Injection moulding - for rubber and silicone components

1 What is injection moulding?

Injection moulding is an established process for the production of dimensionally accurate and reproducible rubber or plastic components. The raw material is plasticised, injected into a mould under pressure and cured or vulcanised there.

With most plastics and thermoplastic elastomers (TPEs), the raw material is plasticised by melting and cooled in the mould. The rubber raw material is only preheated, plasticised by the screw movement and vulcanised after injection in the mould by means of targeted temperature control.

In an industrial context, it is used in particular for the series production of technical moulded parts with complex geometries and tight tolerances. As a manufacturer of moulded rubber parts with more than 35 years of experience, we know that it is particularly convincing compared to the two other common discrete processes for the production of moulded rubber parts, compression moulding and transfer moulding, especially when it comes to high quantities due to its short cycle times and efficient operation thanks to a high degree of automation. Injection moulding also excels in terms of dimensional accuracy and reproducibility.

Design and function of injection moulding machines and moulds

At the heart of every injection moulding production are two centrally linked systems: the injection moulding machine and the mould. Both must be precisely coordinated in order to meet the requirements for precision, cycle time and product quality - especially in rubber processing, where thermal and rheological characteristics play a central role. As a rubber moulded parts manufacturer with our own in-house mould construction, we have the expertise to combine these factors in a solution-oriented manner.

2. injection moulding machines

An injection moulding machine is usually designed as a screw-type machine, in which the raw material is plasticised, homogenised and conveyed via a rotating screw. The screw also moves along its longitudinal axis: during plasticising, the screw moves slowly away from the nozzle opening and is then guided quickly and under pressure to the nozzle opening, whereby the plasticised material is injected into the mould.

In rare cases, the plunger-type machine version can also be used, in which a plunger is used instead of a screw, e.g. to build up higher pressures. However, this is the exception due to the poorer homogenisation and temperature control.

Injection moulding machines for processing elastomers often have a combined screw and injection plunger design. The screw is responsible for plasticising and dosing the rubber compound, while a downstream plunger injects the preheated material into the mould at high pressure. This principle combines the good homogenisation of the screw with the precise dosing and injection control of the plunger and has proven itself in elastomer processing in particular.

Figure 1: Schematic representation of the injection moulding process with screw and raw material feed as granulate, with open (top) and closed (bottom) mould.

In contrast to the schematic diagram in Figure 1, the raw material feed for rubber injection moulding is not usually in the form of granulate, but rather by feeding in a rubber strand. This is plasticised directly by the screw without intermediate granulation.

Depending on the arrangement of the components, injection moulding machines are also divided into horizontal and vertical machines. The differences in design and the respective advantages and disadvantages are compared in Table 1.

Table 1: Differences between horizontal and vertical machines in injection moulding.

A special form of injection moulding is 2K injection moulding (two-component) or multi-component injection moulding. Different materials are usually injected into the mould in sequential order. Injection moulding machines with several screws are generally used for this, which are usually designed to swivel in order to inject into the same sprue. Alternatively, the two screws can also inject into different sprues. In very rare cases, only a single screw with alternating raw material feed is used, but this results in components that can only be controlled to a limited extent and run into each other.

Moulding tool

The moulding tool typically consists of two halves. A distinction is usually made between the nozzle and ejector side or the fixed and moving side. In horizontal injection moulding, the nozzle side where the plasticising unit attaches is the fixed side. In vertical injection moulding, on the other hand, the ejector side is usually the fixed side. The mould halves usually have the following elements:

• Duct or distribution system: Feeds the plasticised raw material into the cavities under pressure.
• Cavities (also mould cavities): Define the geometry and surface of the part to be produced as a negative mould.
• Ventilation ducts: Conducts trapped air and gases out of the mould.
• Heating or cooling systemDepending on the material being processed, heating elements (e.g. for temperature control during vulcanisation of rubbers) and/or cooling channels (e.g. for rapid cooling of TPE) are usually integrated.
• Centring and guide bushes: Ensure precise closing position and repeat accuracy.
• EjectorFacilitate the removal of components after opening the mould.
• VacuumAn integrated vacuum ensures that the cavities are filled largely free of air pockets. The controlled evacuation of the mould halves before injection prevents the formation of air pockets or burns, which significantly improves the surface quality and dimensional accuracy of the moulded parts.

