Mold Making Glossary

The vocabulary of two-part mold design, in plain language. These are the terms the tool uses in its analysis panels and the ones you will run into in any casting or injection molding workflow.

Cavity

The hollow space inside a mold that forms the outer shape of the part. In a two-part mold it usually refers to the top half.

Core

The half of the mold that forms the inside or underside of the part. In a two-part mold it is usually the bottom half.

Parting direction

The axis along which the two mold halves separate. Every surface of the part is evaluated relative to this direction.

Parting plane

The flat surface where a mold splits into two halves, perpendicular to the parting direction.

Parting line

The visible seam left on a finished part where the two mold halves met.

Draft angle

The taper applied to walls parallel to the parting direction so the part releases without sliding contact. One to two degrees is a common default.

Undercut

Geometry with negative draft that hooks behind the parting plane and mechanically locks the part into a rigid mold.

Demoldability

How easily a part releases from a mold, determined by draft angles and undercuts relative to the chosen parting plane.

Sprue

The channel through which casting or injection material enters the mold cavity.

Runner

A channel that carries material from the sprue to one or more gates. Used when a cavity needs more than one fill point.

Gate

The narrow opening where material enters the cavity itself, at the end of a runner.

Vent

A small channel that lets displaced air escape as the cavity fills. Without one you get short shots and trapped bubbles.

Flash

The thin fin of material that squeezes into the gap between mold halves when they are not clamped tightly or aligned properly.

Short shot

An incompletely filled cast, usually caused by trapped air, a material that set too early, or a sprue that is too narrow.

Alignment pins

Matching pins and sockets on the two mold halves that register them to each other so the seam stays tight and the two sides of the part line up.

Shot volume

The amount of material a machine can inject in one cycle. The Mold Backer Compatible envelope caps part volume at 45 mL.

RTV silicone

Room-temperature-vulcanizing silicone rubber, poured around a master and cured to produce a flexible mold. Available in tin-cure and platinum-cure chemistries.

Cure inhibition

Failure of platinum-cure silicone to set where it contacts sulfur, latex, some resins, or incompletely cured photopolymer, leaving a permanently tacky surface.

Master

The original object a mold is made from. Often a 3D printed part.

Release agent

A wax, PVA film, or spray applied to a mold surface so the cast part does not bond to it.

Degassing

Pulling a vacuum on mixed material before pouring, to remove entrained air bubbles.

Pressure casting

Curing a cast under pressure, typically 40 to 60 psi, so remaining bubbles collapse to invisible size.

Shore hardness

A measure of a cured elastomer's firmness. Shore 10-20A silicone releases undercuts easily; Shore 30-40A holds dimensions better.

Watertight mesh

A closed mesh with no holes or non-manifold edges. Required for reliable boolean operations when carving the cavity.

B-rep

Boundary representation, the exact mathematical solid description used by CAD and by STEP files, as opposed to a triangle mesh.

Floating section

A disconnected island in a cross-section slice, indicating geometry that would sit in the mold with no path out.