Plastic Injection Moulding via Contract Manufacturing

Plastic injection moulding is the most common manufacturing process to obtain plastic parts, it’s widely used in the world for variety of industries includes automotive, medical, home appliance, consumer products, sports, etc.

As we know, the Bronze Age lasted around 2,000 years – but the people using bronze, for almost every task, did not think of theirs as the “Bronze Age” – it took an historical view point, and long Iron, use to coin that name.

Welcome to the Plastics Age. It may not last as long as the Bronze Age, but that’s how important polymers are. In fact, the Plastics Age may already have given way to the Composites Age. We are living through history, as it happens.

OK, that may seem a little grandiose – but polymers are everywhere and understanding them is key to understanding manufacturing.

Polymers and processing

What are plastics?

Technically? They’re organic materials that can be taken from a liquid state to a solid state by heat or chemical action, and therefore moulded/cast/sprayed/painted/ into rigid or elastic shapes, to suit the widest range of applications.

Generally made of long chains of simple molecules (monomers), they belong in three basic families –

Those that can go from solid to liquid repeatedly, by heating (Thermoplastics);

Those that can go from solid to liquid and back to solid just once, by heating (Thermosets); and

Those that can go from liquid to solid just once, by chemical reaction (Epoxies, Silicones).

These last two tend to be tougher or more elastics, because the molecular chains are chemically bonded (cross linked).

Injection Moulding

Our purpose here is to review and understand the Contract Manufacturing implications of these materials when used in Injection Moulding. There are many variations and categories of moulding process that are, or are akin to Injection Moulding. This is a general definition so it’s clear what is meant;

Injection Moulding is a manufacturing process that creates parts, generally in large volumes because of high initial costs. Injecting liquid or molten material into a cavity under high pressure, and allowing the air to escape, forms that material into a precise copy of the cavity. The material then solidifies, so the cavity can be opened and a rigid or elastic part can be removed. However complex the cavity shape is, the part will generally be a perfect reproduction of it, down to the fine surface details too small to be seen unaided. There is a beauty and precision to Injection Moulding that even Daedalus would have appreciated.

Typically this process is applied as a very high volume manufacturing method, to produce thousands/millions of identical components. Injection Moulding is a term reserved for high pressure processing of polymeric materials – despite glass, metal and low pressure polymer processes being essentially indistinguishable.

To manufacture an Injection Moulded part, you need a cavity – usually referred to as a Mould Tool. And this is the expensive ‘machine’ that makes this process only become cost effective for high volumes of parts.

Moulding as a Contract Manufacturing service

Injection Moulding is not an in-house process for any but large, established businesses – and even then, it requires a pretty special business case, to keep this work in-house.

Mould tooling manufacture is a foundational aspect of the process, and the same rules apply.

In both cases, the specialist knowledge and up to the minute skillsets, in this rapidly developing sector, need practice, study and steady effort to build and maintain the ability to do it right, do it well and do it at minimum cost.

OK, enough background. You have parts, you’re sure they should be injection moulded, you need to find a Contract Manufacturer to undertake the work for you. Before you do this, there are a few boxes you need to tick, to save ‘iterations’ (common and easily avoided mistakes);

Have your parts and assemblies reviewed by someone familiar with tooling and with injection moulding. At least, be very aware of;

Mouldability – not every component can and should be moulded

Unnecessary undercuts – they cost money and reduce lifespan in tooling, as bits of tooled steel have to move to make them work

Section analysis – sudden changes in wall thickness will make shrinkage marks that are never a good look (sinking). Sections that are too thin will not ‘fill’ properly (short shot).

Assembly issues – will your assembly assemble without the need to damage parts? Will snaps that close it work, will screw pillars withstand the torque of screwing? If you’re glueing parts, is there a anding that will not influence cosmetics?

Finding the right plastic parts contract Manufacturer

With this review done and the learnings applied, talk to Contract Manufacturers. Working your contacts may get some recommendations – as will RFQs on and – and respondents will already be filtered for language issues.

When you’re experienced and confident, you can bend some of the rules – but when you’re new at this, follow a few basic steps and you’ll get a great result;

Find one-stop-shops; you will get the best results by buying a full service solution, where the moulding Contract Manufacturer takes responsibility for the entire process – from final mouldability review and materials sourcing, through tool design and moulding process optimisation, to flash cleaning and bulk packaging. You buy clean and acceptable parts in your hands, rather than detailed management of the process as a weight you carry.

Get a recommendation of potential suppliers from the Contract Manufacturers you are evaluating for the product assembly. When the Contract Manufacturer owns the supplier relationship, they own the problem solving processes – so you don’t have to. Most Contract Manufacturers of Box Build (finished product) will have partners or in-house tooling and moulding services on-hand. You may pay a little more this way – but you’re buying knowledge and security.

Validating that this full service supply chain is what it appears to be, can deliver what you need and will be low-pain for you to deal with requires experience – there are several things you can do to get greater confidence in this;

Get images of tools they’ve made, their tooling facilities and a recommendation/reference from a client outside their region. Most suppliers will be eager to share the experiences of their best clients. Get the information reviewed by someone with deep knowledge – use your networks and pay for the service, if you need to. One issue identified can save you big time and big money.

Get parts from the suppliers and get them reviewed.

Hire a contract auditor, local to the Contract Manufacturer to review their work. If you can, visit the supplier with expertise accompanying you – nothing works better than face to face and it will help you learn about the process.

Proceed to order, managing the process

When you commit to a supplier, don’t pay too much up front – stage payments according to milestones – usual stage payments are;

Tooling design

Tooling manufacture

T0 or T1 parts signoff (terminology for ‘first shots’ varies)

Tooling completion

50% deposit with order to cover these aspects

Final tool trial – your pre-production parts not included generally

50% balance on final signoff of parts

This gives you some insurance against bad workmanship or timetable slippage.

You should expect the Contract Manufacturer to operate professionally, on time, at high quality – help them with fast, clear communications, timely and drama free payments and a partnerly approach. Building a strong relationship with this supply chain will give you strong foundations – and your future projects will get priority treatment, as you become a valued client.

Management of the process should not be a burden on you – but it’s good to monitor closely, make their schedule part of your scheduling – and make this clear by asking for updates and on-time confirmations so you can plan effectively.

A single order for tooling and moulding trials will get you;

A handful of T0 or T1 parts – cosmetically incomplete and with a variety of moulding issues to be addressed, but possibly usable for an R&D build. Nothing builds team/investor confidence like a product that looks like the thing you’ve been working towards. But be careful – these parts will NOT be perfect, so don’t let the complaints in-house diminish the importance of the achievement – prepare the ground, set expectations!

A small box of T1 or T2 parts – problems identified at the first trial addressed, but tools/parts not textured. There’s a reason for this – texturing is the LAST STEP  – removing/repeating texturing is a pain you want to avoid! These parts WILL be good enough for an R&D build – same expectations issues as at the first tool trial.

A bigger box of final parts, which you can expect to be production standard – and acceptance/review/signoff of these parts triggers the final tooling payment and sets the tools/parts as ready for mass production.

The final step in the process is a standalone ‘tool trial’ to mould enough parts for your R&D and pilot build needs. Our recommendation is, order more than you think you need! The trial setup cost will be the same, the extra parts will not cost much and HAVING them is good insurance.

To conclude, Contract Manufacturing of Injection Moulded components is not easy, but the risks can be managed – it is one of the most commonly contracted aspects of manufacturing and the supply chains in China and South Asia are very well adapted to handling overseas relationships. Above all, you should be able to achieve high quality and reduced cost outcomes, compared with these services anywhere in the Western economies.