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MRJ06301
2-2.
PARISON CONTROL
2-2-1. HISTORY
Many different products are produced by the Blow Molding process. Any food, drug or toy store is filled
with items using blow molded products. Many automobiles use blow molded windshield cleaning fluid.
From a small beginning many years ago, the blow molding process has grown to a major industry.
The value of parison wall thickness control was recognized when the industry was young. Some of the
early machines used two position hydraulic control of the die-mandrel gap. Others used heavy cams in
strong structures to withstand the large forces involved. The first electro-hydraulic blow molding system
replaced the heavy cams with light cams that were easy to modify. This was quickly followed by a readily
adjustable electronic master cam used to determine the correct cam shape. Electronic interpolation then
eliminated the cam. Today’s parison programming systems provide reliable programming of parison wall
thickness and in addition, may provide control of mold motion; screw speed; injection barrel heater tem-
peratures, etc. or the capability to control all machine functions.
2-2-2. PARISON PROGRAMMING BENEFITS
Parison programming provides improved container quality, higher production rates and increased profits.
Control of parison wall thickness as a function of parison length results in constant container wall thickness
after the parison is blown to conform to the mold.
Quality tests determine the ability of a container to withstand drop tests without bursting or leakage of its
contents. Parison programming provides constant wall thickness throughout the container, insuring im-
proved mechanical endurance at minimum weight.
Container wall thickness control reduces the container weight, eliminates the hot spots, resulting in de-
creased parison cooling time. The parison programmed blow molding machine’s shorter cycle time results
in increased production rates, combining with the decrease in material cost to ensure higher profits.
2-2-3. CONTINUOUS EXTRUSION MACHINES
A continuously rotating extruder screw pressurizes the granular plastic material, driving it through a heated
tubular barrel. The resulting molten plastic is then extruded through the mandrel die gap, forming a con-
tinuous tubular parison.
As the parison never stops being formed, multiple molds are required to receive the parison in turn. One
mold is in the cooling position, where the plastic cools until the container can stand alone when the mold
is opened. The open mold has been moved to a position surrounding the parison, and when the parison
is long enough, the mold is closed. The parison is then pressurized with air through the blow pin, causing
it to expand and take the shape of the mold walls, after which the mold is moved to its cooling position.
The other mold is now open and is moved to the position where it surrounds the parison and the cycle
repeats.
The machine and parison program cycle is started by the knife cutting the parison. Program start Delay
time may be used to properly position the programmed parison profile relative to the mold profile to insure
constant wall thickness.
2-2-4. ACCUMULATOR MACHINES
Accumulator blow molding machines are normally used to make large containers. Plastic is extruded into
an accumulator until the amount required (Shot Size) to make the container is available. When the mold
is positioned to receive the parison the accumulator piston moves, extruding the plastic through the pro-
grammable die gap to form the parison.
A position transducer measures the accumulator piston motion and causes the parison profile to be con-
trolled as a function of the volume of plastic extruded through the die gap. During filling, the transducer
also causes the accumulator to stop filling when Shot Size is achieved.
Un-programmed
parison
