Custom-Engineered Shock-Absorbing Characteristics
Using patented manufacturing technology, Stabila Messgeräte Gustav Ullrich GmbH, Annweiler, Germany, produces amongst others spirit levels with accuracy that lasts a lifetime. As part of a new development, it was decided that end caps of the tool, previously molded from a single plastic, should be designed as a two-component part in order to provide a shock-absorbing function that would absorb impact during a fall. A hard component that ensures firm attachment of the cap to the spirit level would be combined with a second, soft component.
The performance and dimensional stability of several prototypes were tested under everyday conditions, such as exposure to acids and alkalis as well as wide temperature fluctuations. Various parts produced by means of conventional injection molding. It ultimately took incorporation of a third material characteristic in the form of chemical foaming to solve the problem. The cellular structure of a foamed plastic makes it possible to achieve outstanding shock-absorbing behavior in conjunction with high dimensional stability. The surface consists almost entirely of closed cells, while the core has a porous, shock-absorbing structure.
An important aspect of the hot-runner system design for this application is uniform temperature control throughout with no dead corners. Processing the melt mixture (TPU with chemical blowing agent) at a low temperature, below the manufacturer’s recommendation) and then raising the temperature slightly just prior to reaching the cavity yielded optimal foaming. Although the gas for the foaming process is already generated during plastication of the melt in the injection unit as the result of chemical reaction of the two blowing agent components, the intentional increase in temperature as the melt approaches the gate increased molded part quality significantly. Exact temperature control throughout the hot-runner system proved especially effective in the case of this approach.
For this project, Witosa GmbH, Frankenberg, Germany, relied on a special hot-runner system with cylindrical needle shutoff. Using a 6 mm wide shutoff needle that forms part of the geometry of the molded part to close the gate is an especially clever feature of the design. This resulted in an almost invisible gate, which enhanced the part design instead of detracting from it unnecessarily. Such a shutoff needle, which in this case is almost functioning as a stamping punch, must be prevented from rotating unintentionally in this case of an asymmetrical contour. Witosa provided special actuating cylinders for this purpose.
It is also possible to incorporate electronic sensing of the piston stroke to send a signal to the injection molding machines control system upon reaching the end position. Even slight deviations of the needle’s movement in response to possible interference such as a drop in air or hydraulic pressure are detected in this way and can be documented in the form of continuous recordings of production conditions. This facilitates sorting of faulty parts. Production of the new end caps for professional-grade spirit levels has been running without problems three shifts per day since 2009 at Stabila.
Witosa GmbH Heisskanalsysteme
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