Rubber parts power many industries, from automotive and medical to energy and consumer goods. Selecting the right forming route determines dimensional stability, surface finish, cost, and throughput. This guide explains the fundamentals, strengths, and trade offs of the Top 10 Rubber Molding Processes (Compression, Transfer, Injection) in a clear, structured way for basic and advanced knowledge seekers. You will learn how each process flows, what materials and geometries suit it, and which quality controls matter most. With practical notes on tooling, gating, venting, cure control, and defect prevention, you can match design intent with a capable method and build reliable production plans.
#1 Compression molding
Compression molding places a precisely weighed preform into a heated open cavity, closes the mold, and applies pressure until the elastomer flows, cures, and stabilizes. It excels with large sections, thick cross sections, low to medium volumes, and compounds with higher viscosity. Tooling is simpler, maintenance costs are modest, and setup is fast once preform weight and shape are tuned. Key controls include preheat uniformity, parting line venting, and press dwell time linked to cure kinetics. It is tolerant of inserts and can achieve durable rubber to metal bonds. Typical defects are porosity, short fill, and flash from poor preform sizing.
#2 Transfer molding
Transfer molding loads a preheated slug into a pot, then a plunger forces material through sprues and runners into closed cavities. It offers better flow than compression, captures finer details, and reduces part to part variation for medium complexity parts. It is popular for insert encapsulation because the cavity is closed before material enters, protecting placement. Crucial settings are pot temperature, transfer pressure, and runner balance. Trapped air is mitigated with vents and vacuum capability. Scrap stems from cull and flash, but can be managed with optimized gates. Tooling cost sits between compression and injection while changeovers remain practical.
#3 Injection molding
Rubber injection molding plastifies compound in a heated barrel and injects it under controlled pressure into closed cavities. It enables short cycles, automatic dosing, precise filling, and excellent repeatability, making it ideal for high volume seals, grommets, bellows, and precision micro features. Cold runner blocks reduce waste and stabilize shot size. Process windows are defined by barrel temperature profile, injection speed, holding pressure, and cure time validated by rheology. Automation supports degating and part ejection. Upfront tooling and equipment costs are higher, but cost per part falls significantly at scale. Robust venting and vacuum options help prevent air traps.
#4 Liquid silicone rubber injection
Liquid silicone rubber uses two low viscosity components metered and mixed before injection into a relatively cool mold that quickly heats the charge to cure. LSR flows into fine details, thin walls, and long micro features with exceptional surface finish and biocompatibility options. It is a leading choice for medical valves, infant care products, wearables, and food contact parts. Critical controls include static mixer efficiency, pigment dispersion, shot accuracy, mold temperature uniformity, and residence time to avoid premature cure. Flashless tooling is achievable with razor shut offs. Post cure may be specified to reduce volatiles for regulated applications.
#5 Vacuum compression molding
Vacuum compression augments classic compression with a sealed chamber and vacuum evacuation just before closing and curing. Removing air and volatiles reduces porosity, knit lines, and incomplete fill in intricate or deep draw geometries. It benefits compounds that trap gas or parts with cosmetic surfaces. Setup focuses on preform design, cavity vents, and vacuum timing to avoid pre cooling the charge. The method yields improved mechanical performance and tighter dimensional control with modest cycle time changes. Tooling remains simpler than injection while quality approaches transfer and injection results. Maintenance centers on gasket integrity and stable press platen temperature.
#6 Cold runner injection for rubber
Cold runner injection isolates the runner system from curing with thermally managed nozzles, preserving unvulcanized compound between shots. This reduces material waste, improves shot to shot consistency, and shortens cycle times by focusing heat and cure within the cavities only. Balanced manifold design and valve gates enable family tools and multi cavity scaling. Essential parameters are nozzle temperature, gate style, fill speed, and cavity cure profiling validated by thermal mapping. The approach suits high volume precision seals and micro features. Although tooling is sophisticated, reductions in cull regrind and stable dosing improve overall equipment effectiveness and part cost.
#7 Rubber to metal insert overmolding
Insert overmolding bonds rubber to prepared metals or engineered plastics for vibration isolators, bushings, pulleys, and mounts. Success relies on surface preparation, primers or bonding agents, and controlled preheat of inserts. Transfer and compression are frequently used because the cavity is closed before flow or accommodates bulky inserts. Injection offers automation at scale. Key risks include bond voids, adhesive degradation from excessive heat, and contamination. Process engineers tune insert temperature, placement fixtures, and cure dwell to maximize adhesion. Quality verification uses peel, shear, or torsion tests. Proper venting prevents trapped gas around sharp insert transitions and edges.
#8 Flashless molding systems
Flashless molding uses precision shut offs, micro vents, and highly accurate metering to eliminate or minimize flash at the parting line. It is attractive where trimming labor, cosmetic demands, or particulate control are critical, such as medical components and miniature seals. Tool steels, platen parallelism, and mold alignment are carefully specified. Processes include injection with valve gates or compression using rigid land seals around cavities. The method demands tight compound viscosity control and consistent preform mass. Benefits include reduced secondary operations, improved edge definition, and less tooling wear from trimming. Capability studies confirm Cp and Cpk meet specification.
#9 Micro molding for rubber
Micro molding creates sub gram parts with micro scale features for electronics, fluidic seals, and minimally invasive medical devices. Injection with cold runners and vacuum is common to maintain dosing precision and replicate tiny geometries. Mold design emphasizes high polish, micro venting, and thermal homogeneity to avoid premature cure gradients. Rheology and cure kinetics are characterized at small volumes to control residence time. Metrology uses vision systems and tactile probes to verify micro features. Cleanroom production may be required to control particulates. Material selection considers low compression set, biocompatibility, and aging stability for mission critical applications.
#10 Two material and multi shot molding
Two material molding combines rubber with a second substrate in one cell, creating integrated seals on plastic housings, soft touch grips, or multi durometer components. Approaches include rotating platens, indexing cores, or robot transfer between stations. Compatibility between materials, adhesion promoters, and mold temperatures must be validated to prevent delamination. Gating and sequence control ensure the first shot remains dimensionally stable while the second shot bonds without distortion. Benefits include part consolidation, improved sealing performance, and shorter assembly time. Robust design of experiments helps map window overlap across both shots and confirms repeatability under production realities.