Machine Design

Finally, A Soft Touch For Acetal

Molding hard and soft materials together is an evolving field with new combinations still being developed. A recent entry is a SEBS grade that overcomes bonding issues with acetal.

Sensor housings for adjusting automobile headlamps come from Dekorsy Co., Radolfszell, Germany. Good bond strength between the Hostaform acetal copolymer and soft Thermolast-K SEBS material makes for watertight seals in this underfloor device.

The technique, reportedly the first of its kind, matches Ticona's Celcon and Hostaform acetal copolymers with an adhesion-modified SEBS called Thermolast-K from Kraiburg Corp., Duluth, Ga. The SEBS material is tailored to form a strong bond to the acetal by overcoming acetal's inherent lubricity. The SEBS combines high elasticity and flexibility at low temperature with resistance to water, acids, and alkalis. It also has a wide range of Shore hardness values (45A to 70D) and a relatively wide processing window.

The two-component molding process involves an acetal melt temperature of 200°C and a cooling time of at least 2 sec. The acetal substrate is held just below its crystallization temperature after molding. The SEBS is introduced via hot runners at 250 to 260°C. Residence time for the SEBS in the machine at these temperatures is kept as short as possible, in part by using small barrels, so the adhesion promoter does not degrade. Since SEBS flows well at 250°C, molders should adjust for jetting and entrapped air as the material enters the mold. The tool should be vented to release trapped air.

This hard-soft technology can be used to add long-lasting seals, gaskets, and nonslip and energy-absorbing elements to acetal parts. Its potential uses include seals for auto door-lock housings, sensors and sensor housings, as well as in medical devices, heating and air-conditioning systems, and audio components.

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