Rapid tooling and it's faster in modeling

the next step in this project was to collect some information about rapid tooling and pom group.inc. this information was prepared by Don.

The "rapid" in rapid-tooling technology originally meant that molds could be made much faster than with conventional machining. That's still true. But in the last few years, RT technology has evolved toward building molds that provide up to 40% faster cycles than are possible with conventional technology. That emphasis on productivity accompanies a shift in RT applications from prototype to full production tooling.
Newer RT approaches can minimize or eliminate warpage and internal stresses caused by uneven cooling while boosting productivity by drawing heat more quickly from thick sections or other hard-to-cool features in the mold such as bosses or ribs. Tests of newer high-conductivity tools by at least two sources reportedly found that cooling time in the molding cycle could be set to zero and the process would still yield acceptable parts.
Conventional tool making is limited by two factors: One is the modest thermal conductivity of standard tool steels like H-13, P-20, or stainless, which are chosen for hardness and/or corrosion resistance. Cooling is also limited by reliance on straight gun-drilled channels in the mold base. Conventional mold makers can attempt to overcome these limitations with inserts of beryllium-copper or other highly conductive copper alloys, secondary cooling channels or "bubblers" in deep cores, and heat pipes or "thermal pins" that can reach into the body of the tool to extract heat.
Suppliers of rapid tool making techniques have developed two routes to improved mold cooling: building conductive metals into the tool from the start and/or incorporating cooling channels that conform to the part surface, no matter how irregular its shape. RT suppliers claim that either or both of these methods can do better than either bubblers or heat pipes. And the newer RT methods typically combine conductive metals with a tool steel to create a bimetallic structure that helps deliver better wear resistance than copper alloys or aluminum alone.
Some conductive alloys for conventional mold making have hardness values as high as 30 to 40 Rc, similar to P-20 steel. There are also methods of prehardening copper alloys and aluminum to similar levels, says metals supplier Bohler-Uddeholm. But today's RT techniques combine conductive metals with tool steels in bimetallic structures that provide high conductivity with hardness of 50 to 80 Rc—as high or higher than H-13 steel.
New developments with hard but highly conductive metals could give a needed boost to acceptance of rapid tooling, which has prompted great interest but only modest adoption by mold makers and molders. For the most part, RT approaches have found use in prototype and low-volume production. Now, so-called "bridge" tooling applications and high-volume jobs are within RT's grasp. "Basically, the goal now is to get the same tool life with the higher cooling rates for the faster cycles," says Jim Knirsch, president of RSP Tooling, a supplier of RT technology.

Bimetal layering methods
POM Group's Direct Metal Deposition technique produces a 99.8% fully dense tool component from metal powder. Commercialized in 1998, the process uses a 5-kw laser and powdered tool steel (D-2, H-13, P-20, stainless, or many other types). To make a highly conductive tool or component, the first step is to fabricate a core out of a conductive metal such as Ampco 940 chromium copper alloy. Then a steel shell is deposited on the core by DMD.
The process starts with a specially designed nozzle with separate orifices for the laser, the metal powder, and the inert gas that creates an oxygen-free environment. The layer-by-layer building of the tool follows a tool path generated from a CAD file. A small amount of powdered metal is injected through orifices in the nozzle onto a spot 1 to 2 mm in diam. As the powder reaches the substrate surface, the laser beam simultaneously melts the metal into a small pool, which cools quickly as the laser moves on. Build rate ranges from 2 to 9 cu in./hr. The shell is built up in layers of 0.5 to 1.25 mm to a near-net shape with 0.007 to 0.010 excess material that must be finished with conventional polishing or EDM. POM claims there is a true metallurgical bond between the core and shell, achieved with the aid of a proprietary metal-based "interface" or bond coat.
POM offers a five-axis DMD machine with a work area of 1 x 2 x 0.75 meter. Depending on the powder-metal alloy used, DMD can produce a steel shell with surface hardness as high as 62 Rc. Because of this, POM can use its technology to repair damaged tools. If a tool is being modified because of a design change in the part, just the area to be changed can be machined away and a new geometry added directly to the mold with DMD and machined to final dimensions. Unlike a conventionally rebuilt mold, there is no need to subject the entire mold to heat treating to harden the rebuilt section. Also, DMD can be used to add conformal cooling channels to slides, inserts, and lifters of old tools to reduce cycle time.
The Rapid Prototyping Center at the University of Louisville, Ky., has a DMD system from POM and is conducting extensive tests with bimetallic tools of chromium copper and H-13 steel to evaluate tool life and overall performance. A single-cavity tool ran PP with zero cooling time, cutting the overall cycle from 27 sec to 22 sec, while still producing acceptable parts. The Center is also testing the capabilities of DMD tools for aluminum die casting, which is a much harsher processing environment, says Tim Gornet, a research associate at the Univ. of Louisville Rapid Prototyping Center.
A second RT process involving bimetallic layering is RSP's Rapid Solidification Process, a metal spray deposition method that produces 99.5% to 99.8% fully dense cores and cavities. The process was developed at the Idaho National Engineering & Environmental Laboratory (INEEL) in a two-year consortium with 11 companies. The process uses a ceramic pattern built with the aid of stereolithography, SLS, or other rapid-prototyping process. Then molten steel is injected into the nozzle where it encounters a channeled flow of inert gas that conveys droplets 18 to 24 in. to the part surface. The droplets cool rapidly in flight and solidify instantly when they encounter the pattern. The nozzle can deposit up to 500 lb/hr of material. The spray deposition process mimics surface detail closely. To produce a skin/core structure, the deposited steel is heated, and then a layer of copper alloy is sprayed over it. Conformal cooling lines can be added to the tool by stopping the spray process, placing copper tubing, and then encapsulating it with more sprayed metal.
Says RSP's Knirsch, "Spraying the steel layer first allows for an even layer to be built. That will give us uniform heat transfer from the molding surface. The copper layer will vary in thickness across the mold. We let the copper make up the full depth of the insert."
The bond between the steel and copper is formed by the rough-spray technique, whereby the open surface of the steel is extremely rough. When the copper is sprayed over the steel, it infiltrates the nooks and crannies forming a strong physical bond, though not a metallurgical bond.
Mold components can be built in minutes to dimensional tolerances closer than ±0.002 in. Parts can be made up to 7 x 7 x 4 in. and a machine with larger capacity is planned.
The RSP process can be used with at least 10 different tool steels and with softer materials. When used with H-13, tool hardness can exceed the 60 Rc of a conventionally heat-treated H-13. This is due to the quick quenching of the sprayed droplets, which affects grain structure and strength. RSP tooling can also be heat treated and tempered.
RSP is evaluating high-chrome- content metals, which can deliver even higher hardness. It is also testing Inconel, a high-temperature, corrosion-resistant nickel-chrome-iron alloy. Future developments will also provide the ability to blend different metals during the spraying process.