When developing small, precision rubber seals for medical devices, the molding process is just as important as the compound. Choosing between injection molding and transfer molding affects part accuracy, flash levels, material waste, assembly fit, and long-term reliability.

At Primo Rubber Co., we work with medical device engineers to select the optimal molding method based on geometry, tolerances, material behavior, and regulatory requirements. This guide breaks down how both processes compare — specifically for small, high-precision elastomeric seals.

Why Medical Device Seals Require Special Consideration

Seals used in medical devices — such as valves, diaphragms, micro O-rings, duckbill valves, septums, and pump components — must meet unusually strict requirements:

  • Ultra-tight dimensional tolerances
  • Low or near-zero flash
  • Biocompatibility
  • Low particulates and smooth surface finish
  • Repeatable performance under pressure or motion
  • Sterilization compatibility (EtO, steam, gamma, plasma)

These demands often dictate which molding method is most suitable.

Injection Molding for Medical Rubber Seals

Injection molding is ideal when precision and consistency are top priorities.

How it works

Rubber is plasticized inside a heated barrel, then injected under pressure into a closed mold.

Advantages

  • Excellent dimensional repeatability
    Great for micro-seals and thin-walled components.

  • Low flash due to high clamp pressure
    Critical for medical sealing surfaces.

  • Fast cycle times
    Makes it cost-effective for medium- and high-volume production.

  • Automated gating and runner trimming
    Reduces manual labor and contamination risk.

  • Gentle material handling
    Prevents pre-curing and maintains compound integrity.

When injection molding is the better choice

  • Parts require tighter than ±0.05 mm tolerance
  • Seals have delicate features, micro-lips, or thin membranes
  • High annual volumes
  • Multi-cavity tooling is needed
  • Regulatory environments require repeatability and low particulate generation

Transfer Molding for Medical Rubber Seals

Transfer molding bridges the gap between compression and injection molding, allowing more control for complex geometries.

How it works

A slug of rubber is placed in a heated pot. A plunger pushes it through channels into the cavities.

Advantages

  • Better for complex geometries
    Particularly small parts with sharp internal corners or deep undercuts (when mold design permits).

  • Lower tooling cost
    Ideal for prototyping or lower production volumes.

  • Good material distribution control
    Useful for minimizing knit lines or stress marks.

  • Reduced shear vs. injection molding
    Helps with shear-sensitive medical elastomers like some fluorosilicones.

When transfer molding is the better choice

  • Short runs or prototyping
  • Exotic materials that degrade under high shear
  • Parts with complex flow paths
  • Seals that don’t require ultra-tight flash control
  • Situations where tooling budget is limited

Comparing Injection vs. Transfer Molding Side-by-Side

Factor Injection Molding Transfer Molding
Dimensional accuracy ★★★★★ ★★★★☆
Flash control ★★★★★ ★★★☆☆
Tooling cost ★★★★☆ (higher) ★★☆☆☆ (lower)
Cycle time ★★★★★ ★★★☆☆
Suitable for micro-seals Yes Sometimes
Material shear Medium–Low Very Low
Best for High-precision, high-volume Complex shapes, lower volume

Special Material Considerations for Medical Applications

Both molding methods can work with common medical elastomers:

  • LSR (liquid silicone rubber)
  • HCR silicone
  • EPDM for pump seals
  • FKM/FPM for chemical exposure
  • Fluorosilicone
  • Custom biocompatible compounds

Injection molding excels for LSR micro-parts, while transfer molding is often preferred for high-consistency rubber (HCR) parts with sensitive geometries.

How to Choose the Right Process

Ask these questions when selecting a molding method:

  1. What are the dimensional tolerance requirements?
  2. What annual volume do you expect?
  3. Does the part have delicate or micro-scale features?
  4. Is flash removal unacceptable or restricted?
  5. Is the elastomer shear-sensitive?
  6. What is the tooling budget?
  7. Do you need a multi-cavity setup?

Primo Rubber Co. helps evaluate each part’s geometry, functional requirements, and regulatory environment to determine the correct molding strategy.

Need Help Designing Medical Seals or Choosing a Molding Process?

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