Polymer mould for casting
Exposing a polymer sheet
You should carry out a few experiments to arrive at the correct exposure time for your equipment; the exposure time is not highly critical, and it varies with the energy emitted by the UV-lamp, the distance between lamp and object, the sensitivity of the photopolymer. With our equipment we obtain good results with an exposure time of 20-40 seconds.
- Take a sample of the photopolymer sheet of appropriate dimensions (we use LSL-148-SB SH Staal, to be obtained via Jet Europe, Zeewolde, NL).
- See to it that the minimal thickness of the sheet ultimately, i.e. when you make the mould, is about 0.7-0.8 mm; this can be achieved by pasting material onto the metal back. Take care also to maintain a flat sheet: the corners tend to curl when you trim a piece of polymer sheet with a pair of scissors; this may prevent close contact with the negative and lead to a poor mould.
- Put the photopolymer sample in your UV exposure unit with the negative face down on top. See to it that there is close contact between negative and polymer sheet, so as to prevent distortion of the image on the polymer.
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| Figure 10 - Copper was used to make an airtight box into which small holes were drilled. Put the polymer mould on top of it, cover it with a negative (face down) and cover the entire top of the box with thin foil. Apply suction to the box via the tube in the front, and you are likely to have optimal contact between negative and photopolymer. |
The Model Master® UV unit 
is functional albeit rather basic. The glass plate used to ensure good contact between negative and polymer sheet is not ideal. We have therefore replaced it with a home-made copper box perforated with small holes at one surface (figure 10). We put the photopolymer or other material (silver or copper covered with photoresist) with the negative face down on the box, cover the entire surface with a thin, very flexible foil and then create a vacuum, so that contact between negative and object is always optimal. The vacuum can be generated with a small pump, the vacuum required is minimal. Our sample of the Model Master® is not equipped with a timer, but such devices are available elsewhere. In fact it is easy and quite cost-effective to make your own UV exposure unit; use two Philips PL-S 9W/10 lamps which can be bought complete with fitting in various stores. UV exposure units available from electronics shops for making electronic prints are a cost-effective alternative.
Washing out the polymer sheet
Use water at about 35 °C (95 °F).
- You may submerge the polymer while you brush the surface, using a soft, wide brush; brush with quick movements, gently or you may wash out exposed polymer. Wash for about 2, maximally 4 min. in one go. If you extend this period without first thoroughly drying the unfinished mould, your run a great risk of washing out exposed polymer and that the material may even detach from its base. You can stop the washing out process if foam is no longer formed when you gently apply a wet brush to the object as it rests on your palm. You may also experiment by first washing the larger surfaces out with a brush, and the small recesses subsequently with a spraying device. With some ingenuity you can transform portable spraying equipment used in gardening into a useful device for these purposes; but do fill it with lukewarm water.
- Dry the polymer with a hair drier after 2-4 minutes; it is properly dried if the polymer no longer sticks to your fingers.
Inspect the mould, if need be with a magnifying glass. If it is still not washed out completely, repeat the washout process. If a line is thin, be aware that you may wash some of it out unintentionally if you go on too long (figure on the right). This may also occur if the black background in the negative was not completely black; in that case the portion that needs to be washed out has received some exposure, needs prolonged washing out to the detriment of the unexposed parts. When the polymer gets soaked for a prolonged time, material will detach. - Curing: if the mould is properly dried with the hair drier, cure the polymer by exposing it to UV light again. The exposure time depends on the UV lamps used, distance to the polymer surface and sensitivity to UV light; with our equipment a final exposure of 1 min. is satisfactory.
Minimal thickness of the mould
When you completely wash out the polymer sheets that we use, the minimal thickness is 0.18 mm. This is far too thin for casting purposes. You should therefore increase the thickness, e.g. by applying acetate sheet with double-sided adhesive tape. You can apply a number of layers, but see to it that the tape covers the entire area, or you will end up with depressions that unfortunately reproduce in the mould, and subsequently in the casting. We found that a minimum thickness of 0.7-0.8 mm worked well for casting cloisonné objects.
The thickness of the Jet ® sheet prior to washing is about 1.50 mm, the metal back and adhesive layer comes to 0.50 mm, so the depressions (and hence the edges of the cloisons) come to 1.0 mm. Do take into account, however, that silver shrinks by about 10% when solidifying.