An edited version was published in the October 2000 issue
of MoldMaking Technology Magazine.
By Philip Dickens, Richard Hague and Terry Wohlers
Two broad categories of rapid tooling (RT) exist. One category involves indirect approaches that use RP master patterns to produce a mold and the other category is a direct approach, where the RP machine builds the actual core and cavity mold inserts.
The demand for faster and less expensive tooling solutions has resulted in an impressive number of RT methods being developed worldwide. Many companies are pursuing the development and commercialization of RT because of its market potential. Each of the processes comes with a set of strengths countered by limitations. Typically, this results in solutions that cater to niche applications. Yet, because of their possible impact, these developments are causing a flurry of inquiries from companies in the Americas, Europe, Asia and other developed regions. Meanwhile, countless manufacturing companies are working hard to determine if the time is right to phase in one of these approaches.
Several pattern-based processes have been developed for creating a mold rapidly, with varying costs, leadtimes and process capabilities. The accuracy of these processes depends in part on the accuracy of the RP process used to create the pattern.
One of the most popular tooling applications for RP is the production of room temperature vulcanizing (RTV) silicone rubber molds. Silicone is a versatile material (although somewhat expensive) that can be molded around a master pattern to produce a cavity. With the advent of rapid prototyping techniques, master patterns are often an RP model. Silicone rubber molds are used to produce urethane or epoxy prototypes.
The process of making a rubber mold consists of making a master pattern, finishing the pattern to the desired appearance and casting RTV silicone rubber around the pattern to form the mold. Using the transparent material the model is suspended within a box and silicone rubber is poured to fully surround the model. After the silicone rubber has solidified, the parting line is cut with a scalpel and the model removed, leaving the required cavity.
It is then possible to mold two-part thermoset materials within the cavity. One of the most popular is polyurethane, which is available in a variety of mechanical properties and can mimic the mechanical and thermal properties of elastomers, ABS, nylon and other popular thermoplastics. Polyurethane is usually poured into the silicone rubber cavity under vacuum to avoid air bubbles in the molded component. The silicone rubber tool will generally produce about 20 polyurethane parts before it begins to deteriorate. This will depend on the amount of detail in the tool and the type of polyurethane being molded. Flexible polyurethanes require longer post cure times within the mold, which is placed in the oven at 149° F (65° C). This prolonged contact dries out the surface of the silicone rubber and renders it more brittle. Once this occurs, fine detail on the inner surface of the mold starts to break off and subsequent molded parts reflect this loss.
Silicone rubber tooling provides fast, inexpensive molds, excellent part cosmetics, and the option of using multiple materials. The process is suitable for small or large parts. The primary weakness of the process is that the properties of the urethane materials are different from those of the thermoplastic materials used in production. Due to material cost and labor demands, individual part prices are relatively high.
Even with its limitations, silicone rubber tooling can be used as a production process. Bastech (Dayton, Oh) currently uses silicone rubber tooling to make an instrument case that has high cosmetic requirements, including texture, but very little strength requirements. In this project, the customer required only 100 parts per year. Measuring 350 x 300 x 20 mm (14 x 12 x 2 inches), the case would have required a significant investment in hard tooling.
Plastic parts are vacuum cast by placing a silicone tool in a vacuum chamber with a polyurethane resin. The two-part resin is mixed and de-gassed before being poured into the silicone cavity. After pouring, the vacuum is released and the tool is removed to a post-curing oven for up to two hours depending on tool size. Following an exothermic reaction of the two-part resin, the cavity is opened and a polyurethane part removed. The silicone cavity is then closed and the process repeated.
Unlike vacuum casting, the Reaction Injection Molding (RIM) process does not rely on expensive vacuum chambers and mixing units. The process uses a simple resin injection system with two pressurized chambers. Using an injection nozzle, a silicone tool is filled at atmospheric pressure until excess resin is driven up through a series of riser holes. The cure reaction time is much shorter than in vacuum casting. There is no thermal cycling and the contact time between the resin and silicone rubber is much shorter; therefore, the tools can last for up to 100 shots.
