Developing the
process validation around the following four plastic variables allows the
flexibility to move the validated mold and process to any comparable molding
machine.
Plastic
Melt Temperature
Use the material
manufactures recommended melt temperature for process development and verify
with a calibrated melt probe to check the plastic melt temperature. As a
general rule of thumb the heat energy required to melt plastic is 20% from
barrel heaters and 80% from mechanical shear. The melt temperature is a
combination of many factors and settings. As part of Crescent’s start up
and shut down procedure the established melt temperature is verified and
documented.
Plastic
Flow Rate
Inject as fast as
possible, consistent with quality, until the cavity is 95 to 99% full. As
this point, transfer to pack and hold using machine position transfer.
This is the established (plastic flow rate) machine fill time for this
mold. The fill time is a combination of many factors and settings.
The established fill time for this validated process does not change.
Crescent buys
world-class equipment that gives us the confidence of consistent (plastic flow
rate) machine fill time through closed loop control. The CNC electric
molding machine has pushed that confidence of consistency to an even higher
level versus hydraulic machines. As part of our start up and shut down
procedure the established fill time is verified and documented.
Plastic
Pressure Gradient
Pack the part to
finish filling while adding enough plastic to compensate for shrinkage.
Establish the hold pressure that gives the part the desired cosmetic look and
size. Coupled with the established hold pressure the proper amount of
hold time is established. If a post gate cavity pressure sensor is not
present, then use the weight method.
Traditionally for
Crescent’s process development, we are establishing gate seal. We then push the
hold pressure to its high and low limits to establish the range for the
validated process. As part of our start up and shut down procedure the
established hold pressure is verified and documented.
Plastic
Cooling Rate and Time
Cooling rate is the
difference between melt temperature and mold temperature. The mold
temperature is established using the material manufactures recommended ranges
and the customers working temperature of the part. Water flow is verified
and checked using a calibrated surface probe to measure the temperature
difference between the in and the out. The delta T of each water circuit
during process development is established at 4 degrees or lower and documented
on the process sheet.
A targeted cooling
time is already established based on the part and mold design and the material
being used. Fine tuning the cooling time during process development is
directly related to the customers’ expectations of part appearance, size and
desired properties. Once established it is documented on the process
sheet.
没有评论:
发表评论