Blowing Agent or Foaming Agent
Chemicals added to plastics or rubbers that generate inert gases on heating, causing the resins to assume a cellular structure.

Catalyst
Chemicals often used to initiate polymerization. They are supposed to act by their presence and not be affected by the chemical reaction which they induce. In plastics, however, the catalyst can be combined and change its structure.

Compression Set
Measures the resistance of material to permanent deformation. In this test a rubber pellet is squashed to 75% of its original height, kept at that amount of deformation for 22 hours at 158°F, then released and allowed to return to original height. The value reported is the percentage not returned to the original height, so the smaller the number, the better.

Cure
A change of physical properties by chemical reaction; usually accomplished by heat and/or catalysts, with or without pressure. Another term is set.

Cure Time
Cure time is often longer than demold time, sometimes even though a product feels cured and can be demolded, curing may continue for up to a week.

Demold Time
Demold time is the earliest a curing product can be removed from the mold if curing has been at recommended temperature. Generally, longer time in a mold improves chances of obtaining a distortion free piece.

Density
Density is the weight of material that fills a given space. Water has a density of 1.00 and a pound of water takes up 27.5 cubic inches. Thus a pound of material with a density of 2 will only fill half the space or 13.75 cubic inches. When buying mold rubber or casting resin you must consider the price per volume, as the price per pound can be misleading. If the product is dense a pound will not fill as much space as a pound of lighter material and molding and casting materials are usually used to fill a space!

Elastomer (elastomeric)
An elastic, rubber-like substance, as natural or synthetic rubber.

Elongation
Elongation, reported in percent, is the length the material stretches before breaking. A 1” section is marked in the center of the piece being tested. The measurement is the length the marked section stretches before it breaks. Ex. The 1” section stretches to 2” this is 100% Elongation.

Exotherm
Either the temperature/time curve of a chemical reaction giving off heat or the amount of heat given off in a reaction.

Filler
An inert substance added to a plastic to make it less costly, improve physical properties such as hardness, stiffness, and impact strength.

Flash
Extra material attached to a casting along the parting line.

Gate
Opening in a mold through which liquid is admitted.

Hardness
Shore A durometer, Shore D durometer, hardness measures the resistance of a material to indentation. It is measured with an instrument called a Durometer. It pushes a needle-like probe into the rubber as it is pressed against the specimen to be tested. The farther the needle penetrates into the specimen the lower the reading. The Shore A is a dull needle on a weak spring for measuring elastomers. The Shore D is a sharper needle on a stronger spring for measuring rigid materials. Both scales go from 0 to 100. A rubber with a Shore A hardness of 0 is about as soft as warm chewing gum. Shore A-10 to 20 is about as soft as any mold rubbers can be. Shore A-50 is about as hard as a car tire and above Shore A-80, rubbers begin to feel more like a plastic, which may measure on the low end of the Shore D scale (Shore A 100 = Shore D 40). Shore D-90 is about as hard as plastics get.

These devices are excellent for measuring and determining if a cast material is curing properly. Most people, however, use the durometer measures as the first criteria for determining the material they need as "I would like to have a Shore A rubber that will..." The hardness gives an indication of the type of properties to expect from a material but is not always the indicator of performance.

Isocyanate Resins
Based on a combination with polyols (such as polyesters, polyethers, etc.). The reaction joins members through the formation of the urethane linkage.

Lay-Up
Process of placing reinforced material into position in a mold.

Modulus
A key physical property that any user of a material will definitely experience. The modulus is basically the stiffness of the material, or more specifically, the modulus is the amount of force needed to deform a material a set amount. Modulus is measured in psi (pounds per square inch). Modulus can be measured in any mode of deformation, i.e., tension (stretching), compression (crushing), flexing (bending), or torsion (twisting).

A low elongation is good. Ex. 20 psi at 200% elongation is a great product. This means that you can stretch the mold to remove the casting by hand. It does not take a lot of force. The higher the modulus the more effort you will need to remove the casting. If casting resin is green (not fully cured) when removing it from the mold and the modulus is high of the molding material, the casting might break.

Mold Release Agent (Separator or parting agent)
A lubricant used to coat a surface to prevent plastic from sticking to it.

Mothermold
A rigid to semi rigid material used to hold or house a fixable inner mold.

