SLA 3D Printed Investment Casting Patterns

The investment casting process has been around in one form or another for thousands of years. In the early 1990’s its was revolutionized when the first 3D Printing process was adapted for it. That process was SLA 3D printed investment casting patterns. The following article will explain how traditional “lost wax” investment casting works as well as how a process built on SLA 3D Printed investment casting patterns works and why it is the best choice for either low volume or prototype casting production.

What is investment casting and how is it traditionally done?

Investment casting is a process also known as the “lost-wax” process, or “precision” casting. In this process a wax pattern must be made for every casting and gating system; i.e., the pattern is expendable. This process offers the end user good value for money where good surface finish, complex geometry, and cast features are desirable without the necessity of extensive machining or other fabrication/ finishing work required to provide a usable end item. A traditional investment casting process goes something like this:

1. A steel injection mold is shot full with molten wax, after hardening it releases a wax pattern of the part to be cast
2. The wax pattern is then assembled onto a wax “tree” with a wax gate structure
3. The tree is then submerged in a wet ceramic slurry, then dunked in sand that sticks to the ceramic, this process is repeated multiple times to build up the outer shell of the mold
4. The mold is allowed to dry and then is heated in an autoclave to melt all the wax out of it
5. After all the wax has been melted out of the mold it is then heated up in a preheat furnace and poured full of metal.
6. After cooling the mold shell is broken off the casting
7. The individual cast parts and then sawed off the tree / great structure

Using SLA 3D Printed investment casting patterns:

Historically, the reluctance of the Designer or Buyer of a new part design that is considered low volume, short lead, or prototype to consider the investment casting process has been due to the lengthy lead times and high cost required to build the initial tooling. Depending on part complexity, the tool building process alone can take from 8 to 20 weeks and cost $50,000 to $100,000 or more. As a result many potential investment casting users have historically been driven away to other traditional metal working processes. The advent of SLA 3D Printing has filled in this niche for reducing the total turn around time in investment casting. By using Stereolithography 3D Printed parts as expendable patterns for investment casting without the need for tooling. The total turn around time can be reduced to 3 to 4 weeks and potentially the entire production run of patterns can be built for less then a wax tool would have cost. The SLA 3D Printed part is used as a substitute for the wax pattern part in the investment casting process which eliminates the need for low-production-run wax pattern tooling. Traditionally low volume and prototype orders tend to make the investment casting process less effective because of the high tooling costs and long lead times for wax pattern tooling. SLA 3D Printed investment casting patterns flip this age old problem on its head and opens the door for much smaller quantity orders of investment cast parts to begin to make economic sense for manufacturers.

What are the largest part sizes that can be successfully turned into a SLA 3D Printed investment casting patterns?

Why use SLA 3D Printed investment casting patterns to produce prototype metal parts instead of Metal 3D Printing (DMLS)?

Investment casting process when using SLA 3D printed investment casting patterns:

1. First, the investment casting pattern is designed on a 3D CAD system and saved to an STL which is then uploaded to Forerunner 3DP’s SLA 500 machine where it is produced in a matter of hours out of WaterShed XC 11122
2. Each investment casting pattern component is sealed and leak checked once it comes off of the SLA 500 and has its build support removed. 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 the investment casting patterns are secured to a central wax bar with gates, called a sprue
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 the first layer,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 from the shell
7. The mold is preheated prior to pouring in the molten metal
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

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