For what appears to be an industry first, U.K.-based molder Slatebond has successfully overmolded a relatively soft TPE part with a softer TPE material. The breakthrough relies on a new grade of Kraton G polymer used in compounder AlphaGary?s Evoprene TPE material.
According to Derek Fraser, global business manager for AlphaGary, the project?s success came after a year of collaboration between Slatebond, AlphaGary, and material supplier Kraton Polymers involving numerous trials. ?We began with an existing product using a standard Evoprene TPE compound molded in a single-shot process,? says Fraser. ?The customer wanted to go to a two-shot process. As we trialed various options, we realized we needed to use some alternative polymers from Kraton.?
Special Kraton formulations were devised together with the techniques to produce Evoprene compounds with the right rheological characteristics. Slatebond then had to make tool design changes to gates and runners. Major challenges met by the team included adhesion of the two layers, the ability of the top (soft) layer to fully flow and not short shot, and the ability to demold without distortion.
This soft-on-soft technique involves coinjecting a soft TPE compound (30 Shore A) onto a medium-soft TPE substrate (60 Shore A) without any deformation taking place. Slatebond used this method to produce a handle with an even wall thickness and excellent adhesion between layers, overcoming previous adhesion difficulties with other soft, flexible base materials.
According to Fraser, the handle?s substrate is 2.5 mm thick, and the soft top layer is 1 mm thick. ?With this process, the substrate can be any reasonable thickness, and the top layer thickness is largely determined by what is required in terms of final performance,? he says.
For design purposes, Fraser explains that there is a balance between Shore A hardness, wall thickness, and cost. As the wall thickness increases the apparent hardness decreases and the cost rises as more material is used.
Evoprene G compounds are compatible with PP and other polyolefins. Cost is dependent on volume but is comparable with other unfilled (.9 density) SEBS compounds on the market. These compounds offer a wide range of performance properties, including long-term durability, good grip, and oil and solvent resistance. This opens up possibilities for the use of soft-on-soft overmolding in such applications as grips, appliances, packaging, and automotive parts.
?AlphaGary and Slatebond have learned a lot about what is needed to successfully mold and will now be able to bring the next generation, on which work has already begun, to market much quicker,? Fraser says.?Michelle Maniscalco
AlphaGary Corp., Leominster, MA