Rapid Tooling for Casting Patterns

Among other things, rapid prototyping is used to produce rapid tooling.  Rapid tooling is then used to produce patterns destined to create sand or investment castings .  The powerful reasoning for using rapid prototypes goes as follows.

     1) Rapid prototypes are perfect candidates for pattern making since the process is geared to low volume production of one or two pieces.

     2) Traditional wooden or aluminum patterns are expensive to build due to the lack of experienced shops capable of doing high quality manual or CNC work.  Many domestic patternmakers command premium prices for their labor.  Even though CNC pattern making can reduce the manual labor component for simple parts, when the part geometry is complex, the intricacies of undercuts and the difficulty in reaching all of the pattern surfaces with the cutting tool creates problems for the pattern maker using manual or CNC machining centers.  Rapid prototypes bypass this issue by depositing material instead of removing it.  The process is perfectly suited to build complex patterns in one pass, at much lower cost.

     3) Lead times are much shorter when using rapid prototypes as casting patterns when the pattern involves complex surfaces.  However, simple patterns can still be produced on CNC machining centers or manual machines in less time than rapid prototype machines due to the speed of material removal using a cutting tool when the machine has access to all surfaces.  Rapid prototyping lead time for a single piece can range from a few hours to 3 days.  Pre-production tweaking and setup is added to that time.  Conventional pattern building can take weeks or longer when complex patterns are needed.

Investment Castings using Rapid Tooling for Plaster Molds

This process is used for short run production on zinc and aluminum castings where volumes are usually less than 2,000 pcs./year.  The surface finish is good and often approaches that of a traditional die cast part.  Complex configurations present few problems since the rubber molds can accommodate undercuts and fine detail without damage.  Investment casters using the plaster mold process do not use a wax pattern.  The plaster mold process uses a pattern master.  The pattern master is often created using the SLA (stereolithography) rapid prototype (RP) process.  The SLA model of the part is made using the 3D solid modeling CAD which has been modified for shrinkage and other minor factors.  The part master is then used to build a temporary rubber negative mold.  The rubber negative mold is then used to make the final rubber positive mold.  The final rubber positive mold is then covered with plaster slurry to produce the plaster mold. The metal is poured into the plaster mold and the process proceeds to completion in the traditional method.

Prototype Investment Castings using Rapid Tooling

Rapid tooling can save much time and money when only one or several prototype pieces are needed for cast metal alloy parts. There are a few variations, but the process is generally the same.  SLS rapid prototype parts are made from the 3D CAD model with the usual modifications for shrinkage and other minor factors.  The RP media is often a mixture of polystyrene and foundry wax.  The process creates a direct positive which is immersed in the casting slurry directly.  No other molds are required and the rapid prototyping piece is used as the casting pattern.  They are directly used to make the ceramic mold. For every metal casting, one RP investment casting pattern is required.  When the ceramic mold is fired, the pattern is burned out and destroyed.