Response summary: pre-preg technique
Ted Trower
Description
Collection
Title:
Response summary: pre-preg technique
Creator:
Ted Trower
Date:
12/16/2005
Text:
My original question was:
We've been experimenting with the pre-preg materials for orthoses and
have a question about technique. The supplier suggests the use of a
PVA bag on the exterior surface of the material to provide a smooth
surface finish. This works to a degree but in our experience the PVA
partially bonds to the surface and removal becomes extremely
difficult if not impossible. It's just a time consuming mess. Is
anyone aware of a better material to use for this purpose?
Responses follow:
=======================================================
You can give us a call if you like. We are using a technique that
seems to work well. We vaccuum form 3/32 modified polyethelene
around the afo mold. We then lay up our prepreg schedule. We then
place the prepreg'd mold in a pressure vessel that we have made for
an hour. We then use a bagging material to form a vacuum bag and
cook it under vacuum for a number of hours. I think our system is
working well considering we are not using a autoclave.
_______________________________________________________________
One of our 2nd generation CO's, invested in this exact technology
several years ago. He is one of my members and his dad was one of my
mentors, many years ago! He has what I lovingly call the iron
lung autoclave for heat under pressure, the freezer and a technical
staff making custom prepreg orthotics. He would be a great guy for
you to contact and brainstorm with on this advancing technology. I
brought Marlo and John there when we were discussing a prepreg
external frame, but that is way down the wish list at this point.
_______________________________________________________________
We use a tear sheet directly on the surface of the carbon, then a
layer of suction fleece to sick away excess resin, a layer of
stocking for vacuum and then the PVA bag. That seems to leave a
fairly decent surface.
_______________________________________________________________
Peel ply from Euro international!
_______________________________________________________________
You can try low temp vacuum bagging material. It is similar to the
roasting bags to cook in. It is not water soluble like the pva is.
_______________________________________________________________
There are some release films that won't stick, but they are not very
stretchy. Purple release film used by friends of mine in the Indy Car
industry may work, but you may have to have some pleats or over laps.
_______________________________________________________________
I'm interested in what you are doing. What company and what technology are
you using? I assume it is of the LTM (Low Temperature Molding) type,
compared to the high pressure and heat techniques which require autoclaving,
etc. Eventually, with the help of finite element analysis, we'll be able to
propel O&P to the next level in a practical sense and leave all the
want-a-be's in the dust. Too bad I'm on the tail-end of this. Anyway, I
would love to hear what you are doing and would be interested in working and
sharing information with you. I have access to some of the race car
composite guys here in Indy and they obviously know a lot about carbon
fiber. They are very selective in who they show their information with, but
I think I know a few who would be glad to help. I have not entered with
world, just yet, but have doing some of the preamble background work. I've
met with the carbon composite folks, but at the time, we were talking about
the high pressure technology. There are still a lot of benefits to using
these procedures too, but a lot more complicated, manufacturing and
equipment-wise. Thanks for your reply.
_______________________________________________________________
A couple of options for you to consider.
Otto Bock makes PVC bags.
I don't know if they will bond as well.
You might consider using a tire tube.
We've experimented with various urethane laminations with the same problem.
We found that we had to paint the cast (which must be very dry) with
silicone in order to seal it.
That way we didn't have the stuck pva bag on the inside of the afo.
_______________________________________________________________
Use PVC bags instead.
_______________________________________________________________
>as you were writing your request we have been working diligently on
>a more reliable smooth outer peel surface it is still in the testing
>stage and should be released the first of the year. This is a
>stretch membrane with perforations every 13 mm to allow resin to
>escape and be absorbed by the suction fleece. The current method of
>striping the PVA as soon as the part is removed from the oven works
>but this needs to be done quickly prior to cooling.
_______________________________________________________________
It has been my experience that the PVA is about the only material
that I can guarantee will not work. Why do they use that stuff? Check
out any of the better fibergalss supply houses and you will find lots
of alternatives, like green bag (nylon) or my favorite, silicone. all
these other materials handle the heat and are not attacked by the resin.
I use pallet wrap (saran wrap works too) on the inside and silicone
on the outside and get really good results.
_______________________________________________________________
Silicone sheeting is the material of choice. It is the burnt orange
colored stuff. Reusable and available in several sizes. Also know as
bladder material. Also try reducing the heat when using PVA. The heat
makes the resin flow at a certain rate but you can reduce the heat
and just leave it in longer to achieve the same flow. Think of it as
molasses running downhill where the slope of the hill represents the
temperature. You need a certain slope (temperature) to get it moving
but once it is, time is the determinate of flow.
=======================================================
End of Responses
I'll be looking into some of the other separator films
mentioned. I'd love to receive more detailed information on sources
and materials for this job.
We have been pulling thin thermoplastic (PP or PE) over the mold and
delivering the orthosis with this in place. It provides a superb
moisture barrier, a glass smooth interior and improved options if fit
adjustments are necessary post delivery.
We have tried stripping the exterior PVA fresh out of the oven but
that doesn't seem to help us any. I think it's just the wrong material.
Ted A. Trower C.P.O.
A-S-C Orthotics & Prosthetics
Jackson, Michigan, USA
www.amputee.com
___________________________________________________________
CONFIDENTIALITY NOTICE:
This email message, including any attachments, is for the sole use of
the intended recipient (s) and may contain confidential and
privileged information. Any unauthorized review, use, disclosure, or
distribution is prohibited. If you are not the intended recipient,
please contact the sender by reply e-mail and destroy all copies of
the original message.
