Lessons learned from first attempt at casting

[blacksmithn]

My first attempt at resin casting was unsuccessful, in the sense that I didn't get anything usable as an end product. On the other hand, what I did get were a number of valuable lessons. I pass them on in the hope that others may find them useful:

1) Start with a flawless mold. Or, at least, as flawless as possible. In my initial attempt at a 2 part mold, I wasn't careful about laying down a smooth clay surface. I just pressed the clay out with my hands, making for a very uneven bed for the master to rest in. This caused problems later, as the clay pressed up against the sides of the master at different heights in different places.

2) Secure your mold halves with the proper amount of banding. I sandwiched my mold between two pieces of foam board and on my first try used four evenly spaced rubber bands to secure them. When the first try was not successful, for reasons I'll go into later, I retried, using only three bands applied haphazardly. This resulted in leakage from the bottom of the mold.

3) Speed is not your friend. When you hurry, you make mistakes. So, for me, I need to look for resin with the longest possible working time. In my case, what I had purchased has a 2 minute working time. On my first attempt, the resin actually cured before I could empty half of it into the mold. On my second attempt, I got about 2/3 of the resin into the mold before it cured, but about half of that flowed out the bottom of the mold, per item 2.

What I ended up with for my efforts was half a railroad flat car, from the top of the mold (since the bottom half ran out the end). It's quite a nice half, with cast in detail on the deck and underside that is really amazing to see. There's quite a bit of flash, some of which is easily removable and some which will require a lot of carving-- attributable to my error in item 1. But the possibilities that are demonstrated are remarkable. Important experience gained, I'm looking forward to retrying the whole process, starting with a fresh, much cleaner mold.

[RageofAchilles]

Hi,
Don't be too dismayed, as you say you did learn from your mistakes and you also learned about your approach and materials.

I use Smooth cast 305 which has a 10min pot life and a 30min demould, I started out with fastcast resins from Axson which would often go off half way through pouring in the way you have experienced.

As for the 2 part mould, here is something I do that you may find useful


Obviously the gates in this mould are barely adequate but I wanted to show how the imbedding of beads in the first clay half , produces a mould which can hold together tightly, minimising slippage and flash.


You also want a rubber with a shore A hardness of 25-30 when making objects with long straight edges, it will compress (and warp) less when banded up. You also need something really hard as boards on each side of the mould. If your having leakage out of the bottom strap a board to that too or run tape around the seam of the mould before pouring.

[mangozac]

[blacksmithn]

This is the mold together. You can see at the bottom where the resin leaked. To the right, at the top of the bands,you can see the result of my initial casting attempt.



This is the split mold. The wood dowels are a result of my pouring the first half of the mold before placing any keys. These were the best improvisation I could frantically come up with on the spur of the moment. Truth to tell, they actually work! Once I got over my panic, I figured out that I could make keys in the rubber by inserting the end of a round paint brush through the liquid silicone and into the clay base. So you also see silicone keys. You can again see at the bottom where the leaking occurred. As it didn't leak on my first attempt (with 4 bands) I believe the leakage is just a matter of not enough evenly spaced bands.



This is the 1/2 flat car that turned out. As you can see, the edges are particularly raggedy. For example, note the 3rd stake pocket from the left. I haven't figured out how (or if it's even possible) to get clean open pockets. The detail on the top, particularly the wood grain is pretty remarkable, though. That's what gives me hope for a new, cleaner mold.

[mangozac]

The leaking should definitely be sorted out with some more rigid boards and the right clamping force.

I'm not sure why the sides and stake pockets are so raggedy. Did everything align nicely when you put the two halves together? There's no reason you shouldn't be able to halve them come out perfectly...

You know, to use up the quick setting resin you have you could just pour it a bit at a time? The resin will bond to the previous pour just fine.

[RageofAchilles]

[blacksmithn]

I suppose I could try a multi part pour. It should be noted though that I only made a measured cup of resin at a time, although my workspace is in direct sunlight and it was approx. 86-90 degrees F at the time, which undoubtedly had an additional accelerating effect. The alignment appears perfect. Thanks to the dowels, there seems to be zero slippage. As I say, I believe the issue is that the master was embedded in clay that was insufficiently even and flat (indeed, that's really an understatement. You can see by the surface of the surrounding mold that it resembles rolling hills instead of a flat plain), causing the line where clay and master meet to be an undulating rather than a straight line.

I don't believe that I have sufficient room in this mold to cut any but the tiniest of additional pour channels. That's something else I want to try with a new mold.

[mangozac]

[blacksmithn]

Side one....



Side two...



Mold bottom 1...



Mold bottom 2 (Center)...



Mold bottom 3...



Mold top 1, pour vent on left...



Mold bottom 4, blind end...

Hope this is what you're looking for. And thanks.

[mangozac]

Ohhhh I see now. The problem isn't the "hillyness" but rather how raggedy the silicone is along the edge of the part. As you identified, making a new mould and making sure that you clay it nice and smoothly where the clay meets the part will solve that issue.

Actually, the ragged-ness wasn't caused by tearing when you separated the two halves of the mould after your little initial mishap forgetting about silicone release for the two halves?

[RageofAchilles]

[blacksmithn]

Okay, my next question is, when I place the master into the clay, which side should I embed? I'm thinking deck side down and bring the clay even with the bottom of the deck boards, as they overhang the car body, and that far only. That will give me a straight line guide to follow. I tried it the other way the first time and, as you can see, it was kind of messy.

[mangozac]

Yeah that's probably the best way to go

[Blindhorizon]

Before you start over try this, get a piece of foam core and cut it the same length of your mold but make its width about 75% of the mold. Then cut another piece the same length but it's width to half of the last cut you did, basically it's a pyramid when using your rubberbands it will evenly distribute the pressure. You'll need 1 for each side they should look the same. The leakage you have is from the rubberbands pressure hitting the very edges of the mold so it bows the middle Away from itself causing a gap. I use a lot of 2 part molds and have seen this frequently, I created what I described and call them pinch plates.

When creating your 2 part mold first half you want to seat the part just deep enough so the silicone can't push it around. That means it will mostly be sticking up out of the clay, this will cause it to be deeply embedded in the silicone. Once the first half is done and you clean your mold and master the master should be held pretty firmly into the first half of the mould and allow for easy pouring of the second half.

I can post picks when I get back home middle of next week.

Blind-

[blacksmithn]

"When creating your 2 part mold first half you want to seat the part just deep enough so the silicone can't push it around."

Yeah. I think that was my big mistake. I did exactly the opposite, burying most of the master in clay. That also made it hard to figure out where to place the joint between clay and master. I know I can do better, which is why I'm looking forward to a second try.