, powdery discolored deposits on plaster casts caused by excess minerals and salts in the slurry.
Elastomer: Any natural or synthetic elastic substance, such as silicone or latex rubbers.
Elongation: A measure of the ability of material to stretch without breaking. Usually measured with the Die C test (____), and expressed as a percentage of length that is the material's maximum stretch before breaking.
Endocast: 1. (noun) A naturally occuring trace fossil (a naturally-formed mold) formed when a fine-grained matrix solidifies within (most often) the neurocranium (brain-case) of a skull. The endocast can then either remain within a wholly or partially intact neurocranium, or can remain intact and independent of the neurocranium once the latter has been broken away either taphonomically (prior to discovery) or after discovery (either accidentally or intentially during preparation or later handling).; 2. (noun) A mold (or derivative cast) made of an endocranial space in order to better visualize endocranial morphology which might otherwise be difficult or impossible to see directly. Such endocasts are often used to study the morphology of the meningeal, arterial, and venous impressions left by brain structures on the inside of the neurocranium.
Epoxy: Any of numerous synthetic resin compounds, usually thermosetting, which are capable of forming tight cross-linked polymer structures characterized by toughness, strong adhesion and high corrosion and chemical resistance. Used for making casts and as a high-strength glue.
Evaporative Cure: A type of cure (usually of an elastomer) that occurs when a solvent (often alcohol), in which a catalyst or monomeric compound is suspended, evaporates and frees the catalyst or monomer to complete the cure of the material. Because this type of cure depends on evaporation, the speed of the cure is directly related to the ratio of the surface area of the curing material to its volume. Thus, this type of material is best applied, and allowed to cure, in many thin layers rather than one thick layer. This type of cure is distinguished from the addition reaction cure.

Exotherm: The amount of surplus heat produced during a chemical reaction. Materials such as resins which produce heat are refered to as exothermic.
Filler: A compound added to a plastic resin to increase its volume without significantly affecting its properties. Usually done to reduce the cost of the material, or to reduce the exotherm of the material, as the filler acts as a heat sink.
Flange: That portion of the rubber part of the mold that contains the tabs and runs out to the edge of the mold. Does not include either the specimen impression or the reservoir portions of the mold. Syn. with apron, sill.
Flare: see flash.
Flash: A thin, planar remnant of the casting process, which is attached to the cast, and represents a cast of the void of the partline left between two mold halves. Synonymous with flare.
Flashline: The line on a cast which is left when the panes of 'flash' are removed after casting. Syn. with seam line.
Flask: see jacket
Gel Coat: A thin layer of resin that is painted on the surfaces of the impression without any reinforcing material. A gel coat is necessary so that reinforcing materials present in the rest of the cast, such as fiberglass, won't show through to the surface of the cast and obscure the natural features of the specimen.
Gel Time: The amount of time required for a plastic resin to set to a gel-like consistency. Synonymous with pot life, working time.
Gilbert Method

Glyptal®: An adhesive and a sealing agent used widely in paleontology. Is acclaimed for its strong yet fully removable qualities as a glue. Used to penetrate and strengthen fragile specimens, glue broken specimens, seal mold jackets, and coat teeth on casts, among other applications.
Gypsum Cement

Gypsum Hardener: A liquid solution heavy in minerals which is combined with a casting plaster, usually dental stones, which result in the final cast having increased density, strength, and scratch resistance.
Hardener: see gypsum hardener
Hydrocolloids

Impression: That portion of a mold that was formed in direct contact with the specimen. The elastomeric portion of a multi-piece mold will have both an impression and a flange. Also used to refer to one-piece peels or quick molds.
Impression Material

Inner Surface: The surface of the mold towards the specimen. On a multi-piece mold, it is the inner surfaces of the mold pieces which come together at the partline.
Intra-Mold Separator: see separator
Jacket: The outside plaster shell of a completed mold, which gives the mold its support and rigidity, and therefore, its dimensional stability. Synonymous with mother mold, keeper shell, flask.
Keeper Shell: see jacket
Keys: see tabs
Label

