[Cafemondo]

Hello everyone, were new to this and looking forward to getting some forming done soon now. Lots of interesting advice on here thats going to save us some time hopefully. Major learning curve for us coming from a traditional furniture making industry specialising in traditional Welsh country furniture.

Were now building bodykits for beach buggys and at the mold finishing stages now. The bodykits main purpose is to keep sand and water off the users but stuff like UV durability and impact resistance will play some part in design.
The bodykits will be made either from PP or HDPE or even possibly acrylic and formed by us in a vac form machine were hoping to build.

The male/positive molds are made from mdf, plywood and bodyfiller. Once we have our finished & sanded mold do we just leave it raw or coat it in a varnish or polyester resin or something similar to preserve it?

The size of the molds are 19" x 24" with a draw of 8" for the smallest and 36" x 48" x 10" for the biggest piece. They have quite a smooth rolling design with no sharp edges/corners so we're thinking we can go down the shop vac route rather than a vacuum tank system.

Many thanks, this is probably one of the first in many lines of questions, look forward to reading your thoughts.
Mondo & Jam

[gandalf100]

Our pattern maker at work makes patterns out of pine and mdf and casts them in polyurethane resin. He generally coats them with three or four coats of brushing laquer to prevent sticking, sanding lightly after the first coat to smooth them out. I believe he uses laquer primarily because of the fast drying, therefore re-coating time. Hope this helps.

[Cafemondo]

Thanks for the reply Gandalf. Were a bit confused by your reply, when you say 'casts them in polyurethane resin' do you mean he pours or paints the resin onto the mould or into the mould to produce a positive full resin mould. This is then laquered and then used as the thermoforming buck?

So I guess our final finish for our mould will be 3 or 4 coats laquer using Japlac perhaps?

[cod]

The size of the molds are 19" x 24" with a draw of 8" for the smallest and 36" x 48" x 10" for the biggest piece.

I dont think a shop vac will be enough for a piece that big.

[Cafemondo]

What about two vacs?
Theres no high definition corners or creases with our designs, mainly big flowing curves so its just a case of sucking it down long enough for the plastic to cool isnt it?

[gandalf100]

Thanks for the reply Gandalf. Were a bit confused by your reply, when you say 'casts them in polyurethane resin' do you mean he pours or paints the resin onto the mould or into the mould to produce a positive full resin mould. This is then laquered and then used as the thermoforming buck?

So I guess our final finish for our mould will be 3 or 4 coats laquer using Japlac perhaps?

Sorry for the confusion. I'll try to clarify. He makes positive patterns out of pine and MDF, places them into a wooden (plywood) cavity, and pours polyurethane resin around it to form a negative mould. He learned the hard way that failure to seal the wood pattern causes the resin to stick like glue to the pattern.

[stevesplanes]

One good web site to check out RTM (Vaccuum INFUSION) is my bud Tony at www.corsair82.com Consider your molds should be using some sort of RTM, Tony's methods are the cheapest around.


Some people think that the plug is disposable. After time you will find that isn't true. However if you can pull a part off of your molds ofter every so often, making new plug and thus new molds. However there are ways to make your molds to last langer and make them repairable.

Making your Moldswith stiffners very thick will help prolong the usefull life of the mold. Remembering that epoxie resins can only be mixed in small batches, this will take some time and some help from freinds. The wall thickness can be increased by SORIC.
(See Http://www.fibreglast.com/documents/361.pdf)

The gell coat thickness is a large factor in the longevity on many molds. Mainly cracking and other damage done to the mold when removing the part from the mold. Gell coats can be saned, but its not easy or something you would want to do. In general apply multiple coats of gell coat and more coats where the mold is most likely to bend when the part is being removed.

Well I have to get back to work now. Keep doing research, and do consider Vaccuum infusion if you want to keep some of your brain cells. My brother said the people in the aircraft repair business that spend a year or two in composites, stay in composit repair. Brain cells I guess.

[coonhunter]

Cafemondo:
I think your going your going to need more then a shop vac for your vacuum,you have 8"and a 10" draw ,What size vac. holes did you drill and how far apart? I would coat your mdf mold with lacq.,linseed oil,or poly. resin.Note. mdf will dry out and crack pretty quick,depending on the gage of the material and cycle time.