Resinating a WIP

I have regretfully concluded that, after several attempts, turning my apartment kitchen into a woodshop was not going to be practical. Routers and power sanders don't seem to be a good fit with pots and pans. And I live alone, so my mess is my own.

Following earlier threads, I have gotten into doing some urethane resin casting of celluliod scales with fancy designs. I like the old designs and would like to keep the patterns after the celluloid is long gone. With the modern resins, we should be able to create scales for your blade's next 100 years.

I need some advice from those who have done it before. It's as much about finishing as quality control.

I’ve been making molds using RTV platinum silicone and have had middling luck. There are two primary issues, one is crowning above the part line with the urethane and the other is degassing. I can get the mold flat and level but after the pour there often seems to be a crown in the finished product. In order to get an even line on the edges of the scale, the whole back would have to be sanded. Which would mean a belt sander, something I would hope to avoid.

Here are some WB replica's in chocolate brown that have that problem.
Attachment 127420Attachment 127421Attachment 127422Attachment 127423

The other issue is bubbles. In opaque casts, interior bubbles aren’t a problem until they form along the part line. This is today’s work. It has a green translucent base and brown iridescence.

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It has minor bubbles that can be seen if held to the light.

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The issue here is that, like pitting along the bevel when you're honing, they tend to form at the part line and if I sand them completely out, I’ll destroy the pattern, or at least the two sides probably won’t be symmetrical.

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Vacuum degassing will probably solve bubble issue (do you pressure cure?) but the crowning has me scratching my head. A small pump and chamber set up is easier to live with that the dust. Any help?

Spent most of the last thirty years of my work life often pouring resin prototypes and making silicone molds. Vacuum is the only real answer! And 28"Hg or higher. The stuff will foam over so drop the vac to ambient pressure and pull it up again a few times to shake out all the bubbles without having excess resin all over the place. Many resin's hate water and will foam and spew readily with any moisture at all in the mold. It pays to keep molds in a dehydrator before use.
Have fun and use a vent fan! Most of the heavy metals have been taken out of the formulas by now but, I hate going to the MD and getting a MRI and CT scan every year for "shadows." And it is horribly expensive!
~Richard

Thanks Richard. Iowa boy myself, Dubuque County.

Just read the msds. Think I'll move the operation out of the kitchen.
http://www.smooth-on.com/msds/files/...,_326,_327.pdf

Ordered a pump today in fact. I'll build a simple chamber and start from there.

Two questions. In your experience, would you cast scales under pressure or save that for clear resin pieces?

The other question involves the doming effect of overfilling the molds. Any way to avoid that other than sanding? Seems like you would need to slightly overfill rather than underfill in order to get a nice, smooth side to the inside of the scale.

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Originally Posted by Geezer

Spent most of the last thirty years of my work life often pouring resin prototypes and making silicone molds. Vacuum is the only real answer!

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This is what Gary did / does as well.
If you pour resin, a vacuum chamber is a must.

Quote:

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Originally Posted by DGilloon

Thanks Richard....snip...
The other question involves the doming effect of overfilling the molds. Any way to avoid that other than sanding? Seems like you would need to slightly overfill rather than underfill in order to get a nice, smooth side to the inside of the scale.

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In the past, I have done some really weird mold work to get around the problem, however, The stuff sands quickly with wet / dry paper on a flat plate like lapping a stone with running water.

If you do not remember that each and every square inch of surface of your mold box will have about 15lbs of weight upon it, the noise and flying material after the implosion will remind you!. For small items we used 3/8ths" thick plastic that we glued and screwed together with machine screws. A thin layer of RTV will serve as a seal for the opening. There are other methods but that was our safety molding flasks the we used for many years..
Have fun but be safe!
~Richard

Thanks for the advice. One of the things that I have learned (and relearned, and relearned) is about the time value of money. I could spend a get deal of time looking for scrap and second hand shops to cobble together something that might or might not work based on some half assed idea I had in order to save a little money. Or I could spend a littler greater amount of money on something that was designed for the job and will work correctly the first time, saving me a greater amount of time. Time is the one asset that you can never recover. Thus I ordered this today.

1 5 Gallon Vacuum Chamber for Degassing Urethane Silicone Epoxies | eBay

If I may pick your brain a bit more Richard, on one of the forums devoted to making props, a poster suggested that in casting small amount as I would be doing with scales, warming your resins and molds would aid in finishing. Was this your practice?

Another poster contented that you can avoid running into pot time issues by degassing your Part A and Part B separately and then mixing. His view was that this achieved the bulk of that you were trying to accomplish by degassing and you did not start the chemical reaction any earlier that you had to. I would think that mixing would undo most of the degassing and you would have to do it over again anyway, perhaps for a shorter period of time tho. Do you think that this is valid?

Please be sure to let us know how the tank works. It sounds really good from the description.