The removal of components during the injection moulding of rubber is much more difficult than with conventional plastic, as it can be elastic depending on the material and there can be significant adhesion to the mould surface. If ejector systems are not economical or would damage sensitive components, removal can be automated using brush systems, compressed air or robotics with gripper systems.

The runner system of the moulds themselves can, but does not have to be tempered separately from the mould. A distinction is made between hot runner and cold runner systems. While a hot runner system ensures a consistently high temperature for TPEs so that the material remains molten and therefore flowable, a high temperature would lead to vulcanisation in the runner system for rubbers. Accordingly, rubbers and, in particular, liquid silicone rubber (LSR) are cooled and processed using cold runner systems.

As injection moulding is usually used for higher quantities than compression or transfer moulding, particular attention is paid to the service life, i.e. the period of time during which the mould can be used before it has to be replaced or repaired due to wear or damage. The abrasive fillers in elastomers, e.g. carbon blacks, metal oxides or glass fibres, further increase this focus. To minimise wear, wear-resistant coatings such as DLC (diamond-like carbon) or hardened tool steels are therefore often used.

In addition, the mould change, i.e. the installation and removal of the mould in the injection moulding machine, is usually more complex as part of the set-up process than with compression or transfer moulding. Accordingly, attention is also paid to making mould changes as simple as possible, e.g. by using quick-clamping systems or modular clamping plates, in order to minimise set-up times and ensure a high OEE (Overall Equipment Effectiveness).

Step-by-step process of injection moulding

The injection moulding process for the production of moulded rubber parts is a precisely controllable, multi-stage production process in which the control variables must be precisely coordinated in order to achieve consistent component quality while using resources economically.

1. feeding of the raw material
Depending on the form in which the raw material is available, it is fed in differently:

• Granulate (e.g. TPE): Insertion into the hopper, either manually or automatically via a pipe system
• Strand material (e.g. for most rubbers): The strand is usually guided by a roller system, sometimes with its own drive, to ensure even feeding
• Liquids (e.g. LSR): injection via pump system from the storage containers, usually drums

2. plasticisation
The raw material is plasticised and homogenised in the plasticising unit, usually designed with a screw, by applying temperature and shear stress. In the case of rubber or LSR, little or no temperature is applied to prevent premature vulcanisation.

• Closing the mould
  The parts of the mould are friction-locked - usually servo-hydraulically or mechanically.
• Injection
  The plasticised material is injected into the closed mould under high pressure. The material is fed to the cavities via the runner system. The opening at the transition from the channel system to the cavity is also known as the gate.
• Pressing and vulcanising
  After injection, the pressure is maintained in order to prevent shrinkage and air pockets in the cavities. At the same time, vulcanisation takes place in this step through targeted temperature control. The dwell time in the mould, i.e. the vulcanisation time, depends on:
  1. Material and cross-linking system (peroxide, sulphur, etc.)
  2. Wall thickness of the component

Typical cycle times are between 2 and 15 minutes, but can be significantly longer for thick-walled or multi-layered parts. TPE is not vulcanised, as the elasticity in this case is not created by the wide-meshed cross-linking.

Cooling (TPE only)
Cooling takes place exclusively with thermoplastic elastomers (TPE), as these are not vulcanised, unlike rubber and LSR materials, but solidified by cooling. The component is then cooled in a closed mould, usually by cooling systems integrated into the mould. The temperature to which the workpiece must be cooled before removal depends on the material. While rubber parts made of rubber or LSR can be removed much warmer, TPE must be cooled down sufficiently before removal, as it loses its dimensional stability at higher temperatures.

Opening, demoulding and post-processing
Once vulcanisation is complete, the mould is opened. Demoulding is carried out manually, mechanically using an ejector or with a compressed air brush. This is often followed by reworking:

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

Critical process parameters - control of component quality

Optimum component quality can only be achieved by precisely coordinating a large number of process parameters. The most important parameters in injection moulding are

• Temperature - both in the plasticising unit and in the mould and its exact change (heating and cooling) throughout the process
• Pressure - both when injecting and when pressurising
• Time - both during injection, vulcanisation and cooling
• Reduce differences in wall thickness as far as possible

Process simulations are increasingly being used to test possible mould geometries and parameter settings before mould production and to minimise iterations in production, especially for demanding components or materials such as FKM or CR. Tool design, gate position, venting geometries and cycle times can be optimised with the help of filling studies, vulcanisation predictions and warpage simulations.

