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Old 11-28-2019, 09:07 AM
John Buchtenkirch John Buchtenkirch is offline
MetalShaper of the Month October 2012
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Default How to reproduce lens.

If you are involved with restorations this is probably worth watching. ~ John Buchtenkirch
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Old 11-28-2019, 10:20 AM
billfunk29 billfunk29 is offline
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Location: Minnesota
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Default Mold

Thanks John, very interesting process.
Bill Funk
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Old 11-28-2019, 06:46 PM
cliffrod cliffrod is offline
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Seems like a very poor technique to produce a silicone or comparable flexible mold with no durable mother or support, no sprue to charge/fill/vent the cavity of a multi part mold and then simply tell the customer to "be careful" while flexing it open to fill it and then clamping it closed in order to get a good result. Might work if you've got some experience, but if you're new to the process ymmv...

At a glance, their product pricing is not cheap but somewhat comparable to similar two part tin cure silicone products I use at around $100-$150 per 10lbs (about 1 gallon). That means the mold they made, approx 2- 2 1/2 gallons, cost at least $300.00 plus positive casting materials. That's a lot of $$ to gamble on a suggestion to simply "be careful". If you're only selling the product, it is a great way to sell more product.

Molding & Casting isn't rocket science. it is reasonably straightforward to produce great results. imho study the process a little more elsewhere before jumping in if you prefer to not waste your time, your money or worse.
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Old 11-29-2019, 08:30 AM
Chris_Hamilton Chris_Hamilton is offline
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Location: Southisde Virginia
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Good post Cliff.
Chris (trying to be the best me I can be)
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Old 11-29-2019, 12:05 PM
crystallographic crystallographic is offline
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Reproducing glass lens parts involves graphite molds and pouring hot glass into them.
Jack's Cadillac build shows him drape molding hot plastic sheet for some of his lenses.
Molding plastic is a process altogether different from each of these, described above.
For nice clean molded lens work I would probably use a process different from the Utoob indicated here.

"All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919.
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Old 11-29-2019, 02:01 PM
norson norson is offline
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Location: Portland, Or.
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I appreciate John's post and learned from it. The other posts add to the conversation, but I would really like to see examples of these other techniques. I was considering purchasing a car a few weeks ago that would need lenses not available. Not metal shaping, but related.
Norm Henderson
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Old 11-30-2019, 06:51 PM
cliffrod cliffrod is offline
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Originally Posted by norson View Post
I appreciate John's post and learned from it. The other posts add to the conversation, but I would really like to see examples of these other techniques. I was considering purchasing a car a few weeks ago that would need lenses not available. Not metal shaping, but related.
That's a tall order. Maybe some keywords and basic info would help you search for information & videos-

Rammed molds use granular media (graphite, French sand, corn starch, etc) that is packed (rammed) with a rammer, whether pneumatic or hand held tool hit with a swung hammer to a desired density against a positive form to be reproduced. Rammed molds are used once. Then the granular media is broken away and either recycled or discarded. The mesh of the mold media in part determines the register of the mold or negative against the positive and the ultimate quality of the reproduced positive. In general, The finer the mesh and the harder the ram, the better the results.

Silicone, urethane, latex or "rubber" molds produce a flexible mold that provides some level of versatility in terms of managing draft (undercut or overhang which can defeat a mold that is not flexible. That very flexibility means the mold can easily deform in use so a inflexible support called a mother is produced around the "rubber" mold before it is removed from the original positive. The mold & mother may be permanently attached to each other or may be easily separated & reassembled. Manufacturers like to sell you gallons of products to pour thick silicone or urethane molds. But they can also be made with less of such products if you're pragmatic about it and then reinforce the $$ product with less costly materials. These molds may be reused, but some fatigue quicker than others.

Some products, like resin, fiberglass, gypsum products, etc can be made to produce an inflexible poured or built-up mold. To manage draft, the mold and any parting or dividing lines need to be carefully planned.

