When Warner Tools sent its designers to the CAD terminal with ideas for a new line of paint scrapers and putty knives, the emphasis was on standing out in a competitive market. The Minneapolis-based OEM focused on what professional contractors wanted from the tools. "In a word, comfort," says David Henke, product development manager for Warner. "After listening to customer feedback and looking at other ergonomically designed, handheld products, we realized a better grip was in order and decided to go with a soft-touch material."
Selecting the right material was a joint effort between Warner, molder Contour Plastics (Baldwin, WI), and material distributor Prime Alliance (Des Moines, IA). Their considerations were abundant. Aesthetically, the grip had to look and feel soft while providing comfort, safety, and value. Because the tools would be used on drywall compounds, which are abrasive, the soft material needed a low wear factor. It had to be chemically compatible with the substrate, a PP rigid base handle, and resist harsh cleaning solvents, water, mud, and temperature. Cost considerations pointed to TPE rather than thermoset rubber for its ability to be injection molded.
For the ProGrip series, the team selected a high-flow TPE (Sarlink 3460 from DSM Thermoplastic Elastomers). The material, an EPDM-based PP mix, solved compatibility and chemical resistance problems neatly. According to Tony Mattair, account manager for Prime Alliance, Warner needed a material with superior chemical resistance because painters often work with mineral spirits on their hands. "Other TPEs can degrade when they contact these solvents, so we suggested Sarlink."
Prime Alliance's Bob Hatch also provided technical support for the project. "In general, for any two-shot molding, my recommendations are to rough up the surface of the base part by texturing the mold. This offers better adhesion between the rigid substrate and the TPE. Also, designers need to watch the flow path and make sure gates are sufficient," Hatch noted (See overmolding tips, below).
Contour Plastics used a two-shot process: molding, then manually loading the PP handles on two 88-ton Battenfelds with closed loop controls. Barry Grant, president, explains Contour has over 10 years of experience with two-shot molding and gave the design team suggestions for moldability and part design. "We also contributed tool design for the four-cavity mold. There were draft and shutoff issues, always prevalent with two shot designs, which we helped to eliminate," he said. Volumes initially were 1.5 million parts/year. Recently, Warner has shifted molding for the Pro-Grip series to its captive operations.
For this project, Bob Hatch of Prime Alliance offered the following recommendations, which are good guidelines for most overmolding applications. Several of the caveats here, along with others, can be found in a presentation titled "Overmolding Sarlink" available from DSM Thermoplastic Elastomers.
- To improve adhesion between a substrate and TPE, texture the mold surface for the base part.
- Run melt temperatures higher for the overmold than if just running regular molded parts.
- Heat the substrate part to 125 to 150F and dry the TPE for 2 to 3 hours prior to molding. (For Sarlink, drying temperatures of 180F are advised.)
- Run barrel melt temperatures in the low- to mid-500F range and mold temperatures around 150F.
- Use a relatively small runner and gates to feed the overmold material on the surface of the substrate, but make sure they are sufficient.
DSM Thermoplastic Elastomers
Phone: (800) 524-1020
Fax: (978) 534-1005
Phone: (715) 684-4614
Fax: (715) 684-4634
Des Moines, IA
Phone: (800) 247-8038