Responses to vacuum forming part 1
Cheryl Lewis
Description
Collection
Title:
Responses to vacuum forming part 1
Creator:
Cheryl Lewis
Date:
3/29/2006
Text:
Thank you to everyone who responded to the vacuum time for thermoforming
plastic. I received a lot of in depth answers regarding the set
temperatures of the plastic and using IR thermometers to ensure that this
temperature is reached. Once the temp is lower than the �set temperature�
(which is outlined in the specifications of the plastic manufacturer) the
plastic formation is complete and vacuum no longer has an effect. This
temp must be reached inside and out ( someone suggested drilling a hole next
to the seam (in the waste section) and immediately using an IR thermometer
to take an internal reading.
The general consensus is that vacuum time is of much less importance than
all the other variables in pulling plastic. Using a warm, dry cast in a
draft free environment with proper wicking and rapid seal to achieve vacuum
are all necessary to prevent springing etc. I understand the principles of
cooling plastic evenly etc. and how important this is for tight molding � my
technicians and I were just hoping to come up with standardized time to turn
off the vacuum!
There are a few ways to see if the plastic has cooled sufficiently to shut
off the vacuum � the most low tech one from Gary Bedard at Becker : �In the
lab at Becker Orthopedic in Troy and at Becker Oregon, we instruct our techs
to touch the double seam with their hands. If they can leave their hand in
place without noxious pain, it is safe to take the plastic off vacuum and
off the mold. This simple test is easier than shooting the plastic with an
IR thermometer� This seems to be the simplest � and it never hurts to leave
it another minute or so after that. Most people still advocate leaving the
mold uncut (off vacuum) over night.
Ultimately there is no set �2x length of time in the oven� rule of thumb,
especially now when we have PDQ ovens, convection ovens as well as the good
ol� pizza oven. There was some agreement that this �rule� is more vacuum
than needed � better safe than sorry I guess!
I have attached the summary of all the responses � including the names of
respondents. I appreciate all the input � it was great to learn some of the
tricks others use!
Cheryl Lewis BSc(HK), CO(c)
Certified Orthotist
RESPONSES
Although people in our profession have used the 2 times rule in shutting off
vacuum the issue is what is going on with the molecular structure of our
plastic. Depending on the temperature you heated your plastic and the time
in the oven is not always the best. Although the melt temp should be the
same the heat distribution in your oven and the accuracy of temperature have
an effect as to how long it takes for the molecules to stop scrambling
around, this is the cause for most plastic springing. If the cast is on
vacuum for 20 minutes and the plastic is cool to the touch or close to room
temperature its usually OK to remove from the vacuum, as long as your
waiting to trim out till the next day or a least sufficient time for the
plastic to be completely cooled. The key is controlling the memory property
of the plastic and that is why it is not recommended to cut off the plastic
too soon. As far as where it was written the most informative info can be
found in the original tech notes from Dupont on the use of polypropylene
sheeting and its forming properties. In addition this discussion took place
a number of years ago when DAW industries introduced their plastic line.
Also good sources are Fillauer , Friddle Orthopedic,Guard Plastics and JMS
Plastics for specific guidelines . All are good sources for proper use of
vacuum forming techniques.
Carey Glass CPO, LPO, FAAOP
You'll see this written in the American Plastics company catalog in the
General/Technical/Information Articles, where they recommend, for their
products, times to remain under vacuum.
Robert MacDonald RTPO(c)
The rule of letting it cool twice as long as it took to cook is kind of
questionable nowadays with our efficient infrared ovens. I do think it is a
good general rule though. If you have a larger cast than add time and
the opposite for a smaller one.
Sounds like everything else is in line... waiting overnight.
The kicker, in my opinion, is the wetter the cast the faster you have to
cool the plastic on the outside. The wet plaster will take care of cooling
the plastic on the inside and when you don't cool both inside and the
outside at the same rate That's when you have springing and contracting.
With a wet cast you should be applying compressed air to the mold after
vacuum is achieved and with a drier cast it may not be necessary to apply
the forced air.
