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Cooling inside the parison shrinks cycles, expands output.

Most blow molding operations leave the task of cooling to the mold. However, a relatively new method of cooling, whereby heat transfer occurs inside the formed parison as well as on the outside, is showing big benefits in reduced cycle times and higher production rates.

Developed by the Austrian parent of Fasti USA Inc., Elgin, Ill., the Compressed Air Cooling (CAC) system uses the blowing operation to cool the part internally through a continuous exchange of chilled and heated air. Combined with inflating the parison against the chilled mold, this creates "two-sided cooling" that enhances the heat-transfer rate throughout the part, says Fasti president Rainer Farrag.

The CAC system typically results in 35% faster cooling and better part definition, Farrag says. He adds that more thorough and uniform cooling means parts are stiffer when ejected, so molders may be able to trim part weight by 5-10%.

More cooling on the inside

Typical blowing systems introduce and then evacuate ambient air in one shot. This keeps the parison inflated, but the air is stagnant during the molding cycle and doesn't take away much heat.

Fasti's continuous internal-cooling system exchanges the air inside the part up to 10 times during the molding cycle. This is said to be especially useful for complex shapes or for multilayer applications, where cooling both the inner and outer walls of a part reduces the chance of deformation. One-sided cooling creates a temperature differential between the inner and outer part surfaces, whereas two-sided cooling minimizes that difference. And even in simple bottle shapes, poor cooling of common trouble spots such as the neck and tail can result in distortions.

Farrag says other firms have tried internal cooling with nitrogen or C[O.sub.2], but because the gas is stagnant, the cooling rate is still unpredictable and not necessarily uniform.

The benefits of higher heat transfer were revealed in a test on a very complex blow molded automotive part from Audi. The Fasti system provided an air inlet temperature of -20 F and outlet temperature of 132 F. For a standard one-sided cooling system, the blowing-air inlet temperature was 68 F. The traditional system had a cooling time of 44 sec and overall cycle time of 61 sec. The Fasti system cut cooling time to 24 sec and overall cycle to 41 sec, yielding 48.8% higher throughput.

Multi-channeled blow pin

Fasti's system consists of a compressed-air chiller and a special blow-pin assembly with air-inlet and exhaust channels. Air from the chiller is distributed radially into channels close to the O. D. of the calibrating sleeve, where it internally cools the bottle mouth and neck. Near the end of the blow pin, some air is diverted through openings in the side of the pin to cool "air-starved" areas around the neck. Heated air returns through six channels in the pin.

Fasti's system costs from $35,700 to $79,400, depending on container size and number of cavities. Increased productivity reportedly can pay for the system in 15 to 180 days.
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Title Annotation:blow molding operations
Author:Knights, Mikell
Publication:Plastics Technology
Date:Apr 1, 1998
Words:503
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