A compression molding press is the most widely used production method for rubber molding. It is ideal for low to medium volumes and can be used for a largest variety of part sizes and materials, including high cost materials, and applications that demand extreme hardness. It is a very useful molding process for forming bulky parts, gaskets, seals and O-rings. It is also a very efficient, low waste method that offers the simplest process, lowest investment, and greatest flexibility. Compression molding generally results in lower amounts of scrap. It does not consume excess rubber in the runner of an injection mold, or in the pot of a transfer mold.
The most commonly viewed drawbacks to compression molding are longer cycle times and costly labor costs. However, both of these can be addressed to equal or surpass the injection molding process. Cycle times for compression molded parts using preheated preforms can be less than for injection molded parts. Automated preform, loading/unloading, and post handling equipment can be integrated with a compression press to nearly equal the labor cost of injection.
Best suited for:
• Low to medium production volumes
• Medium to large sized parts
• Thick cross-sectional parts.
• Low to high durometer materials including very high hardness – Ideal
• More expensive rubber formations, and other high cost materials
• Molders who require quick tooling changeover.
Advantages over other methods:
• Lowest investment for tooling and machinery
• Shortest mold setup times make a perfect match for short production runs
• Internal stress is minimized, producing less warping
• Ability to process very stiff, high durometer materials
• Generates less waste than other production methods
• Ability to process thin to large thick parts
• Greatest flexibility in molding various part sizes and materials
• Less shrinkage of material leads to greater accuracy of parts
• More cavities per mold are possible as lower molding pressure is required
• Requires a preform (a pre-measured slug of material)
• Can produce a higher rate of dimensional inconsistency
• Generally produces the largest parting line
• Flash removal requires a secondary operation
• Typically the most labor intensive, but can be automated to nearly equal injection
Compression Molding Process Applications
• High Durometer Materials – Medium hardness materials (60A-90A) work the best in compression molding. Compression molding is easily capable of molding material of even higher durometers. Higher cavity pressure can be difficult to obtain with other molding processes without degradation of material properties.
• Filled Materials -The compression molding process is well suited for molding materials that are combined with stiffening fillers. Fiber orientations are not altered of destroyed as commonly observed in injection molding.
• Virtually all thermoset materials, from bulky material compounds to very high durometer materials.
• Mostly every molding industry from industrial to aerospace.
Compression Molding Typical Products
Compression molding has the largest range of typically molded products from O-rings, gaskets, diaphragms, seals, bearings, bushings, golf balls, shoes soles, to highly valued critical components used in the medical, fuel cells and printed circuit board industries
Compression Molding Common Materials
SMC – Polyurethane
BMC – Polyethylene
SBR – Neoprene
High Viscosity Materials
Epoxy – Nylons
Resins – Polycarbonites
Polyamide – Acrylics
Melamines – Filled
Polyesters – Materials
High Cost Materials
Compression presses are used in virtually every molding application from various thermoset to thermoplastics, including laminates and composites. There are many others processes that can use compression presses in a stand only condition, or in combination with auxiliary material dispensing systems.