Water Mist Shortens Cycle Times
In conventional stretch blow molding, the blow molded item is cooled primarily from the outer surface that is in contact with the mold. No significant amount of heat is removed from the inner surface. Blow molded bottles, however, have a relatively heavy-walled bottom that transfers heat to the mold very slowly. This extends the cooling time.
A research project conducted jointly by Krones AG, Neutraubling, and the IKV has demonstrated that the cooling time can be reduced by injecting a water mist into the completely formed plastic item. The injected water removes heat from the inner surface of the item, thus shortening the cooling time.
The water mist is injected during the final phase of blow molding, immediately after the product is fully formed (final inflation). A nozzle that injects the mist into the interior of the blow molded part is located at the tip of the stretching rod. The injection time is synchronized with the motion of the stretching rod and selected such that the spray cone wets largely the heavy-walled bottom region.
The water mist injection system is based on a Nessie Plug&Spray unit from Danfoss GmbH, Offenbach. The system is supplied with water from a conventional water connection. A special shutoff valve prevents water dripping from the spray nozzle. According to the information provided, the production output of stretch blow molding machines can be increased considerably.
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