Design for manufacture and injection moulding – getting the best results
24 November 2015
Demand for injection moulding services continues to grow globally and Ireland has been a centre of excellence for injection moulded products over many years. Historically, the automotive sector has been a mature niche-market for toolmakers and the plastics industry in Ireland however in recent years the medtech and pharmaceutical sectors are key drivers, often requiring high volume high speed product runs which demand stringent quality processes and production values.
Automatic Plastics (APL) has been a leading Irish supplier of injection moulding services since its foundation back in 1972 and its services have evolved with market demands. Managing director Al Lawless took over running the business about a decade ago, taking the reins from his father who founded Automatic Plastics. In recent years he has overseen an expansion of the company, securing significant new business contracts both locally and overseas.
Lawless outlines how the company has evolved in line with the industry here in Ireland: “It’s interesting, as if you track back on our history in terms of achieving quality standards, they have pretty much been achieved directly in correlation with the growth of key sectors in the Irish marketplace – the automotive market was important in the early days and TS 16949, the automotive standard, was essential to work in the auto sector.
“With the significant growth in the medical devices and pharma sectors here in Ireland we introduced a cGMP production focus and achieved ISO 9001 and ISO 13485 also; Food packaging has been identified as a growth market and recently we achieved the BRC-IOP Food Packaging Safety Standard.”
As an expert practitioner in tooling and high volume, high speed manufacturing, Al has learnt many practical lessons at the coal face of industry in relation to managing the Design for Manufacture (DFM) process and meeting the needs of multi-nationals.
Effective DFM and cost implications
In this article we outline the key considerations for buyers of injection moulding services in terms of DFM and some useful, practical tips Al can offer as a result of his in-depth expertise in this area. Lawless says: “A change in supplier is often motivated by quality issues, re-shoring or new projects – but there are perceived risks for the purchaser; along with the potential cost of new tooling, possible inconsistencies in the injection moulded product and high rejection rates or worse; the risk of down-time on the factory floor if production scheduling is affected by non-delivery of components from the new supplier.”
Yet he explains that there are considerable advantages for businesses that can make this change effectively: “Cost is a key driver; we’ve found that by being innovative at the DFM stage, the life-time cost of a new tool can be a lot less than expected and ongoing production costs are often typically less than the client was paying previously. Of course for UK buyers we have the exchange rate benefit also.”
The injection moulding process is outlined below and the costs of each stage of the process need to be considered. Costs include inputs such as raw materials, labour and purchases components; equipment, information and tooling are key costs also and of course resources such as energy, supplies and the management of waste all impact price. “Not all costs are equal obviously so although we scrutinise them all, it’s the elements that add significant expense that are worth really analysing – tooling is key.”
As upfront tooling costs have a considerable impact on the life-time cost of production, it’s natural that buyers are wary of committing considerable expense at the outset. “We’ve developed a proven procedure now that allows us to build lower-cost trial moulds that can be put through a series of tests to prove production values, quality and consistency.
“This is a more iterative trialling process than has previously been available to customers and it means tool design can be proven before the full cost of building the tool is incurred. Obviously it’s more cost-effective to spend time at the DFM stage finalising the tool design than it is any of the later stages in the process.”
Lawless makes another interesting point in terms of ongoing costs; he mentions that companies should also consider how stringent their requirements need to be as naturally the running costs of the supplier’s facilities impacts pricing.
“There’s a lot of talk about the need for Clean Room facilities in the manufacture of medical device components, for example. What we have clearly established with many Medtech customers is that this level of environmental control – with all its associated costs – is not always necessary. White room facilities will often meet all the requirements and offer a much more cost-effective production solution.”
Key DFM considerations for tool design and build
The same basic principals of DFM that apply to all manufacturing markets are relevant to injection moulding e.g. the use of proven industry standards, design to specifications, maximise quality and minimise material usage, optimise the use of resources such as energy, machinery, floor space and labour.
Naturally there are many key considerations specific to the injection moulding process also – we’ve given a few key pointers below.
Usually the aim is to standardise the wall thickness throughout the component. If transitions are essential, these should be gentle and from thick to thin regions. Al explains it can be easy to add significant rigidity where required without necessarily adding considerable cost “The rule of thumb here is that an increase of just 10 per cent in wall thickness can deliver a 33 per cent increase in the stiffness of the end product.”