In addition to two-part resin molding, silicone cavities also are suitable for low-pressure injection molding of waxes for investment casting. Using a low pressure, injection system, semi-molten wax is forced into the silicone cavity until full. The cavity is then chilled until the wax has fully solidified. The wax is then removed from the cavity and the process repeated. However, due to the fragility of the investment casting wax material, extreme care must be exercised when removing wax parts from the silicone tool.
The advantages of RTV silicone rubber tooling is that it is quick to produce, reproduces detail impressively well and it's fast for producing a limited number of plastic parts. Disadvantages are poor tool life, cost of silicone rubber and lengthy cycle time. Also, it does not use the final production material (except for wax) or final manufacturing process.
An interesting application of silicone rubber as a mold material is available from a company named Technicast Moulds Ltd. (Watford, Herts, England). The tools used in this process are made from vulcanized rubber with several models located in a radial fashion in a disc-shaped tool. This process makes it possible to cast polyurethane or zinc-based alloys. To aid in the filling of the cavity, the tool is rotated so that the centrifugal force pressurizes the cavity.
This is an ideal process for forming small zinc castings that will ultimately be produced by die-casting. If handled carefully, spin-cast tools can produce in excess of 100 replicated parts before degradation of the tool. Parts also are produced in a range of low melting point alloys.
The advantages of spin-casting include the option of processing a variety of materials that range from polyurethanes to zinc; the process is relatively quick and the equipment is relatively inexpensive. Disadvantages include size limitations, the fact that it is not a production process and the mechanical properties of the zinc castings are not the same as with die-casting.
This is one of the simplest and most economical methods of producing a tool for the injection molding of thermoplastic parts. It consists of mounting a pattern within a mold box, setting up a parting line, and then painting and later pouring resin over the pattern until there is sufficient material to form one half of the tool. After completing the first half, the process is repeated for the other half of the tool. There are many tooling resins available with different mechanical and thermal characteristics – with epoxy being one of the most popular. The resins are often loaded with aluminum powder or pellets to improve the thermal conductivity and compression strength of the tool and to reduce the cost of the resin. Cast resin tools are usually used for 100 to 200 molded parts, although it's possible to get up to 1,000 parts – depending on the material being molded.
The advantages of this process are that it's fast, relatively simple, and can be used to mold common thermoplastics such as polypropylene and ABS. A disadvantage is the low mechanical strength of the molds, especially when the mold includes small thin features. For this reason, this method of rapid tooling is only suitable for relatively simple shapes. Also, the low thermal conductivity of the mold material lengthens the molding cycles times.
Metal spraying is used for the production of soft tooling. It involves spraying a thin shell of about 0.080 inch (2 mm) in thickness over a pattern and backing this with epoxy resin to give it rigidity. Several metal spraying techniques are available. With most RP techniques, the models produced have a low glass transition temperature (i.e., the temperature where the material starts to change to a soft amorphous structure). Therefore, it is important to keep the pattern temperature as low as possible when spraying. If the temperature of the model increases sufficiently it will start to relax and distort, which results in an inaccurate tool.
The most popular techniques for use with RP models are spraying low melting point alloys (lead- /tin-based) with a gun similar to a paint sprayer and metal deposition with an arc system. The arc system feeds two wires into a gun and an electric arc is struck between them. This causes the wire material to melt and then a compressed gas atomizes and sprays it onto the pattern. The higher the melting point of the wire material, the more difficult it is to keep the pattern cool. Therefore, it is common to spray zinc or aluminum based alloys directly onto RP models. It also is possible to spray higher melting point materials onto RP models, but it is necessary to be a little devious. One technique is to apply a metallic coating by using electroless plating or physical vapor deposition. Once there is a metallic coating on the model, heat will be transmitted more readily across its surface.
One problem associated with metal spraying is that it produces shells with high internal stresses. It is possible to counteract these by simultaneously shot-peening the sprayed shell. Steel shot fired at the shell during spraying induces compressive stresses that counteract the tensile stresses.
Metal spraying is typically used on models that have large gently curved surfaces and is indeed most suited to this type of geometry. It is very difficult to spray into narrow slots or small diameter holes. When these types of features are included on the model, it is common to make brass inserts, locate them in the model and spray around them. When the model is removed from the shell, the inserts are permanently fixed into the shell. These inserts also are stronger than the shell material, which is weak and breaks easily if formed as a tall, thin feature.