Release agents can vary from proprietary formulas to paste wax to soap/detergent to Vaseline. Some higher quality products like two part silicones may benefit from release products but they are not mandatory. The product I use can be poured directly onto material like bare plywood and it will not adhere.

Tin cured silicone behaves differently than platinum cured silicone. Some products require ambient humidity to cure or they never will. Some silicones and urethanes have strong etching qualities, usually because of a present acid component, and are thus designed to adhere aggressively. Different qualities may help or hinder your work.

Some mold work requires a core (or interior mold) be produced to limit the cross sectional thickness of the cast positive. Core work is synonymous with more elaborate cast iron work. More elaborate molds require the development of sprues (basically plumbing channels to allow fluid material into the mold) which can be very complicated. These may be fabricated from wax and attached to a positive before the mold is produced. After casting, the sprues are removed. With or without sprues, Trapped gases or bubbles must be easily vented. Simple draft can cause this in very basic molds.

The ceramic shell process used for the lost wax process (common in art bronze casting) uses a thin wax positive (reproduction of the original work, often using either a waste mold or silicone mold) that is dipped repeatedly into a ceramic slip to build up a usable mold. For thin work, Both sides are slipped. The wax is then melted out so molten metal can fill the cavity. The ceramic shell is then broken away from the casting, so the mold is destroyed.

Another type of basic waste mold can be made from plaster or similar material. The soft original work (usually clay or wax) is encased in plaster with appropriate parting lines. The first coat of plaster may be tinted in color as an indicator of nearing the cast positive when later wasting the mold away from the positive. Other layers of plain plaster is then added to strengthen the mold. After curing, the mold parts are separated. The soft original is evacuated. The mold interior is released, reassembled and charged with the casting media. When all is set, the mold is carved away from the cast positive. This is a one time process, usually the cheapest method but the least tolerant for any mistake.

Pattern making is really significant, because the reproduction will not be better than the original. You'll also learn things like factors or coefficients of shrinkage, which are necessary because making a pattern that fits perfectly but casting it into a media that shrink 2% may mean a fail... It's also why you cannot always just pull a mold from an original part and then cast it without fitment problems. The mold materials may shrink and the reproduced casting may shrink as it cures or hardens. The possibility of two rounds of shrinkage must be factored into the process. It may not be a big deal to address (or overlook) or it may mean a total failure.

There's any number of hybrid methods now used like styrofoam or foam positives, slumping sheet plastic, kneading cornstarch into tube silicone for simple press molds, exposing certain flexible mold product to specific chemicals to make them grow for casting larger positives, using plain plaster (softest) or hydrocal (harder/stronger) or dental stone (hardest/strongest) depending upon the project, etc., etc...

Trusted suppliers that I would recommend include Smooth On, Freeman Supply, Silicone Industries. Other may be as good or not. For example, A resin product used to easily cast a lens may not be as scratch resistant, uv resistant or heat/cold resistant. For a headlight lens, I would expect most product such products to fatigue quickly because an OEM lens is usually much more than just poured resin.... interior application or maybe a rear-facing taillight lens may be a better use of such materials.

Injection molding and other methods can be addressed by someone else. Not my forte'..

Certainly not a how-to or an inclusive list, but it should help you do some searching.
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Old 11-30-2019, 09:12 PM
Marc Bourget Marc Bourget is offline
Join Date: May 2013
Location: North Ca
Posts: 417

Having worked on a mold to make Stop Light lenses and owned a plastic injection molding business, I am familiar with the process. It's not for one-off parts and the shrinkage, sprue design and venting issues are significant.

Injection molding becomes practical in the 10's and 100s of thousands of parts.

Unless you're a crazy hobbist !

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Old 12-01-2019, 03:03 PM
Mike Motage Mike Motage is offline
MetalShaper of the Month Oct 2016
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I used this company's product and help making the rear lenses for my Jaguar project. They were very nice to deal with. I visited them in Kalamazoo while on vacation. Showed them what I needed to make , they made sure I had all the info and materials to successfully make my lenses. The lenses turned out nice.
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