Molding on a Carousel

Chances are that you have seen a rotary multistation coinjection molding machine at a trade show and you probably thought, ". . . specialty machine . . . some strange application," and walked on. Well, yes and no. It is specialized, but various interesting applications are running on multistation rotary machines right now. The specialty is actually any medium-to-large part with thick sections that needs a good exterior finish and economical volume production to be viable. The design possibilities and production economies offered by a multistation rotary, however, add a lot to that simple definition.

The critical problem is that cooling times of these thick parts are measured in minutes rather than seconds. The toilet seat shown can be up to .78 inch thick, and the molded parts weigh more than 7 lb. Cooling time is around 8 minutes. A multistation rotary machine overcomes that problem by mounting multiple molds on a rotating carousel. As a mold is filled and begins its cooling cycle, moving around the carousel to the last station for demolding, the next mold moves into position to be filled, and the cooling cycles overlap. More than four toilet seats are produced on a five-station Presma multistation rotary during the single-piece cooling time.

Coinjection Contributes

The coinjection process involves two injection units connected to one nozzle that lets the two materials flow as skin and core, not to be confused with multicomponent, multishot, or any technology using independent nozzles. The coinjection process involves no licenses or royalties. Multistation rotary-produced parts can have a Class A appearance, functional shielding, and special purpose skins, and/or use reclaim material in the core.

Other savings come from lower material content per part (generally about 15 percent), use of recycle/reclaim in the core, limiting colorant to the skin, and reduced machine cost because of lower tonnage and lower injection pressure. Low-pressure molding means tooling can be aluminum or lower cost steel. Design options are numerous; for example, a long flat part like a tabletop made by conventional injection molding needs ribbing, while the coinjected equivalent can be flat with no sink marks.

The toilet seats made and marketed by Italian bathroom accessories supplier Maspi use all the advantages of the multistation rotary and the coinjection process. Maspi has 14 conventional injection machines in addition to its two multistation rotaries.

Company president Mario Mazzilli says categorically that solely for economic reasons the seats could not be on the market using single-station injection machines. Far from being low-cost plastic alternatives, Maspi seats compete with polyester-coated wood at the top end of the market. Unlike coated wood, Maspi seats have no coating to delaminate; their appearance is smooth and lustrous without sink marks, thanks to the foam core; the "feel" is solid and substantial; there are no problems with warping or screws coming loose as there are with wood; and the plastic material does not harbor bacteria or germs as wood does. Maspi seats even have a small price advantage. Maspi enjoys the flexibility gained from being able to easily change molds and control inventory in production.

Another molder is making all parts needed to assemble a table and four chairs in one turn of the carousel. Multistation rotary platen sizes can be large, particularly relative to what is normal for a conventional injection machine of the same tonnage. In Germany, a five-station 660-ton Presma machine with a 55-lb shot capability produces tabletops ranging up to 70 by 39 inches at the rate of one every 2 minutes. By contrast, thick-sectioned brushes have been produced for the last 20 years with the use of four- to eight-cavity molds in 10-station, 33- or 55-ton rotary presses. They cannot use gas assist because the bristles have to attach to the inside material; an empty space will not work.

Compare a multistation rotary with conventional presses needed to do the same job. Naturally, cost varies with press size and number of stations, but Presma says the cost increment for a multistation rotary over a conventional press is in the 18 to 35 percent range. Lower tonnage helps lower the cost. A part needing 4-lb shots of each material and 32 inches between tiebars would call for a conventional press of about 500 tons. The multistation rotary would be about 200 tons. A 650-ton multistation rotary could be compared to a conventional 1500-ton machine. A better measuring stick would be parts per hour or shift relative to capital investment. This could be a solution just waiting for someone to find, or create, more applications.