The ExpressTool Process
VIII. PROCESS CHARACTERISTICS
Electroformed nickel–copper inserts with encapsulated CCCs provide a num-ber of important benefits, as well as some limitations. These are discussed in this section.
1. Thermal conductivity. As noted previously, the thermal-conductiv-ity values of nickel and copper are both dramatically higher than any of the various tool steels. Assuming 2 mm electroformed nickel
and 4 mm electroformed copper, the effective thermal conductivity of the insert is about 180 W/m K, or more than six times the thermal conductivity of typical tool steels. Consequently, for the same heat flow, the temperature gradients in the mold need only be one-sixth as great! The result is a more uniform mold temperature distribution and faster cooling.
2. Conformal cooling. A key characteristic of the ExpressTool process is the inclusion of encapsulated conformal cooling channels. The primary benefits are as follows:
• The reduction in the magnitude of ‘‘hot spots’’
• More uniform core and cavity temperature distributions
• More uniform plastic shrinkage
• Less stress induced in the plastic part
• Reduced part strain
• Reduced part warpage
• Shorter cycle times
3. Enhanced productivity. Actual performance data for a range of part geometries have shown Ni–Cu/CCC productivity enhancements, relative to P20 or H-13 tools, ranging from 20% to 75%. The aver-age improvement in overall mold productivity has been about 33%.
Simply stated, enhanced thermal conductivity coupled with the use of encapsulated conformal cooling channels will, on average, enable the production of 133 plastic parts in the same time that a conven-tional steel tool would generate 100 plastic parts.
4. Insert accuracy. Insert accuracy is critical at parting surfaces and at shutoffs. The mandrels are CNC machined, achieving the same accuracy obtained for other CNC-generated objects. Also, electro-forming is atomic in nature, regularly replicating mandrel features within 0.1µm for the production of CD masters. Finally, electro-forming involves almost zero mean shrinkage, so the associated random-noise shrinkage errors are virtually nonexistent.
5. Speed. Faster spindle speeds, improved cutter path software, and better cutting materials have reduced lead times for CNC-generated steel tooling by 30% over the past 3 years. However, 12–15 weeks delivery is still too slow, as product life cycles shrink. Ni–Cu/CCC inserts for production molds require 7–8-week lead times, with 9–
10 weeks delivery for a complete tool with ejectors and frame.
6. Chemical resistance. The active surface of the tool is electroformed nickel, which is substantially more resistant to chemical attack than
all conventional tool steels. The best of the conventional mold ma-terials used when injection molding reactive plastics [e.g., poly (vinyl chloride)] are stainless steels. Indeed, nickel is used as an alloy ingredient in stainless steel to improve chemical resistance.
Experience has shown that Ni–Cu/CCC inserts exhibit virtually no signs of chemical attack during the injection-molding process.
7. Surface quality. Electroformed nickel surfaces can be highly pol-ished and have been used for many years in the injection molding of plastic eyeglass lenses. Optical quality surface finishes as good as Ra⫽ 2 µ in. (⬃0.05 µm) have been routinely achieved on elec-troformed nickel.
8. Textured surfaces. Mold-Tech, Inc. has successfully textured the active electroformed nickel surfaces of Ni–Cu/CCC inserts. Ac-cording to Mold-Tech, the resultant texturing using their standard procedures was ‘‘sharp, well defined, and capable of good depth when needed.’’
9. Mold repair. A truly unique aspect of building production tools through the use of the electroforming process is the capability to
‘‘reelectroform.’’ In the event that a glass-filled plastic has gradu-ally eroded any portion of the active surface of the tool, it is possi-ble to simply mask the unworn portions of the insert and then reelectroform the worn surface. Because the nickel electroforming process adds material at about 1µm every 5 min, it is possible to rebuild worn areas in a very controlled manner. Obviously, if the tool surface is textured, the rebuilt area will also require subse-quent texturing. Of course, the same would be true for a conven-tional steel tool that had undergone weld repair. One major differ-ence, however, is that weld repair involves considerable heat input and the possibility of insert distortion. Conversely, electroforming is performed in a warm bath involving negligible heat loading and essentially zero insert distortion.
10. Size. The electroforming process is not fundamentally or intrinsi-cally limited in size by any accuracy, plating, or processing step.
Because CNC is certainly capable of producing large mandrels accurately, and electroforming involves essentially zero random-noise shrinkage, the only limit at present involves the size of the vats. The current ExpressTool electroforming vats are about 3 ft wide by 5 ft long by 2 ft deep. This has been more than sufficient for all projects performed to date. Should larger inserts be required,
larger electroforming vats are certainly feasible and could be built, calibrated, and operational within a few months.
There are also a number of limitations to the ExpressTool process. Some of these are cultural and apply to all forms of rapid tooling; others are specific to this process. Among these limitations are the following:
1. Cultural. For many molders and mold-makers, the statement ‘‘If it isn’t made out of steel, it isn’t a production tool’’ summarizes their perception. Clearly, this attitude will slow acceptance of all new forms of alternative production tooling, in general, and electro-formed tooling, in particular. Just as machinists initially resisted CNC, but gradually embraced the new equipment when productivity gains became obvious, this author believes the same shift will hap-pen here. When numerous case studies have clearly and conclu-sively documented the productivity gains and reduced part distor-tion, market forces and global competition will pull manufacturing in the direction of lower unit cost and higher part quality.
