Understanding why material stress occurs is the first step to understanding how to solve the problems that come along with it such as products failing leak, load and drop tests. Attempts to counteract material stress, if approached incorrectly, can lead to higher material costs, longer cycle times and decreases in rates of production, all of which effect the bottom line - profit.

Blow molded products are blown by compressed air and cooled by chilled water in mold cavities. Heat is transferred from the outside surface of the part to the mold surface. The internal surface (hollow) of the blow molded part remains at a much higher temperature during the mold cooling process. The variance in the outside and the inside surface temperature is what causes material stress. The wall thickness distribution is never equal in a blow molded part. The mold cooling is not equal on the mold surface either. Heat transfer from heavy parts of a blow molded product through a limited mold surface is not equal to that of thin walled parts through large surfaces. This in fact causes more material stress and distortion in blow molded products.

Material stress leads to a bad product quality and the product may fail leak, load or drop tests. Blow molders are often forced to increase the wall thickness by up to 10% to pass the tests. Increasing the weight is combined with higher material cost and longer cycle time.

The cooling time, which is the longest part of the total cycle time and the blow molding process, is often extended to get the heat from the part all the way through the wall to the mold, but a difference in the temperature is always expected. Extending the cooling time slows the production and shrinks the profit.

Exchanging chilled air inside the product during the cooling time to withdraw heat from the internal surface reduces the material stress and dramatically reduces the cooling time. It is a fact that better cooling results are achieved with lower chilled air temperatures.

The ideal and most profitable blow molding process is that which includes an internal cooling system with acceptable air flow, acceptable temperature, not higher than 5 °C [41 °F] but not lower than to -35 °C [31 °F], and good, turbulent air distribution. Air chillers with integrated refrigeration circuits are recommended.

Fasti offers two complete systems that can create the ideal blow molding environment. The Blow Molding Booster (BMB) with air temperature at 5 °C [41 °F] and the Blow Air Chiller (BAC) with a temperature as low as -35 °C [-31 °F] are available with a complete set of suitable blow valves and individually designed blow pins or blow needles.