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Understanding the Various Rubber Molding Processes

The molding process of rubber is the means by which rubber is transformed into products. There are various types of these processes, and the choice of which process to use depends on the product’s intended use and our desired outcome. The molding processes of rubber include compression molding, transfer molding, injection molding, extrusion, and blow molding, etc.

Compression Molding

This method involves placing powdered or fibrous rubber compounds into a mold cavity and then applying pressure. The material flows and fills the entire mold cavity under the influence of heat and pressure. The rubber material fully flows and cures upon cooling, ultimately forming the desired rubber product.

Compression Molding

Compression molding is suitable for manufacturing complex shapes and larger rubber products, ensuring uniform material distribution and high-quality molding results. The advantages of compression molding include a mature and reliable process and lower mold investment. However, the drawbacks are low production efficiency and longer molding cycles.

Process:

Accurately weigh raw rubber, additive agents, and some fiber materials according to the formula. Use a two-roll mill or other related machinery to convert the raw rubber into a soft, easily processable plastic state. Then, uniformly mix the raw rubber with various compounding agents to produce rubber compound. The next step is molding. The rubber material is weighed according to the process requirements, and the inserts and rubber materials are accurately placed in the mold cavity. During the compression molding process, the machine applies pressure to the rubber through the mold, opens the mold, and ejects the product, thus completing the production of compression-molded rubber parts.

Compression Molding

Transfer Molding

Transfer molding is a method developed based on improved compression molding, mainly used for thermosetting plastics. The mold in transfer molding has a separate feed chamber connected to the cavity through a runner system.

Transfer Molding

The molding principle involves closing the cavity and preheating it to molding temperature. Plastic raw materials are placed in the feed chamber, initially heated and plasticized. Then, a plunger driven by a press applies pressure, forcing the molten material into the closed mold cavity through the runner system. After a period of heating and pressure, the plastic undergoes cross-linking and curing. Once optimal performance is achieved, the mold is opened, and the molded part is removed.

Process:

The raw rubber undergoes mastication and mixing to attain a certain plasticity and flowability. The formula includes various additives, such as carbon black and vulcanizing agents, to enhance the rubber’s physical properties and mechanical strength. 

The plastic raw materials are placed in the feed chamber, initially heated and plasticized. The press drives the plunger, and the molten material flows into the closed mold cavity through the runner system. After a period of heating and pressure, the plastic cross-links and cures. When optimal performance is achieved, the mold is opened, and the molded part is removed.

Transfer Molding

Transfer molding is mainly used for thermosetting plastics, such as manufacturing tires, shoe soles, and other complex-shaped rubber products. Compared to compression molding, transfer molding can produce deep-hole and complex-shaped products with accurate dimensions, higher quality, and less mold wear.

Injection Molding

Injection molding involves injecting thermoplastic polymers into a closed mold where they cool and solidify.

Rubber Injection Molding

This process includes a clamping system and an injection system. The barrel heats, and the screw rotates, generating frictional heat that raises the temperature of the rubber. When the rubber reaches a certain level of plasticization, the screw plunger advances, forcing the rubber through the nozzle into the closed hot mold. After vulcanization, the mold is opened, and the product is demolded while still hot. This method is similar to the mold vulcanization of rubber products but involves higher pressure and faster transmission speeds.

Injection Molding

Injection molding is suitable for various rubber materials and complex-shaped products, offering better dimensional control, reduced waste, and shorter cycle times.

Ensuring Quality and Consistency in Rubber Part Production

Formulation:

Understanding the grade, hardness, and shrinkage of rubber materials is essential, as these factors influence mold design and material selection. Knowing the specifications, dimensions, shapes, and performance requirements of rubber products helps choose suitable additives. Proper formulation can control the rubber’s hardness, strength, aging resistance, coloration, wear resistance, and processability, enhancing product quality and reducing costs.

Mold:

Mold design and manufacturing directly affect product quality. Precision and consistency in molds ensure the uniformity and stability of rubber products. During early mold development, reference the product prototypes and select appropriate molding processes. Familiarize yourself with the product’s specifications, dimensions, shapes, and performance requirements to ensure mold design meets production needs. Mold design should consider molding process consistency to avoid defects like localized bubbles after vulcanization.

Vulcanization:

Vulcanization is a critical step in producing compression-molded rubber parts. Adding vulcanizing agents (like sulfur) and other additives (such as accelerators) under specific temperature, pressure, and time conditions causes rubber molecules to cross-link and form a three-dimensional network. Different types of rubber have varying vulcanization temperature and pressure requirements. The vulcanization process typically includes four stages: induction, pre-vulcanization, optimal vulcanization, and over-vulcanization. During this process, rubber transitions from a plastic to an elastic state, significantly improving its physical and mechanical properties, such as strength, aging resistance, and temperature range.

Vulcanization Temperature:

Examples：

  • Natural Rubber (NR) vulcanizes at 150–160°C, not exceeding this range.
  • Synthetic rubbers like Butyl Rubber (IIR) and Ethylene Propylene Rubber (EPM) vulcanize at 160–180°C, not exceeding 200°C.
  • Silicone Rubber (SiR) and Fluororubber (FR) vulcanize at 170–180°C, with secondary vulcanization at 200–230°C if needed.

Vulcanization Time:

Vulcanization time varies with the thickness of rubber products. It is determined by performance testing, combined with equipment and process conditions, then confirmed using a rheometer or other experimental methods to establish the optimal vulcanization time, flat curing time range, and temperature coefficient.

Vulcanizing Agents:

Different vulcanizing agents impact rubber properties, including vulcanization effectiveness, hardness, aging resistance, and fatigue resistance. For instance, in flame-retardant silicone rubber, increasing vulcanizing agent reduces tear elongation but increases hardness. The vulcanization system choice also affects aging resistance, such as varying performance in fluororubber under different systems.

Equipment and Process Parameter Optimization:

Precise setting and strict control of temperature, pressure, speed, and other technical parameters during production are crucial for product quality. Optimal process parameters for different equipment, raw materials, and products require extensive practical experience.

Rubber Extrusion

Rubber extrusion involves heating and plasticizing rubber materials using a screw or plunger and then extruding them into desired shapes through a die. The material is heated and plasticized in the extruder, pushed forward by the screw or plunger, and shaped into semi-finished products using forming dies.

1.Extrusion

Rubber extrusion is primarily used for producing various cross-sectional shapes, such as inner tubes, treads, hoses, etc., especially suitable for high-precision sealing products. It offers advantages such as a small equipment footprint, simple operation, and rapid continuous production.

Rubber Blow Molding

Rubber blow molding involves placing pre-formed rubber blanks into a mold and inflating them with compressed air to expand and cool into the final shape. This process includes intermittent and continuous extrusion methods. Controlling blow pressure and cooling time results in products with uniform wall thickness. The process is highly automated, suitable for mass production, but products can only be single-colored.

Rubber Blow Molding

Used for manufacturing various products such as bottles and inner tubes.

In summary, compression molding is mainly used for large and simple-shaped rubber products, transfer molding for complex and high-quality products, and injection molding for small, complex, high-quantity, high-precision, and intricate rubber products. Rubber extrusion is mainly for continuous products with constant cross-sections, while blow molding is used for producing hollow plastic products with good transparency, sealability, light weight, and uniform wall thickness. Each method has its unique advantages and applicable scenarios. The appropriate molding method is chosen based on specific product requirements and production conditions.

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