If you have any sort of lining... it acts as an insulator and therefore you
should not try to cool the outside of the plastic at all. Just let it air
cool. Of course, with the addition of a liner, it does increase the amount
of time vacuum should be applied.
Sorry for rambling.... I'm sure you already knew most of what I emphasized.
Good luck and I look forward to your posting the replies.
Sean Sullivan RTPO, CFO
Open that can of worms! I am old school and prefer overnight. We did a
little in house test afew years ago and now allow 3-4 hours to be
conservative. This is a very old discussion. Gary Bedard of Becker is the
best source of real facts.
Teri Powers-Watts,CPO,CPed
The mold time for thermoplastic depends ot two factors, first the type of
plastic, and second the temperature of the mold. In production process with
polypropylene I have seen mold times as low as 2 minutes. They of course
use aluminum mold and heat them to about 200 degrees F. They also use a
very pure form of polypropylene. You should check with your supplier about
additives in your polypropylene. The mold temperatures helps resist
reorientation, the tendency for the plastic to return to a flat sheet.
The important factor is the difference in tempter of the two surfaces of the
material. If the inside surface cools faster then the outside you will see
more reorientation. Because we use a plaster molds, and almost never allow
them to completely dry we have a greater problem with the material surface
temperature. The correct answer is the time that works best for your
facility, with your material. You should run some test in your lab. BTW it
does not matter how fast you heat the plastic. However, over heating the
material is fatal.
Ed Herman, CPO
member, Society of Plastic Engineers.
Your cooling rule of thumb 2X time to heat up is an old myth in the field.
I use the word myth as I have never seen any technical reference to
support this rule in any industrial thermoforming textbook or O&P reference
for that matter.
Your removal procedure can be based on the set temperature of the material.
The Plastic Set Temperature is: �The temperature at which a part can be
removed from a mold without distortion�. The quote is from the textbook
Technology of Thermoforming, by James L. Throne, Ph.D., who is one of the
foremost experts on heavy gauge, cut sheet, drape, vacuum thermoforming, in
North America. He has several textbooks on the subject and also contributes
to the quarterly magazine of the thermoforming division of the Society of
Plastic Engineers.
Polypropylene for example, has a set temperature of 19O degrees F. Once your
pull has reached that internal temperature, it can be removed from vacuum
and cut off the positive model. Polypro in essence freezes at 190 F.
Leaving the PP on the positive overnight has no macro effect on the
molecular structure of the material. If you claim that you have spreading if
you do not leave it on over night, that is another issue entirely that is
not connected directly to the length of time the material is left on the
positive model.
In the lab at Becker Orthopedic in Troy and at Becker Oregon, we instruct
our techs to touch the double seam with their hands. If they can leave their
hand in place without noxious pain, it is safe to take the plastic off
vacuum and off the mold. This simple test is easier than shooting the
plastic with an IR thermometer. Even the heat gun dead fingers of most
orthotists and techs can still feel pain at approximately 135 degrees F. If
you don't believe that, check the court records on the case of the little
old lady that burnt her lap with spilled McDonald's coffee.
Temperature is the most accurate method of determining your cool down period
or dwell time as it is called in the industrial thermoforming sector. A
time formula can also be used which is; one second per mil of sheet
thickness. Example: A .187 (3/16) thick sheet would cool for 3 minutes and
12 seconds . (I think Gary meant �1 minute� per millimeter? Then again it
may be 1 second per 1 thousandth of an inch ? either one is close and the
millimeter estimate seems to err on the side of caution) Of course all the
variables of room temperature, mold temperature, plastic temperature, air
stability, i.e., no fans blowing on the plastic etc., will effect your
formula, so it becomes much less precise than checking the actual
temperature of the plastic.
If you would like more information on clinical thermoforming, I do have a
DVD based course. Unfortunately, I just gave my last copy to the new O&P
school at St. Pete's College in Florida. All the O&P schools in the US use
this course as does BCIT. Touch base with me in the near future to check on
the availability of the DVD package. If memory serves me correctly, ORTO-PED
INC., our distributor in Canada has a copy of the program.
