The Glossary of Vacuum Casting Terminology

Vacuum Casting

The overall process of using a vacuum to draw liquid polyurethane resin into a silicone mold to create high-quality replicas of a master pattern. The vacuum eliminates air bubbles.

Urethane Casting

A common synonym for Vacuum Casting, emphasizing the use of polyurethane resins as the material.

Polyurethane Casting

Another synonym, highlighting the specific family of materials used.

Silicone Molding

The broader category of manufacturing that Vacuum Casting falls under. It specifies that the mold is made of silicone.

Replication Process

The core concept of the technology: creating multiple copies (replicas) from a single master model.

Master Pattern (or Master Model)

The original, perfectly finished model from which the silicone mold is made. It is typically produced via SLA or CNC Machining.

RTV Silicone Mold

Room-Temperature-Vulcanizing silicone mold. The silicone rubber cures to a solid, flexible state at moderate oven temperatures (e.g., 40-70°C). It is the material of choice for the mold.

Casting Cycle

The complete process of pouring resin into the mold, curing, and de-molding a single part. A single mold can withstand multiple cycles before degrading.

Polyurethane Resin (PU Resin)

A two-part system (Polyol + Isocyanate) that, when mixed, cures into a solid plastic. These resins are the “ink” of vacuum casting.

Two-Part Resin

The standard form of PU resins, comprising a “Part A” (base/resin) and a “Part B” (hardener/catalyst) that must be mixed in a specific ratio.

Mix Ratio (e.g., 100A:80B)

The precise proportion by weight in which the two components of the resin must be mixed to ensure proper curing.

Pot Life (or Working Time)

The amount of time available to work with the mixed resin before it begins to thicken and gel, making it unusable for pouring.

Cure Time

The time required for the poured resin to fully harden and develop its final mechanical properties inside the mold, typically in an oven.

Shore Hardness

A scale (e.g., Shore A for rubbers, Shore D for rigid plastics) that measures the hardness of the cured material. (e.g., A “soft-touch” resin might be Shore A 60).

Simulation Material / “Like” Material

PU resins are formulated to mimic the properties of production thermoplastics. Common examples include ABS-like, PP-like, PC-like, and PMMA-like (transparent) resins.

Sprueing / Gating System

The network of channels designed to allow resin to flow into the mold cavity. It includes the main sprues, runners, and gates.

Pouring Cup / Funnel

The opening at the top of the mold where the resin is poured.

Runners

The channels that carry the resin from the sprue to the gates.

Gates

The small, controlled openings where the resin enters the main part cavity. Gates are designed to be easily removed with minimal marks.

Venting / Air Vents

Small, strategically placed channels that allow air to escape the mold as the resin flows in, preventing air traps.

Mold Cavity

The hollow space within the silicone mold that is an exact negative impression of the master pattern.

Mold Parting Line

The seam or line where the two halves of the silicone mold meet.

Mold Life / Number of Shots

The maximum number of high-quality parts that can be cast from a single silicone mold before it degrades (typically 15 to 25 cycles for standard resins).

K-factor (Shrinkage Factor)

A calculated factor (e.g., 1.004) applied to the master model’s dimensions to compensate for the shrinkage of both the silicone mold and the cast PU resin.

Vacuum Casting Machine

The primary equipment, consisting of a vacuum chamber, a vacuum pump, and often a built-in oven.

Vacuum Chamber

The sealed chamber where the mold is placed during the resin pouring process to remove air.

Degassing

The critical step of placing the mixed resin (and often the liquid silicone before mold making) in a vacuum chamber to remove entrapped air bubbles.

De-molding

The process of carefully opening the silicone mold and removing the finished, cured part.

Post-Curing

An additional curing step after de-molding, where the part is placed in an oven to achieve its ultimate mechanical strength and thermal properties.

Incubation / Curing Oven

An oven used to accelerate the curing process of both the silicone mold and the cast PU parts.

Flash (or Fin)

A thin, unwanted film of resin that escapes into the parting line of the mold. It must be trimmed off in post-processing.

De-flashing

The manual process of trimming away the flash from the finished part.

Overmolding

An advanced vacuum casting technique where a rigid substrate (often a previously cast part) is placed into the mold, and a second, flexible material is cast over it to create a single, multi-material component.

Insert Molding

Similar to overmolding, but a pre-made component (like a metal insert) is placed into the mold, and the resin is cast around it.

Surface Texture / Grain

The surface finish (e.g., matte, glossy, leather, orange peel) that is replicated from the master pattern onto the final cast parts.

Tumbling / Vibratory Polishing

A mechanical finishing process used to smooth edges and surfaces, often used for de-flashing small parts in bulk.

Rapid Prototyping

A primary application of vacuum casting: quickly creating prototypes that look and function like production parts.

Bridge Tooling / Bridge Manufacturing

Using vacuum casting to produce parts for market testing, clinical trials, or early sales while production (injection molding) tools are being manufactured.

Low-Volume Production

The use of vacuum casting for manufacturing small batches of end-use parts (from 10s to 100s of units).

Pilot Run

A small-scale production run to validate the manufacturing process before full-scale production; vacuum casting is ideal for this.

Injection Molding

The high-volume production process that vacuum casting simulates. The key comparison is soft tooling (silicone) vs. hard tooling (steel).