We've been experimenting with the pre-preg materials for orthoses and
have a question about technique. The supplier suggests the use of a
PVA bag on the exterior surface of the material to provide a smooth
surface finish. This works to a degree but in our experience the PVA
partially bonds to the surface and removal becomes extremely
difficult if not impossible. It's just a time consuming mess. Is
anyone aware of a better material to use for this purpose?
Responses follow:
=======================================================
You can give us a call if you like. We are using a technique that
seems to work well. We vaccuum form 3/32 modified polyethelene
around the afo mold. We then lay up our prepreg schedule. We then
place the prepreg'd mold in a pressure vessel that we have made for
an hour. We then use a bagging material to form a vacuum bag and
cook it under vacuum for a number of hours. I think our system is
working well considering we are not using a autoclave.
_______________________________________________________________
One of our 2nd generation CO's, invested in this exact technology
several years ago. He is one of my members and his dad was one of my
mentors, many years ago! He has what I lovingly call the iron
lung autoclave for heat under pressure, the freezer and a technical
staff making custom prepreg orthotics. He would be a great guy for
you to contact and brainstorm with on this advancing technology. I
brought Marlo and John there when we were discussing a prepreg
external frame, but that is way down the wish list at this point.
_______________________________________________________________
We use a tear sheet directly on the surface of the carbon, then a
layer of suction fleece to sick away excess resin, a layer of
stocking for vacuum and then the PVA bag. That seems to leave a
fairly decent surface.
_______________________________________________________________
Peel ply from Euro international!
_______________________________________________________________
You can try low temp vacuum bagging material. It is similar to the
roasting bags to cook in. It is not water soluble like the pva is.
_______________________________________________________________
There are some release films that won't stick, but they are not very
stretchy. Purple release film used by friends of mine in the Indy Car
industry may work, but you may have to have some pleats or over laps.
_______________________________________________________________
I'm interested in what you are doing. What company and what technology are
you using? I assume it is of the LTM (Low Temperature Molding) type,
compared to the high pressure and heat techniques which require autoclaving,
etc. Eventually, with the help of finite element analysis, we'll be able to
propel O&P to the next level in a practical sense and leave all the
want-a-be's in the dust. Too bad I'm on the tail-end of this. Anyway, I
would love to hear what you are doing and would be interested in working and
sharing information with you. I have access to some of the race car
composite guys here in Indy and they obviously know a lot about carbon
fiber. They are very selective in who they show their information with, but
I think I know a few who would be glad to help. I have not entered with
world, just yet, but have doing some of the preamble background work. I've
met with the carbon composite folks, but at the time, we were talking about
the high pressure technology. There are still a lot of benefits to using
these procedures too, but a lot more complicated, manufacturing and
equipment-wise. Thanks for your reply.
_______________________________________________________________
A couple of options for you to consider.
Otto Bock makes PVC bags.
I don't know if they will bond as well.
You might consider using a tire tube.
We've experimented with various urethane laminations with the same problem.
We found that we had to paint the cast (which must be very dry) with
silicone in order to seal it.
That way we didn't have the stuck pva bag on the inside of the afo.
_______________________________________________________________
Use PVC bags instead.
_______________________________________________________________
>as you were writing your request we have been working diligently on
>a more reliable smooth outer peel surface it is still in the testing
>stage and should be released the first of the year. This is a
>stretch membrane with perforations every 13 mm to allow resin to
>escape and be absorbed by the suction fleece. The current method of
>striping the PVA as soon as the part is removed from the oven works
>but this needs to be done quickly prior to cooling.
_______________________________________________________________
It has been my experience that the PVA is about the only material
that I can guarantee will not work. Why do they use that stuff? Check
out any of the better fibergalss supply houses and you will find lots
of alternatives, like green bag (nylon) or my favorite, silicone. all
these other materials handle the heat and are not attacked by the resin.
I use pallet wrap (saran wrap works too) on the inside and silicone
on the outside and get really good results.
_______________________________________________________________
Silicone sheeting is the material of choice. It is the burnt orange
colored stuff. Reusable and available in several sizes. Also know as
bladder material. Also try reducing the heat when using PVA. The heat
makes the resin flow at a certain rate but you can reduce the heat
and just leave it in longer to achieve the same flow. Think of it as
molasses running downhill where the slope of the hill represents the
temperature. You need a certain slope (temperature) to get it moving
but once it is, time is the determinate of flow.
=======================================================
End of Responses
I'll be looking into some of the other separator films
mentioned. I'd love to receive more detailed information on sources
and materials for this job.
We have been pulling thin thermoplastic (PP or PE) over the mold and
delivering the orthosis with this in place. It provides a superb
moisture barrier, a glass smooth interior and improved options if fit
adjustments are necessary post delivery.
We have tried stripping the exterior PVA fresh out of the oven but
that doesn't seem to help us any. I think it's just the wrong material.
Ted A. Trower C.P.O.
A-S-C Orthotics & Prosthetics
Jackson, Michigan, USA
www.amputee.com
___________________________________________________________
CONFIDENTIALITY NOTICE:
This email message, including any attachments, is for the sole use of
the intended recipient (s) and may contain confidential and
privileged information. Any unauthorized review, use, disclosure, or
distribution is prohibited. If you are not the intended recipient,
please contact the sender by reply e-mail and destroy all copies of
the original message.
Citation
Ted Trower, “Response summary: pre-preg technique,” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 5, 2024, https://library.drfop.org/items/show/225918.
