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Space-age coatings protect metal molds.

Space-age coatings protect metal molds

While PET (polyethylene terephthalate) is widely accepted as the material of choice for plastic beverage containers, various complications inherent in the stretch-blowmolding process have always presented seemingly insurmountable problems to bottle manufacturers. Chief among them, according to Johnson Controls, Manchester, MI, has been the detrimental effect of corrosion on the metal molds, along with pitting, wear, and other problems associated with corrosion.

Most of the mold assemblies used at Johnson Controls combine both stainless steel (not hardened) and aluminum (a notoriously "soft" metal in their construction. Moisture condensation during the manufacturing process is one source of corrosive attack on both metals. Galvanic reaction at the interface where both metals meet is another. Both usually result in pitting of the precise, polished mold surfaces--highly detrimental to maintaining clarity in the finished container.

In addition, extensive wear, from the constant abrasion between the plastic and the metal mold sections during the molding cycle aggravated pitting problems. This led to frequent mold repolishing, significantly shortened mold life, increased labor costs, and costly quality-control monitoring. Added to that, the release of the finished container from the mold cavity was severely impeded.

Johnson Controls has solved the problem by using "synergistic coatings" originally developed for solving similar difficulties that had been encountered in the use of both steel and aluminum in the US space program. Created in a multistep process by General Magnaplate Corp, Linden, NJ, synergistic coatings are not coatings in the conventional sense of the word. The process combines the advantages of anodizing or hard-coat metal finishing with the controlled infusion of low-friction polymers and/or dry lubricants.

In the Nedox[reg] process, nickel is first electrodeposited on the metal surface. This deposit contains countless micro-pores which are enlarged in a series of proprietary treatments. Next, the surface is sealed with a controlled infusion of sub-micron size particles of selected polymers. It is then given a final heat treatment which ensures the mechanical cross-linking of the polymer particles to the substrate.

The synergistic coating gives the steel part permanent non-stick lubricity along with improved surface hardness, better abrasion resistance, and exceptional protection against corrosion and chemical attack.

Unlike conventional coatings, the Nedox impregnation cannot chip, peel, or be rubbed off. Important to the avoidance of equipment hang-ups, very few solid substances will permanently adhere to the polymer-impregnated surface of the coated part.

A second General Magnaplate process, tradenamed Tufram[reg], solved the wear problems on the mold's aluminum sections by making the component's soft aluminum surfaces as hard as the surface of steel.

In its first step, the oxide crystals expand and form a porous surface layer. Upon the introduction of sub-micron polymers into the newly formed surface, the polymer particles interlock with the oxidized surface, forming a continuous lubricating polymer-ceramic surface that will not chip, peel, or delaminate.

The result is an aluminum surface which ASTM standard Taber abrasion tests show to have greater abrasion resistance than case-hardened steel or hard-chromium plating.

PHOTO : Highly polished male (steel) and female (aluminum) molds--shown with PET bottle they produce--are protected against corrosion and wear by space-age synergistic coatings.

PHOTO : In the Nedox[reg] coating process, sub-micron, low-friction polymers are infused into enlarged pores of porous nickel plated on steel, copper, or aluminum surface and then heat treated for increased hardness and controlled thickness.

PHOTO : In the Tufram[reg] process, a permanent synergistic surface is created on aluminum by mechanical crosslinking of polymers and oxidized aluminum crystals: 1. surface growth, 2. original metal surface, 3. penetration of metal.
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Manufacturing Solutions
Publication:Tooling & Production
Date:Nov 1, 1991
Words:587
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