Natural and synthetic rubbers can be moulded by various methods – including casting and injection moulding – but the great majority of true rubber components are made by compression or transfer moulding.

The choice of true rubbers – for their greater elasticity, chemical resilience and wear properties – leads down the more traditional rubber moulding technology pathways.

Rubber compression moulding

Compression moulding is the oldest method of manufacturing simple rubber parts. It differs from OEM injection moulding in that the material is introduced by hand, roughly shaped to the tool and with an excess that will be squeezed out and wasted as the tool closes(hence compression moulding).

The process places an excess amount of uncured rubber between two simple tool parts that together represent the cavity that is to be filled to make the required component. The rubber is presented for moulding in a soft and uncured state, and as the tool closes, it is heated which induces curing or vulcanisation.

When is compression moulding the right choice

If your requirement is for a moderate precision, tough, flexible part with a long service life and fast concept to mass production times then compression moulding is likely the right technology choice.

Because of the simplicity of the two part tools, and the in-tool curing process (generally by heating) time-to-product can be very fast and production rates can also be high. Multi cavity tooling allows each tool cycle to mould many parts (depending on the tool size and machine compression capacity.

Key factors that must be considered in choosing the suitability of this production process relate to the cosmetic requirements of the part, its complexity and minimum wall sections and the tolerance for flash. Compression moulding tends to make parts that don’t look great and have only moderate tolerances, so critical dimensions tend to limit the applicability of the process. Compression mouldings are usually manufactured to standards listed here;

Rubber Manufacturers Association, RMA-A2 

RMA Class A O-Ring tolerances

ISO3601-1 O-ring tolerances

ISO 3601-3 Grade N and Grade S tolerances

Compression moulds are capable of producing the widest range of part sizes. Maximum measurements can range from a few millimetres up to a metre in the largest dimension – though thicknesses are limited to 15-25 mm depending on the material choices and the supplier skills.

Many product areas benefit from compression moulding, as it produces the easiest production setup, low cost tooling and moderate to low cost parts – where tolerance requirements are relatively low.

Overall, compression moulding is unsuitable for very high production rates, as cure times can be quite extended, post cure needing up to several hours for completion.

The compression moulding process steps

With the tool open – i.e. the moulding ram retracted along with the upper tooling plate – the tooling is ready for use. Generally, the lower plate will be pre-heated so that the curing process can be completed as quickly as possible.

Uncured rubber compound is hand shaped to fit the cavity – prepared in advance, or while the previous mouldings are curing. This uncured rubber shape is referred to as a preform – ready to be moulded. Each cavity or part will have a different requirement of preform, and this relies on operator skill to get filled cavities (complete parts) with only moderate wastage. Too much material is wasteful and can cause flash to become too thick, while too little material can cause voids in the part.

The preform is placed into the cavity of the pre-heated mould tools lower plate. The mould is then closed, using a hydraulic ram which applies a large force in the closing. Heat and pressure are applied in a very simple compression moulding press which requires little operator skill, costs very little to buy and operate and can have a very long operational life – often measured in decades. Better equipped suppliers use up to date presses driven by a programmable logic controller (PLC) to monitor and control critical parameters like temperature, pressure and time to ensure moulding takes place within predetermined limits. This stage can be long – several minutes – to ensure the part is sufficiently cured to safely be handled without damage.

The mould tool is then opened, or opens automatically, and the cured or vulcanised rubber part is removed along with its flash. At this point the part or parts are cured but they will be attached to a sheet of waste material that was squeezed out of the tool as it was compressed.

The part then undergoes post-moulding processing consisting of deflashing and rest time in a curing oven to ensure that the vulcanization process has completed.

Rubber transfer moulding

The transfer moulding process

Where transfer moulding differs from compression moulding is that the rubber is not placed directly into the cavity of the mould then compressed – it is placed in a small cavity in an intermediate tooling plate and then forced through a gate and into the tooling cavity by a piston that is part of the top plate. This compresses the rubber preform and injects it into the mould cavity.

Some transfer mould tooling requires a preform for each cavity and a very directOEM injection moulding, whereas a galleried sprue system allows the tool to be loaded with a single preform that will fill many cavities.

Advantages of transfer moulding

Transfer moulding provides tighter control of dimensional tolerances than compression moulding – and allows for thinner sections and greater precision/detail in the part design.

