Resin help and trying to get a flat bottom.

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ajaxjones

Member
thanks for reading and hope that someone could help in my attempts at resin moulding parts I've made. I'm doing OK so far at making moulds and casting in resin using various types, but one of things that seems to puzzle me so far is getting a flat surface after pouring a mould. Everything tends to have a slight concave surface and although I can also pour a little extra in and then scrape off the excess the final surface is never really flat. I've bought several resin kits and they seem to be able to produce a very flat finished surface, I've attached a pic of one I have. The larger pieces is perfectly flat and seems to have a slight texture to it, I have tried adding plates and caps to my moulds, but surface tension etc always seems to thwart my attempts. I've tried glass but it seems too easy to get voids under the glass. Also you can see on the smaller parts whoever made them managed to get a very thin flat skin that also keeps all the parts together. Is there some trick or technique to achieve this, or is it down to the resin and its viscosity? I've searched a lot and never found anything that seems to address this part, unless I should just be making a fully enclosed mould although I cant see any pouring flash lines on the pieces I've seen.

Any help or pointers would be most useful as it would save me a ton of time with any sanding.
flat back resin.jpg

this is what I'm trying to achieve.
 

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TazMan2000

Sr Member
Make a rectangular RTV mould a couple of cm thick. Place the rectangular chunk over your open-face mould with the resin poured, in a rolling motion akin to unrolling a carpet. You may still get occasional bubbles, but doing it the aforementioned way will reduce their occurrence.

TazMan2000
 

ajaxjones

Member
Make a rectangular RTV mould a couple of cm thick. Place the rectangular chunk over your open-face mould with the resin poured, in a rolling motion akin to unrolling a carpet. You may still get occasional bubbles, but doing it the aforementioned way will reduce their occurrence.

TazMan2000
Agh,i didnt think of just trying it with a flat slab and laying it on like that. thanks
 

robn1

Master Member
Make a rectangular RTV mould a couple of cm thick. Place the rectangular chunk over your open-face mould with the resin poured, in a rolling motion akin to unrolling a carpet. You may still get occasional bubbles, but doing it the aforementioned way will reduce their occurrence.

TazMan2000
That's exactly how I do it, but I also weigh it down to make sure it makes good contact. I place a rectangle of 1/4 inch plywood with fishing sinkers hot glued on top.
 

TazMan2000

Sr Member
That's exactly how I do it, but I also weigh it down to make sure it makes good contact. I place a rectangle of 1/4 inch plywood with fishing sinkers hot glued on top.
True. You can use weights or use your hand pressure until the resin cures. Be careful when using weights, and not monitoring your mould, because if your casting surface or mould is not level, your rectangular chunk will slowly slide off the bottom part of your mould, since resin is very fluidic before it starts curing. Even if your table is off level by one degree this can happen.

TazMan2000
 

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Duncanator

Sr Member
I've also used a plate of acrylic sprayed with mold release. I lay it on top of the mold like TazMan mentioned.
The clear acrylic lets you see if you've trapped any bubbles.
 

robn1

Master Member
True. You can use weights or use your hand pressure until the resin cures. Be careful when using weights, and not monitoring your mould, because if your casting surface or mould is not level, your rectangular chunk will slowly slide off the bottom part of your mould, since resin is very fluidic before it starts curing. Even if your table is off level by one degree this can happen.

TazMan2000
I use a pressure pot so holding it by hand isn't an option. And I should have addressed the sliding issue, the top does tend to slide around on the wet resin. I make the mold with a raised lip around the edge to contain the top.
 

TazMan2000

Sr Member
I've also used a plate of acrylic sprayed with mold release. I lay it on top of the mold like TazMan mentioned.
The clear acrylic lets you see if you've trapped any bubbles.
That would work too, but perhaps a think piece of acrylic that you can roll and squish out the resin would work better. I will try that, next time I cast, with a weight of course.

TazMan2000
 

swgeek

Sr Member
You can also pour the resin so it just crowns past the top of the mold and let it cure. After it's fully cured, leave all the parts in the mold and sand the resin down until it's flush or nearly flush with the mold. I use a stationary belt sander, but be careful it will grab the mold and throw it if you're not careful. Also, when you get the parts out of the mold and need to do some hand sanding, sand the part against a piece of sand paper glued to a flat surface and sand in a figure eight motion.
 

tdb68

Active Member
lots of great comments here! when hand sanding i use a clip board. its pretty resilient and it allows me to change grits easily.
 

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ajaxjones

Member
I tried some clear casting with a piece of moulded silicone pressed down and it did the trick well enough for my needs, so thanks for all the tips. Still got a couple of bubbles, but I think it will be even better with the F32 resin when I get around to using that, the bases are certainly flat and the moulds exhibit that small thin excess that I observed on the pro pieces. I've been making some fake radio vacuum valves in clear resin with printed inserts. So I'm going to remake those moulds to incorporate a flat plug for the bases :)
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IMG_5928.JPG
 

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