Compression molding is the oldest and most straightforward molding process for thermosetting materials. A pre-weighed charge of material is placed directly into a heated mold cavity, the mold is closed under hydraulic pressure, heat cures the material to its final shape, and the part is ejected. Unlike injection molding, compression molding does not require a complex runner system—material flows under direct pressure from the mold closing force.
This process is the backbone of the global rubber products industry and is widely used for fiber-reinforced composite parts (SMC and BMC). China's compression molding industry is concentrated in Zhejiang, Shandong, and Hebei provinces, with thousands of factories producing everything from automotive rubber seals to electrical insulation panels. For buyers, compression molding offers lower tooling costs than injection molding (often 40–60% less), making it ideal for medium-volume production runs of 500–50,000 parts per year.
| Parameter | Compression Molding | Injection Molding | Transfer Molding |
|---|---|---|---|
| Typical Materials | Rubber (NBR, EPDM, silicone), BMC/SMC, phenolic | Thermoplastics (PP, ABS, PA, PC) | Thermosets with inserts |
| Mold Cost (China) | $1,500–$8,000 | $5,000–$30,000 | $3,000–$12,000 |
| Cycle Time | 3–15 minutes | 15–60 seconds | 2–8 minutes |
| Annual Volume Range | 500–50,000 parts | 10,000–1,000,000+ parts | 1,000–30,000 parts |
| Dimensional Tolerance | ±0.20–0.50 mm | ±0.05–0.15 mm | ±0.10–0.25 mm |
| Flash | Moderate — requires deflashing | Minimal | Moderate |
| Complexity | Simple to moderate shapes | Complex shapes possible | Good for insert molding |
China's rubber compounding industry is massive. Most compression molding factories purchase pre-compounded rubber from specialized compounders or mix in-house. Key parameters to specify: durometer (Shore A), tensile strength, elongation at break, compression set, and heat aging resistance.
| Rubber Type | Temperature Range | Key Properties | Common Applications |
|---|---|---|---|
| NBR (Nitrile) | -30°C to 100°C | Oil & fuel resistant | Seals, gaskets, hoses |
| EPDM | -40°C to 130°C | Weather & ozone resistant | Automotive weatherstrips |
| Silicone (VMQ) | -60°C to 230°C | High/low temp, FDA food contact | Kitchenware, medical seals |
| FKM (Viton) | -20°C to 200°C | Chemical & high-temp resistant | Chemical processing seals |
| CR (Neoprene) | -30°C to 110°C | Moderate oil & weather resistance | Industrial gaskets |
| NR (Natural Rubber) | -50°C to 80°C | Excellent tensile & tear strength | Mounts, bushings |
Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) are glass-fiber reinforced polyester or vinyl-ester composites. They offer high strength-to-weight ratio, excellent electrical insulation, and corrosion resistance. China is the world's largest producer of SMC/BMC parts, particularly for electrical enclosures, automotive body panels, and bathroom components (bathtubs, shower trays). SMC typically achieves tensile strengths of 60–120 MPa with glass content of 20–30% by weight.
Phenolic (bakelite) compression molding remains important for electrical components (switches, circuit breaker parts), pot handles, and brake friction materials. Chinese suppliers commonly use PF-type phenolic molding compounds meeting UL 94 V-0 flammability ratings. Phenolic parts tolerate continuous use at 150°C and short-term exposure up to 250°C.
| Region | City | Specialization | Strengths |
|---|---|---|---|
| Zhejiang | Ningbo / Yuyao | Rubber seals, O-rings, silicone products | Strong export experience, ISO certification common |
| Hebei | Hengshui / Jing County | Rubber hoses, gaskets, bridge bearings | Very low labor costs, high volume |
| Shandong | Gaomi (Weifang) | Rubber tires, industrial rubber parts | Established rubber industry infrastructure |
| Jiangsu | Wuxi / Changzhou | SMC/BMC electrical & automotive parts | Advanced composites, OEM supply chain |
| Guangdong | Dongguan / Shenzhen | Silicone consumer goods, precision rubber | Fast turnaround, high automation |
| Anhui | Wuhu / Hefei | Automotive rubber parts | Proximity to auto OEM plants |
For rubber compression molds (curing at 150–190°C): pre-hardened P20 (HRC 28–32) is sufficient for low-volume runs (<10,000 cycles). For production runs exceeding 50,000 cycles, specify H13 or 4140 steel, hardened to HRC 48–52. For silicone (which can contain acidic byproducts during curing), specify 420 stainless steel to prevent corrosion. Good mold surface finish (Ra 0.4 μm or better) is essential for rubber parts to prevent sticking and ensure clean release.
