|
THERMOSET CONVERSION
Many molders would like the flexibility of molding thermoset materials on their
thermoplastic injection molding machine. Converting their existing machine
can be done successfully by changing injection unit components manufactured from
highly wear resistant tool steels:
COMPONENT MODIFICATIONS
BARREL:
Because thermoset materials melt and
cure at a very low heat, temperature control is critical. This is better
accomplished utilizing a new water jacketed barrel rather than conductive heater
band heat. A short barrel or L/D ratio is also recommended to reduce
residence time.
SCREW:
Heat can be generated by shear from standard screws. To
avoid excessive shear rates, it is necessary to change the compression ratio to
approximately 1.15:1.
SMEAR TIP: In
phenolics, a smear tip with the proper nose angel is required. A free flow
valve assembly may be used for polyester.
END CAP:
Clearance between the smear tip and end cap angle is critical. Too
much and the material can cure and block flow. Incorporating the end cap
and nozzle as one unit is usually required.
ADDITIONAL EQUIPMENT:
A support bracket may be necessary to relocate the hopper
opening closer to the output end when reducing the L/D ratio.
|
