Vacuum Forming.

 

Introduction.

 One possibility I looked at for the cannon was metallic paint. After experimenting with dozens of brands of gold paint. I came to the conclusion that existing paints were not the answer, mainly because they are formulated for indoor use, or are not metallic enough. One of the first paints I tried was Humbrol gold enamel .... which produced a very nice finish, but when exposed to sunlight it turns a shitty brown within three months.  Gold paints for outdoor use do exist and some of the best are those manufactured for sign writing. It was these I eventually used on the cannons. So when I was looking for a metallic finish for life castings, I automatically ruled out gold and silver paint. 

 

Physical Vapour Deposition (PVD) of metal onto a substrata.

The main process's for the deposition of metal onto substrata are ...

Basically the metal is vapourised and deposited on the target surface an atom at a time.  Since the metal coating is very thin, typically in the order of a few atoms,  metal costs are low, which makes it practical to coat objects in pure metal such as gold.  For example, it would be possible to coat a torso casting in pure gold for about 50 pence ... excluding labour costs. However if you send your life castings to a specialist for a PVD metal coating ...  it is expensive and that cost has to be passed onto the client. 

The question is ... are there any other alternatives ?. 

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A simple Vacuum Forming 'machine'.

As Rod Stewart was once quoted as saying ... "Picture wurf a farsend wurds, innit". So lets make a simple vacuum forming machine. The reason I built this was because I wanted to try out some ideas on a small scale, mainly testing adhesion of materials to plaster.

First we need a vacuum source, so I decided to use my workshop 850 watts vacuum cleaner. As vacuum cleaners go this is fairly low powered. UK workshop vacuums are normally about 1500 watts and American ones tend to err towards 2500 watts. The higher the wattage, the stronger the vacuum.  Now at the moment I am only interested in forming very thin sheets of material and my vacuum cleaner turned out to be too powerful for what I had in mind. 

Since I am only doing material testing on it, I will only be using small samples of material, so I only need a small machine. In fact I only need it to form A5 sized samples, but just to be on the safe side I decided to make it big enough to also form A4 sheets.  I had some scrap 20mm MDF  in the workshop so I decided to construct the whole machine from that. The first job was to make up the vacuum box.

 

The hole on the right hand side is for the vacuum cleaner hose. The box is completely sealed and the only way air can get into it is via the hole matrix . The top size is A4. The holes drilled into the top are 3.5mm, which turned out to be too large for thin sheet. On top of the vacuum box we then make a frame that the plastic sheet can be clamped to .....

 

This frame simply sits on top of the vacuum box. The vacuum will hold it in place. Note the black foam rubber gaskets to make the frame air tight. 

Since I had not vacuum formed very thin plastic before, I was very much feeling my way.  For that reason I decided to subject it to the severest test possible and that was thin plastic rubbish bag material. This stuff is only one thou thick and I expected it to highlight any problems. The only heat source at this stage was a heat gun, which didn't help matters at all. I noted the following problems ....

a.    The vacuum box holes were too large.  

b.    I had too much suction.

c.    I need to make up a proper heater assembly.

I decided to make another board using only 1mm holes on a 10mm matrix instead of the 3.5mm holes on a 20mm matrix.  This extra board simply sits between the vacuum box and the material clamping frame, like so ....

Again, the vacuum holds it in place.  I actually have  a suction adjustment on the vacuum cleaner hose which was a slide that uncovered a hole. Even with this fully open I still had too much suction, but this was partly offset by the smaller holes in the vacuum box. I had to have something to copy so I used a plastic measuring scoop as a 'master' to form over. The next step was to try out a bit of plastic sheet, which turned out to be a scrap of grotty old damp proof course plastic. Here it is in the clamp ....

 

I then removed the frame from the former, heated up the plastic with the heat gun ... switched the vacuum cleaner on and then dropped the clamping frame back onto the vacuum box. The vacuum holds the whole lot together while the plastic sheet cools down.

The next thing to do is to make up a master 'plug' to use for the tests. As far as life casting is concerned, probably the best test object would be a simple plaster casting of a small woman's breast with 'overhang'. That would require the greatest 'draw depth' and therefore the worst case situation for lamination. Unfortunately I do not have one. I would probably have to make an extra A4 size clamping frame for that. Donald, I need to borrow your house keepers body for half an hour !. It would give you a wonderful opportunity to see how persuasive you can be Hi!. 

 

Today's conclusions.

I started on this last night. Originally I wanted to build  full sized machine capable of laminating a full torso ( bit bigger than four feet by two feet, I need to find out roll size) but I was concerned about locating the right kind of sheet or foil.  Yesterday I located a  UK  Chinese source for what I wanted, but they turned out to have zero stock here and would have to order from China. I have those small A4 samples and that is what lead me to go down the A5 sample route for test purposes. During today's thinking I have now solved the supply problem and could now tackle a full body casting former. I have a feeling that I am making things difficult for myself by going down the A4 route, because the process would work a lot better life size. On the other hand the A5 route will simplify the adhesion testing. 

