Before any solid silicone part can be molded, the raw material must first be compounded — base gum, curing agent, pigment and additives milled into a single uniform sheet. This step, called mill mixing or compounding, is where a batch's consistency, cure and color are decided. Here is how it works, with real footage from the CYES production floor.

Watch the Process

An operator compounds silicone on the two-roll open mill — milling the base gum and curing agent into a uniform sheet.

What Mill Mixing Is

Mill mixing is the mechanical blending of raw silicone ingredients into a homogeneous, moldable compound. On a two-roll open mill, the raw silicone base gum is fed between two counter-rotating rolls that knead, shear and re-fold it. A curing agent is added to make the compound crosslink during molding, and a pigment masterbatch is weighed in to set the color. Any additional fillers or processing aids are added in the same pass.

The operator does not simply run the material through once. The sheet is repeatedly folded and re-fed through the rolls so the ingredients disperse evenly — an uneven mix means an uneven cure, color streaks or weak spots in the finished part.

The Five Components of Mold-Making Silicone

Mold-making silicone (the RTV-2, room-temperature-vulcanizing grade) is built from five components:

  • Base gum — the silicone polymer that forms the bulk of the material.
  • Crosslinker — reacts with the polymer to build the crosslinked network that gives silicone its elasticity.
  • Catalyst — triggers and controls the cure reaction.
  • Filler — adjusts hardness, reinforcement and cost.
  • Additives — pigments, processing aids or functional modifiers.

In practice these are supplied as two parts: the base gum, crosslinker and filler combined as one component, and the catalyst as a separate component. That is why this grade is called a two-part (or two-component) RTV silicone — when the two parts are mixed, they crosslink at room temperature into a flexible, elastic rubber.

How Pigment Is Added

Color is introduced with a pigment masterbatch, weighed against the batch size to hit a precise ratio. The operator calculates the masterbatch weight from the mixing quantity, then mills it in with the compound so the color is distributed evenly — the same discipline behind our Pantone color matching to ΔE ≤ 1.5.

Mixing Time and the Roll Gap

A typical mixing cycle runs about 30 minutes, but the exact time depends on the compound type and the roll gap. The roll gap is the key process variable:

  • Wider gap — faster, gentler, less shear heat; good for delicate or temperature-sensitive compounds.
  • Narrower gap — more shear and better dispersion, but more friction heat and static charge.

As the silicone is squeezed between the rolls it rubs and shears, generating friction heat and static electricity. A tighter gap produces more of both, which is why operators manage the gap carefully — excessive heat can scorch the compound and even pose a burn risk to hands. This temperature control is part of why the mix is a skilled operation rather than a simple pass-through.

After Mixing: Sheeting, Cutting and Resting

Once the compound is uniform, it is wrapped in plastic film so it stays clean during transfer and storage. The sheeted stock is then cut to size on a cutting machine, marked, and laid out to rest. After a resting period the stock is delivered to the molding shop, where it is shaped under heat and pressure into the final part.

Mill mixing is the quality foundation of solid silicone molding. A well-compounded batch cures consistently, holds its color and performs predictably — which is why we control the mix on our own two-roll mills in-house.

Why In-House Mixing Matters to Buyers

For a buyer, in-house compounding means three things you can verify during an audit:

  • Consistency — the same compound recipe, mixed the same way, every batch.
  • Color control — pigments weighed and dispersed in-house, not outsourced to an uncontrolled third party.
  • Traceability — the mix can be tied back to material lots and process settings for each production run.

This is one part of our broader quality system — see our silicone material guide, Cpk & process capability and full manufacturing process for the rest of the picture.