Today I started a new pattern to make a metrology parallel/level

Actually, Freeman recomends demolding when the Repro is slightly flexible. See the 2:44 point in this video: https://freemanvideos.com/Moldmaking-Casting/
I checked the casting this morning and it is as straight as my eye can tell when laid on a flat surface.

I can see that a flexible mold would obviate this demolding concern. I may well make my next mold from silicone to see how I like that. The fact that wax and PVA would not be needed would be a plus. I did apply my PVA with an air brush which pretty much eliminated texturing from the PVA itself and was fast and thorough.

I will definitely try the silicone. I may trim away the wood frame on the urethane mold used here to take advantage of the flexibility afforded by urethane rubber.

I am sure that the Repro could heat up to unacceptable temps if used in thick section. But, compared to common 1:5 epoxies and polyester resins, the Repro is much less prone to heating. But, its reaction is still exothermic. It is much much less prone to shrinkage and distortion than 1:5 epoxy from which I cast patterns for my 8" straight edges. TO make them, I had to cast in 1/4" lifts. It was a pretty chunky casting.

I am not sure which side it is on! I know it should be obvious, but I tried scraping the cast pattern and the mold and am not convinced which side it is on. Maybe that is because the ari-brushed coat is quite thin?

Your thoughts are appreciated, Kelly

Denis

 

Thank you. The reason for posting my steps and missteps is twofold: It allows me to look back and remeber what was done. That has helped me on several occasions when I was unsure. And it is hoped it will help those wanting to try similar techniques. I feel very indebted to those who have posted their work---it is very very helpful to me. So, I guess we should all return the favor.

Denis

 

Thanks for responding.

 

I made my second Repro pattern this morning. I did it basically the same way as before with one small but modestly significant improvement. Instead of placing the drywall extraction screws head-down in the Repro I just screwed them into the cross pieces in the conventional way so that the threaded part of the screw would embed in the Repro. And I placed a cross piece at each end of the mold so that there were a total of 5 cross pieces rather than 3. After just over an hour at ambient temp of about 60 in my shop the Repro would not dent to thumbnail pressure. That is when I pulled the casting and it separated cleanly and with very little effort. (I had applied 3 coats of wax, an air-brushed thin PVBA coat that barely tinted the mold and one more wax application. The air-brushed PVA did not cause any brush or application marks which I am told is common with brushed-on PVA.)

Once the casting of Repro was removed from the urethane rubber mold, the drywall screws were easily backed out of the Repro. That was good since in the prior casting of Repro the heads were embedded in the Repro and I had to use a small abrasive wheel to cut them off flus and then grind them a little below surface level as I mill off the back of th Repro prior to gluing to the match plate. Hard screws in plastic would cause problems during milling.

I am at a standstill in my matchplate production as the local company that was going to make letters for me has not got to it. I am going to see if I can find another local supplier as I want to finish the job.

I bought some 2-part silicone to use for the second mold. Its advantage is that it should require very little release agent prep. Not a big deal, but a small labor saver. And I just want to compare processes.

Denis

 

Yah, well, I don't know which catalyst is used. I bought Magicfly 32 oz silicone rubber making kit on Amazon.

https://www.amazon.com/Magicfly-Silicone-Making-Rubber-Casting/dp/B09G6YZ7G3

It may say somewhere whether it is platinum or tin. I did not see it on looking at the site and I did not find an SDS for that product. Does it matter? Admittedly, most of the products indicate whether they are one or the other.

The Repro 1 is proving its worth.

I did by a 3/4 sheet of MDO kraft-faced ply for match plate making. Expensive!!! But really nice 7-ply and dead flat as compared to the wonky AC or CDX I am ususally working with. That sheet will likely last me several years.

Since my prior post I've made new lettering arrangements. To be finished mid week.

Denis

 

Tests are in progress. From what I’ve read, oil-based release agents work well with silicones.

We’ll see if a PVA barrier is needed.

Denis.

 

Harrumph! I wish you would stop making me learn stuff, Kelly! Condensation cures and all that hokum.

So, once I saw the term in your post above, I looked it up and found a very interesting and reasonably understandable article on one-part (RTV) silicone systems and two-part silicones. Learning a bit about the process was helpful and explained a few things most of us have probably noticed but maybe not really understood.

(One probably-useful point I learned is that atmospheric moisture is essential to cure of single part silicone rubbers materials. So, my conclusion is that if I store my opened tube of RTV in an air-tight container with a dessicant, the tube should not partially cure over time. Some sort of wide mouth jar with a good seal should suffice.)

