Lost Wax Casting; Reducing Casting Defects

How to eliminate casting defects when casting jewelry.

Almost all jewelry productions worldwide are made via the lost wax casting process. Lost wax casting, while versatile in that it allows you to create small batches and relatively low set up costs per each part, can lead to all sorts of defects. Almost every company casting silver jewelry has experienced bad casts due to shrinkage porosity or incomplete casting fills. While many people have tips and tricks that they find work for them, few are willing to share these with the rest of the industry.

Our good friend Tyler Teague of JETT ResearchProtoproducts, who has acted as a consultant to industry professionals for over ten years wrote such a great articles on this that we felt obligated to share some of his wisdom.

In his article “Fill ‘Em Up” which he originally published on MJSA, Tyler discusses the two extremely important yet little known concepts among casting houses the user of “Risers” and use of “Funneling”. The below excerpts of his original article are re-published with his permission. The original article may be found in it’s entirety at the following link Fill Em Up.

 

In the below general casting tips, Tyler addresses some of the most common mistakes that lead to casting defects:

Tips for improving fill and casting quality

As a general rule, when casting it’s best to use the coolest flask and metal temperatures possible that will allow for complete fill and good structural integrity. Cool system temperatures will create less sulfur dioxide gas at the metal-mold interface during casting, which can damage your castings in many ways, causing rough surfaces, enhanced shrinkage porosity, and copper sulfides and oxides—none of which is desirable.

To reduce the chances of creating too much sulfur dioxide gas, you must feed the metal in quickly and keep turbulence to a minimum. Here are some guidelines to follow for feeding castings:

1. Feed into the thickest part of the casting. This promotes differential cooling (also known as directional solidification).

2. Avoid or reduce turbulence to a minimum by shaping or aiming the metal stream as it enters the mold cavity.

3. Avoid sharp corners. These cause turbulence and can break off pieces of the investment mold and wash them into the mold.

4. Use the fewest number of feeders possible to fill the casting.

5. Be sure the feeder has a greater cross section than the cross section of the area where the gate contacts the casting (at least 1.25 times greater is safe).

6. Thick areas of the casting that cannot be fed from the main feeder should have another method, such as a riser, to feed them.

The next major concept that Tyler covers that we’ve included into our standard operating procedures is the use of what he calls Risers.

Using risers to improve fill in complicated jewelry castings

I don’t envy the job of a contract caster. With the growth of CAD/CAM for jewelry manufacturing, the industry is seeing an increase in jewelry designs that, while aesthetically pleasing, are difficult to cast successfully. Often uniquely shaped (for example, with lots of thick-to-thin sections) and sometimes quite large (such as big cuff bracelets), designer pieces can be especially difficult to work with when contract casters cannot modify models.

When feeding a casting of any shape or size, you must be mindful of the “thermal center”—the area that cools last and is most likely to incur shrinkage. To eliminate shrinkage porosity, you must move the thermal center out of the casting any way you can. It is a given that since the thermal center is in the heaviest part of a casting, you should attach the feeder to that section to better your chances of getting a successful casting. But this method is not foolproof: When they cut off the feeder, many casters are dismayed to find the thermal center manifest itself as a hole in the part beneath where the feeder was attached. There is a little-known trick adapted from the foundry industry that jewelry casters can use to pull the thermal centers out of challenging castings. It’s called a riser.

A riser’s purpose is to act as a reservoir of heat and feed metal. It is designed to prevent cavities that can occur as a result of shrinkage when the metal cools. They are especially useful when you have multiple heavy sections in a part with thin areas in between them. I use risers because they simply work better than an ungainly network of extra feeders running all around a casting and back to the main sprue. More often than not, extra feeders cause more problems than they solve. They use more metal, make mold cutting more difficult, and simply don’t feed a casting as well as they should. In many cases, multiple feeders actually act as heat sinks; rather than aiding fill, they require more heat to fill or simply prevent complete fill altogether.

Riser Tree Diagram

For instance, in the photo above, your inclination to place the feeder at the heaviest part of the casting—the top of the ring—is complicated by the intricate pattern on the ring top. Placing the feeder at the bottom of the shank makes it easier to cut the mold, but it’s inadequate to pull the thermal center out of the casting. As the casting cools, the shrinkage porosity will occur in the top of the ring, the heaviest part. By placing a riser on this part of the casting, and attaching it to the side so as not to mar the detailed top, you create a pool of molten metal that cools last, constantly feeding the heaviest section of the casting and pulling the thermal center out. Any shrinkage porosity occurs in the riser, which solidifies last.

Finally Tyler discussing Tapering and casting pressure. This is something very rarely considered by jewelry casters , but that can make a world of difference when casting intricate 3D printed parts.

Taper Away

Figure 1

Figure 1 is an example of a typical 3 mm round feeder with a cross-sectional surface area of 7.069 square millimeter  that I see in many casting operations.

Figure 2

Figure 2 is basically the same 3 mm feeder, but its end has been hammered into a flared shape.

Figure 3

Figure 3 is ideal: a larger tapered feeder with a cross-sectional area of 19.635 square millimeter on the large end and 7.069 square millimeter on the tapered end.

When casting at low system temperatures, flaring the feeder is beneficial because it increases velocity (speed and direction) of the feed metal as it enters the mold cavity. As the molten metal is entering the feeder and moving toward the gate and casting, the speed into the cavity (velocity) increases proportionally as the cross section of the feeder decreases. It’s the same thing that happens when a fireman puts the nozzle on the end of his fire hose. The water that would pass through a 4-inch hose at a given pressure in a fixed time interval is now passing through a tapered nozzle and out of a 1.5 inch to 2 inch opening. Since the same volume has to move in the same time, the speed has got to go up a lot. The difference between the figures shown is that the velocity of the metal entering in figure 3 will be about 2.78 times that in Figures 1 or 2. If you can fill the parts about 2.78 times faster, then it stands to reason that you can still fill the parts using lower system temperatures that improve the castings because less sulfur dioxide gas will be liberated.

Going back to our example of the firehose, the velocity change allows the fireman to deliver water to the top of the burning building, not just to the windows of the first floor.

How to save money and improve casting fills

It doesn’t matter whether you are 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. Some French guy named Blaise Pascal in the 1600s and a Swiss guy from the 1700s named Daniel Bernoulli described these laws of liquid pressure way back. Read up on it.

Figure 1

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 the metal you throw into the button makes you feel secure but serves as little more than a heat source and money drain.

Figure 2a Figure 2b

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.

 

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