Raw materials and shrinkage
Polymers such as thermoplastics and thermosets are most common and there’s a huge variety of choice available; also materials can be blended to offer desirable or required qualities, for example, polymers can be mixed with alloys. Buyers of injection moulding services should be confident that their suppliers have true expertise in this field and will be able to advise on the best options availability given the application and the strength and functionality required of the component.
The table, right, outlines some common resins suitable for rapid injection moulding services,
Shrinkage also needs to be taken into account at the design stage; when a mould is closed after the polymer is injected into the cavity, a holding (packing) pressure is applied to compensate for shrinkage. In addition to the properties of the materials, the process can also effect the shrinkage and that is why shrinkage rates are not specific and are given as a range. There are long-established rates of shrinkage for different raw materials; here are some common ones,
Acetal (POM) 2.0-2.5%
Acrylic (PMMA) 0.1-0.4%
Nylon 6 (PA6) 0.5-1.5%
Pollycarbonate (PC) 0.5-0.7%
Lawless points out that new materials are coming on stream all the time: “Years ago the options in terms of polymers and raw materials for plastics were much more limited; now we can get more robust materials so it’s worth spending the time early on researching what’s out there at the earliest possible stage in the process.”
Hard tool versus soft tool
Typically more expensive tools made from materials such as hardened steel have been used for high volume production runs and they prove their value over time, ultimately offering a low investment cost per unit produced. Moulds can also be made from softer materials such aluminium and P20 (pre-hardened steel). Aluminium moulds can be cheaper to produce and offer faster lead times – they can also be reinforced to offer more longevity.
“The decision on the type of tool to be manufactured is critical; not only to the costs at the outset but also in terms of productivity, maintenance costs and production values. This is where technical experience and expertise can really add value for clients.”
Gate design and location
The gate is the opening that allows the polymer to be injected into the mould. There are three main options in terms of the type of gate; Hot runner, cold gate manually cut or a cold gate automatically cut and all are appropriate in different circumstances.
The manner in which gates are designed and located has an impact on how they are removed and how they appear on the final part. Lawless explains: “Obviously a small gate will usually look better but are not suitable for parts with thick wall sections and it will take longer to fill the cavity so productivity considerations are key. An innovative approach in terms of gate design can often reap significant benefits.”
There are a number of common problems that can occur in the injection moulding process and some of these are outlined below. “Of course the whole concept of an effective DFM process is to eliminate these defects before production but it’s useful to recognise what might be going on when you are trialling a mould. You need to establish is it a problem with the mould or with the moulding process?”
Raised or layered zone on surface of the Plastic part
Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Black or brown burnt areas on the plastic part located at furthest points from gate
|Tool lacks venting, injection speed is too high.|
|Colour streaks||Localised change of colour||Plastic material and colorant isn’t mixing properly, or the material has run out and it’s starting to come through as natural only|
|Delamination||Thin mica like layers formed in part wall||Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond|
|Flash||Excess material in thin layer exceeding normal part geometry||Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.|
Part of the moulding is missing, usually at the last place to fill
|Many Causes, Shot size incorrect, Packing pressure not adequate, mould too cold, Injection speed too slow.|
Sink Marks Dips in the surface finish Due to thick wall sections, Poor cooling, Inadequate packing pressure, Gate too small.
It’s an exciting time for injection moulders; not only is demand high and the market dynamic, but new technology in the guise of 3D printing is going to change the face of the DFM process. As the magazine Machine Design discusses, it will redefine industry standards in relation to rapid prototyping and related tooling costs.
However, Lawless gets back to basics and points out that new technologies can only be leveraged to best effect when communications are strong: “The best results in the DFM process come when there are clear lines of communications between all parties; not just the engineers, production managers and tool-makers but these days there are often marketing or branding aspects that needs to be taken into account too – this can be particularly relevant in markets such as food packaging.
“So we’re keen to embrace new opportunities but we’re not going to lose sight of the importance of building long-term relationships with clients; not just tools.”http://www.engineersjournal.ie/2015/11/24/design-for-manufacture-and-injection-moulding-getting-the-best-results/http://www.engineersjournal.ie/wp-content/uploads/2015/11/aaaainject1.jpghttp://www.engineersjournal.ie/wp-content/uploads/2015/11/aaaainject1-300x300.jpgMechmanufacturing,United Kingdom