Flawless Cardiac Valves
On behalf of a Russian client, Koebelin Formenbau GmbH has developed a mold with which cardiac valve flaps can be produced from PA 12 without having to undergo any subsequent finishing steps. The particular challenge lies in the need to prevent any deposits, blood clots or disruptions to blood flow from forming on the mechanical flap. This means that the injection molded part is not permitted to exhibit any gate vestige or ejector marks. Until now, any flash that formed was removed by means of laborious manual polishing.
The moldmakers satisfied the requirements for a flawless surface (achieved by polishing to a mirror-finish quality) and the maximum permissible parting line offset of 5 µm by machining the mold surfaces with a Type V33i high-precision milling machine from Makino Milling Machine Co., Ltd., Tokyo/Japan. The cavity as well as the parting lines and locating means were machined in a single machining setup. The V33i vertical machining center provided the key technology for satisfying these extremely challenging demands.
Classic venting of the cavity is not possible, since since it would result in the formation of flash on the valve flap. Instead, the air is removed by a vacuum system. Gating and ejection without any witness marks were also among the requirements. It was thus decided to use of a special cold-runner system with side gating to prevent the otherwise inevitable marks. The cold runner is separated from the molded part in a way that leaves the gate almost invisible and undetectable by touch. After being injection molded, the cardiac valve flaps are placed in a titanium ring. A cleaning process ensures that the finished valve flaps are microbe-free and clean when needed.
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