I am thinking that I need to make the thing micro-processor controlled, which would give better and more consistent results from a heater assembly and suction control.

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7th June 2007

The measuring spoon again, with a highly reflective chrome lamination. This was with vacuum on and no adhesive. I have some adhesion samples drying at the moment. The film is very tough and the problem today was insufficient vacuum, coupled with excessive draw. I need to modify the frame to raise the height of the plug, to get drape equal to draw.  At the moment I am not using the available material extension to the best advantage.  The Chinese company I was talking to have now found me some sheet samples and I hope that they include gold. If not then I can source it elsewhere.

 

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8th June.

Samples still not turned up, so still playing with scraps..  Knocked up a small 'breast' in MDF to use as a more realistic plug ...

 

Although it looks smooth, sanded MDF always has a 'nap' on until it is sealed and 'de-whiskered'. The film is actually reproducing  the detail of the nap, even on the outer surface of the formed film. The highly reflective film highlights this giving the slightly mottled finish seen in the left photo below. 

                                                      

Need more suction. I am beginning to think that the film I am using is not a thermoplastic !

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9th June 2007.

Margarets birthday !.  At last my Chinese 'samples' arrived .... one whole A5 sample !.  At least it is the right material ... although  much too thick. It looks as if I am not going to get many bites at the cherry, so I decided to seal the dummy boob .... to eliminate the nap effect, before I sacrifice the sheet sample  on the alter of my own ignorance !.

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10th June 2007  (Morning)

Wedding Anniversary !. Decided to have five minutes in the workshop and try the Chinese sample.   Now at first glance this attempt looks like a complete failure !. Actually, I am very pleased with it. Sealing the plug nap solved the problem of the mottling, but that was predictable. I also knew in advance I didn't have enough suction, so I only expected a partial draw. Also I am still using the heat gun and since it cannot uniformly heat up all parts of the sheet I knew that at some stage the sheet would blow a hole in itself ... which it did ... stopping the draw. Now, I have lots of resistance wire and all I need to create a decent heater panel is some ceramic or glass stand-offs ... so why haven't I done it ?. Well, I am still feeling my way and There were a few questions in my mind that had to be answered first. I was also sure that to proceed much further I had to start making things a bit easier for myself and that really meant, moving to a full sized vacuum former ... and I wasn't sure if I wanted to do that. After all I am never going to do Life Casting myself and I am only interested in the problems involved.

 

 

OK let me ask you a question Don. Why is the formed sheet still on the boob plug ?.  The simple answer is that I cannot get it off !.  I had already come up with a idea of how to permanently fix the plastic to the plaster but I had no idea if it would work. Accidentally, I have proved that it does Hi!. 

I guess the answer to whether there is an alternative to PVD ... is yes !.

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10th June 2007 (Evening).

The thing I love about research is that it does not matter where you start ....  because you never know where you are going to end up !  ... and if you travel down enough dead ends you will eventually find the 'best solution' that you seek. I call it the 'Eureka moment'. Sometimes it happens in hours, usually days, but often takes years.  In this case I started out by asking the wrong question..... 

"When I was looking at life casting, I was wondering what the kinds of finish could be applied to a plaster cast that would give a realistic metallic look". 

 If I had been thinking laterally I would have been looking for a way of producing ANY kind of finish on ANY kind of material.   As it was, I shot myself in the foot and got bogged down with vacuum forming metalised PETG film  on plaster casts. Silly me !.  However, if I had not looked at PETG and listened to what was being said to me, the 'best' solution would never have occurred to me. In this case it was a throw away remark made by the Chinese salesman, who simply said that he did not know if his film would work in my application because it had a 'special coating' on it.  I let it slip at the time, but later began to wonder what this special coating was for ?. That is how things happen.

 

Holographic glitter finish.

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16th June 2007.

Made up an A4 sized clamping frame. 

Although this move only slightly increased the active forming area, the results are obvious. Drape and extension greatly improved over A5 results .... as predicted !. Again we can see the difference between forming on the  sealed MDF surface near the 'nipple' and the mottled surface  on the unsealed sanded bed of the vacuum box. 

'Polished black granite' finish.

I think the secret of research is knowing when to let go.  I have achieved everything I set out to do and perhaps a bit more, since I am never going to use the end results. Despite my false logic at the beginning, I can now create just about any kind of finish, on any of the normal life casting materials. Taking it  a bit further I can reproduce photographic grade detail and colouring down to skin  pore level, even on soft synthetic rubbers or edible substances.  Time to move on ...... 

This topic is now closed.

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