Regards,
Gary G. Bedard, CO, FAAOP
Becker Orthopedic Company
Sound like your clinic needs to do the Mximimizing Patient Safety Course:
Material Science and P&O design... I'm one of the lecturers, John Michael
is the other....(where are you based?)
Simple answer....3 weeks ..... =-O (Just kidding).
What you have is a material with VERY poor thermal conductivity. This is
(probably ) what's being taken into account for your heat up time.
Now heating up depends on HOW you heat it up....Cooling down depends on HOW
you cool it down. I would suggest that the only common denominator is the
material and NOT the methods of heating and cooling so your rule of thumb is
a little vulnerable (although leaning on the side of caution - which is
good!)
By draping the plastic onto a cast (and there are variables there!) you are
effcetively insulating the inside of that material....even ththough the
cast will have a conductive quench on the surface of the inside of the
material initially. The material will then start to cool on the outside by
pure convection. Chances are you heated it by forced convection.....with
some very simple maths it is possible to show that the time required will
easily be more than doubled if you have a less efficient cooling system and
the material (inside) furtherest away from the cooling system (outside) is
being insulated (by the material wrapped around it)..........make sense???
...and of course , the type of cast and temperature of cast can have a
massive effect.
Buy an infra red thermometer (industrial)....and test some drapes....
Now, when the inside and core is at a temperature BELOW the set
temperature of the material - (do you know what that is?)
then the vacuum will stop having an effect................
THIS WILL TAKE TIME...which is often what the facility fabricating doesn't
have...but from what you are saying (leave it overnight!) you do!
This also means that a lot of rules of thumb (sorry no offense) are
non-sensical
outside, cool enough to touch
dorsal section stops being clear
drill hole in heal
....but I do like
wrap in insulating blanket!?!?!
Classically all of these stages are done poorly, therfore the material
springs out....do it poorly a diiffrent way and it will spring in...weird
huh!
Of course you can use the age old principle (that doesn't work)...buy more
expensive material from a different supplier because their material is
better.. ;-)
Hope this helps a bit
Bill
plastic. I received a lot of in depth answers regarding the set
temperatures of the plastic and using IR thermometers to ensure that this
temperature is reached. Once the temp is lower than the �set temperature�
(which is outlined in the specifications of the plastic manufacturer) the
plastic formation is complete and vacuum no longer has an effect. This
temp must be reached inside and out ( someone suggested drilling a hole next
to the seam (in the waste section) and immediately using an IR thermometer
to take an internal reading.
The general consensus is that vacuum time is of much less importance than
all the other variables in pulling plastic. Using a warm, dry cast in a
draft free environment with proper wicking and rapid seal to achieve vacuum
are all necessary to prevent springing etc. I understand the principles of
cooling plastic evenly etc. and how important this is for tight molding � my
technicians and I were just hoping to come up with standardized time to turn
off the vacuum!
There are a few ways to see if the plastic has cooled sufficiently to shut
off the vacuum � the most low tech one from Gary Bedard at Becker : �In the
lab at Becker Orthopedic in Troy and at Becker Oregon, we instruct our techs
to touch the double seam with their hands. If they can leave their hand in
place without noxious pain, it is safe to take the plastic off vacuum and
off the mold. This simple test is easier than shooting the plastic with an
IR thermometer� This seems to be the simplest � and it never hurts to leave
it another minute or so after that. Most people still advocate leaving the
mold uncut (off vacuum) over night.
Ultimately there is no set �2x length of time in the oven� rule of thumb,
especially now when we have PDQ ovens, convection ovens as well as the good
ol� pizza oven. There was some agreement that this �rule� is more vacuum
than needed � better safe than sorry I guess!
I have attached the summary of all the responses � including the names of
respondents. I appreciate all the input � it was great to learn some of the
tricks others use!
