[bilinghm]

I am making 1/24 scale model car windshields from .030 PETG and I am having problems getting a very clear, distortion free part. The mold is wood covered with thin sheet metal that is highly polished. I preheat the metal mold form before each pull, but the part always has a pattern that looks like the plastic is sticking and spoiling the clarity of the little windshield. Can I use a mold release? Will this help?

Thanks,

Bill
Atlanta

[bilinghm]

I have tried a number of approaches to this problem and I still get a surface that is far from the nearly optically clear finish I require. To answer my own question, Pam cooking spray works pretty well as a relase agent. However, the surface is still poor. I then tried an all wood mold, sealed and highly polished. Still not satisfactory. My next thought is to make a mold where the plastic will not actually touch the surface where clarity is required. Imagine a slightly dished area where the air cannot be evacuated. This seems like it might produce the needed finish. Has anyone experimented with this?

Bill
Atlanta

[redman]

Does you mold have air holes on the winshield surface? If the air can not be evacuated it can cause a pooling effect in the plastic. This is very common on highy polished surfaces.Maybe you can use a very small drill #80 =.0135"
This might help you.

[kayaker43]

You never want a polished mold, the plastic just seals off and traps air pockets. Try wet sanding with maybe 180 grit to put a fine scratch pattern. If the scratches show, go the next finer grit.

You need to be able to pull air out along the surface without the plastic sealing to it. Its a balance between enough scratches to work, but not so course they show. Very fine sandblasting also works. No release agents on clear parts, and blow off any dust.

Having said that, you will never get a distortion free clear part if the plastic touches a mold. Aircraft canopies are free blown and even that's hard to get right. If you play with the surface finish and the surface isn't wavy, you can do pretty good though.

[bilinghm]

I have gone to a matt finish on the tool by molding a piece of white HIPS over the tool, dull side out, and then leaving it in place to mold the PETG. I have found that a very slight coating of pam spray, applied, then wiped off, improves the clarity somewhat over a completely dry tool. However, the comment appears to be correct, there is no such thing a a perfectly clear vac formed part. The parts I am now making do look very clear unless they are held up to the light and examined. There is a slight pattern present on all of the windshilds. But, the way they will be used, mounted in a miniature model car, the flaws will be hard to detect.

Thanks to all for the good advice.

[One of Many]

Bill,

PETG is very soft and marks off easily. You might try a harder material like acrylic(Plexiglas) or polycarbonate. I have had the best luck with a shallow female die which takes much less heat to form the plastic and thereby reduces the chances of the softer hot plastic surface taking an impression from the die. A thin sheet of felt like that used on a pool table as the bottom of the female die can also act as a buffer to prevent marking. For each material choice here, there is an optimum heat in getting the plastic hot enough to form with decent edge detail, but not too hot that it picks up every imperfection on the die in the critical areas. If there is no edge detail and all your part is consists of an arched shape over a male die. Try a felt covered male die, but push the die into the material as a stretching block without vacuum and let it cool.

Secondarily, having an outside surface with minor marking is much easier to polish or buff out, but the material must have decent hardness to take buffing without burning. I have used micro-mesh of 4000-6000grit and a little of their polish with flawless results for custom gage lenses.

While this may produce a reasonably acceptable clarity, these materials are somewhat brittle(do not trim as friendly) and may snap or crack much easier in thin sections with rough cut edges. Not so much with polycarbonate, but cast acrylics can be a real pain.

DC

[bilinghm]

Felt! I would have never considered it but for your suggestion. I will definately give it a try.

[kayaker43]

You can never get optical clarity unless its free blown or otherwise never touches anything, even that's not perfect because of stresses in the sheet. The styrene shell trick produces the best clarity I've seen depending on the eggshell texture, but you can also wet sand the styrene. Somewhere between 400-600 grit works best. Production R/C and slot car bodies use an aluminum mold that's been polished and then blasted with something very fine like glass beads or walnut shells? The right texture is the key.

I doubt if felt will work for PETG, its too impressionable. Acrylic and Polycarb never get that soft.

[bilinghm]

I have experimentally covered a test mold with "craft foam". This is a very smooth, thin, closed cell foam sheet that is sold for craft projects. The results are pretty darn good.

I lightly oiled the surface of the foam with non stick spray and molded some good pieces. These windshields are very clear with no surface defects. There is a slight, but very uniform texture that is probably about as good as it gets. In comparison to other clear vac formed examples (clam shell/ bilster retail packges. etc.) these pieces are as good or better than the best I have on hand.

I am still going to try other experiments with fabrics: felt, micro fiber, etc.

Bill

[cod]

I've been experimenting with felt and PETG also. Definitely helps but not perfect yet.

I was wondering about other fabrics that might be similar, and their resistance to heat.... does the felt have to be real wool? I haven't tried synthetics yet, just real wool felt. maybe a fleece?

[LeeWay]

I bend some tubing using Pet G and the tubing is far from perfectly transparent, but when I heat it with a cheap heat gun, it starts to clear up. This may help the little windshields. Give it a whirl. It won't take very much heat to see results if they are going to happen.

[One of Many]

The problem with marking has to do with the PETG being too soft at forming temp. Flannel, Felt, Fleese of the soft wool/cotton variety had always given us decent results on materials other than PETG. It sure isn't the be all, end all answer, but one option with many ways to alter time, temp and process to get closer to the desired result.

I needed to replace my lens on 12" Mitutoyo dial calipers. I just free form draped some PETG on a short tube in the wifes oven until it had the sag I desired. I made a pair of felt covered mating stubs to mount in my lathe chuch and tailstock that clamped the formed lens so I could turn it for a snap fit into the dial bezel. Perfect results on the cheap!

DC

[spektr]

Use FLANNEL. Tighter weave than felt. I use 3 layers on a piece of plywood to heat sheet stock, transferring it to the drape form molds by hand using fuzzy chore gloces. The yellow ones. No chill marks or sticking. If you get really anal about it, you can grease form the part. Use a high temp grease soaked into the felt or flannel. It flows when the hot plastic hits it, bridging all markoff. Old style aircraft canopies used this process. Scott.