For anyone interested I would recommend this article as a nice summary of condensation curing as relates to silicone rubber systems.

https://www.sciencedirect.com/topics/engineering/condensation-cure-silicone

Now, from the general to the specific: Magicfly's two-part silicone rubber system kit does work very nicely. I poured it in a cooler-than-recommended environment (50F) and it set up partially in 6 hours or so. But it felt a bit sticky. So, I brought it indoors and let it proceed overnight. The result was a very typically tough and very flexible tack-free cure. The material feels just like the wiggly artificial worms and the like sold as baits used in recreational fishing. (I think this is the material used for production of these items.) I did spray and wipe off the lacquered pattern segments on which I tested the the silicone rubber. Separation was easy with no tendency to adhere noted. The material flowed out and self-leveled very well and had no bubbles included. Detail rendition was extremely fine. So, I am happy as can be.

I will be working on the match plate and flasks while waiting to get the letters from the engraver. I went to an engraver yesterday who seems to grasp well what I am trying to do and will be cutting letters out of vinyl that is a little over 1/16" thick. They will not have draft, but I think I can add that with spray paint or adhesives. I will be playing with a spray plastic primer today to make sure I can get paint to stick to the letters and "climb" the side of the letter in response to surface tension. If anyone has a favorite plastic primer or experience painting vinyl, I would appreciate it if you let me know your preferred methods.

Denis

 

Just a couple things to add on the silicone journey.

I do see a good number of 1:1 two-part silicones on Amazon. I think the manufacturers may have realized that such a mixture ratio is a bit more idiot-proof than a 5:1 or 10:1 ratio. (Note which style I chose!)

I tried adding fill agents to the silicone today to see how it would behave and see if doing so might be useful. I used West systems 407 Low-Density Filler and their 403 Microfiber Filler both of which mixed in easily without forming bubbles and seemed to allow as high a ratio of filler to silicone as you might desire. Both thickened the silicone as you would expect and increased the firmness (Shore Hardness) of the cured silicone. I took the fiber to a thickish peanut-butter consistency. At that thickness you could spread it into a corner and build it up vertically pretty well. Cure did not seem to be affected nor did parting which was easy for both mixes. I did not find entrapped bubbles in the small areas I troweled the mixtures onto.

I am not sure I will actually utilize these options right now. But the silicone seems like it will have uses beyond mold making. I can see situations where using a thickened silicone might be useful.

Denis

 

And today I cast a pair of parallels using my very recently completed matchplate. Here are the pics. (My apologies for double posting, but not everyone looks at all the threads and this project has had a subthread and application in other folks' threads.)



These fins are .050 thick and related to match plate use. I am still learning some of the nuances of match plates.

I will relate a "disaster" I had with my matchplate:
I made new flasks to fit my new matchplate from "new" lumber. When I picked up the lumber at Home Depot, it seems a bit on the heavy (moist) side. But, I needed lumber and my usual stoer was closed on the weekend. I carefully made the flasks and made sure they were flat and without twist. And then a few days later I tried them out. BUT, I did not check to confirm that the cope really closed fully on the drag. Because there were some knots on the edges of the flasks, as the wood dried further in my shop, it left some of the knots a bit proud. I did not notice that. Long story short, the flask did not close fully and I had a runout. Ugggghhhh! Discouraging. But, yesterday I got out my trusty hand plane and used my tablesaw table as a flat reference and made danged sure the flasks were flat and that they closed perfectly on each other. And after moldoing the cope and drag and then closing the mold I used printer paper to probe for any gaps. I could find none. And today, no run out. Though there was a small fin. I'll be re-evaluating the flasks before pouring again. And I think I will make it my habit to place 1x3" strips of paper on the drag prior to mating it with the cope. Then, I will check for resistance to withdrawal. That will satisfy me that closure is good. I wonder if there are aother precautions folks take to check tight closure.

Denis

 

You would be right about the sand standing proud of a non-flat flask if I did not clamp my plate to the flask when ramming. However, to prevent shifting of the plate on the flask and to prevent wedging of sand und between the plate and flask when ramming, I use six #9 GRK R4 general purpose 2.5 inch screws to screw the plate to the flask. They have plenty of strength the bend the board so it confirms to the flask and probably deflect the flask some to conform to the plate.

