Investment Casting Design Guidelines

A Simple Nine-Step Process

1. Investment Casting Pattern are designed on a 3D CAD system and saved to an STL file for use with SL equipment. Solid the part geometry and the final design is uploaded to the company’s SL machines where it is produced in a matter of hours.


2. Each Investment Casting Pattern component is sealed  and leak checked once it comes out of the SL machine. The components are no holes are present in the final part. (It is recommended that leak-checking also be completed at the foundry after gating and sprue assembly, prior to first dip.)


3. Once the assembly gets to the foundry, Investment Casting Patterns are secured to a central wax bar with gates, called a sprue, to choose to design the central bar and gates into the original CAD design for upload to the SL machine.


4. A shell mold is created by dipping (investing) the cluster into a very fine ceramic slurry. The first layer, the face coat, allows for the reproduction of fine detailed features.


5. After this coating,the shell is layered with a fine ceramic refractory grain like sand. Upon drying, the process of dipping the shell mold into the slurry and layering the sand is repeated (with coarser grains) to obtain the desired shell thickness.


6. After the shell mold dries, it is typically flash-fired in a furnace to sinter the mold and remove the Investment Casting Pattern style the shell. These processes leave a negative impression of the assembly.


7. The mold is preheated prior to pouring in the molten metal using gravity, pressure, vacuum, or centrifugal force become one solid casting as the metal cools.


8. The ceramic shell is removed from the solid metal through mechanical vibration, chemical cleaning, or water blasting depending on the particular metal used.


9. The original parts are now cut from the sprue and gates and ground smooth so that they are ready for additional processes.


10. Final inspection is performed on each part to assure dimensional accuracy, material density, and other mechanical properties dependent on their final use.

Investment Casting Pattern Handling & Processing Guidelines

Receiving a Investment Casting Pattern
Received patterns must be kept in a room that is maintained cool and dry, away from humidity. Additionally, and to prevent water absorption, the pattern should be kept in a sealed plastic bag with a desiccant.

Checking for leaks
It is always wise to double-check the Investment Casting Pattern for holes, cracks, or leaks of any kind. Even the smallest opening can allow slurry to run into the interior of the pattern, which would create a non-metallic inclusion in the casting. It is advised that both pressure (to locate holes) and vacuum checks (to find hairline cracks) are made to assure the pattern is sealed. Holes may be sealed using casting wax or photocure resin.

Creating specific features
Similar to any mold pattern, a foundry may wish to add wax into a fillet to follow their specific casting design requirements for adequate radii. Undrained sections under 0.060 in (1.5mm) may expand and crack the shell at the burnout temperature. Always be careful to maintain the dimensional requirements of the pattern or expect to machine or grind excess metal to acquire the dimension needed.

Gating and venting
It is recommended to all foundries that they over-gate their patterns. This eliminates the potential for having to remake a casting. The gating area can be cleaned using Isopropyl Alcohol for a more secure attachment of the wax gate. Because some surfaces repel wax, it may be necessary to sand the area using 180-grit sandpaper. This will allow a strong bond between the gate and pattern. Once the gating is secure, it is suggested that both pressure and vacuum checks are made again to assure a good seal.

The most common method used to vent a pattern is to use gating runners that can back drain into the pouring cup.

Shelling the pattern
Most foundries have developed a proprietary process that they use for shelling patterns. Investment Casting Pattern will work well with any of them. The popular shelling materials include fused silica, alumina, aluminosilicate, and zirconium silicate.

Fused silica transforms into cristobalite when heated over 1650°F (899°C), and will happen during the burnout cycle. This limits a shell to only one thermal cycle. This means that if the shell is cooled to clear it of ash, it cannot be reheated without deteriorating.

Fused silica can be used with Ferrous metals because of the extreme heat of the molten metal itself. Not only will the pattern be burned from the shell, but the high temperature will often vaporize any ash as it is poured. This guarantees that it is done in a single thermal cycle, where the shell is not returned to room temperature.

