Plug-Assist Coating Improves Surface Quality & Formability

Manufacturers and users of plug-assist tooling have a new technique to improve the surface finish and cycle time of thermoformed parts. A novel coating reportedly improves tool surface finish and eliminates cleaning and part rejects. It is claimed to be less costly than other options, such as release agents.

BUILT-IN RELEASE
In a slightly different version, this tool coating from Dimension Bond Corp. is already used on glass-reinforced injection molded parts that are used in sliding or rotating applications. The coating reduces friction and covers protruding fibers that cause wear on mating parts. The coating does much the same thing for plug-assist tooling made of glass-reinforced thermoplastics by providing wear resistance for plug durability, as well as low friction and a smooth surface that won’t mar the formed parts. Demand is strong in plug-assist tooling because the industry is seeking labor- and cost-saving techniques, according to Bruce Nesbitt, Dimension Bond president.

The company developed a special two-layer “composite” coating that is applied as a liquid to the plug and cures to form a strong adhesive bond. The coating for thermoform tooling is different from those tailored for injection molded parts because the film needs the added capability to release the product. The new “tooling class” of coatings has a smooth yet ultra-long-wear surface that prevents scratching the plastic sheet. Not only does the coating film cover any reinforcing fibers protruding from the plug surface, it allows formed parts to be removed from the mold with minimal effort and no additional mold release.

The two-layer coating consists of a top layer of PTFE, which provides low friction and release, and an under-layer of specially formulated carbon and ceramic nano-particles in a polyimide or polyamide-imide resin matrix that bonds to the plug-assist. A special silane coupling agent bonds the matrix layer to the plug assist.

The liquid coating is applied and cured on the plug assist using Dimension Bond’s proprietary computer-controlled process. It is applied to the entire plug surface and there is no limitation on plug size. First, the silane solution is applied to the plug and then the matrix layer is applied and then cured at 350 F for 60 to 90 min. The final step is bonding the PTPE top layer using infrared curing. The plug is rotated during curing and reaches a surface temperature of 700 F for about 15 sec. The process creates a permanent film with a maximum thickness of about 50 microns, depending on requirements. No polishing is required.

The coating has a coefficient of thermal conductivity similar to mica, resulting in higher heat resistance than a thermoplastic plug assist. It insulates the tool and enables use of higher sheet temperatures, thus improving formability. Heat transfer to the plug is also minimized.

Dimension Bond offers a coating service to apply its DB-L1714/NC family of nanocomposite coatings with 48-hr turnaround. The firm has worked with packaging manufacturers and several leading plug-assist makers. Dimension Bond is investigating use of PEEK in the coating layers because of its inherently high heat resistance (550 F) and wear resistance

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