Wizard61

My Feedback: (7)

I thought I would give this a try. How about this for a choice as my first vacuum forming project attempt. To describe what I've done so far: First I sectioned a 1/5 TF P-51B canopy, removing the center section. I then cast the two ends. My hope is to make the new center section and then re-join the three mounted on an MDF plate.

Here are some photos of the real McCoy as well as my current progress. I would be interested in what you guys think. My kids have a lot of faith in me and have already pronounced it as an Epic Fail. They have historically underestimated me however.

Thanks,

James

Chris Nicastro

My Feedback: (3)

Ok James, not to worry your going to make it work. I used to design vac form car bodies and other tools for HPI Racing and other companies.

First, decide if this is a Master Model or a limited use tool.

As a Master you can put detail into it and I can explain this in detail. There are a couple things to consider but totally doable.
As a limited use tool you have to consider your materials right now for higher temps or the tool will deform or bubble and make defects in your parts.

So choose a path and I can explain how you go about making this work.

Wizard61

My Feedback: (7)

Hello Chris,
Thanks for the reply. My initial thought was for it to be a limited use tool, but I would be curious in what would be involved in making it a master. My plan was to carve, sand , and finish the center section from basically solid balsa. I would then fiberglass it and make it very smooth At that point I was going to lay up some fiberglass over the plug. Once that had cured, I would remove it and seal the ends in similar fashion that I did for the Top Flite B Canopy ends. Then I would cast the new center section using the same material I used for the first two components. I cast them using Durhams Rock Hard putty. There was virtually no shrinkage, it picked up the surface detail really well, is really smooth, and is very hard. That was basically my thought process on it. I am really interested in hearing from someone like yourself, that has experience with this.

What a great science experiment!

Regards,

James

Chad Veich

My Feedback: (60)

I did something very similar for the Bates 1/5 scale P-51B except mine does not include the glass for the rear windows. This was provided to Vic at www.vicrc.com who used it to make a mold from which a urethane part was cast to make the actual vacuum forming buck. As hinted at by Chris above you might get away with making a few parts from your plug as is but it will probably start to deteriorate pretty quickly from the heat and pressure of multiple pulls. You could save yourself a lot of time and effort and just buy your canopies from Vic but I fully understand the idea of wanting to do it yourself and learn something in the process!

Wizard61

My Feedback: (7)

Thanks Chad,

This is definitely an exercise in learning for me. I am glad to know that I can get a canopy from Vic if it becomes necessary.

James

Chris Nicastro

My Feedback: (3)

Ok sounds like you have a plan but let me explain a couple things.

Master Models have all the detail and geometry designed into them. They are usually built from very stable materials and then a mold is made from them. They are designed undersized from the finished dimensions by the thickness of the sheet to be pulled on them. So every dimension in every direction should be accounted for so the final part appearance is normal on the outside.

The mold can be rigid like fiberglass or solf like silicone. Since your part has an under cut in the middle because of the bubbled glass shape you would have to make the rigid mold in two parts or it will lock. The other way is to make the mold from a castable hard or firm silicone for tools. It's dimensionally stable. This way you could cast your tool from aluminum filled epoxy with a foam block to replace volume and maintain an average wall thickness of an inch.
The tool, however its made, should be hollow and rigid plus capable of handling temperatures up to 300f. The reason its hollow is for cooling, lower cost and to be a plenum chamber to suck the sheet tighly to the mold. You drill very small holes in key detail areas to suck the sheet tighly to the surface and render the detail better.

Contrary to popular belief the tool surface doesn't have to be polished to get a glossy finish. The surface can be slightly scuffed to promote release without the need for release agents or wax.

Since your part has the widest undercut section in the center then when its pulled off the tool the ends will be free as the center is at its widest point. The ends have to be slightly angled in towards the center to help with the timing of the release. The steeper the face the sooner its free and can allow for stretch in other directions.

You will also see whats called webbing from the front and back faces at the corners especially if they are tall with sharp corners. A bigger corner radius helps and adding a blended cone shape at each corner will cure webs.

Materials can be found at BJB Enterprises in CA. They have a lot of good info on the site and I have used their materials since 1994.

Ask anything you like, happy to help.

Chris Nicastro

My Feedback: (3)

By the way, if you made an outside copy of a part then turned that into a tool your finished part would have grown by 2x the sheet thickness or so. So, keep in mind which method your using and the steps needed to make a final part to the size required.

Wizard61

My Feedback: (7)

Hello Chris,

Thanks for your information and your quick reply. I have been on the road for work the last several days and have not had time to do much. I made my pattern from the inside of the existing canopy parts to allow for the thickness of the plastic. It looks, based on your reply, that the best direction for me to go would be to continue on with my pattern and finalize it. At that time I should make a fence around it and use a silicone product like the BJB Enterprises TC-5050 A/B room temperature curing silicone. From this mold, I can then cast the actual tool out of the TC-1650 A/B High Temp Aluminum Filled Casting Resin System with a disposable insert to keep the wall thickness of the tool around 1". This should provide the durable high temp tooling for the desired part.

My next question is in reference to the holes drilled in the tool to aid in the vacuum forming process. I imagine those should be in the corners and recesses? What diameter hole would you consider for this application?

Thanks again for all the guidance. I will post some progress on the further work on my plug hopefully this weekend.

Regards,

James

Chris Nicastro

My Feedback: (3)

No problem James,
Yes, thats the right track. The silicone should be very firm so the weight of the epoxy will not distort the shape.
As for the holes the largest diameter I use is 1/16 inch. Place them in corners and also engrave lines to tighten up details. For example the frame of the canopy verses the canopy glass. You want the frame to have a clean definition so you can engrave lines into the canopy to create an air channel next to the frame. Take away a little material from the frame and remove the radius you molded from the old canopy. You want the canopy to appear to continue under the frame. Using a nice sharp chisel and sanding block you can achieve a tight clean detail in the pulled part.
Always remember the sheet thickness, the master and the exterior pulled part are always related. Anticipate the outter finished surface of the part and whatever final measurement you want is always smaller on the master.
If you want a finished width of .25in on something raised then the master will be 2x the sheet thickness in the vertical narrower than the finished .25in dimension. So consider the height of the detail and the width then estimate the compensation or the sheet stretch.
Since you making a relatively simple part with low details this is more info than you need but important to know.

Post pics of your master before you cast it in silicon.

Ram-bro

My Feedback: (101)

Just in time training....I want to do a plug for a bird cage corsair......