2. Tool life. At present, data regarding the tool life of electroformed inserts for a range of unfilled and filled thermoplastics are incom-plete. Although numerous inserts have already run in excess of 100,000 shots and 1 tool has reached 270,000 shots with no visible signs of wear, none of the inserts has yet been run to failure. The reason, simply stated, is that none of the projects responsible for their development have required larger numbers of parts. Recently, G.E. Plastics (Pittsfield, MA) and ExpressTool signed an exclusive joint agreement intended to document the following:
(1) The cycle time for Ni–Cu/CCC inserts versus tool steel inserts (2) Part distortion with Ni–Cu/CCC inserts versus tool steel
in-serts
(3) Ni–Cu/CCC insert lifetimes, for glass-filled and neat GE plas-tics.
A cycle time as low as 10 s has already been achieved using Ni–
Cu/CCC inserts. Assuming 12 h per day, 5 days per week operation of the injection-molding press at GE Plastics, one can mold about 20,000 parts per week, 250,000 parts in 3 months, or 1 million parts in about 1 year. With allowance for finite press downtime, these in-tervals will probably increase somewhat. As these data are collected and analyzed, it will be made available to the public in the form of publications, mailings, and information posted on the Internet.
3. Deep recesses. A fundamental characteristic of the electroforming process involves the transport of ions along electric field lines.
When the conducting mandrel is connected to a voltage source, an electric field is established. Because electric fields are stronger near external corners and weaker near internal corners, plating occurs more rapidly near the former and more slowly near the latter. The nonuniformity of the electroformed coating is not a problem itself.
However, if inadequate plating time is allocated, then internal cor-ners may not be sufficiently thick to ensure long tool life. Thus, core or cavity geometries involving recesses with aspect ratios (i.e., depth/gap width) greater than 3 require additional nickel electro-forming time. When aspect ratios greater than 6 are essential to part function, ExpressTool will generate a machined steel insert rather than attempt to electroplate such a high-aspect-ratio recess. Thus, the electroforming process is ideally suited to smoothly varying, albeit complex, curved geometries and is less suited to geometries involving high-aspect-ratio recesses with sharp corners.
ACKNOWLEDGMENTS
The author would like to acknowledge the outstanding cooperation of the Pro-cess Modelling and Optimization group at the the National Research Council, Boucherville, Quebec, Canada, under the direction of Georges Salloum, and especially the extraordinarily capable and creative efforts of Michel Perrault.
Mr. Perrault developed the Finite Element Analysis model for the conventional H-13 core and cavity inserts with drilled cooling channels, as well as the FEA model for the electroformed nickel–copper core and cavity inserts with encap-sulated conformal cooling channels. The FEA temperature distributions pre-sented in this chapter were the result of his excellent efforts.
REFERENCES
1. P Engelmann, E Dawkins, J Shoemaker, M Monfore. Improved product quality and cycle times using copper alloy mold cores. J Inject Mold Technol 1(1): 1997.
2. P Engelmann, E Dawkins, M Monfore. Copper vs. steel cores: Process perfor-mance, temperature profiles and warpage. Society of Plastics Engineers, Techni-cal Conference, Toronto, 1997.
3. I Sudit, K Stanton, G Glozer, M Liou. Thermal characteristics of copper-alloy
tooling in plastic molding. Report No. 397, Department of Mechanical Engi-neering, Ohio State University October 1991.
4. B Bedal, H Nguyen. Advances in part accuracy. In: P Jacobs, tech. ed. Stereoli-thography and Other RP&M Technologies. Detroit, MI: SME New York: ASME, 1996, pp. 164–180.
5. P Jacobs. Fundamental processes. In: Rapid Prototyping & Manufacturing: Fun-damentals of Stereolithography. New York: SME/McGraw-Hill, 1992, pp. 79–
110.
6. T Pang. Advances in sterolithography photopolymer systems. In: P Jacobs, tech, ed. Stereolithography and other RP&M Technologies. Detroit, MI: SME/New York: ASME, 1996, pp. 27–79.
7. B Bedal, H Nguyen. In: P Jacobs, tech. ed. Detroit, MI: SME/New York: ASME, 1996, pp. 156–162.
8. T Mueller. A model to predict tolerances in parts molded in pattern based alterna-tive tooling. Proceedings of the 1998 SME Rapid Prototyping & Manufacturing Conference, Dearborn, MI, May 1998, pp. 559–577.
9. K Filipiak. Injection molding thermoplastic parts in days in tooling produced from new composite board. Proc. 1998 SME Rapid Prototyping & Manufacturing Conference, Dearborn, MI, May 1998, pp. 223–243.
10. International Nickel Co. Electroforming with Nickel. American Electroplater’s Society, Inc., 1997, pp. 12–14.