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Better molded parts through optimum mold cooling

DTI Globalwatch : 22 October, 2006  (Technical Article)
Robert Beard of Robert A. Beard & Assoc. Inc. (Kenosha, WI) has seen firsthand the changes the plastics industry is undergoing, and the pain this is causing processors and mold manufacturers alike. In his observation, 80% of the surviving molding companies have taken over the business of the 20% that didn't survive, and now that 20% is going to China. Resin is typically 40% of a molder's cost, and we all know what's happening to resin pricing. For processors to capture and retain work for U.S. plants, increase productivity, and optimize resin usage while reducing costs requires thinking outside the typical moldmaking box.
Beard spoke to a group of mold manufacturers at the annual fall conference of the American Mold Builders Assn. He spent his engineering career in the plastics industry as a general manager for a custom injection molding firm and 10 years at Abbott Laboratories in engineering management. Since 1984, he's been a consultant to processors by helping them optimize their processing capabilities through state-of-the-art plastics technology.

He presented two options for mold manufacturers to become solutions providers to their processor customers: 1) Become a robotics system integrator by partnering with a robotics maker and by providing end-of-arm tooling to automate the molds you build; and 2) design and build molds that run 40% faster.

Cut cycle time with water
Option two is one that all mold manufacturing companies can implement just by doing some homework. First, when it comes to deciding on the number of cavities, “choose cavitation for financial reasons, not capacity reasons,” Beard cautioned. “Moldmakers and molders have two differing viewpoints. The molder is selling time and if he sells a 16-cavity mold vs. an eight-cavity mold, he's not selling enough time.”

Mold manufacturers need to determine how they can help their customers get to the least-cost-manufacturing scenario. One way to reduce unit cost is to increase cavitation. “We need to educate the molder, the OEM, and ourselves on this,” Beard said.

Another way is to decrease cycle time by using high thermally conductive metals for production molds such as beryllium and aluminum. Beard reported that Paul Engelmann of Western Michigan University has proved in testing that this achieves a 20% decrease in cycle time.

However, mold manufacturers can design the mold first to be a better heat exchanger, and secondly for part form and function. “The main function of the mold is to be a heat exchanger, so to maximize that function you have to aggressively improve the design of the water flow,” Beard stressed. “The moldmaker has to take over the cooling aspect of the mold because controlled heat exchange is key to optimized productivity and lower costs.”

There are three forms of heat transfer: conduction, radiation, and convection. Since crystalline resins require 30-50% more cooling capacity in the mold than amorphous resins, Beard advised attendees to calculate the heat input into the mold from the resin.

Water flow affects dynamic heat transfer. Turbulent flow is required for the fastest heat transfer and the shortest cycles, he explained, and offered the following tips:

Moldmakers need to make the calculations for the molder, and calculate water flow that will achieve optimum water cooling temperature.

Build in the cooling channels during the mold build.

Conformal cooling is the key to the whole process.

Balancing water flow is critical.

Additionally, the moldmaker should supply the mold-mounted manifolds with color-coded supply/return lines, and preplumb the mold for the customer to ensure that the mold is always plumbed correctly when set in the molding machine.

One attendee remarked, “The next time a customer calls me and says my mold isn't making parts to spec, I'm going to tell him to check his water!”
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