Post by bruceg on Nov 10, 2018 15:19:17 GMT -5
Greetings
I would like to ask some basic questions about an object that I would like to cast. This is in the area of industrial molding/casting, not life casting.
Imagine a washer-shaped object, a disk with a hole in the middle. In this case, the diameter of the disk is 7 feet, it is 1/2" thick and its edge to edge width is 4" (the hole in the middle is very large). All of these dimensions are approximate and small changes in them don't change my basic questions. Also, small additions will be added to the washer shape making it thicker in some areas and, in general, more complex. But let's not worry about those details just yet.
I need to make a piece that represents a 45 degree sector of the washer, i.e., 1/8 of the circle. Therefore, the piece that I am talking about is approximately 2 feet long, 1/2" thick and is a 4" wide arc on a 3 1/2 foot radius (I realize that 1/8 of a 7 foot diameter circle is longer than 2 feet, but since it's an arc, the 2' 9" arc length fits inside a 2 foot long box).
The final cast is to be 80D urethane, unless someone can recommend a better material. Four of the pieces will be assembled to form a half of a 7 foot diameter ring supported by ball transfer units (BTUs), those things you see workers using to push cargo into airplane holds. The ring rotates on the BTUs. The load being carried by the ring is light, however, and is insufficient to produce any deformation of the urethane pieces. If they are flat to start with, they will stay flat.
I want the flatness of the cast form to be within about 1/16" across the 2 feet of length with no bending forces applied. Flatness along the length of the arc is not terribly critical, since the BTUs are guaranteed to make contact at multiple places, but twisting across the 4" dimension is a bigger concern.
I intend to make the mold using HDPE as a form, so I have something of suitable smoothness and flatness. I have done some mold making and casting, but by no means would be considered to be experienced. I do have vacuum equipment that will allow me to hit 29" of vacuum.
Now the questions. I suppose I could simply stop and ask "How would you people who have experience approach this?". If someone wishes to answer from that perspective, that's fine. What I am asking below is based on my expectations from past work.
Choice of molding compound. I was planning to use a simple poured tin silicone molding compound, though mold flexibility is not much of an issue, I don't expect any difficult shapes and I expect minimal flexing required for release of the cast. I planned to build a rigid box for support to maintain flatness.
1) What would you choose as a molding compound, if not a pourable silicone?
2) What aspects would you consider essential to obtaining a smooth, flat cast from the mold?
3) What durometer would you aim for in the molding material for an object like mine?
4) How would you support it to maintain flatness?
Casting.
1) Would you recommend anything other than an 80D urethane for the part?
2) I expect to vacuum the urethane. I would like to evacuate the mold as well, but it seems that I might start sucking in air along the mold separation line. How do I make sure I don't get bubbles in the corners of my casting?
3) Casting material will have to be injected into the mold at one end or the other of the arc. Both the upper and lower surfaces of the casting are intended as rolling surfaces and need to be kept clean of mold marks (though I suppose they could be sanded off if necessary). Do you see any problems with filling the mold that way?
4) Would you tip the mold slightly for filling or fill it level?
5) Anything I should know about the mixing, vacuum, and mold filling in general that goes beyond normal casting?
6) Anything I should know about mold release (or moisture or other things) to avoid pock marks or other surface defects on the finished part?
Misc.
1) Aside from following the manufacturer's recommendations, is there anything special that you would do?
2) The length of the piece along its inner edge is shorter than along its outer edge. Will this induce any sort of stresses in the part that I need to think about?
3) Are there any post casting treatments that you might do?
I would also appreciate comments on the overall project
Thanks in advance
Bruce G
I would like to ask some basic questions about an object that I would like to cast. This is in the area of industrial molding/casting, not life casting.
Imagine a washer-shaped object, a disk with a hole in the middle. In this case, the diameter of the disk is 7 feet, it is 1/2" thick and its edge to edge width is 4" (the hole in the middle is very large). All of these dimensions are approximate and small changes in them don't change my basic questions. Also, small additions will be added to the washer shape making it thicker in some areas and, in general, more complex. But let's not worry about those details just yet.
I need to make a piece that represents a 45 degree sector of the washer, i.e., 1/8 of the circle. Therefore, the piece that I am talking about is approximately 2 feet long, 1/2" thick and is a 4" wide arc on a 3 1/2 foot radius (I realize that 1/8 of a 7 foot diameter circle is longer than 2 feet, but since it's an arc, the 2' 9" arc length fits inside a 2 foot long box).
The final cast is to be 80D urethane, unless someone can recommend a better material. Four of the pieces will be assembled to form a half of a 7 foot diameter ring supported by ball transfer units (BTUs), those things you see workers using to push cargo into airplane holds. The ring rotates on the BTUs. The load being carried by the ring is light, however, and is insufficient to produce any deformation of the urethane pieces. If they are flat to start with, they will stay flat.
I want the flatness of the cast form to be within about 1/16" across the 2 feet of length with no bending forces applied. Flatness along the length of the arc is not terribly critical, since the BTUs are guaranteed to make contact at multiple places, but twisting across the 4" dimension is a bigger concern.
I intend to make the mold using HDPE as a form, so I have something of suitable smoothness and flatness. I have done some mold making and casting, but by no means would be considered to be experienced. I do have vacuum equipment that will allow me to hit 29" of vacuum.
Now the questions. I suppose I could simply stop and ask "How would you people who have experience approach this?". If someone wishes to answer from that perspective, that's fine. What I am asking below is based on my expectations from past work.
Choice of molding compound. I was planning to use a simple poured tin silicone molding compound, though mold flexibility is not much of an issue, I don't expect any difficult shapes and I expect minimal flexing required for release of the cast. I planned to build a rigid box for support to maintain flatness.
1) What would you choose as a molding compound, if not a pourable silicone?
2) What aspects would you consider essential to obtaining a smooth, flat cast from the mold?
3) What durometer would you aim for in the molding material for an object like mine?
4) How would you support it to maintain flatness?
Casting.
1) Would you recommend anything other than an 80D urethane for the part?
2) I expect to vacuum the urethane. I would like to evacuate the mold as well, but it seems that I might start sucking in air along the mold separation line. How do I make sure I don't get bubbles in the corners of my casting?
3) Casting material will have to be injected into the mold at one end or the other of the arc. Both the upper and lower surfaces of the casting are intended as rolling surfaces and need to be kept clean of mold marks (though I suppose they could be sanded off if necessary). Do you see any problems with filling the mold that way?
4) Would you tip the mold slightly for filling or fill it level?
5) Anything I should know about the mixing, vacuum, and mold filling in general that goes beyond normal casting?
6) Anything I should know about mold release (or moisture or other things) to avoid pock marks or other surface defects on the finished part?
Misc.
1) Aside from following the manufacturer's recommendations, is there anything special that you would do?
2) The length of the piece along its inner edge is shorter than along its outer edge. Will this induce any sort of stresses in the part that I need to think about?
3) Are there any post casting treatments that you might do?
I would also appreciate comments on the overall project
Thanks in advance
Bruce G
