Mold-heating technology raises thermoset molding quality.

Mole-Heating Technology Raises Thermoset Molding Quality

A relatively new heat-transfer technology for thermoset molds has enabled a processor to eliminate a problem of uneven mold heating and reduce cycle times by up to 23%. When Square's D's Columbia, Mo., injection molding plant took delivery of its second 200-ton Bucher injection molding machine 18 months ago, molding engineer Gary Emanuel had strong expectations that the new equipment would yield better cycle times and less waste for the polyester BMC part he had been running on less sophisticated equipment. However, early in the startup process, it became evident that the design of the mold Square D had been using was creating problems that even the best of equipment and material couldn't overcome.

The Columbia plant molds and assembles circuit breakers for household electrical service lines. The mold used for the breaker's plastic housing is both deep and complex in configuration. Production had been plagued by excessively long cycle times, incomplete curing, and haze and blisters on part surfaces.

Through testing, Emanuel discovered a critical 50 [degrees] F differential between the heated mold base and portions of the core face. Because the cavity was so deep, when the mold was heated to a set temperature of 350 F, the reading on some mold surface registered only 300 F.

Process engineer Jeffrey Ison experimented with varying temperatures and preheating routines, with no improvement. Says Isom, "The hotter the mold, the faster the material cures. If you get too hot, you can damage the part, produce a hazy appearance, or, in the worst case, the mold will lokc up on you."

As Emanuel and Ison ran out of ideas, their equipment supplier, Bucher, Inc., Buffalo Grove, III., called to see how the new machine was performing. Emanuel recalls, "Bucher's response when it heard of our mold heating problems was reassuring. They had experience with other complicated molds and found solutions, including a new way to bring heat from the base to the face of the mold directly and instantaneously."

HEAT PIPES DO THE TRICK

The solution Bucher offerd to the mold-heating problem was a relatively new heat-transfer technology for thermoset applications from Acrolab Instrument, Ltd., Windsor, Ontario. Buchers is Acrolab's exclusive agent for this technology in the U.S. Acrolab's "Isobar" heat-transfer system consists of inserting an array of heat-pipe thermal conductors within the cavity and core of the mold to achieve high-speed heat transfer within the mold as well as thermal consistency of the mold faces. The system also employs integrally heated sprue spreader and core pins, which permit active heating and independent control of specific areas of the mold, such as bosses and sprues.

The Isobar system resides completely within the mold itself, and requires no additional external control hardware, nor any special processing or handling consideratins. The principal benefits of the system are exceptional temperature uniformity of all molding surfaces and rapid mold-temperature recovery after each clamp opening.

Acrolab's Isobar heat-pipe system was custom engineered to suit Square D's mold. In all, over 150 Isobar heat pipes of 13 different diameters were installed in holes drilled into the various inserts that make up the cavity and core faces. Acrolab engineers calculated minimum hole depth, location and wallthickness requirements to ensure that the inserts were structurally sound and that the Isobar array would produce the most uniform temperatures at all molding faces. All the heat input for the inserts was conducted by the heat pipes from a bank of cartridge heaters in the mold base.

Square D's mold was sent to Artag Plastics Corp., a Chicago mold builder, for preparatory machining and Isobar installation. Holes had to be drilled, cleaned and dried for precision fitting of the heat pipes. Don Malchow, Acrolab's service manager, flew to Chicago to inspect the mold and install the Isobar system, a service extended to all Acrolab customers.

HOW IT PERFORMED

To evaluate the results of the newly installed heat-pipe system, Emanuel devised initial tests that would eliminate all other variables and provide valid comparisons. He took a computer readout produced a year earlier by the Bucher CNC press controller, and matched the parameters of that process exactly to run the new tests. He selected the same material, stroke, injection profile and plasticating profile. He preheated the mold to the specified temperature, and ran the process. The results were dramatic.

Not only was surface appearance improved, the system made it possible to reduce curing time by 13 sec. The reduction in total cycle time averaged 22-23% over the old mold design, exceeding Bucher's own prediction of at least 15% improvement in cycle rates. And Emanuel says he is certain they can do even better. Mold heating was totally stabilized through the entire depth of the mold. The face of the mold now registered the same temperature as the mold base. As a result, Square D's team was able to reduce setpoint temperature more than 40 [degrees] F, with a corresponding saving in energy costs.