Systems that can be cooled after the burnout cycle and reheated before pouring include alumina, aluminosilicate, and zirconium silicate. After the shell cools, any ash remaining from the Investment Casting Pattern can be cleaned through flushing, vacuuming, or vibration. This is the preferred shell material for non-ferrous materials because it will withstand multiple thermal cycles.

Many facecoat slurry systems contain surfactants to promote wetting the slurry on wax. Some slurry systems do not adhere to all of the resins being used. If this is the case, it’s easy to facilitate adhesion to Investment Casting Patterns by using aerosol glue, and allowing it to dry, before dipping begins.

Three typical types of binders are used today. Water-based binders like colloidal silica, alcohol-based binders like ethyl silicate, and other water based specialty binders all work with Investment Casting Patterns. The first dip exhibits low water content, and is intended to dry quickly, which exposes the Investment Casting Pattern to a minimal amount of water before the shell takes over. Even though subsequent dips have higher moisture content, they are not in direct contact with the Investment Casting Pattern.

Note that if a Investment Casting Pattern creates high stresses on the shell, there are several ways to eliminate the problem. By adding extra backing coats (multiple layers to the shell mold), stresses can be reduced. Another method used is by incorporating wire mesh between coats. Finally, adding chopped ceramic fibers, chopped stainless steel wire, or chopped stainless steel wool can relieve undue stress on the shell.

Burning out the pattern
Some foundries use an autoclave to remove the wax gates and runners from the pattern, but this may not create enough heat to remove the Investment Casting Pattern, which needs the higher temperatures associated with flash-fire furnaces.

Guide for burning the Investment Casting Pattern from the shell:

• Furnace preheat to 1400°F (760°C)
• Melt the wax gating as much as possible before burnout(using a handheld torch).
• Load shell mold and increase heat to 1625°F (885°C) ina 30 minute period.
• Maintain the desired temperature for at least 2 hours. Heat for longer periods of time for large patterns, and occasionally burn a second time for extremely large patterns.

Maintaining an 8 to 12 percent oxygen content inside the furnace prevents oxides from forming inside the shell. Oxides appear as a crystal residue. The oxide will not adhere to the metal casting, but will make the surface pitted.

For non-ferrous alloys, the shell should be cooled and inspected for the ash residue that can cause the quality of the casting to decrease. Since extreme heat is used when pouring ferrous metals, the shell should not be cooled. It is expected that any ash will be vaporized by the higher heat used.

Inspecting the casting
Investment Casting Patterns are built in layers, which means that the build lines may be reproduced in the casting surface, and should not be misinterpreted as defects during the inspection process.

Using a variable heat soldering iron with specialized tips. Torque testing was done with a Sturtevant Richmont model CAL 36/4 torque screwdriver.

The pull-out strength test sample and fixture as shown in figure 3 was built with a pilot hole and post drilled to the correct diameter. Installation of the Heli-Coil inserts was done using the OEM installation tools. Pull-out strength was measured using a United SSTM-20kN tensile tester and an aluminium fixture.

Data: The data represents 600 data points; 5 data points per insert size, type and material. Helicoil and heat staked inserts were installed in 5 LS materials: Nylon 12 AF Nylon 12 GF, Nylon 12 PA, NyTek 1100 and NyTek 1200CF. Inserts sizes include 2-56, 4-40, 6-32, 8-32, 10-32 and ¼-20; Metric inserts were not tested but are considered to be equivalent in strength for each comparable insert size. Table 1 provides an average of the torque limit and pull-out strength for each insert size and type.

An appendix to this document contains the source data and box plots of each size/type of insert separated by LS material.

Conclusions
The data shows important distinctions between Heli-coil and Heat staked inserts; for each insert size and material combination the heat staked inserts have higher torque strength. In terms of pull out strength, the performance of Heli-coils and heat staked inserts is quite similar to inserts size 8-31 and smaller; larger sized heat staked inserts tend to perform better than Heli-coils of the same size. While Heli-coils do offer the advantage of a smaller boss size relative to the insert, heat staked inserts typically outperform Heli-coils if the boss size and hole size recommendations are used.