Spray metal tools can produce more than 1,000 parts depending on the process, material being formed and the amount of tender loving care given to the tool. Clamping and injection pressures for metal-sprayed injection tools are usually less than those for steel or aluminum tools and may affect the mechanical properties of the injection-molded part. And because the shell is very thin and generally backed up with an epoxy-based resin, the thermal conductivity of a metal-sprayed tool is less than that of an aluminum or steel tool. This also will affect the mechanical properties of the injection-molded components and will increase cycle time. Some plastics are much more corrosive and abrasive on tool faces. This can be partially overcome by a variety of techniques, such as plating the tool surface with nickel or chrome, or using aluminum or steel inserts.
Spray metal tools have been used in many applications including sheet metal forming, injection molding, compression molding, blow molding and pre-preg sheet lay up. Various plastics have been molded including polypropylene, ABS, polystyrene and difficult process materials such as reinforced nylon and polycarbonate.
The main advantage of spray metal tooling is that you can produce large tools quickly. The main disadvantage is that it may be difficult or impossible to spray into narrow slots or deep holds – meaning that the part geometry must be relatively simple. Molds are not particularly strong and the process requires special equipment and special operating environment.
The Sprayform sprayed steel process is similar in method to traditional sprayed metal tooling, in that atomized material is deposited using a spray gun. However, the main difference is in the mechanization of the process, as multiple spray heads are involved. The process produces much harder tools and is therefore a much more useful process than traditional sprayed metal tooling
The Ford Motor Company (Dearborn, Michigan) has purchased the technology behind the sprayed steel process, which was developed by Sprayform Holdings Ltd. of the UK. Currently, Ford is licensing the process to other companies.
The primary advantage of the Sprayform process is that it works well for large tools, especially as sheet metal stamping dies. It offers a high deposition rate and is less expensive compared to conventionally machined steel tooling. The cost of the equipment and the licensing fees, as well as the limitation of spraying into holes and slots are disadvantages.
Another steel spray process is currently in development at Idaho National Environmental and Engineering Lab (INEEL) called Rapid Solidification Process (RSP). It differs from other sprayed metal processes in that it can deposit hundreds of pounds of material per hour, while the conventional wire-feed systems deposit approximately 15.4 lbs. (7 kg) per hour. This means that, potentially, the RSP process could be used to build the entire tool as opposed to a thin shell that requires back filling.
Global Metal Technologies, Inc. (Solon, Ohio) has entered into a Cooperative Research and Development Agreement (CRADA) with INEEL (Idaho Falls, Idaho) and intends to use RSP tooling in its production facility. The company claims that RSP tooling lasts 20 percent longer than conventional dies and can lower tooling costs by 20 percent.
INEEL has been successful in spraying H13 and P20 tool steels onto a ceramic take-off of an RP master pattern. A limitation at present is the size limitation of about six inches (150 mm).
Plaster mold casting is a prototype manufacturing process for simulated die-castings. Although there are several variations of this process, it usually begins with a master shape of the die-casting. It is not absolutely necessary to include draft in the walls at this stage, but it can help. A silicone rubber reversal is then molded over the master. A second silicone rubber is molded into the first. This provides a silicone rubber positive of the original model. Plaster is molded around the second silicone rubber positive to provide a plaster cavity. Metal is poured into the plaster cavity. After solidification of the metal, the plaster is broken away.
The rubber version of the master is required so that it can easily be withdrawn from the plaster mold. It is also possible to mold epoxy off of the master and pour plaster over this. The epoxy molds will have a greater life than those made from rubber.
Typical leadtimes from the manufacture of the master model to manufacture of 10 castings is about 8 days, and two weeks to produce 30 to 50 castings. However, three to four weeks is a more typical delivery time. The cost of prototyping with this process is about two to five percent of the cost of a production die, so it is considered to be a good insurance.
The advantages of the process are low mold cost and good surface detail. Also, it's possible to produce reasonably large parts with this process. One disadvantages is lower cooling rates, which means poorer mechanical properties. This can lead to parts with a yield strength that is 20 percent lower than conventional die-casting. Another disadvantage is that you must produce a new mold for each casting. And finally, slightly different alloys are used for plaster casting compared to die-casting
Electroforming involves electroplating a thick shell (several millimeters thick) onto a master pattern. Before plating can commence, the surface must be made electrically conductive. A simple technique to achieve this is to spray a conductive lacquer onto the model. After plating, the shell is removed and then backed with a suitable material.
