R.T.M injection molded fiberglass

[pyroracing85]

I never knew you could mold fiberglass. I seen this in a molded bumper. How do they do this process?

[turmite]

pyro,

The glass fabric is placed dry in the mold, (form), along with a material to assist in the flow of the resins. The mold is then bolted closed and the resin is injected under pressure (RTM) or with vacuum assist (VARTM). Most of the molds I have seen pics of on the internet have been to where you could see the resin flow. The problem I have read ,that is the biggest for the beginner/home user, is not getting enough saturation in the fabric which results in a scrap part.

RTM can be done with machined metal molds or composite molds, but they have to be made right. Unless you need a ton of parts the easiest way to do your part is with vacuum bagging. Do you have a 3d model of the part you want? If so, and if you do not have a way to make it send me either a sketch, screen shot or the file in either iges, dxf, stl or 3ds and I will take a look at it for you. If it is something I can do quickly I'll give you a price if you want it.
Mike Turner

you can link to my email or pm me by clicking on my user name.

[fyffe555]

The part is either made in an enclosed mold like you would plastic injection or on a former with a vacuum bag. A predetermined amount of glass or carbon is layed into the part dry and the mold or bag sealed and put under vacuum. The resin is then infused through the part, a predetermined amount of resin is injected into the part and the resin is drawn through the part by the vacuum. the parts are usuallt heat treated too if it's a composite like epoxy/carbon. Advantages are exact resin / fibre mix in the composite, less material waste, very precise parts produced. Disadvantages are expensive to do, lots of equipment needed. Also called VARTM, SCRIMP and others.

[Splint]

Hi Pyro,
from my inderstanding Lotus Cars first developed this technique (VARI-Vacuum Assisted Resin Injection) and have used it for quiet a long time to make the bodies of their cars, they effectively make a top half outer skin and use a jig saw to cut out the doors and windows. I have a picture of it in a book at home, I'll try and scan it and post it here.
Splint

[ynneb]

The problem I have read ,that is the biggest for the beginner/home user, is not getting enough saturation in the fabric which results in a scrap part.

I can think of two possible solutions. If the glass fabric was pre soaked then the resin may flow through better. OR, What would happen if you had lots of glass fibers mixed into the resin beforehand. Then you should have a uniform mixture. I know they use the latter technique for making glass reinforced concrete, so I cant see why it wouldnt work with resin.

[ger21]

There's a lot of composite info here. http://www.fibreglast.com/contentpag...enter-286.html
Here's the one on infusion http://www.fibreglast.com/documents/361.pdf
I've been doing a lot of composite research recently, and vacuum infusion doesn't really seem much more difficult than standard vacuum bagging, imo. I haven't tried it, though.

[ger21]

ANd here's a cool site with some examples. http://www.corsair82.com/corsair/vacinfs/

[fyffe555]

The Corsair site is interesting. Amazing what people can do when they put their mind to it.

RTM is a little more complex than it appears. It allows you to dry position the fiber cloth in position with the fibers in the direction and layers required for the part. By doing so the part can be 'tuned' to have reinforcement in specific places or strength in specific directions or different fibers (glass & carbon ) in different places. You cannot do this with a chopped strand wet mix for example, and the weight and strengths would be very different.

For more complex or engineered parts the rtm trick is to get the resin to flow throughout the part evenly and in the quantity and time required. If you just inject resin at one point and vacuum from another then spme of the part will not get impregnated. Even when the part is fully impregnated you have to adjust the injection and vaccum scheme to ensure that the ratio of resin to fiber is as required across the part accounting for the amount of fiber and the shape of the part. The benefit is that once it's worked out you can produce a huge number of identical parts with identical mechanical characteristics with minimal material wastage.

As Gerry suggested I'd think that for the sort of parts a hobbiest is likely to make then a 'super' home vaccum bagging system with a bit of trial and error sounds possible.

[svenakela]

I use RTM a lot and it's a lovely method.
The guys at Polyworx creates simulators for RTM, and they assist shipyards moulding UGE yachts. We're talking 70 feet...
http://www.polyworx.nl/

Regards,
Sven

[pyroracing85]

Originally Posted by fyffe555 The part is either made in an enclosed mold like you would plastic injection or on a former with a vacuum bag. A predetermined amount of glass or carbon is layed into the part dry and the mold or bag sealed and put under vacuum. The resin is then infused through the part, a predetermined amount of resin is injected into the part and the resin is drawn through the part by the vacuum. the parts are usuallt heat treated too if it's a composite like epoxy/carbon. Advantages are exact resin / fibre mix in the composite, less material waste, very precise parts produced. Disadvantages are expensive to do, lots of equipment needed. Also called VARTM, SCRIMP and others.

I heard of the scrimp process in boat building. So r.t.m. is scrimp?

[routalot]

While RTM and SCRIMP are useful processes,the ratio of resin to fibre is less good than in more high tech composites.The tooling costs can be high but for high volume production of components where very high strength/weight is not the prime requirement they work well.RTM typically involves a set of matched moulds for the whole component but infusion can be used with a vacuum bag over a mould which can help with tooling costs but then you need to factor in the cost of the consumables,which is to say the bag and the infusion mesh.

[svenakela]

Originally Posted by routalot While RTM and SCRIMP are useful processes,the ratio of resin to fibre is less good than in more high tech composites.The tooling costs can be high but for high volume production of components where very high strength/weight is not the prime requirement they work well.RTM typically involves a set of matched moulds for the whole component but infusion can be used with a vacuum bag over a mould which can help with tooling costs but then you need to factor in the cost of the consumables,which is to say the bag and the infusion mesh.

I don't agree with much here. First of all, please explain to me what "more high tech composites" are. A composite is a composite, period. It can be made of glue and wood OR "high tech" materials. I can't really think of anything that can be more graduated as "high tech composites" than Epoxi and carbon fabrics, the base ingridients of an RTM-setup...
RTM, Resin Transfer Moulding, is the original name of the procedure of laying a plastic film over a single mould. The vacuum applied INJECTS the resin into the fabric. The physical laws helps out here and creates a wetted fabric that is as close to perfect in fabric/resin ratio.
The cost of producing the mould is not higher than making moulds in any soft material such as Ureol or Aluminium and only one mould side is needed. The other half is automatically generated by the vacuum bag. By nature, when the vacuum is applied the pressure into the form will be 1 kilo per sq-centimeter, 1 Bar so to say. Not much, but if the mould surface is 50x50 cm the pressure is at 2,5 ton, and if the mould is placed in a chamber, just multiply the mould load with the chamber pressure. Try to retrieve those values without buying a moulding machine that requires steel moulds...

But I do agree that the cost of moulding with RTM is a lot higher because of the extra material needed (bagging, tubes, transfer net etc). But at the end, a low volume production of bigger parts made of composites, nothing is as effective.


Regards,
Sven