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Some of my recent slingshots have used home-made laminated boards made of cloth in resin. I thought I'd show you how to do it for yourself. I haven't done one of these laminates in literally years and never did more than a few slabs, so forgive me if I am a bit rusty.
First, assemble everything you'll need.
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First cut your fabric into equal and appropriately sized panels. If you keep the corners square and the dimensions the same, you'll not have to trim much off the edges later. The best materials are strong (silk and linen), absorbent (silk, linen, cotton), thin and not too tightly woven. The colour or pattern should not be too dark. The slab will come out roughly the colour that the fabric becomes when wet. For this reason, I'm using pre-washed denim. Denim is a classic material for home-made slabs, but most people choose a denim that is too dark. Regular denim will turn dark-blue and dark denim will go almost black. Sometimes, you may not have enough ofone material. It's possible at a pinch to use another similar material at the bottom or centre of a stack as long as you later remember not to sand down into it, or you'll expose the different material. Some people use layers of contrasting colours to make layers like tree rings in wood.
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Measure out the resin. I used 500ml (17 US fluid ounces) for a slab that was only 0.8 sqft by 3/8 thick. I always over estimate the amount of fabric needed and that I can lay-up in the 20 minute pot life and under estimate the amount of resin needed to bind it. I could (should) have used 600ml (20 floz.)
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Measure out the catalysts. The resin I'm using needs 100:1 ratios, so I should really have scales that measure in smaller increments than 1 gram. It'd be better if it was a glass top than plastic and I should really have a way to pour liquid with more control. HereI am using film cannisters which seemed to work OK for the catalyst (didn't melt into a puddle like the styrene cups).
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You have to add polyester catalysts separately (follow the instructions). My school plastics shop class teacher mixed up the catalysts together without stirring in between .... in 1984 and a cupful created so much smoke that I remember it vividly to this day. If that happens you'll want to get it outside in a hurry. If your cup melts in the vigorous exothermic reaction, that'll be difficult.
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The clock is now ticking so the rest happens in a big hurry. Invert your bag and use it as a work surface cover. Pour the mixed resin on the bottom sheet.
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I spread it with a squeegee (the edge of my hand in a glove). Get this layer really thick with resin and flip it over so the resin is in contact with the bag. The bag must be flat and not kinked and don't trap an air bubble under the fabric.
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Lay up layer on layer, with a puddle of resin in on top of the last layer, then fabric then resin on top. Each layer must be 100% soaked into the fabric from both sides. In the picture below, the fabric has soaked through from the layer below; it's not wet enough yet. As you lay up, this is your best opportunity to make sure there are no dry spots or bubbles. Either will create a dry spot, which will be weaker and will concentrate stresses later too.
If I'm making knife slabs, I lay them all the same way up because the back is glued to the tang, but for slingshots, which are seen from both sides, I lay half the layers one way up and half the other so that the face sides face out.
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You know when to stop when you run out of fabric, resin, or time. You'll know when times up, because you'll notice a slight thickening in the resin as you pour it. You should nevver leave it so long that you get gel lumps. I find judging by eye is better than setting a timer (though I do anyway) because a small difference in catalyst can affect the time.
Finally, I flip the bag the right way round, seal it with the slab inside and gently roll it with a rolling pin from the centre to the outside so that any large air pockets are excluded. You don't want to squeeze out every last micro-bubble; pressing too hard will squash out all the resin.
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Gently clamp the bagged slab between the boards. You only want to ensure the sides are flat and the thickness is the same. Otherwise, it may be bent, uneven, or big air pockets might get in. Again don't clamp too hard or you'll push out all the resin you spent so much time applying.
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First, assemble everything you'll need.
- Ventilation. The resin is noxious. Especially if you mix the catalysts.
- A good table and something to protect your surface.
- Disposable rubber gloves.
- Cloth; I'm using a yard of denim here.
- A pair of scissors.