This is a technique used to produce tools for shoe soles with complicated patterns from original wax models. Nickel is a common material for electroforming and has good thermal conductivity and strength. The process gives faithful reproductions of the master, but can be limited when plating into deep narrow slots or holes. Electroplating builds up more material on exterior corners and narrow slots can be closed at the top before they have plated enough at the bottom. This can be partially overcome by reducing the current, but this increases the time to produce the shell.
Express Tool (Warwick, Rhode Island) is developing an electroforming process that it plans to commercialize. The company typically produces the patterns by machining graphite. This material is an excellent conductor and it machines many times faster than aluminum. Another benefit to using graphite is that it serves as a natural release due to its lubricant properties – making it easier to separate it from the nickel shell.
An advantage of electroforming is that it is very good at the reproduction of detail. Disadvantages are that the process is not particularly fast and it's not possible to do deep slots or holes.
For higher production volumes and more aggressive polymers, it is critical that the tool material has a high degree of mechanical hardness. Using molten metal casting techniques, it is possible to cast around an accurate pattern with either aluminum- or zinc-based alloys with a two-week leadtime.
Given the casting temperature of both aluminum and zinc it is important to replicate the initial pattern into a material capable of withstanding such heat. Using silicone tooling, as detailed earlier, a cavity is produced around the model. However, rather than a resin copy being made, the silicone cavity is then filled with ceramic. After drying, the ceramic facsimile is placed into a bolster and covered with the molten metal.
Cast aluminum and zinc kirksite tooling offer a simple and low-cost method of tooling. Disadvantages include a distortion of tools, accuracy problems due to shrinking, and geometry limitations (i.e., no fine features).
Some companies have used investment casting with RP models to produce metal tooling. Most of the tools cast so far have been in aluminum, but there are some examples of tool steel molds. If a steel or hardened alloy cavity is required, either for mechanical strength and thermal cycling or due to high-volume production, investment cast tooling can offer an alternative to open cast tools, such as the kirksite process. By making a sacrificial RP model of the desired cavity, the lost wax process can be used to replicate the part in a metal.
The RP pattern is first invested in multiple layers of ceramic slurry, which are allowed to dry between coats. After the shell has dried, the ceramic shell and invested part are fired. The firing process sinters the ceramic shell and causes the invested model to be burned out. After firing, any ash residue is washed from the ceramic shell. The molten alloy of the tool material is then poured through a gating system into the void left by the RP pattern. After solidification and cooling, the ceramic shell is fractured and the newly formed metal cavity is removed and post-process machined.
Investment cast tools have been used for injection mold cavities and die casting tools. However, due to the unpredictable contraction of the casting process, it is difficult to maintain a high level of accuracy with this tooling process.
An advantage to the process is that you can get better detail than with cast aluminum or kirksite tooling. Distortion, limited accuracy and the need for finish machining are disadvantages.
The 3D Keltool process typically starts with a CAD design of the core and cavity mold inserts, followed by the creation of the core and cavity patterns with stereolithography or some other RP process. Once these core and cavity patterns have been finished to the desired surface, silicone rubber is cast against them to create molds into which a mixture of metal powder and binder is poured, packed and cured. The metal mixture consists of finely powdered A6 tool steel and even finer particles of tungsten carbide. At this point, the cast core and cavity inserts exist in a green state. These green inserts are fired in a hydrogen-reduction furnace to burn away the binder, sinter the metal particles and infiltrate copper into the inserts. This produces solid metal inserts that are approximately 70 percent steel and 30 percent copper with physical properties similar to that of P20 tool steel. The inserts are finish-machined, drilled for ejector pins and fitted into mold bases.
The tools from this process show very good definition and surface finish. Lead-time is typically shorter than conventionally produced tooling. The primary disadvantage is size limitation. The maximum size of a mold insert is 150 x 215 x 100 mm (5.9 x 8.5 x 4 inches). The length in the z-direction can extend to 145 mm (5.75 inches) when the x and/or y dimensions are shorter. Some toolmakers have press fit two or more inserts side-by-side – in a mold base – to create larger tools.