Laminated Object Manufacturing: A type of Rapid Prototyping Technology in which ___
Latex: A molding, and sometimes a casting, elastomer which is applied as a liquid solution of pre-vulcanized rubber particles. As soon as the solvent evaporates, the rubber particles bond into a solid layer.
Lay-up Molds: Molds which, due to a large amount of topography on the original specimen, especially vertical surfaces, must be created with a non-flowing, highly viscous elastomer. Reinforcing materials such as gauze are also frequently used.
Linear Shrink: see shrinkage
Locators: see tabs
L.O.M.: An anacronym for Laminated Object Manufacturing
Master Cast: An early generation, flawless cast, which is set aside to take the place of the specimen, should it need to be molded in the future. Masters are not painted, are not cleaned of the flashline, and are not handled.
Master Pattern: Function is similar to that of a master cast, but differs in form. The pattern preserves the detail of the mold, as well as the detail of the specimen. There is one master pattern for each side of a mold. Therefore, a two sided mold would have two master patterns. See Gilbert Method
Material Safety Data Sheets

Microbubbles: Similar to bubbles, but much smaller and in greater numbers. Occur as fields of bubbles on the upper side of a cast left static to dry. Occurs only when casting compound is not completely de-aired.
Modulus: Used to refer to a measure of some physical or mechanical property of a material. The modulus is generally constant for any material, and can be used to calculate a material's response to external conditions, such as applied forces. One common example is Young's Modulus, which expresses the elastic response of a meterial.
Mold: Usually a multi-piece rubber and plaster set which preserves a negative impression of a specimen which can be filled with a casting compound to produce a specimen replica. Can be a single piece of rubber to dozens of rubber and plaster pieces.
Molding Plaster

Mold Separator: see separator
Monomer

Morphology

M.S.D.S.: An anacronym for "Material Safety Data Sheets".
Organotin Condensation Reaction

O-Ring

Overhang: On a cast or original specimen, an area of the specimen which protrudes out over a much narrower lower area. i.e. nasal bones overhang the nasal aperture. Overhangs are coincidental with undercuts, and are sometimes referred to synonymously. See undercut.
Part Line or Partline: The curvi-planar surface at which the various pieces of a mold come together when re-assembled. The partline(s) of a mold determines the flashline(s) of the casts to be produced from that mold; any plaster which remains in the space between the partlines of the mold harden into flash.

Peels: Quick, one-piece molds which are usually made by applying an instant-setting material such as vinyl polysiloxane or alginate to a portion of a specimen and removing it as soon as it cures. Usually used for quick molds of areas such as teeth and muscle scars.
Plaster

Plaster of Paris

Plaster Jacket

Plaster Trap

Plastic: A generic term referring to any number of polymeric resinous compounds, including, but not limited to, polyurethane, epoxy, ____.
Plasticine

Plastilina

Pour: An informal term referring to the production of a cast. It is derived from the action of pouring the casting material into the impression on the mold.
Pour Hole: see sprue
Polymere

Polyurethane

Polyvinylsiloxanes

Print Coat: see detail coat
Pull: An informal term referring to the extraction of a cast from a mold. See demolding
Rapid Prototyping Technology

Registration: The active alignment of the various pieces of a mold (especially of the impression) by tabs so that no portion of the mold will stray from the position it was in when it was initially formed around a specimen.
Release Agent

Reservoir Mold: A mold which has been made with one side of the impression depressed into the mold and the other side elevated from the mold. The result is that the depressed side can accomodate an extra volume of casting material as a safety factor against excess loss of casting material during the closing of the mold.
RTV: An anacronym for Room Temperature Vulcanizing rubber.
Room Temperature Vulcanizing

Rubber: An informal term referring to any number of elastomers, including latex, silicone, and polyvinylsiloxane.
Sediment Trap: see sprue
Set-up Board: A clean, flat piece of [generally] wood or plexiglass upon which the set-up block is sculpted and the first side of the mold made.

Set-up Block: The sculpted form of the future mold which contains the specimen, tabs, a label, and sometimes a reservoir.
Shear

Shrinkage: A measure of a material's reduction in size after setting or curing. Usually expresses as a dimensionless ratio of amount of shrinkage over unit of measurement. Dimensionally stable materials have shrinkages very close, or equal, to 0. [insert discussion of the relationship of circle shrink to linear shrink]
Seam Line: see flashline
Separator: A fine coat of material that is applied, usually suspended in some volatile medium, to finished pieces of a multi-piece mold. The material prevents subsequent layers of rubber that form additional mold pieces from bonding with the piece(s) treated by the separator. Some common separators are vaseline desolved in 1.1.1 trichloroethane, workable fixatif, and even hair sprays! Synonymous with intra-mold separator, mold separator, and contaminant layer.
Silastic®: The brand name for a product line of silicone mold making materials manufactured by Dow Corning. Acclaimed for its high detail, long life, and low shrinkage.
Silicone