- A resealable (or non resealable, but tends to be a bit more messy) plastic bag bigger than your intended slab.
- A couple of fairly rigid boards bigger than your plastic bag.
- Clamps.
- Resin; I'm using clear polyester fibreglass resin here, but epoxy and other strong clear two-part resins are fine too.
- Disposable or cheap plastic containers to mix resin in. Note the catalyst melts styrene cups like these. Guess how I know.
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- Something to use as a rolling pin.
- Something to stir the resin with. Ignore the paint brush, I used that as a stirrer.
First cut your fabric into equal and appropriately sized panels. If you keep the corners square and the dimensions the same, you'll not have to trim much off the edges later. The best materials are strong (silk and linen), absorbent (silk, linen, cotton), thin and not too tightly woven. The colour or pattern should not be too dark. The slab will come out roughly the colour that the fabric becomes when wet. For this reason, I'm using pre-washed denim. Denim is a classic material for home-made slabs, but most people choose a denim that is too dark. Regular denim will turn dark-blue and dark denim will go almost black. Sometimes, you may not have enough ofone material. It's possible at a pinch to use another similar material at the bottom or centre of a stack as long as you later remember not to sand down into it, or you'll expose the different material. Some people use layers of contrasting colours to make layers like tree rings in wood.
Measure out the resin. I used 500ml (17 US fluid ounces) for a slab that was only 0.8 sqft by 3/8 thick. I always over estimate the amount of fabric needed and that I can lay-up in the 20 minute pot life and under estimate the amount of resin needed to bind it. I could (should) have used 600ml (20 floz.)
Measure out the catalysts. The resin I'm using needs 100:1 ratios, so I should really have scales that measure in smaller increments than 1 gram. It'd be better if it was a glass top than plastic and I should really have a way to pour liquid with more control. HereI am using film cannisters which seemed to work OK for the catalyst (didn't melt into a puddle like the styrene cups).
You have to add polyester catalysts separately (follow the instructions). My school plastics shop class teacher mixed up the catalysts together without stirring in between .... in 1984 and a cupful created so much smoke that I remember it vividly to this day. If that happens you'll want to get it outside in a hurry. If your cup melts in the vigorous exothermic reaction, that'll be difficult.
The clock is now ticking so the rest happens in a big hurry. Invert your bag and use it as a work surface cover. Pour the mixed resin on the bottom sheet.
I spread it with a squeegee (the edge of my hand in a glove). Get this layer really thick with resin and flip it over so the resin is in contact with the bag. The bag must be flat and not kinked and don't trap an air bubble under the fabric.
Lay up layer on layer, with a puddle of resin in on top of the last layer, then fabric then resin on top. Each layer must be 100% soaked into the fabric from both sides. In the picture below, the fabric has soaked through from the layer below; it's not wet enough yet. As you lay up, this is your best opportunity to make sure there are no dry spots or bubbles. Either will create a dry spot, which will be weaker and will concentrate stresses later too.
If I'm making knife slabs, I lay them all the same way up because the back is glued to the tang, but for slingshots, which are seen from both sides, I lay half the layers one way up and half the other so that the face sides face out.
You know when to stop when you run out of fabric, resin, or time. You'll know when times up, because you'll notice a slight thickening in the resin as you pour it. You should nevver leave it so long that you get gel lumps. I find judging by eye is better than setting a timer (though I do anyway) because a small difference in catalyst can affect the time.
Finally, I flip the bag the right way round, seal it with the slab inside and gently roll it with a rolling pin from the centre to the outside so that any large air pockets are excluded. You don't want to squeeze out every last micro-bubble; pressing too hard will squash out all the resin.
Gently clamp the bagged slab between the boards. You only want to ensure the sides are flat and the thickness is the same. Otherwise, it may be bent, uneven, or big air pockets might get in. Again don't clamp too hard or you'll push out all the resin you spent so much time applying.
