Last updated: 2026-09-05 · 5 min read

Injection Molding vs. Transfer Molding — When Does Each Process Make Sense for Rubber and Thermoset Parts?

Thermoplastics go through injection molding — melt injected into a cool mold at 20–80°C, solidifies in 10–60 seconds, runner can be reground and reused. Thermosets and rubber go through transfer molding — material cured in a heated mold at 150–200°C for 60–300 seconds, crosslinked permanently, cannot be recycled. The material chemistry decides which process to use; there is no overlap.

Key Facts

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  • Transfer molding per-part cost is 2–5× higher than injection molding — not because the machine is more expensive, but because the cure cycle is 3–10× longer and the cured material cannot be reground. On a 120-second cure cycle vs. a 20-second injection cycle, you lose 80% of throughput.
  • Three material families require transfer molding: silicone rubber (LSR and HCR for seals, gaskets, O-rings), thermoset encapsulants (epoxy and phenolic for electronic protection), and bulk molding compounds (BMC/SMC for electrical insulators and appliance handles). No thermoplastic process can substitute.
  • Transfer mold tooling costs 20–40% more than an equivalent-size injection mold because the mold must include heater cartridges, thermocouple zones and a transfer pot with plunger — all components absent from a cooled injection mold. Expect $3,000–8,000 for a transfer mold vs. $2,000–5,000 for a comparable injection mold.

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

    What Are the Cost and Tooling Differences Between the Two Processes?

    Cost Factor Injection Molding Transfer Molding
    Mold base cost $2,000–5,000 $3,000–8,000 (includes heater cartridges)
    Cycle time 10–60 seconds 60–300 seconds
    Parts per hour (single cavity) 60–360 12–60
    Runner waste Reground and reused (0% waste) Cured, cannot be recycled (100% waste)
    Per-part cost multiplier 1× (baseline) 2–5× (from cycle + waste)

    The per-part cost gap comes from two sources: throughput and material waste. A transfer mold running at 30 parts/hour vs. an injection mold at 180 parts/hour means six times fewer parts per hour — and the cured runner adds material cost on top because it cannot be reground. For high-volume programs, this gap compounds quickly.

    What Design Rules Are Specific to Transfer Molds?

    1. Mold must be heated uniformly. Heater cartridges are installed in both the A and B plates, controlled by separate thermocouple zones.
      Temperature uniformity must be ±3°C across the cavity — hot spots cause premature cure (short shots), cold spots cause under-cure (tacky surface, poor mechanical properties).
    2. Transfer pot volume must match shot weight. The pot holds the material charge before it is forced into the cavity. If the pot is too large, excess material cures in the pot and is wasted. If too small, the plunger cannot deliver enough material to fill the cavity.
      Size the pot at 1.1–1.3× the shot weight.
    3. Cure time is fixed by material chemistry, not process tuning. Unlike injection molding where cycle time can be shortened by adjusting cooling, the cure time in transfer molding is determined by the crosslinking kinetics of the specific compound. Silicone at 170°C cures in 60–90 seconds; at 150°C it takes 120–180 seconds.
      Raising mold temperature 20°C roughly halves the cure time — but risks scorching.
    4. Flash control requires different parting line design. The material flows at lower pressure than injection molding (5–20 MPa vs. 50–150 MPa), but the mold is at high temperature where thermal expansion affects parting line seal. Parting land width must be wider (8–12mm vs.
      5–8mm for injection) to prevent flash at elevated temperature.

    Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds.
    Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    Transfer molding: mold cost $3,000–8,000 (heater cartridges + transfer pot), cure 60–300s, 12–60 parts/hour, 2–5× per-part cost vs. injection. Injection molding: mold $2,000–5,000, cycle 10–60s, 60–360 parts/hour, runner recyclable. The material decides the process — thermoplastics inject, thermosets transfer.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Can LSR (liquid silicone rubber) be processed on a standard injection machine?

    Not without modification. LSR requires a cold feed system to prevent premature curing in the barrel — the screw has a cooled section that keeps the silicone below its cure temperature until it enters the heated mold. Standard thermoplastic injection machines do not have this cooling section.

    LSR injection molding is faster than traditional transfer molding for silicone and produces less waste — but the machine and mold are specialized and more expensive than standard thermoplastic equipment.

    What is the difference between compression molding and transfer molding?

    Compression molding places a pre-measured charge directly into the open cavity, then closes the mold to shape and cure. Transfer molding places material in a heated transfer pot, then forces it through sprues and runners into the closed cavity under pressure.

    Transfer molding produces better dimensional accuracy and is suitable for parts with inserts because the mold is closed before the material flows. Compression molding is simpler and lower cost for large, flat parts like gaskets and pads where dimensional precision is less critical.

    Why can’t thermosets be processed by standard injection molding?

    Thermosets cure by chemical crosslinking — once cured, they cannot be remelted. Standard injection molding melts the material in a barrel and injects it into a cool mold. If a thermoset were processed in a standard barrel, it would cure inside and destroy the screw and barrel.

    Transfer molding avoids this by keeping the material below the cure temperature until it enters the heated mold — the crosslinking reaction only starts at the mold’s elevated temperature of 150–200°C.

    Key Takeaways

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  • Material chemistry decides: thermoplastics inject (cooled mold), thermosets transfer (heated mold).
  • Transfer mold costs 20–40% more; per-part cost 2–5× higher from longer cure and no recycling.
  • Cure time is fixed by material chemistry — raising mold temp speeds cure but risks scorching.
  • LSR injection is a faster silicone variant but requires specialized cold-feed equipment.

    • Send the part drawing with the material specification — the quotation confirms whether the material is a thermoplastic (injection molding) or thermoset (transfer molding) and provides the correct process, mold design and per-part cost.

    Sources: Plastics Technology — Thermoset Transfer Molding vs. Thermoplastic Injection · Society of Plastics Engineers (SPE) — Rubber and Thermoset Molding Process Comparison · MoldMaking Technology — LSR Injection Molding and Silicone Tool Design · Rubber News — Transfer Molding Process Guidelines for EPDM and NBR · ASTM International — Thermoset Molding and Cure Test Standards

    — Arthur, Tooling Engineer at aCatMould