These slow, quiet days of summer are always a good time to review your shop practices before the busy holiday season kicks into high gear. To help you prepare to successfully cast all of the pieces you hope will be big sellers this year, we’ve compiled some of the best casting tips that have filled these pages over the years. From easier wax carving to minimizing metal waste, these tips are bound to have you casting like a champ just in time for those holiday orders.
When drawing a layout on carving wax, I use black brush markers, which are available from arts and crafts supply stores: If I don’t like the pattern, I can wipe it off and draw it again. However, a marker’s ink will tend to bead up on the wax surface. To break this surface tension, I wipe the marker across a bar of soap before I begin drawing. Now the ink will evenly cover the surface of the wax.
If you are working on darker waxes, instead of a marker, use white tempera paint with a drop of dish washing soap in it. However, make sure you wash off all of the paint before using a wax pen: Tempera paint, mixed into molten wax, can cause porosity. When the design looks acceptable, I scribe through the pattern into the wax, then fill the scribed line with the marker or paint.
Variances in the chemical composition of metals and water can ruin a casting. To establish some basic control, make sure you use de-ionized water, and always measure the pH balance. With a simple pH meter, you’ll want to look for a consistent reading from your water supply—rather than a particular pH number—and adjust your casting processes in accord with that consistent pH level.
If the pH in your water source changes significantly from day to day, such as from 6.9 to 7.5, and you don’t know it, you’ll struggle with unexplained variances in your casting output. You should also take samples and analyze your raw materials. Do they have too much oxygen or trace elements that will affect the quality of the casting? The best way to know this is to have the supplier or an outside contractor do an X-ray fluorescence (XRF) test on a sample. (You can also invest in your own small XRF machine, which can now be had for around $15,000.) An XRF does not create a compositional profile, however. You tell the testing company what substances to look for, and it will tell you how much the metal contains in parts per million. For example, you might have sterling silver analyzed for oxygen and phosphorous content— both can contribute to poor castings. With purple gold, an extremely difficult alloy to cast, there’s no room for variance in its gold and aluminum content.
An XRF for likely contaminants will also alert you to future adjustments that may be necessary as you reuse metals from previous castings. Contaminants may build up over successive castings, requiring higher ratios of fresh metal for each batch. At $20 to $30 per XRF test, the cost is cheap compared with the casting problems prevented.
Many companies use spring-operated scales to weigh their investment in pounds or kilos. However, these scales can often be inaccurate when measuring weights of 20lbs. or more.
In our shop, we use a large digital scale similar to the ones used by meat departments in grocery stores. These scales are very accurate and can easily accommodate the required weights necessary for most home and commercial casters. Place a simple plastic bucket on the scale and “tare” or zero the scale (to account for the added weight), then weigh your investment according to the investment manufacturer’s guidelines.
When it comes to weighing water, most investment manufacturers tell you to measure your water in cubic centimeters (cc’s) or milliliters. Most casters will buy a graduated cylinder for this purpose— not realizing that they can weigh the water. In the metric system, 1 cc = 1 gram, so if you need 182 cc’s of water, you will also need 182 grams.We use an inexpensive digital postal scale to weigh our water, which will measure up to 10 lbs. (over 4,500 grams) and is perfect for small shops. Weigh the water in a large plastic container (again, account for the added weight). This way, you can use a sponge to easily add or reduce water as needed, without removing the container from the scale.
Properly cleaning and curing resin-based models, and using the right investment, are musts. After that, since resin-based models don’t melt, but rather expand, shrink, and finally burn, it’s critical to modify your burnout process to reduce ash and improve casting quality. Here are a few tricks I use:
It doesn’t matter whether you’re casting large production trees or small ones, using vacuum-assist or simply gravity casting: The use of large buttons is a waste of money. It’s pure superstition that a large button exerts enough pressure on a casting to improve fill. Fact is, the pressure in the flask is relative to depth and not to volume, and only the metal that is directly over the main sprue exerts any pressure at all.
Basically, depth rules. In Figure 1, the pressure at all points marked “P” are the same regardless of the shape or size of the container. This means that all that metal you throw into the button makes you feel secure but serves as little more than a heat source and money drain.
If you really want a good fill, use a tall main sprue with a funnel-shaped pour basin, as shown in Figure 2; the drop of the metal from the crucible to the bottom of the flask takes advantage of both the kinetic energy gained during the pour and the inertia of the molten mass, leading to a better fill. I prefer to use full-size sprues and flasks when doing this technique to ensure I get those added advantages. The couple of dollars for the extra investment is cheap compared to the cost of the metal for a button or a non-fill casting.
Remember, you want to cast only as much metal as you really need. To help you figure that out, here are my step-by-step instructions for the fearless casting of a buttonless tree, free of superstition: