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11-25-2008

Foams with High-Quality Surfaces

(1) Partial filling with unfoamed melt. (2) Gas and melt flow into the cavity simultaneously. (3) At the end of the injection cycle, only pure melt enters the cavity to seal off the gate and provide an injection molded part surface.

(1) Partial filling with unfoamed melt. (2) Gas and melt flow into the cavity simultaneously. (3) At the end of the injection cycle, only pure melt enters the cavity to seal off the gate and provide an injection molded part surface.

The new SmartFoam foaming system is suitable for numerous injection molded applications, since it can be incorporated into any conventional injection molding machine effectively and inexpensively in conjunction with a licensed beside-the-press unit and a mold modification. The primary feature of this technology is installation of a fluid injector and a static mixer directly in the hot or cold runner system.

The foaming process occurs in three stages. In the first stage, a conventional injection unit fills a small portion of the cavity with unfoamed material. This material forms the skin and thus a bubble-free surface on the molded part. Following this, the fluid is injected into the melt stream via the injector, and both components are dispersed under pressure in the static mixer. The foaming material passing through the gate is now forced into the cavity as the core component. In the final stage, addition of the fluid is stopped and the cavity is sealed off with pure melt. In this way, the next cycle starts with unfoamed material.

Gaseous nitrogen, but also water or liquid carbon dioxide can be employed as the blowing agent. The absorption of heat upon expansion of the physical blowing agent is so great that the melt loses more heat as the result of internal expansion of the foam than can be withdrawn from the mold from the outside. Depending on the fluid, the cycle time can be reduced by as much as 50% through use of the physical blowing agent. The reduction in molded part weight varies between 10 and 30%, depending on the application.

A Frisbee disk was exhibited at the K-2007 as the first semi-commercial product manufactured by means of the SmartFoam process. Since the melt is initially injected into the cavity in an unfoamed state to form the skin and the fluid is introduced only later during further filling, the molded part has the surface quality of a solid injection molded part.

The system can be incorporated into already existing molds by modifying the runner system, if necessary, and taking measures to seal the mold and allow buildup of a gas back pressure. The licensing fee for the process involves only the physical equipment. The license is not based on the number of parts produced, nor are there any other recurring charges. Every customer receives a mold insert for his mold with an encrypted pixel code that contains all necessary information to verify the license. Patents were issued in the USA and Europe two years ago.

Dr.-Ing. Harald Sambale
sambale <AT> kunststoffe.de

contact
Stieler Kunststoff Service GmbH
Wittenstrasse 12/14
DE 38640 Goslar
Tel: +49 5321 33455-0
Fax: +49 5321 33455-9


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