Compression Molding

Process Overview

Silicone Compression Molding

Compression molding places a pre-weighed charge of uncured silicone compound into an open mold cavity. The press closes under controlled tonnage and heat, forcing the compound to fill the cavity. The part cures in-mold, then is demolded and deflashed.

SIKING operates both flat-plate and vacuum compression presses. Vacuum presses use extraction hoods during the press cycle — this evacuates trapped air from the cavity before the charge flows, which vacuum-assisted compression reduces entrapped air and surface defects relative to standard flat-plate methods.

HTV (high-temperature vulcanising) solid silicone as primary material
Hardness range: 30–80 Shore A, wider range with custom compounding
In-house compounding: 0–90 Shore A achievable
Vacuum capability: 12 of 20+ presses equipped with extraction hoods

20+

Compression presses

200–400T

Tonnage range

800×1200mm

Max part size

Compression Press Floor

Equipment Specifications

Compression Molding Specs

Parameter	Specification
Press Tonnage Range	200 – 400 ton
Number of Presses	20+ (including vacuum and flat-plate)
Large-Format Press	400 ton — up to 800mm × 1200mm parts
Vacuum Capability	Extraction hoods — reduces trapped air
Primary Material	HTV Solid Silicone
Hardness Range (standard)	30–80 Shore A
Hardness Range (compounded)	0–90 Shore A
Dimensional Tolerance	±0.05mm – ±0.35mm (per SK9002A)
Material Suppliers	Dow, Wacker ELASTOSIL, Shin-Etsu (100% imported)

Buyer Implications

What This Means for Your Project

Large Parts Without Outside Sourcing

The 400-ton press handles parts up to 800×1200mm in-house. Industry standard typically stops at 200–250 ton. For large gaskets, panels, or pads, this eliminates a subcontracting step and the quality risk it introduces.

Vacuum-Assisted Process for Cosmetic Parts

Vacuum-assisted compression reduces trapped air, which matters for parts with visible sealing faces, backlit keypads, or cosmetic requirements. Not all presses are vacuum-equipped — SIKING routes jobs to the appropriate machine.

Tolerance Traceability

Tolerances are specified per SK9002A, SIKING's proprietary enterprise standard in force since 1996. Dimensional acceptance criteria are documented per part number, not applied generically.

Process Flow

Compression Molding — Step by Step

1

Material Preparation

Compound selected and pre-weighed per job traveller. Batch traceability recorded.

2

Mold Loading

Mold heated to cure temperature. Pre-weighed charge placed in cavity. Inserts located where specified.

3

Press & Cure

Press closes under controlled tonnage. Vacuum cycle applied (vacuum presses). Dwell time per compound specification.

4

Deflash

Parting line flash removed. Cryogenic or manual deflash method per part geometry and cosmetic requirement.

5

Inspection & Pack

Dimensional and cosmetic inspection per SK9002A / customer drawing. CCD visual inspection on high-volume lines.

Best Suited For

Large gaskets and seals requiring a single-piece form
Thick-section parts (wall thickness >5mm) where injection doesn't fill evenly
High-durometer silicone (60–80 Shore A)
Low-to-medium volume runs where tooling investment must be minimised
Parts with wide flash tolerance or where deflash is acceptable
Anti-vibration mounts, industrial dampers, floor pads
Remote control and panel keypads with standard parting lines

Not Suited For

Micro-precision parts requiring <±0.05mm — use LSR injection instead
Thin-wall structures (<0.5mm wall thickness)
High-volume production where cycle time per press is a bottleneck — LSR injection is faster
Parts where flash is structurally or cosmetically unacceptable and cannot be deflashed
Complex undercut geometries — consider transfer molding
Liquid silicone rubber (LSR) — requires dedicated injection equipment

Common Questions

Compression Molding — FAQ

What is the industry-standard maximum tonnage, and how does SIKING compare?

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Most silicone contract manufacturers operate compression presses in the 200–250 ton range. SIKING's fleet goes to 400 ton, with the 400T press capable of forming parts up to 800×1200mm. Above 250T is a rare capability that eliminates the need to source large parts from a second factory.

How does vacuum-assisted compression work, and when is it specified?

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Vacuum presses use extraction hoods that lower ambient pressure inside the mold cavity before and during the early press stroke. This evacuates air that would otherwise be trapped as the compound flows to fill the cavity. It is specified for parts with cosmetic sealing faces, backlit surfaces, or tight porosity requirements. Not every job requires vacuum — SIKING routes based on part specification.

What tolerance should I specify on my drawing for compression-molded silicone?

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Compression molding tolerances depend on part geometry, wall section, and post-cure shrinkage. SIKING's SK9002A enterprise standard defines tolerance bands by feature type: ±0.05mm is achievable on small critical features; ±0.15–0.35mm is typical for overall dimensions on medium-sized parts. Specify your function-critical dimensions and SIKING will confirm achievable tolerances at DFM review. Specifying ±0.02mm on a compression-molded part without discussion creates unrealistic supplier expectations — that tolerance is LSR territory.