Part 2 of Vacuum time for thermomolding
It has nothing to do with length of time under vacuum. The inner portion of
the plastic making contact with the mold cools at a different rate than the
portion exposed to the air. If the inner portion cools faster than the
outer, which is typical, the plastic will tend to spring off. If the outer
cools faster than the inner as when you have an aliplast liner serving as an
insulator, the plastic will tend to curl. Vacuum should be left on long
enough for the inner and outer portions of the plastic to cool below the set
temperature. Once the plastic (inner and outer portions) cools below the
set temperature, the internal stresses are already set. Leaving the plastic
under vacuum, on the model or in a bucket of water is not going to change
what happens when you cut it off the model.
All plastic has a thermoformable range. The slower and more evenly you heat
and cool the plastic the more uniform the results. The materials we use are
the same that are used in many other plastic products and because the
production is so controlled the process is very fast, predictable and
reproducible.
Many of the plastic products made for automobiles are made through plastic
injection or blow molding which are both forms of thermoforming. The
heating of the plastic is controlled to within a few degrees and mold
temperatures are controlled to within similar tolerances, yet cycles are
producing an entire car dashboard in 2 minutes and a gallon milk container
ever 7 seconds. To say it is 2 times the heating time I think is
oversimplifying the process.
Win Gasperson, CPO
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Please note this response was sent to me as it is written it seems to be
a conversation between Bill and John
Hi John and Cheryl,
John Michael wrote:
Bill-
I understand how an IR thermometer will measure surface temp but how can we
determine core temp? Would it be possible to punch a hole in a waster area
of uniform thickness and then immediately aim the IR beam at the edge to
approximate core temp?
Kaaaching!…Probably best to do fair few trials (and get to the “Quality
and repeatability ” Model we talk about).
A quick check of all the aspects including what/where you can measure as
well as cast temperature as well as “my old friend”, gridding material and
do a few trials to determine handling.
Cheryl raises a good question that I wish had a more definite practical
answer….
Don’t forget, KNOW the critical temperatures of the material you are using
as well as your oven as well as the cast temperature as well as the ambient
temperature!…
All of this will give you a very good indication of the cooling process of
the material.
Cheers
Bill
–John
Cheryl -I’m sure you will receive many responses to your inquiry, some
providing facts and research regarding “crystallization of molecules” and
other technical reasoning behind their answers, my input will be much
simpler .I’m currently an RTO who’s been pulling plastic for 18 years and
I’ve come to these conclusions that result in a 99.9% success rate.
Regardless of plastics, oven type, temperature or other equipment involved
in the process , there are a few critical factors . First is time, from tray
to full vacuum must be no more than fifteen seconds (ten or under is ideal).
You can leave the plastic on for a week and it will spread if you didn’t get
full vac quick enough. Second is cast temperature, the warmer the cast the
more time you have to reach full vac.before plastic starts to cool. Third is
wicking,vacuum has to reach the whole cast.Use two layers nylon hose making
sure the gap between pipe and cast is filled with any material that will
prevent plastic from pulling into the pipe. A vac gauge can mislead you into
thinking you have full vac on the cast when all you have is vacuum at the
pipe opening, verify vac by pulling on the trimmed portion of the plastic it
should not come away from the cast. Every step in the process is important
but these three are the “make or break” factors to a good pull. Assuming
I’ve achieved a “good pull” I will leave the vac on until the plastic is
just warm to the touch ( fifteen to twenty minutes) and as soon as it is
cool to the touch I can cut it off and finish it without any adverse affects
. Here’s my process in a nut shell….For AFO out of co-poly / poly-pro
Place modified cast in oven at same time as plastic, set timer for 10
minutes. Remove cast and prepare w/ two nylons add waste nylon or dacron
felt at top of cast/pipe, extend two nylons over pipe and tape around pipe
,fold excess back toward cast . Wrap warm Volara around pipe over taped
nylon. tape over Volara sealing it to pipe leaving some Volara exposed to
seal plastic to. Pull plastic ( using two people is faster, one at the
pipe one at the foot section), trim excess, as soon as full vac is achieved
I “shock” the plastic by blowing air on it from air hose (aprox.ten seconds
just till it turns cloudy ) NOTE; if you use a venturi system for vacuum
any draw on the system may reduce your vac pressure at the cast resulting in
a “bad pull” coordinate your pulling with other AIR functions .
Check that plastic is cool then cut off and finish as usual SUCCESS 🙂
If I’m not in a rush I will leave poly-pro on the cast at least an hour
before trimming though I have found it not absolutely necessary . Sorry for
the long reply but I feel people tend to blame a bad pull on anything other
than human error when that is the main contributor. If you follow these
steps you should have consistently good results .
Good luck , Chris Long RTO
