How to: Plastic lens mold

[level1]

I wanted to know how, and what I would need to make a plastic lens mold for automotive purposes.For example. A mold to make a taillight pastic lens.Does anyone have a link to where I get buy all the stuff? .

 

[insideman]

A typical taillight mold is made by assembling "pins", which are steel pegs with the ends cut at appropriate angles. Nickel is then electrodeposited on the pin assembly to a thickness of about 1/4 inch. This is then backed up with poured metal. Zinc, I think. Then any machining is done. Then the whole thing is fitted into a mold base for injection molding. This is not a process for the casual home craftsman. Lotsa bucks!

 

[patprimmer]

Are you talking about an injection mould for serious production, or a mould for casting resins for one or a few off.

Regards
pat pprimmer@acay.com.au
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[level1]

I think its called casting, I wanna make lens like this guy, if you can check the website, he makes low production lens...

http://www.cardomain.com/memberpage/241461...his

makes mold out of the lens only not the housing. I would guess I would need to take a donor lens out of its housing, make a mold of it, I just dont know what I need to make plastic lens, from what I see, he uses clear and just adds tint to it.

 

[patprimmer]

You can make a mould from an original part by casting with something like Devcon or similar filled polyester, polyurethane or epoxy.

The resin needs to have minimal shrinkage and a good surface finish.

You will need to make the mould in at least 2 pieces that fit together to form the inside and outside surfaces. The mould will need to be accurate so the section thickness is correct.

Once you have a mould, you can cast the parts from acrylic or Poly Methyl MethAcrylate or PMMA.

It should be partially polymerised before you start so you can control the exotherm and avoid forming bubbles due to the monomer boiling.

PMMA is the resin that is used for most original equipment taillight lenses and in sheet form is often known by the trade names of "Perspex" or "Plexiglass".

PMMA is very clear and transparent. You could add colours to the resin, but for multi colour lenses, you would need to cast them in individual colour pieces. If you want to do them in one piece, do them in clear then paint the inside surface with a transparent tint.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[doogie48084]

I think you are looking to make small quantities. if so, you can make a silicon rubber mold and then cast polyurethane into it. clear polyurethane is avaiable from a large number of companies. You can get it for about 8-10 dollars per pound. and since a lense doesnt weigh a lot it makes it releatively inexpensive except for the silicon rubber mold. That runs around 10-15 dollars per pound depending on what volume you buy.
You can get tints and add them for differsnt colors also.

 

[patprimmer]

How would you suggest sufficient precision is maintained in a silicon rubber mould so that the part fits and seals.

what advantage does PU have over acrylic.

How will you control yellowing after prolonged exposure to UV light.

Why are most OEM lights acrylic.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[toolman801]

As a tooling engineer for a major automotive supplier in headlamps, I have order moulds for this use that costs "Prototypes" = $85,000.00 and production tooling costing over $525,000.00 for a 2 cavity mould.
All headlamps and taillamps have UV protection additive put into the material before molding.
Even the prototype tooling for the lense moulds are made via the "Growing of the Pins" type tooling.
Now if you are looking at a lense to go on a garden tractor or such, then a "CUT" tool either alum. or steel can be used. You will still need to put the UV additive into the molding process or else the lense will turn yellow with time.

Regards

 

[jevverson]

for amateur/hobby purposes (like mine and yours) i would recommend alumilite. alumilite is a plastic casting company with a variety of casting resins. for an introduction to plastic casting, they have a "super casting kit" available with everything you need to learn, for about $70.

i'm not sure the difference between alumilite and other plastics, but alumilite is most available to consumers. their website (alumilite.com) has lots of support material and step by step instructions for various casting problems. also, they answer most email inquiries (usually referring you to a FAQ on the website).

for your project you will need a CLEAR plastic resin, and some (orange) dye. alumilite offers ALUMILITE CLEAR, which you would have to purchase seperately.

here is your bare minimum shopping list:
$5 for orange dye. $25 for 32 oz. alumilite clear. 2 lbs of rtv silicon rubber, to make the mold, might be enouph (2 X $28).

additionally you will need some superglue, some material for housing the mold while the silicone cures (cardboard base and corrugated plastic walls is good), and you probably want some modeling clay, at least for sealing the mold box during cure, if not for helping to make a 2 PART mold.

so that is a minimum of probably about $100. however, i would recommend getting the kit too, to start with and learn ($70). also, if you want your lens to be really clear and free of obscuring air bubbles, you will have to PRESSURE CAST the piece during cure. i have never done this myself, but have looked into it. alumilite sells a "pressure pot" for this process, for $185. (i am not sure if this pot is large enouph for your purposes though). i'm sure there are other alternatives for a pressure pot, but homemade ones are not safe, as pressure is dangerous. to use the pressure pot you will need (according to alumilite): "an air compressor of at least 3.5 H.P. and a 1/4"coupling/adapter to attach to your air compressor". i will estimate that to be about another $100.

so pressure casting will probably add about $300 to you project cost, unless you already have those tools.

here's the diff between pressure cast and regular cast:

http://www.alumilite.com/howto.cfm?ID=18

 

[MackTheHammer]

what about reproducing a lot of copies of auto lens no longer avaiable - this I assume would be a CNC mold and plastic injection, but with some of the mold prices tosed around it would be impossible to recover the orginal investment - there must be some other way ?
Cheers

 

[patprimmer]

There is

They are described above, along with some one eyed comercial promotion and absolute garbage.

Casting PMMA in a filled (low shrinkage) epoxy mould is the way to do this.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gallen29]

Ive been making models and prototypes for more than 20yrs.
Ive probably done close to 200 different len's both for customers(in Lighting and Automotive) and with friends in custom fab shops.
If you already have a lens cover you want to modify, take it and modify it. When its ready, you can make a RTV Mold (silicone), and cast parts to your hearts content. One of the len's sets I did is still on a car after 4 yrs (still looks good). The tooling is still good (we even sold a few sets to recoup most of the cost).
There are many ways to get there from here. Dont let anyone scare you off.