To summarize the burnout process:
Ensure that vents are opened and there is a path for airflow through the shell to bring fresh air to support pattern combustion. If you opened the vents to get them through the autoclave, that should be sufficient.
Ensure that there is adequate oxygen in the furnace atmosphere. There are several ways to do this as explained in the text.
Burnout in a furnace with a temperature of a minimum 1500ºF or higher. Higher temperatures may speed combustion and could result in a more complete combustion.
Burnout for a minimum of two hours for small patterns, three hours or longer for larger patterns. These times are conservative. Residual ash testing shows that it takes a minimum of 90 minutes burn time to achieve low ash levels.
After autoclaving, all the wax will have been eliminated from the mold. However, the QuickCast patterns are still intact within the shell. The stereolithography resin used to build the pattern will not melt and must instead be burned out of the shell. The burnout process must not only result in the complete combustion of the pattern, it must also not compromise the integrity of the shell. Unfortunately, the process of heating the pattern in the mold to achieve combustion also causes some dimensional expansion which can exert forces on the shell causing cracking or even full failure if not done correctly – primarily by following the procedures outlined in the earlier sections of this guide. Typically, if the shell was not compromised during the autoclave step, then then the burnout will not cause failure either. The plastic material will have become softened during the autoclaving from the heated steam. Therefore if the burnout process is done while the mold is still very warm, without a complete cool down period, the plastic will collapse inward as heated further, instead of outward against the shell walls.
In order to achieve complete combustion, and leave only a minimal amount of ash requires the right conditions. Once done, the shell is every bit the equal of one built using molded wax patterns.
Burnout under the wrong conditions, however, can result in incomplete combustion, leaving a tarry substance in the shell that can be very difficult to remove. Several variables affect the burnout process. They include:
Time in the furnace
Oxygen Content of the furnace
Foundries who have been successful with QuickCast patterns have generally had to experiment with the above variables until they found a process that worked for them. In addition, they generally have held that process close to the vest, being reluctant to give competitors any information about it. Over the years, we have picked up bits and pieces of information about what has worked and what has not worked, but never have been able to provide a complete recipe to foundries.
3D Systems has worked with a number of foundries to more completely understand how each of the above variables affects the burnout process and to be able to provide specific process recommendations to the foundry. In one test, 36 patterns were burned out in an attempt to determine optimum temperatures and times for burnout. The QuickCast pattern used in the test was the 9-wall part, a test part that has been used since the mid 1990's in developing the ability to successfully cast QuickCast patterns. The 9-wall patterns contained 9 thin walls ranging from 0.020 inch to 0.100 inch. At least two of the thinnest walls would be extremely difficult to build hollow so there would be two or more solid thin walls in the pattern. Consequently, to eliminate the possibility that the wax gates and sprue could affect the results, the pattern was built with a gate and sprue attached as shown in the photo. The patterns were not vented.
36 such patterns were shelled using a fused silica shell system.
Two different furnace temperatures (1500F and 1800F) and 6 different times(30, 60, 90, 120, 150 and 180 minutes) were chosen for 18 distinct temperature/ time combinations. Two patterns were burned at each combination. The furnaces used did have plant compressed air added during firing to add oxygen. The air was not directed into the shells.
18 shells were placed in each furnace. The pattern material burst into flame almost immediately and burned quickly, as shown in the photo. The flame went out within a few minutes, so the material apparently burned out rather quickly.
At each half hour increment, the furnace was opened and two shells were removed. Over the period of three hours, all 36 shells were removed from the furnace. After the shells had cooled, one of each pair was cut open and inspected for signs of incomplete combustion.
We expected to find incomplete combustion at the lower furnace temperature and shorter burn times. However, we found no sign of incomplete combustion in any of the shells. The photo shows part of the shell which was burned at 1500F for 30 minutes. Even the 0.020 inch wall section (where the pattern was solid) burned out cleanly. We could then conclude that, for the set of conditions used in the test, and assuming that there is adequate oxygen available to support resin combustion, patterns will burn out adequately at any temperature above 1500F and for any time longer than 30 minutes.
Foundries have reported successfully burning out patterns at temperatures as low as 1100F.
Oxygen Content: Sufficient oxygen is necessary for proper combustion of the QuickCast pattern. If there is not enough oxygen, ash content increases significantly and will look like small pieces of charcoal rather than the white feathery ash normally associated with QuickCast patterns. In extreme cases, a tarry residue will be left in the shell.
