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PET compound molds faster, is less moisture-sensitive.

PET Compound Molds Faster, Is Less Moisture-Sensitive

New data on a recently introduced 30% glass-filled PET compound backs up the supplier's claims of improved processability relative to other injection moldable PET's. Polysar Inc.'s Engineering Plastics Group in Leominster, Mass. (which will soon be changing its name to reflect its new ownership by Nova Corp. of Alberta), has provided PLASTICS TECHNOLOGY with data showing that its Petsar PD-8201 compound, introduced last spring, flows better and molds faster than other 30%-glass PET's, as previously reported (see PT, April '89, p. 13). These data also illustrate three advantages not previously publicized: that PD-8201 is less moisture-sensitive than other PET's; that it can be molded at lower mold temperatures--similar to those of PBT; and that its dimensional stability can permit it to be molded in existing PBT tools, in some cases. Polysar marketing manager Paul R. Boulier attributes these advantages to a proprietary additive package in the compound.


Figures 1 and 2 illustrate the superior flow fo Petsar PD-8201 compared with two competing 30% glass-filled PET compounds. Using identical molding conditions, PD-8201 yields longer spiral flow over a range of melt and mold temperatures. Also noteworthy is the ability of PD-8201 to be molded at relatively low mold temperatures, which has been borne out in several molding trials at customer plants. According to Polysar's Boulier, PD-8201 can be processed with mold temperatures around 140 F, similar to those used with PBT, whereas competing PET compounds are commonly run at higher mold temperatures (180 F and up) in order to develop optimum crystallinity and properties.

A half-dozen field molding trials of small and medium-sized parts averaged 21% faster cycles with PD-8201 than with competing PET's or PBT. In one case, a 1.5-g, four-cavity carburetor sensor housing cover was molded in 7 sec with PD-8201, vs. 12 sec with PBT, a 42% reduction. In this application, the PET compound yielded part dimensions equivalent to those of PBT molded in the same tool. The PET trial parts snapfitted together perfectly with a mating PBT part, according to Boulier. Even two days after molding, the PET and PBT parts could be assembled with no problems. The ability to mold PET in existing PBT tools in some applications could facilitate switching to the lower-cost PET, says Boulier.

In another application, PD-8201 reduced injection cycle time for a 3-g, four-cavity computer connector by 13% compared with another PET compound. And with a 205-g, single-cavity vapor-seal cover for underground gasoline tanks, PD-8201 gave a 20% shorter cycle than a competing PET.

In general, these trials showed that PD-8201 permits a wider molding "window" than other PET materials, Boulier reports. In the past, determining the optimum mold temperature and cooling time has been one of the critical subtleties of PET injection molding that has caused problems for inexperienced molders of this material (see PT, Oct. '88, p. 73).


Another critical aspect of PET molding that has led to difficulties has been its extreme sensitivity to residual moisture (see article referred to above). PET typically must be dried to a moisture level not exceeding 0.02%, or loss of strength and toughness will result.

A laboratory study of PD-8201 showed that its proprietary additive package reduces hydrolysis and maintains fairly uniform physical properties when molded at a range of higher moisture levels that produce significant degradation in other PET's. This is clearly exhibited by the flexural strength and notched Izod impact data in Figs. 3 and 4. Results for tensile strength and modulus were similar, says Polysar. This factor again makes PD-8201 unusually "forgiving" of processing variations. (CIRCLE 1)
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Title Annotation:polyethylene
Author:Naitove, Matthew H.
Publication:Plastics Technology
Date:Jun 1, 1990
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