All of the previous methods of rapid tooling involve the indirect production of a master pattern from which the tool is produced. One of the concerns of producing a tool is the time it takes to produce and finish this pattern. Also, replication techniques, such as these can lead to inaccuracies. Ultimately, companies want to produce the tooling directly, although most of the direct tooling methods are not without limitations.
Using additive "layer manufacturing" techniques, it is possible to include additional features in the tool that are impossible to achieve with conventional tooling techniques. The most significant of these is conformal cooling (or heating) channels that allow the cooling or heating of the tool at points where it is required – not only where the channels can be conveniently drilled, as in conventional cooling. Investigations have shown that conformal channels can cut injection mold cycle times by up to 40 percent.
Rather than making a master stereolithography pattern around which a material is cast, it also is possible to build the cavity directly on the stereolithography machine. 3D Systems (Valencia, CA) has named this process Direct AIM. (AIM stands for ACES Injection Molding. ACES stands for "Accurate Clear Epoxy Solid," which is a stereolithography build style.) Although not nearly as strong or hard as conventional tooling, it is possible to inject a range of thermoplastics into these cavities and produce useable parts. At present, only less abrasive and lower melting point polymers are being molded, although research is underway to improve this application.
Stereolithography tools are generally produced with the standard commercially-available stereolithography resin. Up to 500 parts have been molded from a single tool, although 10 to 50 parts is more typical. Research into the development of high temperature and filled resins also is being undertaken by several organizations.
The process is quick and it produces parts using production thermoplastics. Low tool strength and the risk of failure are disadvantages.
In the same way that a cavity can be generated directly by stereolithography, it also is possible to build tool cavities directly using the laser sintering process. With DTM's RapidSteel (also referred to as RapidTool), digital models of the core and cavity geometries are created and sent to a Sinterstation machine for fabrication in RapidSteel powder. This material consists of particles of mild stainless steel that are coated with a thin layer of a polymer binder material. The Sinterstation produces green parts that are then fired in a furnace. The furnace removes the polymer binder and infiltrates bronze into the mold inserts through capillary action. This process produces a fully dense tool that consists of about 60 percent steel and 40 percent bronze. The inserts are then finished, drilled for ejector pins, and fit to a mold base.
The process produces a durable mold that can be used for injection-mold tooling, as well as die-casting applications. RapidSteel molds have been used to cast hundreds of aluminum, zinc and magnesium parts. The process allows for complex geometries and RapidSteel molds can withstand the conditions of injection molding. However, RapidSteel requires finish machining and polishing that can be time consuming
The advantages are speed, good tool strength and its use for injection molding and die-casting. Disadvantages are equipment cost and size limitations.
The Copper Polyamide tooling process from DTM (Austin, Texas) involves the selective laser sintering of a copper and polyamide powder matrix to form a tool. All of the sintering is between the polyamide powder particles.
The process boasts an increase in tool toughness and heat transfer over some of the other soft tooling methods. These characteristics are provided by the copper and can give the user the benefits of running a tool with pressure and temperature settings that are closer to production settings. The primary disadvantage is the low material strength.
Direct Metal Laser Sintering (DMLS) from EOS involves the direct processing of metal powders in a laser sintering machine. Typically, the machine is used for the production of tool inserts, but it also is possible to produce metal components. Two materials are available for the DMLS process: 1) bronze-based materials, which are used for injection molding of up to 1,000 parts in a variety of materials, and 2) steel-based material, which is useful for up to 100,000 plastic injection molded parts.
This process was used to produce injection mold tooling for a Germany appliance manufacturer. Seven mold inserts were produced in 20 hours using the bronze-based material. Several thousands molded parts were produced in 30 percent glass-filled polyamide. The tool took two weeks to produce compared to 10 weeks for a machined tool and cost about $6,800 compared to $8,200 for the machined tool.
DMLS offers good feature definition, although the surface definition of the steel-based powder needs improving. Also, the steel material builds slowly.
