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issue: November 2003 APPLIANCE Magazine
Plastics Buyers' Guide
Family Cavity Molds - A Viable Means |
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Molders don't like to hear the term "family cavity mold." |
Typically, the larger number of different sizes and shot weights of cavities in a family cavity mold makes it nearly impossible to accurately gauge the precise amount of material needed to fill each cavity to the proper volume at the same time. Accordingly, an excess of material is "packed" into some cavities to assure that each is adequately provided. While packing may allow each one to receive enough material to fill the cavity, the packing not only wastes resin, but may also lead to warped products. Packing can also lead to early damage of the mold parting line.
If there's one reject, there is an uneven number of parts that can grow. The choice is either to warehouse or to plug the mold runners, evening the count. Either way, it is an additional expense. While it would be desirable to reduce the number of cavities to allow more accurate control, this would require a trade-off between accurate filling and the added cost of multiple molds.
There is a way to address sequentially molding a plurality of plastic articles within one mold, giving each cavity its own molding parameters. Family Mold Sequential Molding* makes the molding of smaller quantities of smaller parts cost effective and practical. This is accomplished with specific proprietary software that closely monitors and controls the entire machine. Each cavity of a family cavity mold can have its own set of molding parameters:
individual plasticizing
individual injection speed
individual injection pressure
individual shot size
Smaller parts do not suffer due to the larger cavities to fill. With no packing, there should be substantial resin savings, and there is no flash to trim.
Each cavity is provided with a hydraulic valve gate connected by a hot runner. The first gate is open until the cavity is filled. This gate closes as the hydraulic valve gate for the second cavity opens and fills and continues until all the cavities are full, but not packed.
By injecting the largest part first, it should be ready to demold as soon as the smallest provided wall thicknesses are somewhat similar. Because cavities are injected one at a time, full clamp tonnage should never be required. Parts may be removed individually or robotically. Thermoplastics and thermosets work equally well with Family Mold Sequential Molding.
The savings of Family Mold Sequential Molding can be substantial over a project. To review:
less mold costs; one mold rather than several
less injection molding machines
less floor space
less clamping tonnage
less utilities
less auxiliary equipment
less resin used
less rejects
*Family Mold Sequential Molding is a patented technology.
This information is provided by Jeanine S. Hettinga, president/CEO of Hettinga Equipment, Inc. and Hettinga Technologies, Inc. (Des Moines, IA, U.S.).
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