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What's new

New Featherweight Prism Design in the Works! 12" Length.

Maybe I missed, but what filament are you using to print patterns like this? I know some plastics don't play well with lacquer paints and fillers.

I appreciate your detailed posts on the process. I have a few projects needing one-off castings reproduced and that expensive pattern shaped gap between a paper print and the foundry gets a lot more manageable with tools like CAD and 3D printing. I have at least one that has so much draft and geometry causing errors in the model, that I think printing what I can and using paint and filler to manually smooth out the rest will make it a lot simpler.

It is amazing how often it is the simple things that can drive you crazy.
I have only experience with PLA and PETG filaments both of which have worked fine for my pattern printing and for related parts.

I use Rustoleum Lacquer, a true lacquer and not an acrylic lacquer, because it goes on nicely, dries quickly and I can build it up by carefully making heavy (ish) coats and letting it flash off and repeating.

And I have found what you are saying about draft and geometry errors being possible if one tries too early in a drawing to apply draft or fillets. I have pretty much limited myself to getting the drawing generally complete except for draft and fillets and then applying them in that sequential order. One of the things I so appreciate about CAD drawing is that draft and geometry accuracy is so much better than even my careful woodworking. Yes, woodworking did work quite well. But I almost always had to fix up the wood pattern with Bondo and and sanding followed by more Bondo when molding revealed yet another subtle draft or geometry error. After making patterns with CAD and printing for almost a year now, my CAD error rate has diminished. Printed patterns typically pull from the sand more easily than "good" wood prints. I think that is because even well-made wood prints have some inherent "wow" or twist in them since the wood is non-homogeneous.

Even with a well-printed filament print I have to do some light sanding for good paint adhesion. I have just recently started using a matte filament, though, and it seems to really like paint even without sanding. And, on certain areas of prints where there are surfaces nearly tangential to the printer head travel there is inherent roughness that needs a few extra lacquer coats and/or judicious Bondo.

The other wonderful thing about CAD is the ability to "nest" parts accurately. I am making a core for this print even though I had hoped to be
to mold it in green sand only. That is out of reach of my abilities. So, the core involves several dimensionally inter-related parts that were much easier to fabricate with cad compared to the "woodshop method."

I may make another post about the core box and core print later. But, I need to get to work on what I hope are final steps.

Well, it has been 8 days since posting on this project. But that does not mean I have been idle for 8 days---hardly! I have been trial building core boxes using various materials and methods in an effort to figure out which will work best, will be durable, and be feasible given the various materials available.

I have decided to print a 3D pattern of the recess in the prism from PETG and then cast a 2-part silicone negative of the printed pattern and then use the silicone negative to cast a working positive of the pattern in Freeman's Repro One.

Link to Repro One

If that seems roundabout, I agree. But it is necessary I believe. That is because, much as I love printed patterns for short runs, this pattern is likely to be used many many times over several years. And if I took perfect care of it, the pattern would likely be fine. But, if just once I leave it in my car in the summer with the sun shining through the window and onto the printed pattern it will almost certainly deform. Same would be true if it were left in the hot sun in the foundry. The likelihood of that happening given my track record is very high. Bummer! So, I will do all this rigamarole to produce a more durable pattern set. An added benefit is that the platinum-cure silicone I use will last decades. So, if a year or two down the line I break or otherwise damage the Repro pattern, I can just cast another one overnight and be back in business. Also Repro fettles more easily than PETG and it is nearly certain that the sand paper and files will come out at some point.

Here are a couple pics of the pattern pieces that I printed in the last couple days and the silicone negative I cast earlier today. (The silicone is amazing in its reproduction of the minutest details in the pattern. And Repro also does a great job. Both are products that have been around for a while and have been tweaked to meet industrial tooling demands. In short, they are a pleasure to work with.)

The PETG pattern was printed in two nesting parts to shorten print time. The little "mouse ears" at the corners are just there to help hold the print on the build plate as it prints.:
Assembled and held together with hot glue.

A close look so you can judge the print quality---love my Prusa XL.

Here is the silicone negative pattern and the mold. Trust me that every detail that can be seen in the PETG pattern are reproduced faithfully in the silicone. Being translucent it just does not photograph easily.

Tonight I will allow a couple teaspoons of Repro One to setup on the silicone and on the box I built around the silicone just to be certain all the components are chemically compatible. I am nearly certain they are. But mistakes at this stage cost time.

And now the hard plastic core box is nearly ready. I cast one yesterday and one today. They are mirror images of each other as the way they will be set up in the flask requires that arrangement.

Here is a pic

I previously mentioned that the silicone-to-Repro system faithfully reproduces the details of the 3D print to a degree that always amazes me.


And here is the back of the plywood frame. I poured the Repro in from this side and remembered to pre-wet the letters and experienced practically no bubbles. I think there are two pin-point bubbles in all the lettering together. The repro is pretty thick stuff and I poured the frame just a little more than full. So the light grey areas are the result of sanding down the high spots.


I do want to apply some paint in such a way that the letters will be filleted and therefore release more easily from the core when I do make my cores.

Thanks for bearing with me as I work my way through this process.