Molding and Casting Questions.

Hello mold making experts of the RPF.

I am editing this post rather than starting a new one. After several months, I have sorted out my issues and now have two excellent molds for my project.

I am also going to answer some of my own questions sharing what I learned, so hopefully this is useful to others as well.

HERE is a link to my BLOG about how I made the molds and HERE is a link about degassing.

EDIT. In the end I went from Pinkysil to Vario40. Pinkysil is a great product. It is 1:1 and quick setting. It is user friendly and generally anyone can make a mold using this stuff. The problems I encountered with it included small bubbles forming on my parts surface and splits and tears from a bad mold design.

Vario40 is a better product IMO but you need to take a bit more care with it. It mixes at 10:1 and it is translucent and has a higher shore or A40 Vs Pinky's A25. It also has a longer cure time, so can be degassed in a vacuum. Like all other silicones, it sticks only to itself, so molds can be built up or kept separate using a release agent. I used good old Vaseline. It stopped the parts sticking together when required and is easy to clean up in warm soapy water. It has also been suggested that Baby Oil is good as well as being a liquid, gets right into fine details Vaseline might not.

Both PinkySil and Vario40 are additive curing silicones. What I discovered is that Pinky reacted with products like ShoeGoo (air curing). The Vario40 does not appear to react with products like Shoe Goo.

I used Plasticine (most people use clay) and the type I used is the same as used in schools - kid safe = No Sulfur. Always pays to test some first though. I did use clay and actually like the Plasticine because whilst is slightly harder, it is easier to clean up and get out of details on the master part. It does not dry out and if it picks up debris, you can pick those parts out easily.

Degassing the Vario in a vacuum was interesting because this stuff rises much higher than the elastomer I cast in. I found I could mix a maximum of 250g in a 2 litre ice cream container without overflow. I can mix 500 grams of resin in the same sized container without overflow. Unlike resins, the Vario rises and falls at least 3 times before all the air is removed.

You see the images of my molds in the first link, but basically I ran the part line along the seam of the original part. I also made the main body of the mold like a bowl with the lid fitting into that like a plug. This means I should have solved my leakage issues. Water test confirm this anyway.

So I hope this helps others new to molding.


Original post: I am about to embark on my first 2 piece mold and want to know if I can use Plasticine instead of clay for the build up around the part. Is there any reason not to use Plasticine?

The product I intend to use to make the mold is PINKYSIL

Thanks in advance.

EDIT: Title has been changed and I will simply use this thread for all my questions rather than start new threads if I can not find answers.

Re: Plasticine instead of clay?

Good question, I did try it and found it did not hold the shape for very long (had a tendency to reshape itself slightly). Mind you I was trying to cast detailed parts from a model sprue.
Might be OK for larger simpler parts, also I was just using a simple children's plasticine...quality of material could be a variable as well.

Re: Plasticine instead of clay?

Thank you for you prompt response. The part I want to make is sort to U shape in cross section. I intend to pour the mold in a plastic container and use the Plasticine to prevent the Pinky from creeping under the part. Once the Punky is set, I should be able to pull that half of the mold out (it has tapered sides), remove the Plasticine, build up the sides and pour the second half. Just was not sure if the oil in the Plasticine would react with the Pinky. I would like to think it won't as I have read about guys using vaseline as a mold release agent.

Re: Plasticine instead of clay?

If you are using plasticine you have to be careful to get one that does not contain sulphur. If it does it will inhibit curing in silicon and any parts in contact with it will never fully cure and remain semi liquid. As far as I am aware most plasticines do contain sulphur so if you aren't sure test a small amount and see how it goes. Having experienced silicon not curing on a part myself it is not fun having to clean it off and it can take a long time if it has a lot of details.

Re: Plasticine instead of clay?

I've used plasticine from an art shop....the big blocks, no problem.....silicone cures no problem

Years ago when I used to do a bit of sculpting I used plasticine & used latex to make moulds.....the latex draws the oil out of the plasticine making the sculpture less susceptible to loose shape

J

Re: Plasticine instead of clay?

NeonHybrid said:

If you are using plasticine you have to be careful to get one that does not contain sulphur. If it does it will inhibit curing in silicon and any parts in contact with it will never fully cure and remain semi liquid. As far as I am aware most plasticines do contain sulphur so if you aren't sure test a small amount and see how it goes. Having experienced silicon not curing on a part myself it is not fun having to clean it off and it can take a long time if it has a lot of details.

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Jaitea said:

I've used plasticine from an art shop....the big blocks, no problem.....silicone cures no problem

Years ago when I used to do a bit of sculpting I used plasticine & used latex to make moulds.....the latex draws the oil out of the plasticine making the sculpture less susceptible to loose shape

J

Click to expand...

Conflicting POVs, so I might be best to buy some clay today then. The Plasticine came from my Wife's (she's a teacher) Year 1 class, so I am not sure if it contains sulphur or not. It was freebie so jumped at the chance to cut a cost corner.



Here are two of the four parts that need casting. I made a start on this last night before wondering if the Plasticine would be OK or not.
The idea (and what I started on last night) was make a bed of Plasticine that is level with the top (inner) surface to capture the "wings" first. I also have some marbled to push into the Plasticine to make indents for registration points.

After the first layer has cured, flip it over, remove the Plasticine, clean up any mess and build it the walls for the next pour.

Off to buy some clay then.



Even though the oval parts probably could be open back cast, I want to use a 2 part to ensure that the thickness remains constant.

Re: Plasticine instead of clay?

Since we're on the topic of plasticine, I have a small block of the Van Aken brand, which looks like this...



