OEM plastic molding is the main process to obtain plastic components or plastic parts. As plastic molding could be high efficient, cost effective, and high output. Nearly every products consist of plastic parts. This is why OEM plastic molding services become so common. OEM plastic molding includes plastic injection molding, blowing molding, rubber molding, and so on. Here we’d like to introduce them in details below.
OEM injection moulding
No doubt you’re reading this article because you need your plastic or rubber components mass produced for you and you’d like to understand the opinions and pitfalls in getting these parts OEM injection moulded.
Injection moulding suppliers take your component designs and outsource or build the tooling to make them – or less commonly take your existing tools from your previous supplier – and use these tools to mass produce your parts. It’s a highly specialist service, with large machine, space and staff-skill overheads and it is one of the last capabilities that OBM (Original Brand Manufacturers) such as yourself should look to take in-house.
Tool design and manufacture often originate with standalone OEM injection moulding suppliers, as the nature of the specialisation does not always sit well in a busy plastics factory – the overhead requirements are large, so it needs a VERY big factory to support a fully equipped and fully capable toolmaking service. Moulding company in-house tooling services are often good, rarely exceptional. We will not address the tooling development and manufacturing process further, here.
Plastic and rubber OEM injection moulding is the process whereby components or products are pressure-cast by injecting molten polymer into a custom shaped cavity, such that when the injected material cools, it will faithfully reproduce that cavity.
The OEM injection moulding sector
The world market for OEM injection moulded thermoplastic components is more than US$250 billion. OEM injection moulding rubber parts make a small fraction of this. The expected CAGR of the sector for the next decade is at least 2.6 percent. Expanding demand comes from all sectors – both for long term use and single (packaging) use. An increasing range of engineering polymers and additive modifiers is widening the range of applications in which OEM injection moulding serves.
The majority of polymer materials processed in this sector are derived from crude oil and natural gas feedstock. They generally offer extreme environmental stability – though many are vulnerable to UV degradation. Virtually none of the millions of tonnes of polymer materials disposed of worldwide are recycled. In reality, most polymers cannot be re-used for high quality product, even under ideal conditions.
There has been a shift in recent years towards organic feedstocks for plastics made for single use applications – through materials such as PLA (PolyLacticAcid) made from corn starch to replace PET (PolyEthyleneTerephthalate). These offer poorer properties and higher pricing than the materials they replace, and they consume food in their manufacture – uptake has been limited. The trend forOEM injection moulding moulded rubber is the opposite – away from natural feedstocks.
Widespread use of plastics is creating environmental and regulatory difficulties that will eventually be addressed – and these materials are not going away, as they solve the widest range of engineering, product, food, transport and medical problems easily and at low cost.
What is OEM injection moulding
This article mostly relates to the moulding of rigid and flexible thermoplastics – materials that can remelt when heated. The same processes are used for some synthetic rubbers. Very similar processes are also rigid thermosetting polymers, glass, metal (die-casting etc.), foodstuffs, cleaning products and water soluble functional chemistry
Injection moulding involves pushing liquid polymer into a steel cavity within a tool or die. It offers many significant advantages over other processes and material families;
Volume production. Fastest processing to finished components ensures that the process is the key to mass production. Cycle times of seconds (for small parts) and a few minutes for the largest mouldings, and the ability (with multi-cavity tooling) to ‘shoot’ multiple parts at the same time make this the ideal manufacturing method.
High material utilisation. Extractive processes, such as CNC machining, cut away material from a blank which results in significant (often 90%) wastage. Moulding achieves 80% utilisation at the worst – and for high volume applications it is to achieve 95% utilisation (using hot-runners).
Complexity of parts. Intricate parts can be produced ready-to-use without secondary processes, reducing cost compared with any other process. There are few limits to the complexity of designs, allowing integration of functions and potential parts into a single component – including integrated springing, fixings and multiple materials (by co-mouding and over-moulding).
Low part costs. The most extreme engineering polymers are still priced below US$30/kg – and many materials cost ~$10/kg. Single use plastics and synthetic rubbers are low (initial purchase) cost.
Consistency/repeatability. The automated repeatability of the process results in the first part being indistinguishable from the millionth (assuming tool maintenance and operation are of high quality).
Where is OEM injection moulding used
The adaptability of the process and the wide range of materials results in a huge range of sectors and markets being heavy users of OEM injection moulded parts;
Consumer products. The majority of consumer devices use plastics. High cosmetic standards, unlimited complexity, resilience, chemical/stress/dimensional stability and hygiene are all make OEM injection moulded parts the solution in domestic and consumer products.