Oxygen is not necessary for preheat. Consequently, most preheat furnaces do not have the capability to increase the oxygen level in the furnace and much of the oxygen that was originally in the furnace is depleted in the process of heating the furnace. Without somehow adding oxygen to the furnace atmosphere, it is difficult to achieve complete combustion in a conventional pre-heat furnace.
Oxygen is not effective unless it is present in the far reaches of the shell cavity where pattern combustion needs to take place. The shell used for conventional investment casting makes it very difficult for oxygen to reach the pattern and for combustion gases to escape. While the pouring cup is fairly open, the passage to the QuickCast pattern narrows considerably at the gate. Since the cavity is a closed vessel, there is minimal airflow into the cavity, increasing the difficulty of getting oxygen directly to the combustion site and of getting combustion gases out.
Fortunately, there are relatively simple solutions to this problem. The vents that were added to help get the shell through the autoclave safely will also be of tremendous value getting oxygen to the pattern and allowing combustion gases to escape.
Some foundries simply make a long nozzle for an air gun (stainless tubing works well) and use it to blow air directly into a vent, forcing plant air into the shell. Oxygen is supplied by the plant air and consequently it is not necessary to adjust the oxygen level of the furnace atmosphere. There are some potential downsides to this method, however. First, the plant air could cool the inside surface of the shell and introduce thermal stresses that could cause some local cracking. This effect will be at least partially offset by the heat generated by combustion. Second, air must be blown into each cavity, which can be time consuming for an assembly with multiple patterns. It might also be difficult to get to all the patterns if they are spaced around the assembly.
A better process may be to allow the vents to act as “natural chimneys” for the gases of combustion to escape as illustrated in the figure. As combustion gases escape through the vents, air will be drawn naturally through the pouring cup. This configuration will provide good burning provided that:
Air can get in through the pouring cup. It will not work if the pouring cup is set on a flat floor of the furnace, effectively closing off the flow of air. Most foundries will set the assembly on a grate above the floor of the furnace. Alternatively, the assembly can be placed on top of two bricks placed an inch or two apart. The space will allow air to flow between the bricks and up into the pouring cup.
The furnace atmosphere has sufficient oxygen. If the furnace has no provision for adding oxygen to the atmosphere, there will probably not be sufficient oxygen in the furnace. Some foundries run a line with plant compressed air into the furnace and turn it on when they are burning out QuickCast patterns.
One foundry came up with a novel solution. They drilled a hole through the door of their furnace and covered it with a simple swinging plate. They use the same steel rack for all QuickCast shells and the shell is always located on the same spot on the rack. They have locating pegs in the furnace so that when the rack is inserted, the pouring cup will be positioned in exactly the same spot each time. They also have a locating bracket on the floor of the furnace directly below the pouring cup. To deliver the air, they simply attach a long pipe with a 90 degree elbow on the end to the exhaust of a simple shop vacuum cleaner. During the burnout process, they insert the pipe through the hole they cut in the door and push it in until it reaches the bracket. It is then located directly below the pouring cup with the elbow pointing right into the pouring cup. When they turn on the shop vacuum, the furnace is provided with a steady source of low pressure, oxygen rich plant air to support combustion. The setup is illustrated in the figure below.
Shell Type: Alumino-silicate shell systems are stronger and do not have a transformation at higher temperatures allowing them to be cooled to room temperature and reheated when ready to pour metal.
Fused silica shells, however, are a little more complicated to work with. Fused silica undergoes a conversion to Cristobalite at temperatures above about 1650F. In general, foundries prefer to avoid the cristobalite conversion because shell strength decreases dramatically when the shell is cooled to remove burnout residue. The shell may not have sufficient strength upon reheating to safely allow pouring. To avoid the conversion to Cristobalite, many foundries burn out the shell at a lower temperature, generally around 1500ºF, although we have heard of some burning out as low as 1100ºF.
Many foundries, however, only have a single preheat furnace which complicates processing QuickCast patterns. If the temperature is lowered to burn out a QuickCast pattern, the furnace can’t be used to preheat regular production shells. Consequently, production stops to burnout QuickCast patterns. Of course, it is possible to schedule QuickCast burnout around regular pouring schedules, but it increases scheduling complexity and may increase the time required to process QuickCast patterns
We know of at least two foundries that burn out fused silica shells at 1800ºF routinely and have not had significant concerns with shell strength. However, we suspect that the strength of the shell after cristobalite conversion may vary significantly with the particular formulation of the shell system. Burnout at temperatures above the conversion temperature may work well with some shell systems and not at all with others. In addition, the fluid pressure on the shell will vary with the size (primarily height) of the casting. The weakened shell may be able to withstand the pressure of small castings, but not larger ones.