Compression moulding of rubber moulded parts

1 What is compression moulding?

Compression moulds. Compression Moulding), also known as compression moulding. Pressure Moulding) is an industrial manufacturing process for the production of moulded rubber parts in which a rubber blank is placed in a heated mould, pressed into its final geometry under pressure and then vulcanised. The component geometry is completely defined by the mould, while pressure, temperature and pressing time control the curing process.

The process is one of the oldest and most robust moulding processes for elastomers and is preferably used for small to medium quantities, for rubber composite parts and for highly viscous materials that are only suitable for injection moulding to a limited extent. It is characterised by simple mould concepts, low set-up costs and a wide variety of materials.

The common alternatives to compression moulding are the closely related transfer moulding and injection moulding.

Design and function of compression moulding tools

As a shaper and reaction chamber for vulcanisation, the mould performs several critical functions: It moulds, guides, transfers pressure and temperature - and ensures the reliable production of even complex rubber parts.

Compression tools are generally made from high-strength, temperature and pressure-resistant tool steel. Heat-resistant, hardened or unhardened steels with good thermal conductivity are often used. The thermal conductivity of the material is just as important as its hardness, as this is the only way to realise uniform temperature profiles and replicable processes. Aluminium tools are also used. These are suitable for small quantities, complex geometries and materials with good flowability. Aluminium or copper alloys are often used as heat carriers and conductors for the heating and cooling plates, supplemented by electric or steam-heated systems.

As a rubber moulded parts manufacturer with our own tool shop, we know that a tool for compression moulding must look like this:

• Upper and lower mould (mould halves): Contain the negatives of the moulded part.
• Heating and cooling plates: Heat the mould halves to the desired vulcanisation temperature (usually 160-200 °C) and are thermally insulated from the pressure plates. Cooling channels are often integrated directly into the mould halves instead of separate plates.

Figure 1: Schematic representation of compression pressing with the mould open (left) and during pressing (right).

The tools themselves generally have the following elements:

• Cavities (also mould cavities): Define the geometry and surface of the part to be produced as a negative mould.
• Venting channels: Guide trapped air and gases out of the mould.
• Centring and guide bushes: Ensure precise closing position and repeat accuracy.
• Ejector: Facilitates the removal of components after opening the mould.
• Overflow channels or defined tear-off edges

Even with a specified component geometry, tool design and production is not only a cost factor, but above all a quality-determining element of the overall process, among other things through:

• Mould parting: The choice of parting line not only has an influence on the production process, e.g. the removability of the components or the effort required for deburring, but also on the quality of the components, e.g. due to changed flow paths.
• Geometry of the cavities: This must be selected and manufactured precisely, but does not necessarily correspond exactly to the component negative if, for example, shrinkage has to be taken into account.
• Surface quality of the cavities: The surface quality of the cavities is the key factor influencing the surface quality of the finished component. While milled mould surfaces are often sufficient for simple components, erosion is predominantly used as a manufacturing process, particularly for visible parts.
• Positioning of inserts: In the case of composite parts, the inserts, i.e. the non-rubber components, must be precisely fixed but also easy to insert. The type of positioning has a correspondingly large influence on process reliability and costs.

Particularly in the case of complicated geometries or large quantities, it can be useful to adapt the component design for the production process, for example to avoid undercuts or to facilitate demouldability by means of demoulding chamfers.

Step-by-step compression moulding process

a) Inserting the rubber blank
The process begins with the insertion of the prepared rubber into the opened mould. This so-called blank is usually available as:

• Cuts: Tape or sheet material pre-cut to the correct quantity
• Preforms: Material already geometrically pre-adjusted for optimum filling

The aim is to achieve a stress-free, symmetrical insert for even material distribution. In the case of rubber composite parts, the metal, plastic or fabric insert, which is usually pre-treated with primer, is also inserted in this step.

b) Closing the mould and initiating the pressing phase
After insertion, the press is closed with a defined Clamping force. Depending on the component size and material, this is typically in the range of 20 to 200 tonnes. The contact pressure ensures that the rubber is pressed into the cavities. It is important to ensure that the pressure is evenly distributed to avoid material build-up and air pockets.

c) Temperature control and vulcanisation
The mould halves are heated via separate heating plates or integrated heating cartridges, steam or oil, depending on the material and cross-linking system. 120-220 °C depending on the material. The dwell time in the mould, i.e. the Vulcanisation time, is based on:

• Material and cross-linking system (peroxide, sulphur, etc.)
• Wall thickness of the component
• Mould design (mass ratios, thermal inertia)

Typical cycle times are between 2 and 15 minutes, However, this can be significantly higher for thick-walled or multi-layered parts. Cross-linking turns the rubber into a rubber/elastomer.

d) Opening, demoulding and reworking
Once vulcanisation is complete, the mould is opened. Demoulding is carried out manually, mechanically via ejectors or using compressed air. This is usually followed by post-mould processing:

• Removal of burrs
  • Thermal e.g. freeze deburring
  • Manually, e.g. with scissors/scalpel/etc.
  • Mechanical, e.g. barrel finishing
• Coating, e.g. talcum coating
• Visual inspection and, if necessary, dimensional check and hardness test

Critical process parameters - control of component quality

Three central physical process parameters directly influence the result:

• TemperatureToo low temperatures lead to under-vulcanisation (sticky surfaces, reduced strength), too high temperatures lead to over-vulcanisation (embrittlement, dimensional distortion).
• Print and materialToo low pressure/too little material causes air pockets or filling errors, too high pressure/too much material can damage the mould or lead to increased burr formation.
• TimeThe holding time must be sufficient for complete cross-linking without unnecessarily extending the cycle time. Under-crosslinked parts are mechanically unstable and cannot be used.

