Moldmakers can now use rapid-prototyping techniques to make cavities for plastic and wax-injection molding. The advantage of such techniques is that they can produce mold cavities quickly, and frequently at less cost than conventional CNC mold cutting.
One reason RP techniques have become more widely used is that their accuracy and surface finish has improved dramatically in the recent past. Good Stereolithography (SL) parts, for example, can often hold 1 to 2-mil tolerances on half their dimensions, and 3 to 5 mils on the rest. As recently as two years ago, best-case SLA tolerances were in the 5 to 10-mil range.
Perhaps the most surprising way of obtaining mold tooling quickly is to use an SL part directly as the mold. This technique is noteworthy because normal injection-molding temperatures are on the order of 450ºF, but ordinary SL epoxy parts have a melting point of roughly half that.
Theoretically, mold tooling made of SL epoxy should quickly become a puddle. In practice, however, this is not the case. Judicious use of cooling and cycle time allows such molds to last long enough to shoot several dozen parts made from nonabrasive resins such as polypropylene. One project, for example, used cycle times on the order of 10 min. as a means of letting heat dissipate before the next shot.
One caveat about using SL parts directly as molds pertains to the size of minimum dimensions. There are obvious difficulties in cooling SL molds containing thin walls. Thus, parts requiring such features generally must either be handled in some other way or be overmolded and machined to final shape in a postprocessing step. Also, the minimum dimensions that are practical depend on the resin used. And there are compromises in the mold design. One is that an SL mold cannot contain moving parts.
A more traditional way of obtaining molds from RP parts uses RTV as the molding material. The RP part serves as a master from which an RTV mold is cast. These molds can comfortably provide features that are within ±5 mils of the pattern. They can also handle thin-wall dimensions on the order of 20 mils wide or more.
Until recently, urethane has been the material of choice for RTV-molded parts. The difficulty is that urethane incurs lengthy curing times -- only two or three parts daily from an RTV mold is typical. But processes perfected within the past year address this limitation. One called thin-wall reaction injection-molding (TW-RIM) can produce copies of rapidly prototyped parts at a rate of about three or four/hour. The short cycle time comes from use of fast-setting polyurethanes and other materials that gel in less than a minute and which can be demolded after about 10 min.
One factor that makes such fast cure times possible is the development of inexpensive disposable mixing heads. They automatically mix two-part resins in a vacuum and shoot the material into the mold. Previously, molders had to mix the materials by hand, put the mixture in a vacuum chamber to de-air it, then send the material to an injector head. These time-consuming steps made it impractical to shoot material that gelled in less than a minute.
The cost of injection equipment for fast-setting resins has dropped, boosting the technique's popularity all the more. When it first debuted, the least expensive injection system ran about $20,000. Newer equipment for handling fast-setting resins carries a price tag of around $4,000.
An additional benefit of quick curing is that it can substantially lengthen the life of a mold. This is because the curing process plays a significant role in disintegrating the mold. Thus a mold that might normally last through 25 to 50 shots could be expected to provide 50 to 100 shots when used with quick-curing resins.
Materials used in such systems have properties similar to those of thermoplastics such as polypropylene. For example, new resins are strong enough to handle parts containing snap fits, providing a flexural strength of 6,400 psi and 4,450-psi tensile strength.
Applications requiring a few hundred parts are candidates for aluminum-filled epoxy molds. These consist of a two-part epoxy mixed with aluminum powder to promote heat transfer. They provide feature dimensions, tolerances, and part finish comparable to those of RTV molds and are constructed in a similar manner. But they're more durable, providing molds capable of shooting a few hundred parts or more.
Two relatively new tooling processes now address applications demanding tens of thousands of parts. RapidTooling is a term coined by DTM Corp. for a process that turns RP parts made on its Sinterstations into metal mold tooling. The technique starts with the creation of a mold cavity geometry on a Sinterstation. When it exits the Sinterstation, the cavity is comprised of carbon steel pellets coated with thermoplastic binder. The part is then soaked in an aqueous acrylic emulsion bath and dried in an oven to prevent distortion during ensuing steps.
