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August 20, 2002

8 Min Read
Making the case for copper alloys

Tool_Copper_fig1.jpg

A manufacturer of large appliances eliminated scrap and rework and reduced cycle times by 20 percent by using copper alloy cores. Compare the absence of corrosion on the copper alloy to the effects of vented gas on the stainless steel mold base.

Thermal conductivity, hardness, corrosion resistance, toughness—the wish list is long. No single material of construction can provide all of the properties needed for optimum mold quality and performance. Choices have to be made. With correct combinations of mold materials, molders can run with a winner. But choosing the wrong stuff can make a mold an also ran.

Aluminum's got great thermals, but some say it leaves a lot to be desired in mechanicals for long-run jobs. Tungsten carbide's another good candidate for thermal properties, but its use reportedly is restricted by mold size limitations.

A noted industry consultant, Bob Dealey of Dealey's Mold Engineering Inc. (Williams Bay, WI), says there's a family of contenders out there that has fallen off the radar screen to some degree after earning a bad rap from what he dismisses as old wives' tales and misconceptions about safety. He's talking about wrought copper alloys. He says copper alloys have the right stuff to solve a number of big molding problems, such as these:

  • Long cycle and cooling times. 

  • Sink marks. 

  • Adverse shrinkage caused by high mold temperatures. 

  • Improper coolant channel placements. 

  • Warpage. 

  • Distortion caused by shrinkage and warp. 

  • Excessive mold and coolant line sweating.


    Tool_Copper_fig2.jpg

    The use of aluminum-bronze copper alloys as wear surface protection components is becoming commonplace in long-running, high-speed, Class 101 molds. This copper alloy ejector sleeve is used in an unscrewing mold.

    Still, lubricated and nonlubricated aluminum bronze copper alloys are quite popular. They are used as an interface between moving parts to reduce friction and wear and to eliminate metal transfer failures. These alloys are used in a variety of components, including slide gibs and wear plates, ejector sleeves, rotating core interfaces, and leader pin bushings. Lifters and guided ejector pin and plate bushings are other applications.

    Fast Heat Management
    t the Molding 2002 conference and exhibition earlier this year, Dealey told attendees that copper alloys have a good mix of advantages to offer, including superior thermal conductivity, adequate hardness, good tensile strength, excellent corrosion resistance, exceptional part quality improvement, and high ROI.

    He identified the big three copper alloys that are specially tailored to provide a good combination of mold properties: C-17200, a high-hardness BeCoCu alloy; C-17510, a high-conductivity BeCu; and C-18000, a NiSiCrCu high-conductivity alloy. Pure copper offers the best thermals, but Dealey says it's too ductile for use in molds.

    Copper alloys have outstanding thermal conductivity, as shown in Table 1. Heat management—transferring heat from a high-heat region to a low-heat region—is the major reason for choosing copper alloys, according to Dealey.


    Tool_Copper_fig3.jpg

    The exceptional thermal conductivity of copper alloys can reduce cycle times by accelerating cooling. Copper alloys were used to cool the intricate details of parts formed on slides in this mold where the waterlines could not reach.

    "The temperature differential between the mold surface and plastic dictates the rate of heat transferred. An even mold surface temperature, consistently controlled at the desired temperature, is the ideal condition for achieving consistent part dimensions and controlling warpage," he says.

    Dealey contends that filling has gotten a lot of play as being an extremely important phase of the molding cycle, thanks to the successful promulgation of Decoupled and scientific molding techniques. But he reminds us that cooling is the most time consuming.

    "The cooling phase of the cycle is where the biggest bang for the buck can be obtained, in both cycle reduction and part quality dimensional consistency," says Dealey.

    Better Parts
    Dealey offered a number of impressive examples in his Molding 2002 presentation in which the use of copper alloys improved part quality. In one case, a Tier One auto molder was unable to meet the dimensional and quality specs of a PP B-pillar cover. Cycle time shot up to more than 3 minutes when the molder tried to air-cool the cores. Mold surface temperature control improved when the mold was converted to incorporate copper alloy inserts. Warpage was just about eliminated. The parts were accepted. Assembly was much easier.

