Frequently Asked Questions

Injection Moulding or Vacuum Casting. Which one should I use?

Injection moulding is often dismissed as being too expensive, requiring long lead times and being inflexible, 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.

Vacuum cast partsIn 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.

If aesthetics are important...

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. Any inserts can be carried over and the master reused, but the silicone element will need re-casting. The silver lining, is that this can represent an ideal opportunity to incorporate design changes if required.

Injection moulding plastic part

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.

Quantities can affect process choice

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.Vacuum casting

There are many factors that influence the break even point

Of course we're not dismissing vacuum casting. As has already been noted, design modifications can be introduced with each re-tool and a large variety of resins tried very easily. However, with the right infrastructure injection mould tool modifications can be done in days and depending on the level of tooling being trialed, with little or no long term consequence.

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.

Which solution is right for you?Injection moulding

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. Contact us for support and advice.

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