Cheryl Lewis BSc(HK), CO(c)
Certified Orthotist
RESPONSES
Although people in our profession have used the 2 times rule in shutting off
vacuum the issue is what is going on with the molecular structure of our
plastic. Depending on the temperature you heated your plastic and the time
in the oven is not always the best. Although the melt temp should be the
same the heat distribution in your oven and the accuracy of temperature have
an effect as to how long it takes for the molecules to stop scrambling
around, this is the cause for most plastic springing. If the cast is on
vacuum for 20 minutes and the plastic is cool to the touch or close to room
temperature its usually OK to remove from the vacuum, as long as your
waiting to trim out till the next day or a least sufficient time for the
plastic to be completely cooled. The key is controlling the memory property
of the plastic and that is why it is not recommended to cut off the plastic
too soon. As far as where it was written the most informative info can be
found in the original tech notes from Dupont on the use of polypropylene
sheeting and its forming properties. In addition this discussion took place
a number of years ago when DAW industries introduced their plastic line.
Also good sources are Fillauer , Friddle Orthopedic,Guard Plastics and JMS
Plastics for specific guidelines . All are good sources for proper use of
vacuum forming techniques.
Carey Glass CPO, LPO, FAAOP
You'll see this written in the American Plastics company catalog in the
General/Technical/Information Articles, where they recommend, for their
products, times to remain under vacuum.
Robert MacDonald RTPO(c)
The rule of letting it cool twice as long as it took to cook is kind of
questionable nowadays with our efficient infrared ovens. I do think it is a
good general rule though. If you have a larger cast than add time and
the opposite for a smaller one.
Sounds like everything else is in line... waiting overnight.
The kicker, in my opinion, is the wetter the cast the faster you have to
cool the plastic on the outside. The wet plaster will take care of cooling
the plastic on the inside and when you don't cool both inside and the
outside at the same rate That's when you have springing and contracting.
With a wet cast you should be applying compressed air to the mold after
vacuum is achieved and with a drier cast it may not be necessary to apply
the forced air.
If you have any sort of lining... it acts as an insulator and therefore you
should not try to cool the outside of the plastic at all. Just let it air
cool. Of course, with the addition of a liner, it does increase the amount
of time vacuum should be applied.
Sorry for rambling.... I'm sure you already knew most of what I emphasized.
Good luck and I look forward to your posting the replies.
Sean Sullivan RTPO, CFO
Open that can of worms! I am old school and prefer overnight. We did a
little in house test afew years ago and now allow 3-4 hours to be
conservative. This is a very old discussion. Gary Bedard of Becker is the
best source of real facts.
Teri Powers-Watts,CPO,CPed
The mold time for thermoplastic depends ot two factors, first the type of
plastic, and second the temperature of the mold. In production process with
polypropylene I have seen mold times as low as 2 minutes. They of course
use aluminum mold and heat them to about 200 degrees F. They also use a
very pure form of polypropylene. You should check with your supplier about
additives in your polypropylene. The mold temperatures helps resist
reorientation, the tendency for the plastic to return to a flat sheet.
The important factor is the difference in tempter of the two surfaces of the
material. If the inside surface cools faster then the outside you will see
more reorientation. Because we use a plaster molds, and almost never allow
them to completely dry we have a greater problem with the material surface
temperature. The correct answer is the time that works best for your
facility, with your material. You should run some test in your lab. BTW it
does not matter how fast you heat the plastic. However, over heating the
material is fatal.
Ed Herman, CPO
member, Society of Plastic Engineers.
Your cooling rule of thumb 2X time to heat up is an old myth in the field.
I use the word myth as I have never seen any technical reference to
support this rule in any industrial thermoforming textbook or O&P reference
for that matter.
Your removal procedure can be based on the set temperature of the material.
The Plastic Set Temperature is: �The temperature at which a part can be
removed from a mold without distortion�. The quote is from the textbook
Technology of Thermoforming, by James L. Throne, Ph.D., who is one of the
foremost experts on heavy gauge, cut sheet, drape, vacuum thermoforming, in
North America. He has several textbooks on the subject and also contributes
to the quarterly magazine of the thermoforming division of the Society of
Plastic Engineers.
