Autoclaving the Assembly

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Generalize

For conventional investment casting, autoclaving is the most common way of de-waxing, or eliminating the wax patterns from the shell. The QuickCast pattern, unlike wax, will not melt in the autoclave. However, there is typically a good amount of wax in the shell in the sprue, runners, gates and vents that must be removed prior to pouring. It is very desirable to be able to autoclave the shell to remove this wax prior to going into the furnace to burn out the QuickCast pattern.

Many foundries have had difficulty attempting to autoclave QuickCast patterns. Shells tend to crack during the autoclave cycle, often beyond repair. As a result, many foundries skip the autoclave step altogether and melt the sprue out with a hand torch as shown in the photo. Any wax remaining in the shell is later burned out in the furnace. While this work-around avoids cracks in the shell, it typically requires 30 minutes or more of labor per shell to melt the sprue. Any residual wax generates a great deal of smoke in the furnace.

In addition, this manual step is a disruption to the normal process flow of the foundry and therefore increases the chances that a processing error will be made. It would be preferable to directly autoclave the shells, avoiding labor, smoke, and any disruption to the regular foundry process.

Some foundries have reported that the shell in the sprue area gets red hot before all the wax is melted out. In some cases, that area of the shell gets hot enough to go through cristobalite conversion and as a result, the strength of that area of the shell is seriously compromised when reheated prior to pouring. If the shell gives way during pouring, a very dangerous situation will then result.

3D Systems ran tests in conjunction with two customer foundries to determine the cause of shell failure during the autoclave cycle and potential solutions. It was never previously clear whether it was thermal expansion of the pattern material or expansion of the air inside the pattern that caused the cracking. We designed the following test to identify the cause:

In the first test, eight QuickCast patterns were built. Four trees were then created with two patterns per tree. A hole was drilled into the pattern on the surface of the gate that would be attached to the sprue and the pattern was attached to the sprue using sticky wax.

Shells were then built using a fused silica shell system. The four shells were then prepared as follows: Shell 1 had no additional processing. In Shell 2, the sprue was melted out with a torch, exposing the hole drilled on the gate surface. Shells 3 and 4 simply had holes drilled through the shell into the pattern, simulating a vent. The wax sprue was left in place.

All four shells were placed in the autoclave at once.  The autoclave was pressurized to 106 psi with 350ºF steam in 2.87 seconds. Shells were held under pressure for 20 minutes. All four shells were inspected after the autoclave cycle. Shell 1 had cracked. The remaining three shells had no damage.

From this test, it is clear that venting the interior of the pattern before autoclaving reduces the likelihood of shell cracking. The shell that cracked was the only one of the four assemblies that had a sealed pattern at the beginning of the cycle. All the patterns that were vented survived the autoclave cycle intact.

 

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While venting had a significant effect, the cause of cracking was not obvious. It had always been assumed that thermal expansion of the pattern material was responsible. In fact, the design of the QuickCast build style was an attempt to allow the pattern to collapse inward as it expanded rather than exert excess pressure on the shell. However, if the root of the problem was differential thermal expansion of the pattern material, all the shells would have cracked.

One theory was that internal pressure resulting from heating of the air inside the pattern caused the cracking. That would explain why the vented patterns did not crack. However, it was calculated that the pressure rise inside the pattern from a temperature rise of 275F would be approximately 8 psi. Considering that this internal pressure is resisted by an external steam pressure of more than 100 psi, it doesn’t seem likely that the pressure would crack the shell.

A second theory was that the greater external pressure resulted in a compressive failure of the shell. With vented patterns, the internal pressure would be the same as the external pressure, eliminating any net forces on the shell. Sealed patterns would have a lower internal pressure and the resulting net external force on the shell could potentially result in shell failure. However, the cracks we observed were very clean breaks as would be expected in a shell tension failure. We did not see any crumbling of the edges of the crack, or multiple crack lines as would be expected in a shell compression failure.

The second test in another foundry confirmed that external pressure was not the culprit. In that test, four 9-wall patterns were shelled. Each pattern was vented in three places, yet all four patterns cracked in the autoclave. In each case, it was clear that the crack initiated at the 0.040” thick wall – the thickest wall that was built solid. This failure points to thermal expansion of the pattern material.

In the first test, we broke apart the shell that cracked to inspect the pattern. We were somewhat surprised to find that the pattern was very rubbery and could easily be compressed. With this information, we theorized that upon exposure to the superheated steam, the resin softens quickly, and even though it expands with heat, it can deform and collapse inward, instead of exerting pressure on the shell. Venting allows the steam to reach the interior of the pattern, accelerating the softening of the honeycomb structure and skin of the pattern. Conversely, if the pattern is sealed, heat must be transferred through the skin of the pattern before reaching the internal hexagonal honeycomb structure, significantly slowing the softening process.

The mechanism of softening, however, is not entirely clear. Is it heat alone that softens the material? Or is it primarily absorption of humidity with the rate of absorption is accelerated by the heat? It may be a combination. Further testing will be done to learn more.

Any wall that is not drained properly, and is therefore solid plastic will potentially exhibit thermal expansion and forces beyond the strength of the shell, resulting in shell cracking.

 

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As a result of these tests and our current understanding of the process, we recommend the following procedure for avoiding shell failure during the autoclave process.

  1. Include at least one vent on every pattern on the tree!. A simple piece of spaghetti wax will suffice.  A hole can be drilled under the vent so that as soon  as the wax vent is melted out, there will be a path for pressure release from the pattern. However, it may be prudent not to drill the hole. If the vent is dislodged during a dip, the hole would allow slurry to enter the pattern, ruining the casting. If the hole were not yet drilled, the pattern could still be salvaged. More than one vent will provide a path for steam to flow through the pattern, accelerating the softening. An alternative to using a wax vent is to include the vent in the pattern model and it will be built as part of the QuickCast pattern. This approach will minimize downstream labor.

  1. After the shell is built, cut off the portion of the shell at the end of the vent to expose the vent. The end of the vent can simply be ground or cut off.

  1. Melt out the wax in the vent. This can be done with a lance heated in a flame as shown in the photo.  If a QuickCast vent is used, there is no need to melt out the vent.   It   may be worthwhile, however, to poke through some of the honeycomb structure to increase the area available to flow of steam into the pattern.

  1. Puncture the skin of the pattern. To be really safe, you can use a thin drill bit, but the hot lance used to melt the wax will work fine. If a QuickCast vent is used, this step will not be necessary.

  1. If there are solid areas on the pattern, melt out the sprue by hand. There is little that can be done to ensure that the substantial solid areas of the pattern will soften quickly enough to avoid cracking the shell from thermal expansion. Extra coats on the shell will strengthen the shell, but it is hard to predict how much it can resist the thermal expansion.

With the above steps, most shells can be successfully autoclaved.