Injection moulding vs Vacuum casting

Injection moulding is often dismissed as being too expensive, requiring long lead times and being unflexible, but that's not always the case. We take a look at some of the advantages the technology offers with a real case that proves that is not always the case.

In the early stages of design when we expect things to change we are looking to produce minimum quantities. An effective prototype will highlight errors or prompt alternative solutions. However, as we progress into the project, the demands grow; quantities start to rise and the methods we use to produce parts have serious cost implications.

When looking at the options for producing a batch of parts, the first decision point relates to aesthetics. If these are important then we can immediately dismiss the direct RP processes as they all involve layers and an associated need to refinish, dress and paint. Whilst technically feasible the costs are non competitive. If aesthetics are important then the contenders are vacuum casting or injection moulding, where process colouring and texturing are conferred by the tooling.

Vacuum casting offers a lower front end cost with a higher unit price, whilst injection moulding demands a higher front end cost, but a significantly lower unit price. Production rates are also significantly different - vacuum casting allows a drip feed of parts whereas injection moulding usually has a longer wait with a fast finish.

Vacuum casting tooling will last between 10 and 25 parts (or lifts) depending upon the geometry of the part and the material being used. After this if the tool hasn't been configured as a multi-cavity tool at the outset it will need replacing. The part unit price remains constant during this time, but the replacement tool will require a second master, costing the same as the first tool, unless you use an early casting or a duplicate master. By comparison, injection moulding costs come from the tooling, the set up on the moulding machine (per batch) and the part price - a combination of material cost and moulding time.

To show how quantities will influence the decision on what process to use, we cite a recent example that uses a part measuring 210 x 145 x 32 mm. We costed this part as both a vacuum casting and an ABS injection moulding.

Based on a batch of '50 off' the injection moulding cost was 10.5% higher and the lead time was five days greater. However, a subsequent requirement for another 100 brought the injection moulding tooling in at 40% less than the vacuum casting solution.

On this geometry the break-even quantity is 55 parts - any more than this and the injection moulding solution comes in cheaper. Whilst subsequent parts are unlikely to fall as low as production moulding unit prices, due to substantially smaller batch sizes, they remain significantly lower than vacuum casting unit prices. Subsequent orders for parts were processed in a fraction of the time to run vacuum castings and ultimately reached over 500 parts.

Of course we're not dismissing vacuum casting. It has the advantage that design modifications can be introduced with each re-tool. That said, all but the most radical changes can usually be incorporated into hard tooling, especially if the potential insert witness lines are deemed acceptable. Furthermore, the turnaround time on incorporating modifications into injection moulding is usually less than the cost of retooling for vacuum casting.

In the case quoted above the injection mould tooling could have been totally reworked several times over, and still shown a financial advantage.

There are many factors that influence where the break-even point on any project will ultimately fall, but injection moulding is worth serious consideration.

In the right hands and applied to the right part geometries it is a serious prototyping tool. Then of course there are benefits like the real production intent material, the option to have some spares and the possibility of picking up production moulding issues much, much earlier in the design sequence.

One of our clients even sold their P20 prototype tool to production, recouping their development budget, giving production a 'zero lead time' option and satisfying all involved.

What solution is right for you?
Injection moulding depends on the number of parts you need and their geometry, but it does offer an option where you can economically have additional parts without a high premium.

At the front end, injection moulding quotes don't take any longer than prototyping quotes to produce and in-house analysis with Moldflow helps us to identify potential design issues ahead of tool design. Moulding parts as a single batch can save multiple set-ups and reduce lead time further. A well thought out control drawing can allow the part to be checked on press at first trial (T1), permitting the immediate production of the first batch.

If your project is not going to benefit from injection moulding, we are in a strong position to recommend alternative solutions.

If you have a project you wish to discuss call Tim Plunkett on 01452 386608 or email tim@plunkettassociates.co.uk

Author
Tim Plunkett - Plunkett Associates

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A very fair assessment Tim. As a leading producer of vacuum casting parts I have watched as the break-even figure has fallen over the years from around 300 to - as you say about 55 in some cases now.

We do a fair degree of short run production work using vac casting still, often on parts which would need complex tooling - a factor which would still push the break-even figure back into the hundreds, but although often a good solution for production, vac casting's real advantage comes from its speed and relatively low start-up cost, which enables thorough testing of a design before progressing to hard-tooling.


Comment Mike Harvey, 04/02/2013
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