Pot Life and Gel Time
These two terms are sometimes used interchangeably but really have two different meanings. Pot life is the amount of time, after two materials have been mixed, that the material remains workable, i.e., pourable for a liquid, trowelable for paste. Most materials should be in place well before the end of this time to allow good flow into detail and air bubbles to rise and break. Gel time is the time after mixing when the materials become a continuous mass.

Rebound
A ball is dropped onto the sample from a height (100%). Rebound is the percentage of how high the ball bounces up after striking the sample.

Rebound measures how the material throws energy back or how alive the material is. A high rebound represents a “lively” material. Anything above 50 is pretty alive. It lets energy out faster. How does this relate to chosing a material for your application? For one, you would not want to use a “lively” material in vibration dampening applications. You would want a “dead” material. Also, when removing a casting from a mold if you have to stretch the mold, the rebound will tell you how long it will take for the mold to get back to its original shape. The higher the number the faster it will return. This impacts how quickly production casting can take place. And finally, a dead material absorbs energy which causing heat to build up in the material. Friction = heat. This is important for applications such as motor mounts. A lively material will throw energy out.

Reversion Temperature
The temperature at which urethane begins to change.

Shrinkage
The reduction in size which can occur with some resins during cure or with some rubbers after prolonged use. Casting materials draw oil-like materials out of the rubber causing it to shrink.

Products that develop heat during cure may gel while warm and as with nearly all materials, shrink as they cool, thus the amount of shrinkage in this case depends on the temperature reached during cure. Some resins get warmer in the center than against the cool mold surface and tend to cause the still liquid resin against the mold to sink. This can be corrected by using warm molds or resins that develop less heat during cure in the mass you are casting.

Slurry
Mixture of a liquid and a fine filler material.

Slush Casting
A method for casting thermoplastic in which liquid resin is poured into a mold where a viscous skin forms.

Specific Gravity
The specific gravity of water is 1. When a resin has a lower specific gravity than water, less than 1, it is lighter, will float, and has more volume for less weight. Conversely, a resin with a higher specific gravity than water is heavier, will not float, and has less volume.

Tear Properties
The amount of energy needed to initiate a tear in a piece of material that is cut in a specific way. Measured in pli (pounds per linear inch).

Tear Strength
Reported as the force needed to tear a split one inch long in a piece of material. Some mold rubbers, particularly silicones, tend to tear very easily if a split is started.

Tear Die C
The section tested looks like a “C.” The “C” mimics contours in a mold. So as you stretch the mold to remove the piece, if a lot of force is required at certain areas of the mold (i.e. undercuts) and the material has a low Die C, the material may fail.

Split Tear
There are two pieces attached at one point (looks like a pair of pants). This test mimics a tear in the mold (i.e. clam shell mold). Lower numbers represents a material that will tear easily. The higher the split tear, the longer your mold will last. When you take a casting out of a mold, you bend the mold to get the casting out. So the higher the split tear, the longer the mold will allow you to cast even when you have a tear in the mold.

Tensile Properties
Ultimate tensile strength, elongation at break, 100% modulus. Ultimate tensile strength is the force, measured in psi, needed to stretch a material until it breaks. Elongation at break is how much the material stretches before it breaks, as a percentage of its original dimensions. 100% modulus is the force, measured in psi, needed to stretch the material to twice its original dimensions.

Higher number = tougher product

Thermoplastic
Material that softens when heated and hardens while cooling.

Thermosetting
A substance that becomes a solid when heated.

Thixotropic
The ability of a liquid to resist the pull of gravity. Materials that are gel-like at rest and fluid when agitated.

Undercut
Protuberances or indentations that lock a solid form into its mold and prevent its removal.

Vent
A shallow channel or hole cut into a mold to allow air to escape as it is being deplaced by a liquid.

Viscosity
Simply the resistance of the material to flow measured in centipoise (cps). A material with a low viscosity, low cps, will flow easily. Up to 3,000 is pretty pourable. Water's viscosity is 1 cps. Materials with a high viscosity, high cps, will not flow easily and will tend to capture air. Peanut butter has a viscosity of about 250,000 cps. Temperature affects the viscosity of most of our products. Low temperatures cause higher viscosities. For best flow into detail, materials should be at least room temperature so the viscosity is low.

Viscosity also tells you how much filler you can add to a product. If the material is already thick, it will not accept too much filler before entrapping air.