Polypropylene for example, has a set temperature of 19O degrees F. Once your
pull has reached that internal temperature, it can be removed from vacuum
and cut off the positive model. Polypro in essence freezes at 190 F.
Leaving the PP on the positive overnight has no macro effect on the
molecular structure of the material. If you claim that you have spreading if
you do not leave it on over night, that is another issue entirely that is
not connected directly to the length of time the material is left on the
positive model.
In the lab at Becker Orthopedic in Troy and at Becker Oregon, we instruct
our techs to touch the double seam with their hands. If they can leave their
hand in place without noxious pain, it is safe to take the plastic off
vacuum and off the mold. This simple test is easier than shooting the
plastic with an IR thermometer. Even the heat gun dead fingers of most
orthotists and techs can still feel pain at approximately 135 degrees F. If
you don't believe that, check the court records on the case of the little
old lady that burnt her lap with spilled McDonald's coffee.
Temperature is the most accurate method of determining your cool down period
or dwell time as it is called in the industrial thermoforming sector. A
time formula can also be used which is; one second per mil of sheet
thickness. Example: A .187 (3/16) thick sheet would cool for 3 minutes and
12 seconds . (I think Gary meant �1 minute� per millimeter? Then again it
may be 1 second per 1 thousandth of an inch ? either one is close and the
millimeter estimate seems to err on the side of caution) Of course all the
variables of room temperature, mold temperature, plastic temperature, air
stability, i.e., no fans blowing on the plastic etc., will effect your
formula, so it becomes much less precise than checking the actual
temperature of the plastic.
If you would like more information on clinical thermoforming, I do have a
DVD based course. Unfortunately, I just gave my last copy to the new O&P
school at St. Pete's College in Florida. All the O&P schools in the US use
this course as does BCIT. Touch base with me in the near future to check on
the availability of the DVD package. If memory serves me correctly, ORTO-PED
INC., our distributor in Canada has a copy of the program.
Regards,
Gary G. Bedard, CO, FAAOP
Becker Orthopedic Company
Sound like your clinic needs to do the Mximimizing Patient Safety Course:
Material Science and P&O design... I'm one of the lecturers, John Michael
is the other....(where are you based?)
Simple answer....3 weeks ..... =-O (Just kidding).
What you have is a material with VERY poor thermal conductivity. This is
(probably ) what's being taken into account for your heat up time.
Now heating up depends on HOW you heat it up....Cooling down depends on HOW
you cool it down. I would suggest that the only common denominator is the
material and NOT the methods of heating and cooling so your rule of thumb is
a little vulnerable (although leaning on the side of caution - which is
good!)
By draping the plastic onto a cast (and there are variables there!) you are
effcetively insulating the inside of that material....even ththough the
cast will have a conductive quench on the surface of the inside of the
material initially. The material will then start to cool on the outside by
pure convection. Chances are you heated it by forced convection.....with
some very simple maths it is possible to show that the time required will
easily be more than doubled if you have a less efficient cooling system and
the material (inside) furtherest away from the cooling system (outside) is
being insulated (by the material wrapped around it)..........make sense???
...and of course , the type of cast and temperature of cast can have a
massive effect.
Buy an infra red thermometer (industrial)....and test some drapes....
Now, when the inside and core is at a temperature BELOW the set
temperature of the material - (do you know what that is?)
then the vacuum will stop having an effect................
THIS WILL TAKE TIME...which is often what the facility fabricating doesn't
have...but from what you are saying (leave it overnight!) you do!
This also means that a lot of rules of thumb (sorry no offense) are
non-sensical
outside, cool enough to touch
dorsal section stops being clear
drill hole in heal
....but I do like
wrap in insulating blanket!?!?!
Classically all of these stages are done poorly, therfore the material
springs out....do it poorly a diiffrent way and it will spring in...weird
huh!
Of course you can use the age old principle (that doesn't work)...buy more
expensive material from a different supplier because their material is
better.. ;-)
Hope this helps a bit
Bill
Citation
Cheryl Lewis, “Responses to vacuum forming part 1,” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 25, 2024, https://library.drfop.org/items/show/226404.
