Help with resin casting.

Predatoj

Well-Known Member
So this is going out to folk with experience with resin casting in silicone molds. I'm casting resin into two part silicone molds but having casting errors which are expensive to discard and difficult to fix. The molds are too complicated and large to inject into a closed mold so I'm layering a 'beauty coat' into the mold using micro batches of resin, when it's thick enough I put the 2 halves together and through pre sculpted pour holes inject 60ml of resin with syringes and manually rotate the mold to join the two halves. When I de-mold I get horrible errors that are like doubling casting over casting and flaky delaminated layers. I hope that all made sense. I've attached photos. Question is, what is happening and how to avoid it? If it was just for me I'd just fill with model filler but these are for sale and can't sell them like that.
I'm using resin from a company called Tomps who are otherwise good.
IMG_20220113_141311514_HDR.jpg
IMG_20220113_141259976_HDR.jpg
IMG_20220113_141330275_HDR.jpg
 

Attachments

  • IMG_20220113_141259976_HDR.jpg
    IMG_20220113_141259976_HDR.jpg
    1.1 MB · Views: 12

TazMan2000

Master Member
I think the "micro batches of resin" are your problem if I understand your explanation. The "beauty coat" is delaminating from the mould, probably due to the curing heat of the new resin. The only way to avoid this, is to ensure that the initial outside layer is thick enough and to fill up the inside with the other resin, or just use one resin for your entire casting.

TazMan2000
 

yaris

Sr Member
You should never have to do 'micro batches' if your mold is planned correctly, then single pour should turn out near perfect...it's all in the prep work...I struggled with similar issues, but changing the mold set up solved all off them...eventually.
 

Valor

Sr Member
RPF PREMIUM MEMBER
Layered casting really only works if you're pouring the second layer while the first layer is barely cured. Otherwise things will delaminate. I think you could probably solve this with several evacuation vents in the mold to pull resin into the tight spots
 

Predatoj

Well-Known Member
Layered casting really only works if you're pouring the second layer while the first layer is barely cured. Otherwise things will delaminate. I think you could probably solve this with several evacuation vents in the mold to pull resin into the tight spots
Evacuation vents? I installed sprues during the mold stage so I don't have problems with air pockets. The problem is cost and weight. If I cast with the mold closed I don't have a way of getting the resin in quickly enough. The holes are too small I made them large enough for a syringe. But even if I could it's wasteful to use so much resin and the piece would then be far too heavy.
 

Predatoj

Well-Known Member
You should never have to do 'micro batches' if your mold is planned correctly, then single pour should turn out near perfect...it's all in the prep work...I struggled with similar issues, but changing the mold set up solved all off them...eventually.
The only way I could do this is if I bought super large syringes, like 200ml.
 

Valor

Sr Member
RPF PREMIUM MEMBER
Evacuation vents? I installed sprues during the mold stage so I don't have problems with air pockets. The problem is cost and weight. If I cast with the mold closed I don't have a way of getting the resin in quickly enough. The holes are too small I made them large enough for a syringe. But even if I could it's wasteful to use so much resin and the piece would then be far too heavy.
Probably another name. Vents from undercut areas of your model up to the top. They pull out air and pull in the resin. Also doing a quick roll around each time you add a bit more resin can help get it in the nooks and crannies.
 

Valor

Sr Member
RPF PREMIUM MEMBER
Sorry, I see, you don't want to do a solid cast. Could you show us the object you are trying to cast and the open face molds you are working with? It might help with problem solving.
 

Predatoj

Well-Known Member
Probably another name. Vents from undercut areas of your model up to the top. They pull out air and pull in the resin. Also doing a quick roll around each time you add a bit more resin can help get it in the nooks and crannies.
Yes sprues. I use cocktail sticks on all the raised areas. I've never had problems with trapped air.
 

Valor

Sr Member
RPF PREMIUM MEMBER
So there's a number of things that could be causing be causing your problems. And I'm not an expert. But based on our conversations here's how I would troubleshoot this if I were you:

1) Try a different resin. Not saying yours is wrong, but it's the easiest thing to switch. Some resins cure hotter than others. Smoothon 320 is a great rotocast-friendy resin. It has a 10 minute cure time which works well with roto work.

2) Dust your molds. Not sure if you are, but baby power will help things stick and keep air off the surface.

