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SammymatiK (Automotive)
20 Apr 03 11:09
I would like to mold some type of clear plastic sheeting to fabricate a headlight enclosure for a car.  

 What material would be easiest to mold and maintain a clean and clear appearance when exposed to sunlight and the elements for long periods of time?  

 And as far as molding, the piece would be relatively flat with only minor curves, cold i heat the plastic in an oven and set place in the mold to form it?

thanks
chaving (Industrial)
21 Apr 03 6:38
The most suitable material aould be Polycarbonate, however forming it could be quite hard as it must be heated to about 250 to soften it.

Depending on the thickness it may be possible to vacuum form your shape, the best company to perform this would be a prototype house or maybee a local University with a lab unit.

If you are only looking for one off you may be best machining and polishing.
patprimmer (Publican)
22 Apr 03 7:43
By far the best material for clearity, UV light stability and scratch resistance is acrylic, sometimes better known by the trade names of Plexiglass, Oroglass or Perspex.

It can be heated in an oven until it becomes rubbery (about 180 deg C if I remember cporrectly), then it can be laid onto a simple mould (a pice of curved wood for example) and formed over it.

It has 2 potential problems, one it is reasonably brittle, and two, it is quite flamable.

It was developed during WW11, where it was used to replace glass in aircraft canopies and windows. It is still used for this today.

It is also very popular even today as automotive wethershielsd, headlight and bonnet guards, badges and tail light lenses, and windows in race cars.

It is also used extensivly in the sign industry.

Polycarbonate, while much tougher, is poor to scratch resistance, and moderate to UV, although it can be modified to improve both. It is also quite difficult to mould, as it absorbs moisture from the air, and if heated to moulding temperatures, before drying, it generally blisters or bubbles, and loses most of it's impact strength.

Unless you specifically need the extra impact strength, or higher temperature resistance of polycarbonate, acrylic is by far the easiest way to go, and generally has by far the best ballance of properties, unless you use very specialised, expensive, modified grades of polycarbonate.

Standard polycarbonate, also has very poor resistance to some very common automotive fluids, ie the aromatic hydrocarbons found in virtually all modern fuels.

Regards
pat

SammymatiK (Automotive)
22 Apr 03 11:35
Thanks for the great advice.

 Sounds like acrylic is the best choice for this project.  I'll pick some up to do some experimentation.  

  Thanks again for advice :D
TomH (Automotive)
22 Apr 03 21:38
Acrylic IS best for light covers, but it absorbs a tiny amount of moisture and must be dried in an oven at 215 degrees for a while before raising the temp or you will get steam bubbles.  Drape forming is the process of heating a sheet of plastic until it sags and then draping it over a form shaped like the finished article. First you build the form from a heat resistant material (like Ultracal 30 plaster) and also make 2 flat metal frames that will clamp the edges of the acrylic sheet in a sandwich while it is being heated.  The oven must be set up so the plastic fumes don't ignite (!) and as soon as the acrylic sags in the frame you pull it out of the oven and press it down over the form which was covered with white cotton flannel cloth.  Hold it in place until it cools as it will try to straighten out.  The best book I have found is The Prop Builder's Molding & Casting Handbook, which outlines this process in detail.  Get it used at ABE books.

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