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Now you see it, now you don’t

02 April 2012

When it comes to new product development, the importance of the prototype simply cannot be underestimated. Gordon Styles, the Middlesbrough-born owner of Chinese firm, Star Prototype talks about the processes that put the product in the hands of the designers before it goes into production – including one that transforms polycarbonate surfaces in the blink of an eye.

The delivery of a prototype is one of the key stages in the development of virtually any new product. After months, sometimes years, of work it’s the moment the designer finally gets the product – or at least a very close approximation of it – in their hands. Giving them the chance to test it fully and iron out any design flaws prior to production.
Therefore, the aim for companies like ourselves is to deliver a prototype that is as close to the production specification as possible. In fact, any major deviation from the intended end product can be counterproductive, both in terms of the product design process and for the prototyping company involved.

As you’d expect, the rapid prototyping market is extremely competitive, with the smallest things making the difference between winning and losing a contract. Whether this be the speed at which the prototype can be delivered or the manner in which it is finished.

What makes the industry particularly complicated is that the prototype can be for virtually anything. For example, in recent months we’ve delivered prototypes for a modular penknife with 18 different functions, a device that stops bar tables wobbling, a hand-held hard skin remover and even a woman’s shoe.

What this all means in practice is that the successful rapid prototyping company tends to be the one with the most strings to its bow, meaning it has the flexibility to deliver the best solution every time. This is an approach that I used successfully with Styles Rapid Prototyping in the UK in the 1990s and adopted again when I came to China to set up Star Prototype in 2005. Our service currently covers everything from Stereolithography (SLA) and Selective Laser Sintering (SLS) through to CNC machining.

SLA and SLS tend to be used for the delivery of fast one-off prototypes. SLA uses a laser that converts a liquid photopolymer into a solid plastic layer by layer. Each layer is different and a 3D model is built up on a perforated plate in the bath of photopolymer. SLS is another laminated manufacturing process, the main difference being that it uses a much higher power laser to sinter plastic powder together to form a 3D prototype.
Meanwhile, CNC machining enables us to use the actual production material of the product as opposed to delivering a prototype in an SL or SLS process material. Using aluminium, copper based materials, and even hardened tool-steel, we use heat-shrink cutter holders for our high-speed spindles to ensure cutting accuracy, and also have a portable Faro laser scanning arm that can be positioned next to the machine to inspect 3D parts prior to them being removed from it.

Vapour polishing
These processes, plus polyurethane casting, tend to be used as standard by most leading rapid prototyping companies. Meaning the key differentiating factors are often found in the detail – and one particularly interesting example is the use of vapour polishing.

This highly dangerous process, which requires the use of toxic Weldon 4 gas, dates back to World War 2, when it was developed specifically to make Spitfire cockpit canopies clear again after repairs. Today, it is most frequently used by technicians seeking to quickly repair damage to polycarbonates.

The process works by heating the solvent to 43o Celsius and allowing the resultant gas, which is only in contact with the surface for one or two seconds, to flow over the surface of the polycarbonate. When it hits the surface it melts it at a molecular level and turns it clear. This happens instantly and is, without any exaggeration, the closest thing to magic you are ever likely to see in the rapid prototyping industry.

What is particularly interesting about the whole process is that it doesn’t actually change the general surface finish. In fact, if the material had milling marks prior to being vapour polished, then they will still be evident afterwards, but the material will be clear. In fact, you can even turn a hacksaw finish clear with vapour polishing.

One drawback is that Weldon 4 gas is extremely toxic and so vapour polishing must be carried out in a strictly controlled environment with active carbon masks being absolutely essential. The dangers of the process are such that when we carry it out we always have a team of four involved. One person manages the Weldon kettle, one does the painting, one is continually inspecting, and another is outside the room, also wearing a mask, watching in case of an accident. The sealed room fills from the floor up with gas and after the vapour polishing is finalised the room is fully ventilated with extraction fans, which scrub the gas through active carbon filters.

One of the most recent projects where we opted to use vapour polishing was the development of a prototype for a finger vein biometric door access system, the pads on the keypad being made from polycarbonate. At first PMMA was considered, but this would have required a lot of manual sanding and the parts were too small for this to be realistic. Additionally, and despite Weldon 4 being a PMMA Acrylic solvent that is used widely for bonding/melting pieces of PMMA together, it is not recommended to use it to vapour polish PMMA as it may craze the surface.

Instead, the decision was taken to make the parts from polycarbonate and vapour polish them - the end result of which was a clear demonstration of the value of the process. The parts were far clearer than they would have been had we persisted with PMMA and manual polishing, and they were finalised in a fraction of the time.
Interestingly, despite its formative Spitfire roots, vapour polishing is not a process that is currently readily available in the UK. Instead, those wishing to reap its benefits have to look further afield – with a number of companies here in China, as well as in the USA, Germany and France offering it.

In my opinion the benefits of vapour polishing far outweigh its dangers and it will be interesting to see how quickly this attitude becomes the prevalent one in the UK. If it doesn’t happen quickly then I do feel the rapid prototyping industry will leave those operating in these regions behind.

If you would like to see vapour polishing in action, click here.

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