Rear quarter windows and Lexan in general

[LateApex]

Of course, of all the silly things I cracked my DS rear quarter window at 2 screw locations. It is very easy to over stress the plastic windows with any irregularity in mating surfaces. You are being gentle tightening down a screw and have an "Aw S#!t" moment.

Has anybody on this forum built a mold form for "forming" Lexan. I had already been considering molding a Lexan rear hatch. It is reasonably straight-forward if you already have a mold, such as an existing glass rear hatch, to heat and form Lexan (or non-trademarked variants). Of course, one cannot form Lexan over a Lexan or plexiglass mold. Just wondering if anyone has made a form from a plexiglass window (like a side-scoop) that has worked for molding plexiglass sheet, and if so, what material you used for the mold.

Thx!

[burchfieldb]

Of course, of all the silly things I cracked my DS rear quarter window at 2 screw locations. It is very easy to over stress the plastic windows with any irregularity in mating surfaces. You are being gentle tightening down a screw and have an "Aw S#!t" moment.

Has anybody on this forum built a mold form for "forming" Lexan. I had already been considering molding a Lexan rear hatch. It is reasonably straight-forward if you already have a mold, such as an existing glass rear hatch, to heat and form Lexan (or non-trademarked variants). Of course, one cannot form Lexan over a Lexan or plexiglass mold. Just wondering if anyone has made a form from a plexiglass window (like a side-scoop) that has worked for molding plexiglass sheet, and if so, what material you used for the mold.

Thx!

We use Renshape at our office, but you would have to machine it. This site has some good suggestions.
https://www.empirewest.com/thermoforming-mold-design-and-materials.html

[John Thuman]

173813 It does not conform to the body shape but is sealed fairly well and removable.

[lance corsi]

I made an epoxy mold for my rear window scoops from blocks large enough to accommodate the window opening contours. I have a friend in the vacuum forming business so I took it to him. I asked about lexan and he said it was possible but more extensive process. Lexan absorbs moisture and must be dried for 24 hours prior to forming. Acrylic is cheaper and the process easier. I had several sets made to account for attrition. If they don’t last, I may resort to carbon fiber. My rear quarter windows were useless for viewing thru anyway. Since the windows on my 818 are deeply set, I configured my scoops to mount flush with the surrounding roof material.

[LateApex]

We use Renshape at our office, but you would have to machine it. This site has some good suggestions.
https://www.empirewest.com/thermoforming-mold-design-and-materials.html

This is interesting. Given the curved nature of the side scoops, I am wondering how readily blocks like Renshape 5169 can be shaped by hand. Going down the CNC route involves a) capturing the shape of a scoop, b) machining and c) finishing the mold. I am guessing that slight imperfections in the surface finish of the mold would not transfer to a sheet of acrylic being thermoformed. Am I close?

Thx!

[LateApex]

I made an epoxy mold for my rear window scoops from blocks large enough to accommodate the window opening contours. I have a friend in the vacuum forming business so I took it to him. I asked about lexan and he said it was possible but more extensive process. Lexan absorbs moisture and must be dried for 24 hours prior to forming. Acrylic is cheaper and the process easier. I had several sets made to account for attrition. If they don’t last, I may resort to carbon fiber. My rear quarter windows were useless for viewing thru anyway. Since the windows on my 818 are deeply set, I configured my scoops to mount flush with the surrounding roof material.

Do you have any photos of that work in progress Lance? Did you make your mold on the male or female side of the scoops? I assume the female side?

Time to dig up my vacuum bagging rig :-)

Thx!

[Blueblood]

Maybe 3D printing would yield good results with the correct material.

[LateApex]

Maybe 3D printing would yield good results with the correct material.

3D printing with correct material (PLA has too low a melting point for example) may produce a rough mold. Due to the XYZ of such printers, curved surfaces are only smooth to the precision of the extruded filament. Maybe an overcoat at that point that can be sanded to a finer finish would work. The first problem though is generating the G-code for the printer, which assumes one has a reasonable design file for the mold. However I have no experience with 3D scanning of an object (for example an existing window scoop) and inputting that into a CAD file in some format.

The brute force approach seems to be a) either cast an epoxy mold of the window scoop (in this case), using vacuum bagging as mentioned above and methods used for boat building and repair for years, or b) start with a block of suitable material (perhaps the Renshape 5169) and manually shape it using a scoop as a guide.

This thread has some legs IMO. 3D printing is creating some very interesting things these days, from apartment buildings to high power rocket fin cans (Iconel is 3D printed metal for example).

In the category of "thought experiments", imagine being able to scan an interesting object like an aluminum extrusion of some interest to a car builder, and generating a mold for that object with a few keystrokes and a 3D printer. Also in the category of good hobby gone bad - Haha

[lance corsi]

The epoxy I used was given to me by my friend doing the forming. In my case, I suppose you’d say that I used the brute force method. Lots of die grinding, sanding, and measuring, however I wasn’t trying to fit to the window mounting surface since my main objective was to create a scoop of sorts. Trimming is quite another story. Then the drilling, countersinking, and brackets. It was a good project.

[J R Jones]

lateapex
I have hand formed numerous plug/models from pine wood, body fill and occasionally epoxy. One time use but still hard to throw away.
To your first comment, there are good and bad ways to fasten plastic windows. Counter sink heads are the worst, they wedge like a log spliter.
Plastic screens for motorcycles have used plastic screws and nuts resulting in light clamp loads.
Pan head screws and nylock nuts work well with moderate clamp load. Loctite is not compatible with poly-carbonate.
A gasket and/or oversized hole under the window can allow the glass to deform and crack under clamp load.
jim

[lance corsi]

JR is correct about securing acrylic with screws. In my case, I wanted a smoother look, so I used countersunk screws, very lightly tightened using nylock nuts on the inside of my L brackets. The brackets were individually fit for each location. No two L brackets were the same due to the curvature of the pieces being joined.
As far as imperfections being transferred from the mold, it is a problem unless your mold is highly finished. I took my molds down to 400 grit wet-or-dry paper. The optical clarity of the resulting part is a function of the degree to which your molds are finished. My first set of molds was made with mdf screwed and glued together, then body worked with filler, etc, but the heat caused the plastic to adhere to my molds and pulled some bondo off with the parts. Also, even though body worked, faint transfer lines showing the mdf layers was present. I ended up making my second set of molds from the epoxy blocks.

[lance corsi]

This project soaked up around 2 months of my time, start to finish. Maybe I work slow. Anyway, it’s just one reason why my car has taken more than 7 years to complete. I’m still working on it.

[LateApex]

I came up with a solution for my Lexan "Big Ears" rear quarter windows. I decided to remove them: Reasoning is

The Rear Quarter Windows are not functional. These were originally designed for rear brake cooling. If this was a track car, I may have used these Scoops / Big Ears with some pipe routing to the rear brake. I have this aversion to non-functional bits, especially if they knit my brow. As another example, I did not attach bonnet scoops (any of the three of them in the kit). If they aren't doing anything useful, they are put on the shelf
The rear quarter windows impact aerodynamics and "scoop" sound. I have plenty of airflow without the scoops in place.
Less is more, and one less problem to solve. Don't get me wrong ... I LIKE problem solving :-)


Happy New Year to you all!