Prototyping process provides tooling flexibility

Space Puzzle Molding is, at the same time and using the same tooling, both a rapid prototyping and a production process.

When Behr, systems partner for the international automobile industry, designed a sophisticated plastic moulding as a radiator tank header for its customer Paccar, the working CAD file correctly showed the component scaled exactly to the size and shape required for the finished product.

The very nature of plastic injection moulding, however, is such that shrinkage and warping potentials need to be accurately optimised.

To make a tool of this, ribbed and three-dimensional, complex part directly from the original CAD data with only a simple overall shrinkage allowance would be a recipe for disaster.

Only an experienced and competent mould designer has the expertise and judgement needed for optimising these important details.

Injection mouldings obviously follow faithfully the cavity created in the tool as the molten resins are injected, but invariably change shape after demoulding.

The trick is to make sure that the shape they end up at is precisely in line with the design intent for the product.

For very simple components shrinkage and warpage is not usually a problem but when long or complex items are involved, such optimisation is a highly skilled "black art" involving both calculation and a degree of "intuitive magic" based on previous experience.

Also, if several alternative materials are to be evaluated for suitability, allowance has to be made in the optimisation for a subtle dimensional balance, based on the combined estimate of these effects for each of the materials.

In considering its options for prototype mouldings and subsequent production of the items, Behr visited Protoform of Fuerth Hardhoehe near Nuremberg airport for technical discussion and a quotation for the work.

Behr had discovered the Space Puzzle Molding process, patented and exclusively operated by Protoform, via reports from other users and were familiar with the advantages of speed and cost, SPM moulds, being typically half the cost and half the timescale to produce compared with conventional single cavity unhardened prototype tool.

The responsibility for tool optimisation is a service offered to clients and welcomed by designers as valuable expertise.

Space Puzzle Molding is, at the same time and using the same tooling, both a rapid prototyping and a production process yielding small volumes for trials and, subsequent to any tweaking to the specification required by the client, the easily modified SPM tooling can go on to produce thousands of components in series-identical batches.

Using the shrinkage-optimised CAD data, aluminium and in some cases steel, tooling of almost any complexity can be created at Protoform, using CNC and multi-axis machining on state-of-the-art high-speed milling machines.

The volume of the final tool from this process is rarely more than 25% larger than the product envelope.

This is made possible by placing the finished set of Space Puzzle mould-halves with their respective inserts into a patented custom bolster mounted on a conventional injection moulding machine.

Each time the mould is filled the loaded assembly is removed from the bolster skilfully, and manually demoulded.

Per-item costs naturally reflect this element of labour content but to the skilled operatives at Protoform, this is the norm for production and the procedures established for each product are slick and swift involving the creation of unique demoulding tools and accessories for each product.

Other processes in the marketplace for size-and-shape prototyping are considerably more expensive on item price and usually have the disadvantage of being in non-production-intent material.

SPM on the other hand offers production quality parts immediately, in any injection-grade plastic, with and without reinforcements inserts and outserts, surface-finished by coating if required.

Also, the tooling is easily modified on-the-fly for design corrections or niche-market variants.

In the case of products not intended for true mass-production it is becoming increasingly the case that the SPM tooling can fulfil the entire production requirement at consistent quality up to a typical figure of 3000 items plus before tool, or insert, refreshing may be needed.

Because of the required length of the item for Behr at around one metre, Protoform had to re-assess its normal size criteria.

The largest moulding previously produced was approximately 800mm in length, the normal maximum envelope being 800 x 400 x 200mm, the size of a medium suitcase, with a maximum shot-weight of 2.2kg.

As the product, although complex in detail, was essentially linear, it was possible to push the envelope to just beyond the metre point.

The component and the areas requiring adjustment to preload the shape for warpage were mainly the long plane defining the interface and some internal details.

It involved progressive adjustments to wall and web thickness along the length of the inner walls and base.

Initially Behr's requirement was for low quantities of three versions of working prototypes to prove the fit-and-function of the design.

Subsequent to this stage, the concept of the plastic tank header proved extremely successful such that a closely similar item was introduced for a second project.

The initial tooling was modified to accept two new interchangeable inserts and the main parts were able to be used for this project also, saving costs very significantly.

The tooling is in current production for batches of each of the two final designs and, if required, can also revert to any one of the three initial prototypes giving the tooling set a five-product potential.

This represents the ultimate in economic tooling for low to medium volume products with niche market variants.

The parts were basically designed for the trucks, Peterbilt 387 and Kenworth T2000.

Meanwhile another module was designed for the Kenworth T800W where these radiator tanks will also be used.

Peter Griesheimer of Behr was the Senior Customer Engineer on this product and comments: "Designing for the automotive market requires expertise in all materials and the opportunity to use plastic injection moulding for this innovative application offered many advantages over the traditional metal formings for prototypes".

"We knew that we were pushing things to get a one metre moulding out of the SPM tooling system but the advantages of lower cost than full production tooling, flexibility to modify the prototypes, coupled with the realistic chance to run on into medium volume production without further major investment were the drivers, and the SPM engineers were confident of a successful outcome".

"For the initial evaluation we needed three different versions of the prototypes and this was simply achieved with variable mould inserts".

"Protoform's response was excellent".

"It was fascinating to learn how much preloading was needed to bring the warping correction into or original design".

"The optimisation was something we preferred Protoform to take responsibility for and they were happy to do this".

"The series-identical end products proved very acceptable on final inspection, and we have subsequently been able to satisfy the market demand comfortably for both the first version and a niche-variant for another application".

"Having surveyed the alternative production methods and associated costs it seems to me that the claims made by Protoform about the cost, timescale and effectiveness of their process are entirely justified and born out by our experiences with them".

"Between us we got this one right, and furthermore we got it right first time thanks to the optimisation exercise".

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