| Cost Component | Share | Notes |
|---|---|---|
| Raw Material (compound) | 25–40% | NBR ~$3–5/kg, EPDM ~$2.5–4/kg, silicone ~$5–8/kg |
| Mold Amortization | 8–20% | Lower for high-volume orders; single-cavity molds typically $2,000–$5,000 |
| Labor — Press Operation | 12–20% | One operator can run 2–4 presses simultaneously |
| Labor — Deflashing & Trimming | 8–15% | Manual trimming is common; cryogenic deflashing adds $0.02–0.10/part |
| Curing Energy | 5–10% | Heating 150–190°C for 5–15 minutes per cycle |
| Quality Control | 3–5% | Hardness testing, dimensional checking, tensile testing |
| Packaging & Logistics | 5–10% | Packaging type and incoterm choice affect this significantly |
| Factory Margin | 10–18% | Wider margin for complex geometry or tight tolerances |
| Stage | Simple Rubber Part | Complex Rubber Part | SMC/BMC Part |
|---|---|---|---|
| Mold Design & Fabrication | 10–18 days | 18–30 days | 20–35 days |
| Material Procurement / Compounding | 3–5 days | 5–10 days | 7–14 days |
| Mold Trial & Sample Approval | 2–4 days | 4–7 days | 5–10 days |
| Mass Production | 7–15 days | 15–25 days | 10–20 days |
| Total from PO to Shipment | 22–42 days | 42–72 days | 42–79 days |
| Criteria | Weight | Evaluation Method |
|---|---|---|
| Quality System Certification | 15% | ISO 9001 (min), IATF 16949 for automotive, ISO 13485 for medical |
| In-house Testing Capability | 12% | Do they have a tensile tester, durometer, rheometer (MDR), and compression set tester? |
| Compound Formulation Capability | 10% | Do they mix their own compounds or buy pre-mixed? In-house mixing allows better quality control. |
| Mold Making Capability | 8% | In-house mold shop reduces lead time and communication complexity. |
| Previous Export Experience | 10% | Ask for export documents (bill of lading, commercial invoice) from the past 3 shipments to your region. |
| Capacity Matching | 10% | Press tonnage: 100–200 tons for small parts, 300–800 tons for medium parts, 1,000+ tons for large SMC panels. |
| On-time Delivery Record | 10% | Request their OTIF (on-time in-full) metrics for the past 12 months. |
| Language & Communication | 5% | English-speaking sales engineer preferred for overseas buyers. |
| Price Competitiveness | 20% | Compare 3 quotes for the same part. Be wary of quotes more than 30% below the average. |
Q: Why does compression molding take so much longer per part than injection molding?
A: Thermoset materials require a chemical curing (cross-linking) reaction, not just melting and cooling. This reaction takes time—typically 3–15 minutes depending on the material thickness. Faster-curing compounds are available but reduce mechanical properties. For high-volume production (>50,000 pieces/year), consider injection molding of thermoplastics or injection-compression molding.
Q: How do I handle flash on rubber parts?
A: Deflashing methods include: manual trimming (lowest cost, suitable for soft rubber), cryogenic deflashing (tumble in liquid nitrogen + shot blast — best for complex contours, adds $0.03–0.15/part), and waterjet cutting (for large, flat parts). Specify the maximum acceptable flash height in your technical drawing.
Q: Can I use compression molding for prototypes?
A: Yes, and it's often the fastest way to produce rubber or thermoset prototypes. A simple aluminum flash-type mold can be CNC-machined in 5–7 days for under $1,500. This is significantly faster and cheaper than injection molding tooling for thermosets.
Q: What is the minimum wall thickness for compression molded parts?
A: For rubber: minimum 0.5–1.0 mm for small parts, 1.5–2.0 mm for larger parts (material flow becomes difficult below this). For SMC/BMC: minimum 1.0–1.5 mm for flat panels, 1.5–2.5 mm for ribbed structures. Thinner walls risk incomplete fill or fiber non-distribution.
Q: How do I ensure compound consistency across orders?
A: Request that the supplier seals a master sample of the compound