Pharmaceutical and medical devices. Providing good content protection, precision and flexibility, plastics are optimal for packaging, disposable/consumable treatment tools, instruments and equipment, surgical and implant tools etc.
Automotive and aircraft parts. The transport sectors get lightweight, durable components tha serve in drive train, fluid handling, electrical and safety roles. Plastics are suited to vehicle interiors and passenger contact, providing hard wearing and cleanable surfaces.
Food packaging.The hygiene, cost and functional advantages in food handling and packaging are impossible to achieve by other means.
The OEM injection moulding process
Once a tool has been produced and validated, moulding parameters such as temperatures, pressures, dwell times etc have been assessed, OEM injection moulding can be divided into five steps:
Clamping. An OEM injection moulding machine first applies pressure, clamping the tool closed by hydraulic or electrical rams that resist the injection moulding pressure. This ram pressure is retained until the finished part is ejected.
Injection. Plastic is introduced into the tool galleries/cavities at a controlled temperature, pressure and flowrate.
Dwell time. Pressure is retailed through a dwell period as the moulding cools, to counteract the shrinkage that results from the change of state as the plastic solidifies.
Opening and Ejection. A sequence of opening that extracts the finished part from one side of the tool (the ‘cavity’ side), followed by further opening that then operates integrated ejector pins and stripper plates that push the finished part off the other side of the tool (the ‘force’).
Gate trimming. Most OEM injection moulding tools introduce molten plastic through galleries (sprues and feeders) that then carry the material to the injection moulding point, the ‘gate’, where the part proper starts. Sprues and gates require trimming to release the finished part from the waste. Some gate types lend themselves to self trimming, meaning that a finished part is ejected separately from any waste material.
Materials for OEM injection moulding
The range and diversity of moulding materials and their particular applications and properties is too large for a short article – even before considering the additives that can be used to modify and enhance required characteristics. This is a brief introduction to the most common families of materials;
Acrylonitrile-Butadiene-Styrene (ABS, AES), the most important family of industrial polymers, used widely in consumer products, automotive, architectural, cosmetic and intricate component manufacture.
Nylon, a wide family of chemically similar polymers that are highly versatileOEM injection moulding moulding materials. Used for high strength parts, electrical insulators, common in automotive applications. Nylons are elastic and wear resistant, self lubricating, highly electrically insulative – but generally affected badly by moisture. Poor to no recyclability
High-density polyethylene (HDPE) is a widely used material which is resilient. Often used for consumer product bottles, toys, recycling bins and bottle crates. HDPE is among the lowest cost polymers and has found uses in every sector. High recyclability
Low-density polyethylene (HDPE) is a softer and more flexible material. Used for bottles, plastic bags and plastic wraps as well as public space and outdoor furniture. It offers moisture and chemical resistance and it is low cost and food safe. High recyclability
Polycarbonate (PC) is a high strength, generally transparent and hard material, used in engineering and automotive. It has susceptibility to organic solvents and oils, which degrade it quickly. Used for safety helmets, bulletproof glass and automotive light covers. Poor to no recyclability
Acrylic (PolyMethylMethacrylate) (PMMA) is common as a low cost alternative to glass – though istrength is limited. Used in windows, spectacles and vehicle lights. It is resistant to weathering/UV and offers high gloss and good abrasion resistance. Poor to zero recyclability
Thermoplastic rubber/elastomers (TPR/TPE) can be used wherever rubber is required – although this family have poorer characteristics than natural or synthetic rubbers they can serve very well for protective buffers for equipment such as instruments and mobile phones and they are used in making gaskets and seals, as well as footwear. Chemically less stable than many polymer groups, they are often not used for long life products.
PolyEthyleneTerephthalate (PET) – widely used and tough, low cost polymer. Used extensively for fluid containers and other single use applications. It is chemically stable and has a long environmental life – but it is susceptible to creep, making its engineering use limited. Recyclability is good – but generally as fibres rather than as prime material.
Polypropylene (PP) is the lowest cost polymer and it is very widely used, holding over 34% of the polymers market . It’s mostly common in the food storage and packing industry because of its chemical stability and minimisation of cross contaminants. It has high impact strength and good moisture resistance. Poor to no recyclability.
Synthetic rubbers such as Nitrile and EPDM. These materials are commonlyOEM injection moulding moulded (despite their inability to re-melt), where part precision and intricacy are more important than price. In terms of volume, a very small part of the market for these materials is injection moulded – most is processed by lower pressure, lower tooling cost methods such as compression moulding and transfer moulding.
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 (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
Injection, compression and transfer moulding summary
Whatever your engineering and product component requirement, if you need moderate to high volumes then an OEM injection moulded polymer component or a 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.