When designing a new rubber part, one of the biggest sources of confusion for engineers is tooling—how molds are made, how cavities are shaped, and what limitations are imposed by rubber molding processes. Good tooling design impacts everything from part quality and flash control to cycle time and long-term mold durability.

In this post, Primo Rubber Co. breaks down the fundamentals of rubber tooling so you can design manufacturable parts, reduce iteration cycles, and avoid costly surprises when quoting your next project.

What Is Rubber Tooling?

Rubber tooling refers to the metal molds used to shape elastomers during molding processes such as:

  • Compression molding
  • Transfer molding
  • Injection rubber molding

The tool defines the part’s geometry, tolerances, surface finish, and gating system, and must handle high clamping pressures, heat, and repeated cycles over thousands or millions of shots.

Key Components of a Rubber Mold

1. Cavities

The cavity is the negative shape of the final part. A mold can contain:

  • Single cavity (for large or high-precision parts)
  • Multi-cavity (for high-volume production)

Cavity design impacts:

  • Flash location
  • Parting line shape
  • Mold temperature uniformity
  • Cycle time

2. Core Pins and Inserts

Core pins create holes, voids, or internal features. They can be:

  • Fixed
  • Removable
  • Floating
  • Replaceable inserts

In rubber molds, pins must account for thermal expansion, pressure, and elastomer flow characteristics.

3. Parting Lines

Parting lines are where mold halves meet. They determine:

  • Flash placement
  • Ease of demolding
  • Tool complexity

Good part designs place parting lines in low-visibility, low-tolerance areas.

4. Sprues, Gates, and Runners

The gating system controls how rubber enters the cavities:

  • Compression molds: rubber is placed directly into cavities
  • Transfer molds: rubber flows through sprues and runners into cavities
  • Injection molds: rubber is injected through a sprue into a runner network

Gate placement impacts material flow, molding pressure, cure uniformity, and cycle throughput.

5. Ejection / Demolding Features

Unlike plastics, rubber often adheres to tool surfaces as it cures. Tools use:

  • Air ejection
  • Stripper plates
  • Push pins
  • Draft angles
  • Mold-release coatings

These features ensure consistent, flash-free parts and reduce cycle time.

How Rubber Tooling Differs from Plastic Tooling

Although both use steel or aluminum molds, rubber tooling differs in several important ways:

1. Higher Temperatures

Rubber cures at 150–200°C, requiring stable and thermally balanced molds.

2. Squeeze Flow and Compression

Rubber flows under pressure differently than molten plastic. Tools must handle:

  • High tonnage
  • Viscous material flow
  • Parting line squeeze

3. Flash Control Is Critical

Rubber seeks gaps. Precision machining, shutoffs, and surface finishes must be extremely tight to prevent excess flash.

4. Venting Is Essential

Air must escape during curing. Tools include micro-vents or intentional flash points to ensure full cavity fill.

Designing Parts With Tooling in Mind

1. Include Draft Angles

Even small draft angles help free parts from the mold and reduce sticking.

2. Avoid Deep Undercuts

Undercuts dramatically increase tooling cost by requiring side actions or collapsible cores.

3. Place Parting Lines Strategically

Choose locations where flash is acceptable or easy to trim.

4. Maintain Uniform Wall Thickness

Uneven walls cause curing inconsistencies and warping.

5. Provide Radiused Corners

Sharp internal corners trap rubber and stress the mold steel—radiuses improve flow and longevity.

Types of Rubber Molds

Compression Molds

  • Lowest tooling cost
  • Ideal for simple shapes or large parts
  • Longer cycle times

Transfer Molds

  • Better control of flash
  • Ideal for moderately complex shapes
  • Good for medium production volumes

Injection Molds

  • Highest tooling cost
  • Best for small, precision parts
  • Fastest cycle times
  • Consistent quality and minimal flash

Tooling Materials

Most rubber tooling is made from:

  • P20 tool steel (balanced cost/longevity)
  • S7 steel (shock resistance)
  • H13 (high-temp durability)
  • Stainless steel (corrosion resistance)

Aluminum is rarely used due to heat and pressure requirements.

How Long Does Rubber Tooling Last?

Tool life depends on:

  • Elastomer type (abrasive fillers shorten life)
  • Cure temperature
  • Clamping pressure
  • Surface finish
  • Maintenance and cleaning

A well-built steel mold can last hundreds of thousands to millions of cycles.

Summary: What Engineers Should Know Before Starting Tooling

  • Rubber flows differently than plastic—design with compression and flash control in mind.
  • Tooling complexity depends heavily on part geometry.
  • Strategic parting line placement saves cost and reduces trimming.
  • Injection molding offers the best precision, while compression molds are most economical.
  • Early collaboration with a rubber molder leads to better results and fewer tool revisions.

Need Help Understanding What Options You Have When it Comes to Tooling?

Primo Rubber Co. specializes in injection molding custom rubber components. Contact Primo Rubber Co. today to discuss your project and find the ideal material, design, and tooling solution for your rubber parts.