Some details emerge on injection molding the new 'plastomers.' (Technology News: Polyolefins)
The new resins are very low-density linear ethylene copolymers and terpolymers, which Exxon and Dow refer to as "plastomers." Up to now, most discussion of these plastomers has been for extrusion applications. Both resin companies are keeping quiet about the injection molding processing behavior of their new materials and are having molders sign unusually long-term secrecy agreements of up to 10 years, in case the grade of resin involved isn't commercialized. Nonetheless, some information has leaked out.
Molded applications of neat plastomers to watch for include tough, clear lids for household containers; flexible colored parts for small appliances; and clear disposable oxygen face masks for hospitals, which want alternatives to PVC. SLOWER CYCLES?
Injection molders who have processed these materials say cycle times are as much as 50% slower and parts can be tricky to demold. "As the comonomer content (butene or hexene) goes up, the crystallization temperature drops quite drastically--as low as 120-140 F at over 15% comonomer. At high comonomer levels, these materials can become sticky and may create challenges to demold. But this is no problem with grades having less than 10% comonomer," says Dr. Lecon Woo, R&D director of Baxter Health-Care Corp., Round Lake, Ill., who has published several technical papers on these materials. Molders expect the cycle-time penalty to disappear as they gain molding experience with plastomers. Cold molds, for example, improve cycle speed: 50 F on the core and cavity is optimum, Dow says, advising use of the coldest water and highest circulation rate available. In warm climates, however, 50 F molds would sweat, so air conditioning or local mold dehumidification may be necessary.
Dow recommends fast injection--0.4 sec at 1500-2000 psi injection pressure--to achieve maximum part density and surface quality. "Fast injection reduces viscosity," Dow says. "It's easier to push these resins faster than slower." Hot runners also improve cycle time, though cold runners will work. Dow recommends 30-mil gates or bigger, 1.5-mil vents, hot tips (50 |degrees~ F less than melt temperature), and feed-throat temperatures of 250-290 F.
Molders say the materials process over a broad range of melt temperatures from 350 to 550 F. And parts must be blown off using air ejection, not ejector pins, which have a tendency to stick.
Blends of plastomers with PP tend to follow the melt behavior of PP. Compounders and PP producers are developing compounds with up to 40% plastomer. In a technical paper at the Schotland SPO '93 conference in Houston last September, Exxon describes an injection molded PP automotive part in which Exact plastomer modification improves stress whitening and impact.
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|Author:||Schut, Jan H.|
|Date:||Feb 1, 1994|
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