Silicone Rubber: A two-component synthetic rubber capable of curing at room temperature by chemical means into a solid elastomer used in molding.
Sill: see flange
Slit Mold: A mold that is fabricated in one piece around an enture specimen. To remove the specimen, the mold must be cut open. For casting, the slit is then partially glued back together.
Slurry: A thin mixture of a liquid, which in casting is often water or gypsum hardener, mixed with a finely divided substance, such as gypsum cement, plaster, or dental stone.
Slush Casting: Get from K.C. Parsons (1973)
Spatula: A small implement with a broad, flat, flexible balde that is used to mix plasters, elastomers and similar substances. Artists' spatulas are usually finer and more flexible, while cement spatulas are stouter and stiffer.
Specific Gravity: The measure of the density of a material by comparing the mass per volume (density) of the compound to the mass per the same volume of distilled water at 4 degrees Celsius. Materials denser than water have numbers higher than 1.00, and those less dense have numbers lower than 1.00.
Sprue: The opening, or hole, through which the casting medium is poured into some molds; also refers to the waste material which hardens in the opening and often adheres to the cast. Alternately, the term can also apply to the piece of material which the mold maker originally places on the specimen to form the opening in the mold. Synonymous with vent, pour hole.
Strength: See also tear strength.
Tab forms: (noun) Features sculpted into (or onto) the set-up block which determine the shape and position of the elastomeric inner tabs of the future mold. The tab forms are essentially molds (more precisely, waste molds) for the inner tabs. Tab forms are most simply made by pressing an object into the set-up block to make a depression. Molding material will fill this depression and cure, thus forming the tab.

Tabs: Projections incorporated into a mold which ensure proper registration of the mold when it is closed. Tabs can either be between the elastomeric parts of a mold (inner tabs) or between the elastomer and the plaster jacket (outer tabs).
Tabs, inner: Small raised knobs, incorporated into the final layer of elastomer on a mold, which correspond exactly to specific depressions on the jacket when the mold is reassembled correctly. They serve as a locking mechanism to keep the jackets aligned with the elastomeric portions of a mold. Synonymous with keys, locators, registration.
Tabs, outer: Small raised knobs, incorporated into the flange of the mold, which correspond exactly to specific depressions on other pieces of the same mold when the pieces are reassembled correctly. They serve as a locking mechanism to ensure proper alignment of mold pieces. Synonymous with keys, locators, registration.
Tamp: To pack down tightly by a succession of blows or taps. Usually used to refer to the placement of the gauze layer on a mold.
Tare: To ensure that a balance or scale weighs in at zero. For instance, you may wish to tare the balance with an empty container on it so that the scale will only display the weight of the container's contents.
Tear Strength: A measure of the force necessary to continue a tear in a material once a tear has been started. With most elastomers, the Die B test (___) is used, and the measurement is made in units of pounds per inch (ppi).
Tensile Strength

Thermoplastic

Thixotropy

Total Cure Time: The amount of time required for all curing processes to terminate. Is usually several times longer than the demold time, often taking up to a week.
Undercut: That part of a mold, cast, or specimen which deviates from a sloping or vertical surface and turns back onto itself, such as orbits or the occipital below the protuberance. Coincident with overhang.
Urethane

Vacuum

Vent: see sprue
Vinyl Polysiloxane: An instant-setting, addition-reaction RTV elastomer developed for use in dentistry. Useful for peels and quick molds.
Viscosity: A measure of a compound's resistance to flowing. Usually measured in units called poise. Materials such as water have a low viscosity, while many elastomers have a much higher viscosity.
VPS: An anacronym for Vinyl polysiloxane.
Vulcanization

Walls: see containment walls
Washing: A method of cast coloration which emphasizes highlighting details rather than making the cast appear realistic. It is usually a single coat of neutral pigment suspended in water or an appropriate medium.
Waste Mold: A temporary mold that is broken or chipped away to remove the cast. Usually encountered when both the mold and the cast are made of rigid materials, such as plaster.
WEP Resins: An anacronym for Water Extended Polyester resins. Polyester resin can be mixed with about the same volume of water catalyzed with hydrogen peroxide to form an economical, heavier cast. These resins are hard to paint because the water will continue to evaporate from the cast for months.
Working Time: see pot life
Young's Modulus

Contents

Epilogue ]
Safety

Last modified February 2, 2002

michael.black@duke.edu