After you establish the fill time for
first stage, you then have to figure out 2nd stage or
hold. You have to determine three things for 2nd stage:
Whether to run with or without gate seal.
How much time to put on the hold or 2nd stage timer
How much pressure to use.
To establish these three issues you first do a gate
seal study to find out whether to run with or with out
gate seal. The complete procedure to do a gate seal
study correctly is in the last section of my seminar
manual. Basically, set hold time to a much longer time
than you think is necessary for gate seal. Then start
with a low pressure and bring it up slowly until you
have a pressure that makes the part Look like it might
be OK. This is only a pressure to do the gate seal study,
it is NOT the pressure you will run production at. Now
weight this part. Take 1-2 seconds off of 2nd stage
time and add it to the cooling or mold close timer to
keep the cycle constant for this experiment. Weight
this part. Repeat this procedure until there is only
0.5 seconds of hold time. Weigh all the parts and plot
part weight vs. hold time (2nd stage time). Here's
what the plot should look like. If it is not flat
for cold runner molds at the look hold times you did
not use enough hold time. Hot runner tools will not
flatten out.
Now take parts that you know have gate seal and parts
you know were made without gate seal and test them for
function. NOT Size. Run the part with whatever gives
you the best strength, warp, or function. If running
with gate seal always choose a time a bit longer than
the actual gate seal time. This will provide a more
robust cycle. If you have to run with no gate seal it
will be CRITICAL to have constant cycle time. Cycle
time is still important for running with gate seal but
not nearly as critical for a process running without
gate seal. Bottom line this establishes hold time.
Once hold time is established you now revisit the pressure
question. Start all over at very low pressure maybe
200 psi on a normal intensification ratio machine.
Increase pressure until you think you have parts the
may be OK. Make as many as QC needs for testing. Now
start raising the hold pressure until you get flash
or parts begin to stick. Drop the hold pressure just
a bit and run another set of parts for quality control
at this high hold pressure. Now run a third set in between
the low and high pressure limits you found. Send all
to QC and find out where you need to run the pressure.
There is no rule as to how much hold pressure you use.
It has nothing to do with first stage.
Your Expert
About
John Bozzelli Competent in resin characterization and analysis, John's
specialty is practical, hands-on injection molding training with both small and
large machines. Learn
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About
John Bozzelli