Laminated tooling is an alternative to building cavities directly on an RP machine. Using the similar principles to the Laminated Object Manufacturing (LOM) process, layers of sheet metal are cut to replicate slices through a CAD model. Laser cutting or water jet technologies generally produce the profiles.
To produce a mold tool, the CAD model must take the form of the required cavity. By cutting all of the slices of the cavity in sheet metal, a stack of laminates can be made to replicate the original CAD model. Using either clamping or diffusion bonding, it is possible to create a pseudo-solid cavity in hardened tool steel without the need for complex post process cutter path planning. Due to the use of relatively thick laminates – typically 0.040 inch (1 mm) – the surface finish of the tools is generally poor; therefore, some form of finish machining is generally required.
Laminated tools have been used successfully for a variety of techniques including press tools, blow molding, injection molding and thermal forming. Research also is being performed into the use of laminate tools in pressure die-casting. Tool life is a function of the initial sheet material, which can be hardened after cutting and lamination. However, part complexity is bounded by layer thickness.
One significant advantage of laminated tooling is the ability to change the design of parts quickly by the replacement of laminates (if un-bonded). Conformal cooling channels also are easily incorporated within the tool design and laminated tooling is good for large tools as well. The need for finish machining to remove the stair steps is the main disadvantage of this process.
The Laser Engineered Net Shaping (LENS) system from Optomec (Albuquerque, New Mexico) –originally developed at Sandia National Laboratories – builds parts using a metal powder feed into a laser, essentially laser cladding. The LENS process injects metal powder into a pool of molten metal created by a focused Nd:YAG laser beam. The fabrication process occurs in a low-pressure argon chamber for oxygen-free operation. A motion system moves a platform horizontally and laterally as the laser beam traces the cross-section of the part being produced. After forming a layer of the part, the machine's powder delivery nozzle moves upward prior to building the next layer.
Like other RP techniques, LENS is an additive fabrication method – although it produces fully dense metal parts. To date, parts have been fabricated in 316 and 304 stainless steel, in nickel-based super-alloys such as Inconel 625, 690 and 718, H13 tool steel, tungsten, Ti-6Al-4V titanium alloy and nickel aluminides.
The primary advantage is 100 percent dense parts. The disadvantages are poor surface finish and small feature definition.
Albrecht Röders GmbH & Co KG (Soltau, Germany) has commercialized a process called Controlled Metal Buildup (CMB). The basic technology was originally developed at the Fraunhofer Institute for Production Technology – IPT (Aachen, Germany). Last year, the company sold three systems.
The process involves laser cladding and milling that results in 100 percent dense parts. CMB deposits the material from a steel wire and a 1-2 kW HDL laser welds the steel onto the surface of the work piece. A high-speed cutter flattens each layer before a new layer is deposited.
ExtrudeHone's (Irwin, Pennsylvania) ProMetal Rapid Tooling System – named RTS-300 – is the commercial realization of MIT’s Three Dimensional Printing (3DP) process for manufacturing metal parts and tooling.
The machine is capable of creating steel parts up to 12 ´ 12 ´ 10 inches (300 ´ 300 ´ 250 mm) in size. ProMetal applications include tooling for plastic injection molding, vacuum forming, blow molding, lost foam patterns and the direct fabrication of powder metal components
ExtrudeHone sold its first commercial RTS-300 to Motorola, which was installed in early 1999. Motorola joined a collaborative effort consisting of several industrial members, all part of MIT's Three Dimensional Printing Consortium. Although early reliability problems delayed the implementation effort, recent advances have provided acceptable results.
Many rapid tooling methods are available. Most of them require a master pattern, although a growing number offer a direct path to fabricating the tooling. In the short term, indirect methods of RT will continue to flourish because these methods are the most developed. However, in the long term companies will lean toward direct methods of tooling because they eliminate a step – the use of a pattern – that can help reduce the time it takes to produce the tooling and improve the accuracy of the process.
For more information contact Dr. Philip Dickens, a professor in the Department of Engineering & Technology at De Montfort University (Leicester, England); Dr. Richard Hague a senior research fellow in the Department of Engineering & Technology at De Montfort University at +44 (0)116 257 7689; or industry consultant Terry Wohlers, president of Wohlers Associates, Inc. (Fort Collins, Colorado) at (970) 225-0086.