These parameters have an interdependent effect: an increase in temperature, for example, can compensate for a shorter time, but only within the material-specific permissible range. Accordingly, precise process control with validated recipes and stored pressing cycles is essential, especially for quality-critical applications.

2. economic key figures in the compression moulding process

In addition to component quality, economic aspects are also decisive for the evaluation of a compression moulding process. Typical key figures are

• Cycle time [s or min]directly dependent on heating phase, vulcanisation time and demoulding
• Mould utilisation factor [%]Ratio of cavities to total mould area
• Reject rate [%]Defective parts per batch
• Material yield [%]Proportion of the raw material that remains in the good part
• Burr content [g/part]Indicator for material loss and reworking costs
• Set-up time [min/lot]Effort for mould change and start-up

The aim is to realise high repeat accuracy with minimum rejects through process-stable parameter control and precise tool technology.

3. materials for compression moulding: Which elastomers are suitable and why?

Compression moulding is suitable for processing almost all elastomers. The only exceptions are thermoplastic elastomers (TPE), particularly low-viscosity elastomers such as liquid silicone (LSR) or elastomers with particularly short pot lives. These are usually processed in the injection moulding process.

Compression moulding is particularly suitable for materials with high viscosity, long vulcanisation times or for compounds that cannot be processed by injection moulding. Common elastomers for compression moulding are

Common elastomers for compression moulding

• NR (natural rubber)
  Good mechanical properties, high rebound resilience and abrasion resistance. Used in dynamic applications, such as vibration elements or dampers.
• NBR (acrylonitrile butadiene rubber)
  Resistant to oil, grease and fuel. Universally applicable in seals, diaphragms and hoses in hydraulic systems.
• HNBR (Hydrogenated NBR)
  Combination of chemical resistance and high mechanical strength. Used in demanding sealing and bearing applications.
• EPDM (ethylene propylene diene rubber)
  Excellent resistance to weathering, ozone and ageing. Particularly suitable for outdoor, construction and automotive applications.
• VMQ / FVMQ (silicone / fluorosilicone)
  High thermal stability, flexibility at low temperatures, excellent insulating properties. Ideal for medical and food technology as well as applications with extreme temperature gradients.
• CR (chloroprene rubber, e.g. neoprene)
  Flame retardant, resistant to ageing, good adhesion to metals - frequently used in rubber-metal bonds.

Influence of material properties on the moulding process

The choice of material not only determines the properties of the end product, but also has a significant influence on the moulding process:

• Viscosity of the mixture influences the flow behaviour - highly viscous materials require higher closing forces.
• Reactivity of the cross-linking system (e.g. peroxide vs. sulphur) influences the cycle time and temperature control.
• Thermal behaviour has an effect on demoulding and shrinkage.

The combination of material, moulded part geometry, tool design and process parameters must therefore be systematically coordinated - especially in regulated industries where validation and component approvals are based on exact material parameters.

Material selection as a strategic lever

The right choice of material is not only technically decisive, but also economically relevant. Factors such as material price per kg, reject rate, reworking costs and shelf life have a direct impact on unit costs and process stability. For this reason, material optimisation is often used in practice, for example through:

• Substitution of expensive fluoroelastomers with optimised HNBR or EPDM compounds
• Use of multi-component joints (hard/soft composite parts)
• Adaptation of the recipe to specific cavity geometries or sizes

4 When is compression moulding the right choice?

Compression moulding is just one of several established processes for manufacturing technical rubber components. The three most common discrete processes are compression moulding, transfer moulding and injection moulding. Each of these processes follows its own principles, requires specific moulds and offers different advantages depending on the component geometry, quantity, material properties and economic context. The most common continuous process, however, is extrusion.

Compression and transfer compression moulding are closely related. In contrast to compression moulding, in transfer moulding the raw material is placed in a separate chamber and from there pressed into the cavities via channels.

In injection moulding, on the other hand, the material is plasticised and injected directly into the mould under high pressure. This makes it ideal for large quantities, but is associated with high mould costs and is only suitable for a limited range of elastomers.

Detailed technology comparison

Advantages and disadvantages of compression moulding at a glance

Advantages:

• Lower tool costs
• Wide range of materials can be processed
• Well suited for thick-walled components
• Very suitable for rubber composite parts (e.g. rubber-metal connections)
• Easy to convert when changing parts

Disadvantages:

• Greater manual effort for material insertion and removal
• Limited cycle time optimisation

5. conclusion

Compression moulding is a standard process in rubber production. It is the right strategic choice especially for

• small to medium quantities,
• Rubber composite parts and
• Materials that are not suitable or only suitable to a limited extent for injection moulding.

It offers robust process reliability with comparatively low initial mould costs.