Quote:

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Originally Posted by DGilloon

Thanks for the advice. One of the things that I have learned (and relearned, and relearned) is about the time value of money. I could spend a get deal of time looking for scrap and second hand shops to cobble together something that might or might not work based on some half assed idea I had in order to save a little money. Or I could spend a littler greater amount of money on something that was designed for the job and will work correctly the first time, saving me a greater amount of time. Time is the one asset that you can never recover. Thus I ordered this today.
If you can make it work well for you it is a good thing. We made our own some places I worked so we could look onto the top of a larger part that was lying down. We had HF pots at one place that worked well for end filling molds. If you are just filling an open top mold, then you may wish to place the pot on its side and secure it well. It would be nice to make a hinging system to get the top into place more easily.
We finally bought a bit bigger tanks! 4FT DIAMETER! And a 25 HP vac pump!
http://i756.photobucket.com/albums/x...VACUUMTANK.jpg

If I may pick your brain a bit more Richard, on one of the forums devoted to making props, a poster suggested that in casting small amount as I would be doing with scales, warming your resins and molds would aid in finishing. Was this your practice?
I totally agree with them! it aids the mixing and pour-ability of the resin. And heating the molds in adehydrator will remove any trace of moisture which is often the main cause of bubbles.
Do Not take the resin or molds over 100ºF / 38ºC. That may over cook the reaction of the material.

Another poster contented that you can avoid running into pot time issues by degassing your Part A and Part B separately and then mixing. His view was that this achieved the bulk of that you were trying to accomplish by degassing and you did not start the chemical reaction any earlier that you had to. I would think that mixing would undo most of the degassing and you would have to do it over again anyway, perhaps for a shorter period of time tho. Do you think that this is valid?
In some cases, yes it is valid. If the resin in a container is separately vac'd only one component at a time it may help. If you are in a high humidity environment, it could add moisture you do not want. We had a problem with some resins in Houston!!!!

I would recommend thinking of a vertical easily filled sandwich mold. You could cast many scales at once..however, only one color at a time. A reservoir is a necessity then and filling from bottom to top is required! Yup, there are ways! You will have to go from here. I cannot give all possibilities of thirty years experience to answer for each change in process.
~Richard

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HAVE FUN! be safe!

Thank you very much for the information and the encouragement. I'll need to get a little more comfortable with the equipment and the process before I start to pepper you with questions again. I'll need to get at least a couple done right before I can start to think about mass production. Might be a good retirement job.

I'm thinking of a non-functioning wine refrigerator with a low wattage lamp inside for a dehydrator. I make my yogurt in the oven the same way but the temp is well over 100 degrees.

There is also a need for a proper work bench now. I can make room in the extra bedroom but in order to create the square footage, somethings got to go.

Anybody interested in a pair of seats from old Yankee Staduim? Right price but local pick up only.

Quote:

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Originally Posted by DGilloon

One of the things that I have learned (and relearned, and relearned) is about the time value of money. I could spend a get deal of time looking for scrap and second hand shops to cobble together something that might or might not work based on some half assed idea I had in order to save a little money. Or I could spend a littler greater amount of money on something that was designed for the job and will work correctly the first time, saving me a greater amount of time. Time is the one asset that you can never recover.

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Exactly!

That is why in the end, I bought a professional belt grinder, rather than building one myself. That way I can use the time I would have spent building it, into using it and then recovering the money by selling some of the things I've made.

Bye the way a pressure cooker with a good seal is a good vac tank. Fergot about making one like that. Cheap air tool parts work for valves etc. Boston reminded me.
~Richard

Still at it. Vacuum chamber, check.

In my checkered past, I worked in a foundry that did investment (lost wax) casting so when Richard suggested a sandwich mold that filled from the top, I could see what the next step ought to be. Research on various forums led me to this set up using Legos. Sounds stupid but seems to work well.

I make my mold box with Legos and let the scale masters stay about a half an inch from the edge of the box. Using the Lego base has the advantage of the surface having keys built in so the mold will always lock securely together.Because the base is actually lower than the scales, the the finished bottom mold has the scales encased in silicone.
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I then trim away the silicone from the back of the scale making sure not to disturb it. This should allow a good part line because when the reverse is poured, the silicone will fill in the edges cleanly.
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In investment casing, the gates and sprues are made of wax and when heated in the oven they burn off. Here I used clay to create the gates and sprues. I wanted them to be located where the scales would be drilled out for the pins.
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The larger of the two pieces of clay is the gate and the smaller is the vent. In theory.

So far, so good. Poured the top half of the mold and cut some plexi to act as plates to clamp the mold together when curing. This helps to prevent blow outs on the seams.

Vacuumed the resin separately, Smooth On 326 with the longer pot life, and mixed. Attempted to pour but the viscosity of the the resin seemed to shut off the pour and there wasn't enough venting. Added vents thru the back of the master but there still seems to be points in the mold where air is getting trapped and creating voids.

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What you are seeing is not the first iteration of the molds. Assorted rods and channels have been cut into other pours and have still run into these problems of voids. There don't seem to be bubbles on the face of the scale so that part is doing alright. Small bubbles on the inside of the scales is something I can live with for the moment. Voids, however, are another issue.

The lego molds do create a nice part line and trimming them is easy while still soft.

Going thru a lot of expensive silicone to get this right.

I'd suggest you PM Geezer - He did a lot of plastic moulding during his career, and may have some suggestions about vacuuming you mould process.