One-Shift Shell: Speed up the Shell Room for Profit
Cycle time reduction has direct impact on profitability for several reasons. Quick response and delivery also makes it possible to get on the “Other” customer list: Most Improved Supplier. A unique shell system is presented that can take a part from assembly Monday morning, apply 6 coats in one shift, dewax the next morning, and have a metal casting by noon on Tuesday. The only special equipment needed is a low humidity final dry cabinet. A slurry consisting of a 35% silica sol and a fiber enhanced dry blend of fused silica powder was found to be able to meet the requirement of wax to metal in less than two days. Using super absorbant polymer coated stucco techniques pioneered by the University of Birmingham (UK), this cycle time can be reduced to less than one day, but the shell may not be a robust.
In today’s business climate, speed of manufacturing is important for many reasons. Customers expect and demand short times from order placement to order delivery. Some of the many advantages for the manufacturer, besides satisfying the customer, are:
- Ability to increase through put without additional factory space
- Lower inventory in process
- Reduced time span needed to ship parts
- Shorter development times
- Lower scrap rates should a problem occur (less product at risk)
One of the opportunities for reducing span time for investment casting is in the shell room. Typically, investment casting shell rooms take 2-4 days to shell parts. Very little of this time is direct labor. Most of this time is needed for drying between dips and final drying before dewax.
Buntrock Industries looked at several shell systems and binders to develop a shell that would reduce the shelling time to only one day from wax assembly to dewax. One customer had asked that dipping would occur on day shift only. So, this challenge was to receive the wax or rapid prototype pattern Monday morning, have it completely shelled by the end of day shift, dewax Tuesday morning, and pour metal Tuesday afternoon stabilized Alpha and Beta phases of Titanium. The Oxygen content in the surface layer is much higher than in the base metal. This layer is hard and brittle compared to the base metal and must be removed from many castings, especially those that have critical mechanical property requirements. Alpha case can be removed by machining, blasting, and chemical milling.