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 of the blow molded (hollow) part remains at a much higher temperature during the mold cooling process. The big difference between the outside and the inside surface temperature 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.

Lowering the chilled water temperature in the mold leads to a limited improvement. It is suggested to use pure chilled water at a temperature not lower than 6 ºC [43 ºF]. The chilled water flow rates are to be at a high rate to create turbulent water flow in the mold cooling channels.

Adding antifreeze to the chilled water to achieve a very low temperature has its disadvantages. Antifreeze agents normally have low thermal conductivity which lowers the heat withdrawal from the product in the mold and the majority of them have high viscosity which lowers the water pump performance and reduces the water flow rates. Lowering the temperature under the dew point of the ambient air causes condensation on the mold surfaces adding one more problem to the process (See Mold Area Protection - MAP).

Post cooling with internal exchange of air is applied in some cases to get rid of excessive heat inside the part after the molding process. This in fact requires more equipment and one more step in the production. It also requires more floor space in the manufacturing facility. Some of the stress could have taken place during the mold cooling and in the transition between the mold and the post cooling station.

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. The proper air distribution inside the product is very important to achieve the desired improvement. Blow pins and blow needles can be specially designed for individual products to guide the air to areas with thicker walls and areas which are not very well cooled by the mold. Turbulent air flow inside the product is also a very important factor. Blow valves can be designed to form the product with the highest air pressure available for the process and then drop the air pressure while chilled air is being exchanged inside the product. Sufficient pressure must be kept inside the product during the entire cooling time to keep contact between the product and the mold. Increasing the air flow improves the results but the relation between air flow and cooling time is not linear. Exchanging the air volume inside a product 10 times might lead to a production increase of 10% but a 15% production increase might be the result when the air is exchanged 20 times during the cooling time. Limiting factors such as limited size of the blow pin or the blow needles might not allow for a high rate of air exchanges. Compressed air cost must be taken in consideration. It is a fact that better cooling results are achieved with lower chilled air temperatures. However, the relation between air temperature and cooling time is not linear either. Lowering the temperature from 20 °C [68 °F] to 5 °C [41 °F] might lead to a production increase of 10% but a production increase of 15% might be the result when the air temperature is further lowered to -10 °C [14 °F]. Air temperatures blow -40 °C/°F are proven to be disadvantageous.

A system injecting liquid Nitrogen or liquid Carbon Dioxide in a form of mist inside the product has proven to be very expensive and not ideal for internal cooling. It is difficult to guide the mist to the desired areas in the product and the accuracy of the injected amount of liquid is very difficult to achieve cycle after cycle. The system is also hazardous and complicated. The dependence on liquid supply and the increasing liquid prices are also factors to be considered.

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.

Two complete systems are offered. 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.