Complex, hollow design? Consider metal core molding

Most designers associate lost core molding solely with automotive air intake manifolds. And while it is true that this process serves admirably in these applications, says CoreTech Assoc.'s Mark Battista, it has also piqued the interest of designers in other market segments where complex, hollow parts are the norm. "Although the process requires capital investment relative to part size, volumes, and the amount of automation involved, there are certain parts that cannot be made using any other process," Battista told IMM in a recent phone interview. "More designers in nonautomotive markets, especially sporting goods, general industry, and aerospace, are looking at what CoreTech trade named Metal Core Technology (MCT) to mold one-piece, internally complex hollow parts."

CoreTech offers a range of services, from initial components development and prototyping to turnkey MCT workcells. Stand-alone core casting and melt-out equipment are available for those who wish to integrate their own production cells. Battista and partner Tom Kidd created CoreTech in 1995, following the acquisition of the division from former employer, Electrovert MDD, where they were responsible for business development and application engineering, including production of the first high-volume U.S. plastic air intake manifold for GM's Northstar engine.

For the record, Battista refutes misconceptions about this process. "MCT is both cost-effective and environmentally safe. Tin-bismuth and other metal alloys cause no harm to operator, plant, or environment. Also, cores are completely recovered after they are melted out of the part, then reused continually without degradation. The only environmental issue is handling a small percentage of heat transfer fluid created during melt-out, and that is done effectively using equipment built into the workcell."

CoreTech staffers have found that OEMs and designers ask the same questions about MCT: Can it produce quality parts at a required rate? Can prototypes be evaluated before investing in new equipment? Are the plastic, metal, and other necessary materials available and proven? In all cases, Battista and company offer an unequivocal "yes."

As director of operations at CoreTech, Battista has identified key parameters that must be satisfied before considering MCT molding. To determine if your application might benefit, take a look at this checklist:

Is the one-piece hollow configuration beneficial?Adding value to a former metal or multiple-piece plastic part is the best way to utilize this process. MCT molding offers a competitive edge for one-piece hollow components in several cases - if they can be made only by this method, if there is a materials savings over other processes, or if significant finishing and assembly costs are eliminated.Can the part be designed for injection molding over a low-melt alloy core?A new design can take advantage of the fact that MCT permits undercuts without the need for core pulls or sliding cores, highly controlled internal surface finish, and tight tolerances on wall thickness. Battista cautions, however, that parts must be compatible with the process. For example, metal cores are accurate to .0015 inch per dimension for creating hollow spaces to that tolerance. Also, the metal core must survive the temperatures and pressures of the molding process, and still be melted out at a temperature and cycle time that doesn't negatively affect the plastic part.Will the volume justify the investment?Although each case is unique, air intake manifold volumes can serve as a guide. Tier One automotive molders are justifying systems based on annual production rates as low as 65,000, knowing that the equipment can be used for original parts and other models over a 10-year period. Most cells are constructed with standard injection molding machines already on hand - the additional investment comes from core-making systems, robotics, conveyors, core melt-out tanks, and finishing equipment.

For a lower price tag, cells can be designed with less robotic and material handling equipment. Battista adds that an MCT cell can be tooled to make various short runs of different parts. For example, volumes for high-tech bicycle wheels were initially low, but the part needed the structural integrity and light weight MCT provided. After the '96 U.S. Olympic cycling team used the wheels, the OEM geared up for a full range of general, mountain, and racing wheels using the same workcell.

To make the answers a bit simpler, CoreTech offers an Application Analysis Study to provide OEMs and molders all the information needed for a decision. This includes a manufacturability assessment, cost projections for prototyping, and a guide to establishing in-house or subcontracted production. Battista also advises bringing a team together that includes the OEM, MCT process company, toolmaker, molder, and resin supplier. "Parts must be optimized for MCT to add value to the part. GM's Northstar manifold, as a positive example, can be processed off the line in less than 60 seconds, thanks to the interaction of all team players."