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    Another case involved an OEM molder of 155-pin unfilled-PBT connectors. Cycle time was 135 seconds. The connectors had thick walls and the molder couldn't properly control the temperature of the .075-inch-diameter steel core pins—they turned blue and only lasted a few hundred shots on average. And sinks and internal voids compromised the integrity of the parts.

    These problems were solved by using copper alloy core pins to transfer heat away from the connector deck. A chill plate with cooling channels made with the same grade of alloy was inserted under the core pin contact area to remove the heat from the core pins. Molding cycles were reduced from 135 to 35 seconds. Core pin breakage was cut by more than 80 percent.

    Sky-high ROI
    In addition to better performance and quality, Dealey says copper alloys can provide fast and fat returns. In one of the cases he covered, a major appliance OEM was having recurring warpage and distortion problems on a PVC part. The part has a 1-mm tolerance running under a constant deviation of ±1.5 mm that was never met. As if the $996,000/year wasted on a 12 percent scrap rate, sorting, and assembly problems wasn't enough, the parts looked terrible.

    Copper alloys were selected to solve the warping problem in new model year molds. Scrap and rework were eliminated, and Dealey says the parts ran within tolerance. Cycle times were reduced by 20 percent, and the parts looked so good you'd think they had plastic surgery. A 588 percent ROI was achieved.


    Tool_Copper_fig4.jpg

    Plating copper alloys with nickel or chrome adds wear protection. Western Michigan University tested this mold built with plated copper alloys and P-20 steel cores at more than 1.6 million cycles to document the life expectancy of components when running 33 percent glass-filled nylon.

    In another case, a manufacturer of a dollar-a-dozen writing pens had problems holding tolerances on a special part feature because of the distortion created by hot cores in its 144-cavity molds. Output was about 450,000 parts/day, but there were assembly problems. A pulled undercut rejected an entire day's production.

    New 128-cavity molds were built with copper alloy cores. Output is now close to 1 million parts/day. Although the incidence of core breakage slightly increased over the special alloy steel cores once used, rejected lots for pulled cores were eliminated. Copper alloy cores added about $2560/mold, but the molder achieved a 668 percent ROI.

    "A point that should be made is that, generally, copper alloys are used to resolve a problem after the mold is built," says Dealey. "Engineering these items into the initial mold build gets you where you need to be faster and with less pain."

    Editor's note: Dealey's presentation and the complete Molding 2002 proceedings can be purchased from Executive Conference Management of Plymouth, MI, (734) 737-0507, www.executive-conference.com.

    Are copper alloys safe?
    That question comes to mind in any discussion of beryllium-copper alloys. Beryllium is not toxic when alloyed with copper and handled in its final part shape, according to Bob Dealey. Beryllium and most other metals used in moldmaking present a health hazard when small particles are inhaled in the lungs. No known health hazards are associated with making physical contact with a completed mold component manufactured from copper alloys. Of course, it's important that a safe work environment be provided for moldmakers and molders.

    "Research at OSHA indicated that there have been no reported incidences of any moldmaker or toolmaker ever coming down with acute or chronic symptoms of an allergic reaction to beryllium," says Dealey. "To get this you would have to be exposed to large amounts of vaporized particles in very small sizes and ingest these minute particles into the lungs by breathing the beryllium for long exposure times."

    Dealey says the only cases on record, according to an interview with OSHA, are of workers in primary metalworking processes who were exposed to beryllium in particle sizes of less than 10 µm when melting or vaporizing the material. Though such exposure also could occur in welding, polishing, dry grinding, and wire or sinker EDM work, it would be rare in machining where chips are generated. Good exhaust ventilation and in extreme cases, breathing protection reduces exposure possibilities.

    Dealey concludes, saying, "There's probably more beryllium in a big head of lettuce than what's contained in a beryllium-copper core pin. Those concerned should contact their copper alloy manufacturers with any questions on safety and health issues. As long as the copper alloy is handled properly it should pose no problem greater than some of the other materials used in molds. Also, there are copper alloys that do not use beryllium as an alloy for hardness that offer similar performances in molds."

    For more information, Dealey recommends visiting www.brushwellman.com and www.befacts.com.





    Contact information
    Dealey's Mold Engineering Inc.
    Williams Bay, WI; Bob Dealey
    (262) 245-5800; www.dealeyme.com
    [email protected]

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