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Lesson 2.6ShapeChapter 2 · Molding plastics and rubber

Compression molding (SMC, BMC, rubber)

A charge pressed and cured in a hot mold

Compression molding places a measured lump of material in a hot, open mold, then closes the mold and squeezes it until it fills the shape and hardens. It is the classic way to make rubber seals and gaskets, and fiber-reinforced car panels. The material cures (sets permanently by a chemical reaction) instead of simply cooling.

Compression molding (SMC, BMC, rubber): a real photo
Photo: Sam D. Wilbur, CC BY-SA 4.0, via Wikimedia Commons
A heated mold squeezes a lump of material into shape and cures itCompression molding of rubber, SMC or BMC, side cross-sectionLoad a chargeheated top halfchargeheated cavityPresspress forcematerial flows outwardCureflashheat cures it forabout 1 to 10 minutesOpen and ejectejector pinflash trimmed off1234rubber or composite charge, then partheaterssteel mold and pin
Compression molding · 4 steps in a heated moldCircled numbers match the steps below. The four panels show one cycle in a heated two-part mold.

How it works

  1. Load a charge. A weighed lump of material, called the charge, is placed in the heated lower mold. It can be raw rubber, SMC (sheet molding compound: sheets of resin with chopped glass fibers) or BMC (bulk molding compound: a putty-like resin and fiber mix).

  2. Press. A hydraulic press lowers the heated upper half. The pressure, typically tens to a few hundred bar on the part, makes the material flow out and fill every corner of the cavity.

  3. Cure. The press holds the mold shut at typically about 140–180 °C. Heat triggers cross-linking (chemical bonds that join the molecules into one network), so the part sets for good. A thin film of excess material, the flash, squeezes out at the joint.

  4. Open and eject. The press opens and an ejector pin pushes the part out. The flash is trimmed off, by hand, by tumbling in freezing conditions for rubber, or by machining for composites.

Because the material barely travels, long glass fibers stay intact, which gives SMC and BMC parts high strength for their weight.

Key facts

Materials
Rubbers (natural, EPDM, silicone, fluoroelastomer); thermoset composites SMC and BMC; also melamine and phenolic resins
Typical parts
O-rings, gaskets, keypads, car body panels, truck hoods, electrical boxes, melamine dishes
Part size
From small seals of a few grams to car panels over a meter long
Precision
Typically about ±0.1 to ±0.3 mm; thickness depends on the flash at the joint
Surface finish
Copies the mold; SMC panels can come out smooth enough to paint as car body parts
Batch size it suits
From hundreds to hundreds of thousands of parts
Tooling and machine cost
Molds cheaper than injection molds for the same size, from a few thousand dollars for rubber to hundreds of thousands for large SMC panels; presses from tens of thousands to millions of dollars
Speed
Typically 1–10 minutes per cycle, set by the cure time

Strengths and limits

Strengths

  • Simple, robust molds with no runners or gates
  • Little material waste; keeps long fibers intact
  • Large, strong, stiff composite parts
  • Works with thick and filled rubbers and thermosets

Limits

  • Slower cycles than injection molding
  • Flash must be trimmed from every part
  • Hard to make very complex shapes, deep thin ribs or tight tolerances
  • Charges must be weighed and placed, often by hand

Where it's used

  • AutomotiveHoods, tailgates and body panels in SMC; rubber seals, bushings and grommets
  • ElectricalSwitch boxes, meter housings, insulators in BMC
  • IndustrialO-rings, gaskets, diaphragms, valve seals
  • ConsumerSilicone keypads, kitchen mats, melamine plates
  • AerospaceHigh-strength composite brackets and fittings

How to recognize it

  • Rubber parts with a thin ridge or trimmed edge along one line, where the flash was removed.
  • No gate mark or sprue, since material was never injected.
  • On SMC panels, a slightly speckled or fiber-patterned back side and thick, even walls.
  • A hard, heat-resistant feel: the part will char rather than melt when heated.

Compare with

Injection molding2.1injects melt into a closed mold; faster and more detailed, but costlier molds and shorter fibers.Calendering2.7also handles rubber, but makes continuous sheets between rolls instead of shaped parts.Resin transfer molding and infusion10.2another way to make composite parts, by pushing liquid resin into dry fibers in a closed mold.

Watch it

Elastech Solutions · A beginner explanation of rubber compression molding

Sofeast · The steps and vocabulary, filmed in a factory

IDI Composites · From an SMC charge to a finished composite part

Taitian Press · A giant 4,500-tonne press making car parts

See real photos

Remember

  • Load a charge, press, cure with heat, eject and trim the flash.
  • It shapes materials that set by chemistry: rubbers and fiber-filled thermosets.
  • Simple molds and long fibers, but slower cycles than injection molding.

More in Molding plastics and rubber

All 100 processes
2.1ShortInjection moldingMelt forced into a closed steel moldOpen the lesson2.2Overmolding and two-shot moldingA second plastic molded onto the firstOpen the lesson2.3Blow molding (extrusion and injection blow molding)Hot tube or preform inflated in a moldOpen the lesson2.4ThermoformingHeated sheet sucked onto a moldOpen the lesson2.5Rotational moldingPowder coats a hot, turning moldOpen the lesson2.7CalenderingCompound squeezed into sheet between rollsOpen the lesson

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