Sometime in the future, if the casting is popular, I’ll make aluminum or steel welded flasks milled flat. Then, trueness and flatness of the flasks should be certain not to mention elimination of flask wear and charring. But, for now, I’ll keep my plane handy and keep a close eye on flask fitup.

I bought 2x4’s yesterday from my favorite yard (before the long weekend) to make a second flask set. I picked through quite a few boards before I found 2 with close grain, not center cut, no edge knots, nearly straight, not twisted, and fairly dry. And, yes, I neatly restacked the pile.

I will also be letting 1/8” steel plates into the areas around my match plate registration holes as I feel the current wood holes allow too much potential play.

It is interesting working to make theory of match plate use converge with actual practice.

It feels good to be working on tweaking improvements, though, rather than just getting a working system together.

Denis

 

And I did modify the registration system on my match plate. Initially, using my mill to ensure verticality and precise spacing, I simply drilled holes 18 inches apart on one side of the flask pair and plate. (I like to have the pins on one side of the flask as it is easier for me to see the pin and hole as I mate the cope to the drag. Most of my flasks are pretty long and are suspended on trunnions (bolts really) located on each end at a midpoint so the flask can be easily rotated about its long axis and balances on the bolts.) The problem with my original setup was that the holes in the plate were a bit sloppy as I had loosened them up a bit to make drawing the plate from the cope and drag easier. That sloppiness caused more alignment slack than I liked and also made drawing the plate precisely to avoid sand damage tricky. So I decided to let steel 2X2X1/8" plates into the matchplate in the location of the original holes. I drilled and tapped the plates so that they formed a sandwich of plate-match plate-plate. I bedded them in epoxy and used paper shims to get them flush with the match plate surface. Once the epoxy was set, I through-drilled the plates on 18" centers using a "W" size drill and elongated one hole 15 thou each direction along the long axis of the flasks. That allows some slight tilt of the plate without locking up the plate on the pins which are a couple though under .375.

This system works a lot better for me. I draw the plates very carefully using 4 wedges and gradually advancing them under the four corners as I lift first the left and then the right side of the plate, I make sure the pins extend far enough to guide the lift. Most likely I could just lift the plate and be successful most of the time . But after packing up the flask, I am willing to spend a couple extra minutes extracting the plate if doing so will increase success rates.

Here are a few pics of the plate and the inset 2X2 plates.


You can see I stamped the distance between holes on the plates. That makes life easier should I decide months from now to make new flasks. Without the spacing stamp I'd be wondering if the right distance really is 18 even or some slight variation. But then I try to "always" do even numbers.

Here are the plates seen from the side.


This new system is nice and crisp.

Denis

 

Today I rammed up a new flask set with my match plate. I was very careful making the mold to be sure the plate sides of the cope and drag set flat on my tablesaw top and also on each other. To be sure I had a good tight fitup I placed 6 scraps of paper towel on the drag before closing the mold. I was very happy when all six scraps were firmly held by the sand. That should make the risk of significant fins or a runout pretty unlikely. The weather looks good for a two-mold pour this afternoon. I should know more by morning. Hopefully, I'll have four good castings. Time will tell...



The top cover comes off for pouring but eliminates risk of sand dropout when the cope is flipped for drawing the filter/sprue pattern and the blind risers. THsi is a pretty wide by fairly shallow 12X3.5X27" mold. So, there is a very real risk of dropout otherwise.

Denis

 

I just poured and got home a few mins ago. That filled nicely without any leak of any sort. Now, will it have good castings inside? That is the next question.

Denis

 

The suspense! Looking forward to seeing the results.

 

There was a gaggle of reporters and satellite trucks out front this morning jostling to hear how the castings turned out. (That might be a slight exaggeration.)

Anyway, the results were really good. ZERO fins! And no sand inclusions, no shrinks, good finish. Yahoo.

The other set of castings done from the original flask set shows I need to work on them just a bit. The castings were just fine with a fin formation on one side about 2X6" and 50 thou thick. I suspected an imperfect closure on it as I could slip a scrap of paper between the cope and drag on one corner and that was the corner with the fin. So, I brought it home and will go over it meticulously with my hand plane to make sure that the cope and drag are a very close fit.

Here are pics of the castings from the new flask set just brushed off partially.



I am now convinced that careful flattening and fitting of cope and drag are essential for match plate work. Of course, I had previously read that, but seeing side-by-side differences when only slight variations in fit are present makes me a believer. My prior match plate work was "charmed" I guess in that they worked just fine right out of the box. I think someone once said (Maybe a few thousand times) that little is learned when all goes well.

Denis