How to make a polymer mould for casting
Introduction
We have always been much attracted by the cloisonné and the champlevé techniques. Both of these presented with some problems.
With the cloisonné technique we would bend 0.4 mm silver wire, shape the design on the plaque, glue it with Klyrfire or tragacanth and then fill with flux, or embed only in flux and subsequently fire in the kiln. There were the usual setbacks caused by the cloisonné wire having changed position, so that the design was no longer entirely satisfactory. Also, if cloisons are not completely closed any more, it is more difficult to prevent differently coloured enamels from mixing.
Engraving techniques are similarly not simple to apply and time consuming; it is certainly not simple to obtain tight lines. Etching is certainly a possibility, but when etching silver to a depth of 0.3-0.5 mm nitric acid eats both vertically and sideways, so that lines may not be clear in the end.
On that account we have explored ways and means of solving the above problems by the use of casting techniques. One of the requirements was that the basic 'plaque' (i.e. the casting minus the height of the edges of the cloisons in the cloisonné technique, or the thickness of the depressions with the champlevé technique), would be less than 1 mm thick. It turned out that using materials that are extensively used in printing industry it is easy to quickly produce moulds, provided one is happy with small objects. This can then be combined with the Delft casting technique®; the latter only leads to reproducible and satisfactory results if one takes due account of factors that come into play when casting such thin plaques.
Each black and white drawing can be the starting point for your design. We often use the computer in designing; an advantage is that it is easy to turn your design into a negative (a mandatory step) and mirror it; also one can experiment to obtain the line thickness that will lead to the best result in the casting.
Materials used
- Computer, image editing software, e.g. CorelDraw® or Adobe® Illustrator®, Adobe® Photoshop®
- Epson Stylus Color 800 printer or similar printer
- Océ InkJet Overheadfilm IJ130 (Art. 99896154) or similar material
- Model Master® or similar UV exposure unit
- Jet® UV-sensitive polymer sheet (we use LSL-148-SB SH Staal, to be obtained via Jet Europe, Zeewolde, NL)
- Water
- Soft brush, paint brush
- Double-sided adhesive tape, acetate sheet, pair of scissors
- Talcum powder
- Delft Casting technique® (clay and set of rings)
- Iron ruler, hammer with a flat surface
- Silver, borax, melting pot and holder
- Propane gas and torch
Required skills
- Use of computer and application of image editing software
- Good casting technique.
Line width for cloisons
We have used CorelDraw!® and Adobe® Illustrator®, but other editing software is obviously also suitable; some of our comments relate specifically to the above software.
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| Figure 8 - Negative used in making a mould for champlevé work. |
When making linear images, curved lines may be drawn in which the line thickness varies with line curvature. It may occasionally be very difficult to observe this calligraphic effect on the screen, and it can also be easily overlooked in the printed version. The difference in line thickness is, however, reproduced when you create the mould. Part of the thinnest lines in the mould may then be washed out, so that the edge of the cloison is not sufficiently high, or is corrugated. The problem can be circumvented by working with the 'Freehand tool' (CorelDraw!) or 'Pencil tool' (Adobe Illustrator) with fixed line width. The line width will not be the same as in the mould, because it is also determined by exposure time, variations in washing out the polymer, closeness of the contact between negative and polymer, and by light scattering. Some people routinely scale the line width in proportion to the final size of the image. We generally turn that feature off as it usually wreaks havoc if the image is resized.
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| Figure 9 - Negative used in making a mould for champlevé work. |
In the champlevé technique line widths are often not so critical. It does matter, though, when the lines form the limit of narrow depressions that need to be filled with enamel. In the example to the left the butterfly's antennae (figure 9, logo of the Netherlands Asthma Fund) had to be made quite thick to end up with something useful in the casting. The white area around the antennae give rise to relatively much light scattering, so that the antennae themselves tend to become slimmer. The antennae can be made thinner if the butterfly in figure 9 is white against a black background.
Black and white
Unexposed photopolymer is washed out, exposed polymer turns hard and cannot normally be washed out. For that reason the image that you are going to reproduce must be in black and white. Use only black and white, shades of grey cannot be reproduced. Black and white should also be inverted relative to the original (see fig. 8 and 9), and you should use a mirror image since the negative will be put face down. Be sure that you produce a negative with a very dense black, so that no light will be transmitted through black areas.
Printing
The negative may be produced by a laser printer on transparent paper. We use an inkjet printer. This does not create sufficiently clear-cut illustrations on transparent paper, as the ink tends to spread. We have, however, found that overhead sheets are quite suitable. This is the procedure we have adopted with the Epson Stylus Color 800 printer:
- Use e.g. Océ InkJet Overheadfilm IJ130 (Art. 99896154).
- Select Photo Quality Glossy Film
- Resolution 1440 dpm, otherwise the black background is unsatisfactory
- Turn Halftoning off (Advanced settings)
- Turn colour printing off.
The sheets are relatively costly, but the moulds used with the Delft casting technique are small, so that one sheet can be used to create many moulds. However, if you use the same sheet too often, the repeated passing through the printer adversely affects the sheet. Finally, if you proceed immediately from printing to making the mould, allow the ink to really dry; we usually apply a hairdrier briefly.