3) Consider opening up your pour spouts to you can actually pour instead of inject with a syringe. Even if you have a bit more to patch on the model at the end.

4) Thinner layers and good timing. I think brushing in a beauty layer before you close the mold sounds fine. But after that use multiple thin layers, timed to JUST as the previous layer flashes. Go too thick and things can start to pull away from the previous layer, which looks like what is happening to your. Maybe.

Anyway, hope something works for you. Good luck!
 

Predatoj

Well-Known Member
So there's a number of things that could be causing be causing your problems. And I'm not an expert. But based on our conversations here's how I would troubleshoot this if I were you:

1) Try a different resin. Not saying yours is wrong, but it's the easiest thing to switch. Some resins cure hotter than others. Smoothon 320 is a great rotocast-friendy resin. It has a 10 minute cure time which works well with roto work.

2) Dust your molds. Not sure if you are, but baby power will help things stick and keep air off the surface.

3) Consider opening up your pour spouts to you can actually pour instead of inject with a syringe. Even if you have a bit more to patch on the model at the end.

4) Thinner layers and good timing. I think brushing in a beauty layer before you close the mold sounds fine. But after that use multiple thin layers, timed to JUST as the previous layer flashes. Go too thick and things can start to pull away from the previous layer, which looks like what is happening to your. Maybe.

Anyway, hope something works for you. Good luck!
Yeah. Some interesting points I never considered. I'm in England so not sure I can get Smooth-On here. But maybe a roto friendlier resin is what I need. I didn't know about baby powder tip either so that's something else I'll try.

I just need to pour in the right amount of resin and rotate to get even coverage with a thick enough wall without needlessly wasting too much, but when you can't see what's happening, easier said that done.
 

swgeek

Sr Member
In my opinion what's happening is the fresh resin is getting behind the already cast parts and causing trouble. If you cast a part in the mold it's going to shrink, even if it's a super small amount. Then when you pour in fresh resin it will find a way to get between the mold and the part. My suggestion would be cast it solid or rotocast it.
 

13doctorwho

Sr Member
RPF PREMIUM MEMBER
Valor is one of the best, take his advice. I've experienced the trouble your having and fixed it using the techniques Valor listed. I use smooth on 65D for rotocasting and it works great. I do three to five layers. As Valor pointed out when a layer starts to kick start pouring the next... that fixes the delamination problems. The resin will not bond to cured resin!
 

Friko

Well-Known Member
Hi.
You seem to have a good deal of great advice already. But I do have a lot of trial and error experience.
I have to agree that Smooth on has some damn good products. However, I see that they don't sell some of their own products via their site anymore. Idk why.
I'm lazy when it comes to molding and would mold it as one piece and slice it out. Minor cleanup in the end but way easier than what you are currently doing.

It does suck that it's so costly but these issues will only give you a better understanding of what to do and more importantly, what not to do.
Best of luck.
 

Predatoj

Well-Known Member
Hi.
You seem to have a good deal of great advice already. But I do have a lot of trial and error experience.
I have to agree that Smooth on has some damn good products. However, I see that they don't sell some of their own products via their site anymore. Idk why.
I'm lazy when it comes to molding and would mold it as one piece and slice it out. Minor cleanup in the end but way easier than what you are currently doing.

It does suck that it's so costly but these issues will only give you a better understanding of what to do and more importantly, what not to do.
Best of luck.
yea for this rotocasing is the way to go. For this piece being so complex cutting a one piece mold would have been disastrous I would never get a good cast. I agree with swgeek fresh resin meant to thicken the wall is finding it's way behind previously casted resin. I just can't fathom how it's happening.
 

Friko

Well-Known Member
yea for this rotocasing is the way to go. For this piece being so complex cutting a one piece mold would have been disastrous I would never get a good cast. I agree with swgeek fresh resin meant to thicken the wall is finding it's way behind previously casted resin. I just can't fathom how it's happening.
I believe you have some good suggestions as to why this is happening. What is the size of this part and how big are these "micro" batches.
I've done more complex single molds. It's also a matter of prep working knowing where to pour the resin in and where to cut a seam line. I just wouldn't rule it out considering that doing a proper 2 part mold is typically harder.
Have you considered the silicone thickness to make sure it doesn't need a mother mold or jacket?
 

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.
Top