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April 1, 1997

4 Min Read
Downcosting? Time to try PP

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The safety cover for a string trimmer was formerly molded in ABS. Aesthetic, impact, and structural requirements were easily met by substituting a scratch-resistant PP from Ferro Corp. at lower cost.

Molders used to call ABS the poor man's engineering thermoplastic. Lately, however, polypropylene is becoming a serious contender for the title. According to Bill MacIver, director of marketing for Ferro Corp. (Cleveland), the current state of the industry is fertile ground for PP substitutions. "We are replacing engineering thermoplastics in cases where PP offers both value and cost reduction," MacIver told IMM in a recent phone interview. "Many times, engineering resins are specified originally in a metal replacement because the designer, unfamiliar with plastics and their properties, wants to ensure a broad safety margin. Years down the road, however, the OEM asks molders for cost reductions. As a result, smart molders are filling their portfolios with a variety of products so they can hold onto an application longer." This is known as the cascade effect: an application that starts at PPS can then be cascaded down to PP so that a molder can hold the application for years.

These substitutions address a big issue in the custom molding industry, namely that before they issue a purchase order, customers want assurances that part costs will go down annually. At the same time, resin suppliers are not backing down on prices. Molders wind up in the middle, caught between demands for cost reduction and rising resin prices. Because material costs are such a large part of a molder's overhead, one solution is to substitute less costly resins that perform comparably.

One such product that Ferro intends to offer is PP with improved scratch resistance. That means it will scratch less than conventional PP. On the pencil hardness scale, standard ABS is rated B, and standard PP is 4B, which is softer. "The product we're trying to achieve is an HB, one notch higher than standard ABS, but comparable to specialty ABS," he said. "We've got three formulas that target ABS, glass-filled nylon, and PC. We're sampling them in test markets, with commercial grades expected to become available by fourth quarter this year."

How comparable is the new PP? Comparing HDT, a workhorse grade of ABS has 180F. This product has HDTs from 220 to 300F. Its Gardener impact ranges to 250, while standard ABS reaches only 80. MacIver also mentioned a point concerning impact measurements: "More material suppliers are going to Gardener as a means of measuring impact resistance. It offers a more accurate reflection of a resin's capability in real-world applications. Charpy and notched Izod tests, on the other hand, give designers a beginning place to compare resins, but these tests were originally developed for metals. When the plastics industry inherited them, it found the tests could give misleading results depending on the polymer being tested."

ABS is being used in many cases for aesthetics, according to Ferro's market research. Its new scratch-resistant PP can offer the same gloss characteristics for products that don't need high impact with scratch-resistance to ABS. "Substituting PP at $1/lb for ABS at $1.15/lb can achieve the cost reduction customers want without trading aesthetics," says MacIver.

Another driver for PP that doesn't scratch as easily, he notes, is that point of purchase is very important for consumer applications. In some cases, companies have to add special handling so that products don't scratch in transit.

MacIver hails PP's strong points. It is chemically inert, and thus can be used in appliances with aggressive detergents, lawn and garden products that are exposed to fertilizers and pesticides, and other chemically unfriendly environments. PP can also be easier to mold, doesn't have to be dried as ABS does, and has comparable cycle times.

Ferro purchases plain vanilla PP from several suppliers, then compounds specific grades at its Evansville facility. "We add value by adding glass, mica, talc, UV stabilizers, and custom colors," says MacIver.

There are a few drawbacks to substituting PP. For one, it has a greater tendency to warp than semicrystalline resins. But Ferro is confident that products can be designed to minimize this characteristic. "Each one of our market development specialists is well-versed in tooling, design, and processing aspects to support customers in this regard," he notes.

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