Although some transfer mould tooling produces flash, flash-less tooling is increasingly common. This requires a moulding setup that applies compression of the two cavity parts before and during the transfer process, so the tool faces are tightly closed. Flash-less tooling is preferable in a transfer process, as post processing is reduced. If transfer moulding produces flash, manual, punch tool or cryogenic trimming for deflashing can be used to remove it.

This process for moulding rubber components makes it well suited for moulding more intricately  shaped parts and for securing inserts that are embedded in a product – which must be manually pre-loaded into the tooling cavities before moulding.

High Cavity Count. In many cases, transfer moulded rubber products require fewer and simpler pre-forms than compression  moulded parts – where pre-form can potentially fill hundreds of cavities. This can save a significant amount of labour in the moulding process and results in very high material utilisation, as galleries are very limited and the entire pre-form is transferred in the operation of the tool.

Design Flexibility allows for sharper edges and smaller features. Micro vents reduce the need for overflow material, allowing for almost flash-less parts.

With standardized ‘pot and plunger’ tool design, simplified preforms allow for standardisation and lower cost – where most aspects of the tool design are preconfigured and only the cavities must be unique, the processes of tool design and tool production can be very fast and be largely completed in pre-made platen parts.

Short production cycles and shorter cycle times than compression moulding are advantages that can compensate for more expensive tooling and higher equipment costs.

Transfer moulding can provide more consistency than transfer moulding, as the cavity is pre-closed and the tool plate separation is not controlled by flash.

Disadvantages of transfer moulding

Relatively complex moulds cost more and can take longer to produce – especially where parts do not lend themselves to pre-existing and ready made cavity plates.

Waste material can be variable, as transfer pots (when excessively loaded) can produce higher volume waste than traditional overflows in compression tools. Transfer moulding typically produces a large pad with sprues – and this pad can vary from near zero thickness (in a well managed process) to considerable height. Scrap and trimming is not reusable, since the rubber is cured in processing.

Mould maintenance is more intensive than for compression mould tools. Typically, inserts must be removed and reset to maintain free movement over time and accommodate wear. Cleaning the tool between cycles can be time consuming, though special equipment such as dry ice blasters can speed up the intricate transfer insert clearing process.

Materials options for compression and transfer moulding

Neoprene (Chloroprene)- good weathering resistance, flame retardant, moderate resistance to petroleum-based fluids.

EPDM (Ethylene-propylene) -excellent ozone, chemical, and ageing resistance, poor resistance to petroleum-based fluids.

Buna-N (Nitrile-butadiene) – excellent resistance to petroleum-based fluids, good physical properties.

Silicone (Polysiloxane) – excellent high and low temperature properties, moderate physical properties.

SBR (Styrene-butadiene) – good physical properties and abrasion resistance, poor resistance to petroleum-based fluids.

Butyl (Isobutene-isoprene) – very good weathering resistance, excellent dielectric properties, low permeability to air, good physical properties, poor resistance to petroleum-based fluids.

Hypalon (Chloro-sulfonyl-polyethylene) – excellent ozone, weathering and acid resistance, good abrasion and heat resistance, fair resistance to petroleum-based fluids.

Urethane (Polyethylene-apdate, Poly(oxy-1, 4, butylene) ether) – good ageing and excellent abrasion, tear and solvent resistance, oor high temperature properties.

Viton, fluoro-elastomer (Hexaflouropropylene- vinylidene fluoride) – excellent oil and air resistance both at low and high temperatures, very good chemical resistance.

Fluoro-silicone (Fluorocarbon) – superior heat and cold resistance, resistant to oils and solvents of fluorinated rubber, good for applications where general resistance to oxidising chemicals, aromatic and chlorinated solvent bases is required, narrower temperature range than silicone but better fluid resistance

Compression and transfer moulding summary

Whatever your engineering and product component requirement, if you need moderate to high volumes, high resilience and elasticity, high chemical stability and low cost, then an OEM synthetic rubber compression or transfer moulding may be the first and lowest cost options to serve.

Care is needed in the selection of materials and processes, to ensure the best possible component outcome at a budget and to a schedule that meets the wider needs of the product manufacture. The range of options in materials, processes, tooling methods and supplier types is a complex Venn diagram, but the narrowing down of options is not as difficult as it might first seem – one property or process characteristic will be of overwhelming importance and is liable to be the main driver of selection.