Once it dries, the part enters a furnace to burn out the polymer and for infiltration with copper. The infiltration process produces a mold that is 60% iron and 40% copper and has properties similar to 7075 aluminum. After furnace treatment, the mold is polished and finished to final dimensions. This postprocessing gives RapidTool cavities a tolerance of ±10 mils. Turn-around time for the entire sequence is about a week.
DTM says RapidTool cores and cavities can produce tens of thousands of parts, depending on the type of plastic, pressure, and temperature used during the injection-molding process. It has tested the process with molds handling 68,000 shots so far.
RapidTool is a relatively new process. Consequently, there are only two service bureaus that have so far installed the special furnaces needed to handle it. These furnaces must be capable of holding temperatures to a tight ±25º over a range of 1,600 to 2,100ºF.
A similar technique that produces molds quickly comes from Keltool Inc. Several years ago the firm bought rights to a process known as Tartan Tool that was devised by 3M. News of its advantages are now spreading rapidly. Its chief benefit is rapid production of mold cavities that can stand up to millions of shots when used with nonabrasive resins such as polystyrene. The cavities are made of A-6 tool steel. These can be annealed or heat treated to Rockwell C40-42 or C48-50 hardness. The Keltool process begins with a rapidly prototyped model which serves as a master part. The part is shipped to Keltool in St. Paul, which builds a thin-metal box around the master. Technicians there fill the box with RTV silicone rubber to make a mold. After the RTV cures, the master is extracted. Into the RTV mold goes a proprietary powder-metal/plastic-binder mix to duplicate the exact shape of the master. The resulting "green" part is cured and then removed from the rubber mold.
Next, the green part undergoes firing in a furnace to sinter the metal and remove the plastic binder. The resulting fused part consists of 70% powdered metal and 30% void space. An ensuing furnace treatment infiltrates the voids with copper alloy to produce 100% solid mold tooling. Most such parts then go through a machining step for final dimensions. A similar sequence of steps can also be used to produce EDM electrodes.
The Keltool process has the advantage of providing ±1 mil tolerances per linear inch. At this point, the process is most accurate for parts that fit in a 5 X 5 X 5-in. envelope, though it will eventually be able to accurately handle larger objects. Keltool molds for objects of this size tend to cost between $2,000 and $5,000 and can be constructed in about four weeks. For a small premium, turnaround times of two weeks and even one week are available. In some cases, savings over typical machined molds can amount to as much as 50%.
Perhaps one potential drawback is that the Keltool process can be obtained only from Keltool Inc. itself. However, the firm says it is in the process of licensing its technology to other companies. Nearly al commercial rapid-prototyping systems can produce models that can serve as acceptable masters for the cavities. The resulting molds may also run faster than all-steel molds because of their good thermal conductivity.
Regardless of the rapid-tooling process used, there are a number of advantages over conventional techniques that relate to business considerations. In many cases, the new techniques do more than just cut costs. They can, for example, make it possible to build two mold cavities for the same price as one done conventionally, this boosting productivity and slashing manufacturing overhead.
It is also interesting to note that rapid-tooling approaches are most applicable to parts that are fairly complicated. Their biggest payoff is with molded components containing complex curves that are time consuming to replicate with CNC. Other new technologies, in contrast, are applied to simple problems first.
One caveat is that rapid tooling has just begun to be applied in complex molds. For example, it is difficult or impossible to fabricate rapid tooling that contains movable slides. The typical approach to handling such features is to build in a nonmovable core that is manually removed after demolding. There can be other complications involving cores that are handled in similar ways.
Thus, when discussing rapid-tooling technology, it is important to distinguish between complicated molds and molded parts having complicated geometric shapes. The latter is a candidate for rapid tooling while the former may not be.