Process stability is ensured by modern process control systems that monitor and control parameters such as mould temperature, injection pressure, injection time and holding pressure curves in real time.

3. economic key figures in the injection 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 injection 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.

Materials in rubber injection moulding

The choice of the right material is a decisive factor in the injection moulding of rubber parts, not only for the component itself, but also for its processability. Three groups of materials in particular are used in the injection moulding of rubber elastomers:

• Thermoplastic elastomers (TPE): Plastics that are flexible and elastic like rubber at room temperature, but can be moulded and repeatedly processed like thermoplastics when heated. They are usually supplied as granules and the raw material is melted in the plasticising unit as in classic plastic injection moulding, thereby liquefying it.
• Rubbers including solid silicone rubbers: During injection moulding, rubbers must be processed in the cavities at a reduced temperature, typically around 40-80°C, before vulcanisation in order to prevent premature vulcanisation. Depending on their flow behaviour, viscosity or thermal stability, the various rubbers are differently suitable for processing in the injection moulding process:
  • Well suited: EPDM, NBR, SBR, VMQ, CR, FKM
  • Less suitable: NR, BR, HNBR, IR, IIR

The feed is usually in the form of strand material.

• Liquid silicone (LSR, Liquid Silicone Rubber): The material is already available in liquid form as a two-component raw material, which is homogeneously mixed from A and B components in a dosing and mixing unit before injection. There is no plasticising in the screw unit, only conveying and mixing before the material enters the heated moulding chamber where it is cross-linked.

For a rubber compound to be suitable for the injection moulding process, it must meet specific Processing properties fulfil: A defined Viscosity curve with moderate heating is necessary to completely fill complex geometries without creating air pockets or flow disturbances. At the same time, the Vulcanisation kinetics must be coordinated in such a way that complete cross-linking takes place within the specified cycle: neither too early (blockage in the channel system) nor too late (rejects due to under-cross-linking).

4 When is injection moulding the right choice?

Injection 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.

Injection moulding vs. transfer moulding

In transfer moulding, the rubber raw material is inserted or injected into a separate chamber of the mould, the so-called transfer chamber, and pressed into the mould cavity via a channel system. This process is used in particular for medium quantities, for the production of rubber composite parts and for materials that are not or only to a limited extent suitable for injection moulding. However, the cycle time is usually longer than with injection moulding and the degree of automation is limited.

The Injection moulding on the other hand, offers advantages especially in the Series production:

• Higher process speed through automated cycle sequences and parallel pre-plasticising
• Uniform mould filling, even with complex geometries or multi-cavity moulds
• Reduced manual handling, which allows integration into interlinked production lines
• Less material loss, as no raw material residues remain in the transfer chamber
• Reduction of seamability (low burr, burr-free): Thanks to the high dimensional accuracy and reproducibility of the injection moulding process, manual post-processing steps such as deburring or trimming are largely eliminated in series production.

Injection moulding vs. compression moulding

Compression moulding is the simplest process: A rubber blank is inserted directly into the cavity and the mould is then closed under pressure and heat. This method is cost-effective in terms of mould construction, but is slow in the cycle and involves a great deal of manual effort. It is particularly suitable for

• Prototype construction
• Small series
• Simpler geometries with low dimensional requirements

Injection moulding is generally only economically viable for medium to large series. The higher expenditure on mould technology and machine technology is compensated for by productivity, reproducibility and automation.

Detailed technology comparison

Advantages and disadvantages of injection moulding at a glance

Advantages:

• Short cycle times thanks to automated cycle sequences and parallel pre-plasticising
• Efficient operation thanks to a high degree of automation
• Very good dimensional accuracy and reproducibility, even with complex components
• Very little material loss, especially when using a cold runner system

Disadvantages:

• Processable material variety more limited than with compression moulding
• Comparatively high tool costs
• Difficult handling of inserts, especially during horizontal injection moulding

5. conclusion

Injection moulding is a standard process in the manufacture of moulded rubber parts. It is the right strategic choice especially for

• large series where short cycle times and high efficiency are important
• particularly high demands on dimensional accuracy and reproducibility
• Materials that generally cannot be processed during compression moulding, such as TPE or LSR.