I don't know how many here can confirm how well its used, but according to their FAQs, it states the following in relation to the sulfur question: Sulfur is not an ingredient in the formulation. It may be present in trace elements that can be measured in parts per million. However, the amount is so small that it does not affect the mold making process. These trace elements of sulfur may be found in the various raw materials used in making non-hardening modeling clay.

Can anyone who's used it confirm that the Van Aken's non-hardening plastalina is good for mold making? I primarily picked up some of this stuff so to make two-part molds with it.

Re: Plasticine instead of clay?

I tried using it to bed in a model for casting. It's too hard and it's a nightmare to remove all traces of it.

Re: Plasticine instead of clay?

So I went shopping for no-sulphur modeling clay and could not find anything that even mentioned it on the label. Then I tried to contact the distributor of the Pinkysil I am using and no nothing from them either. Then I contacted a local RPF member (sleepless) who uses this same Pinkysil all the time and he said that the pinky and the Plasticine will work as they are not in contact long enough for any leeching to occur. I've seen his work and given his results are excellent, I decided to go with the Plasticine and here are the results.



All ready to pour the first layer.



First layer poured.



2nd layer ready to go.



And the final result. A bit rough, but it worked out fine. So one down, three to go.

Re: Plasticine instead of clay?

Nice job, Mark... And "Happy Birthday", for today...

Re: Plasticine instead of clay?

Thank you Just bought myself a B'Day present - 2KG of Pinkysil and 2KG of clear elastomer...

Plus a RAMBO IV knife (proper Hibben version to replace that crappy MC one).

Moving and my first FAIL today.

Rule 1 - DO NOT MIX different types of Silicone or it won't cure.

How did this happen? On my master (clear sole for the NIKE MAG project), the outer part leans in to recreate the "outriggger sole" and I had used some Shoe Goo (which sets in air) to smooth over the join. Now maybe this Shoe Goo was not 100% cured, I don't know, but the Pinkysil did not cure where it came in contact with the Shoe Goo.

I am testing the mast with a small sample now to see if I get another reaction. If not all good. If so, I don't quite know what I can do here.

Is it possible to cut this bad part out, re-insert the master and fill in the gap?

2nd question - does any one know how much vacuum is needed to extract bubbles from this stuff? I can't seem to find anything in the data sheet relates to de-gassing. The people at the store suggested 30cm of vacuum for 4 min but that seemed to do nothing. Yeah some bubbles popped but it did not raise and fall like I was told it would.

DATA SHEET

After 6+ hours curing time, I demolded to find this.



Did I boil the urethane under Vacuum?

Pinkysil is a platinum silicone, which has issues with compatibility with other materials. Even when I tested it for compatibility with certain paints, it still failed when I made a mold. I stick to tin silicones now.

30cm of mercury isn't much vacuum at all. Most recommendations are for 28-29 inches. I don't bother with vacuum myself though, I rely on pressure to eliminate bubbles.

robn1 said:

Pinkysil is a platinum silicone, which has issues with compatibility with other materials. Even when I tested it for compatibility with certain paints, it still failed when I made a mold. I stick to tin silicones now.

30cm of mercury isn't much vacuum at all. Most recommendations are for 28-29 inches. I don't bother with vacuum myself though, I rely on pressure to eliminate bubbles.

Click to expand...

Thank you responding here. Frustrating to say the least.

I used Pinkysil because it is easy to use and readily available. It seems to have worked except where it came into contact with the Shoe Goo but I am now thinking that Shoe Goo was not properly set because it is rock hard now. It did seems to have some give yesterday.

On the data sheet it says - "The cure will be inhibited if cast against a tin catalyzed silicone RTV." If I understand this, I can't use other types of silicone?

Yes I agree, 30cm is not very much pull . My Vac Chamber will pull about 27.5 inches. How long should it be under Vacuum?

Thing is, I did run a small test at this vacuum and all that happened was the stuff foamed up, but it did not slump, so it just looked as if I boiled the stuff. I did this with some poly resin a few years back and it went off in the pot because under vacuum. Apparently water will boil under vacuum at room temp.

I have seen many videos of degassing silicone and it raises and slumps. This did not slump. I am at a loss here.

That's part of the incompatibility of platinum. They say it won't cure against sulfur or tin silicone at all. Some resins are a problem too, so you have to be careful what you make you master patterns with. I like using Tamiya spray paints on my masters, but Smooth-On platinum silicones had problems with it. A six hour cure rubber takes three days to cure against it. It's a shame too because I loved that rubber, 1-1 by volume mix, low shrink and long life. But I'll be damned if I can find a paint to use on my masters that it can work with.

I've been using Mold Max XLSII and it seems to be good stuff. No curing issues with any paint I've tried, and it's also low shrink and long life. Only problems are the 10-1 by weight mix and 24 hour cure time, but hey it works, and one day is better than three.

Like I say I've never used vacuum, I can't give much advice there. I do know the rubber is expected to rise several times it's volume while boiling, then fall back down when the air is released. How much vacuum and time is needed will depend on the volume and viscosity. I just mix it, pour into the mold box, and pressurize it. This is very effective at eliminating bubbles. But the master has to be pressure worthy, it won't work if the master will collapse under pressure.

Just browsed over this thread quickly, The rubber or casting material will look as though it's boiling, that's normal, its the air or gas coming out of the material. Usually you only need to use the vacuum until the bubbling subsided, indicating that there is little air or gas left in the stuff. Then you should pour it. I don't use a vacuum, i use a pressure chamber or pot for a couple reasons, the equipment is easy to get and use and the casting material does not bubble up and over flow the mold, then slump down not leaving enough material to fill the mold. That's why I would recommend de-gassing it in a large paper cup or mixing bowl, not the mold itself, then slowly pour it into your mold.
Also before doing any thing with your rubber or casting material it's a good idea to do small tests. Some Plasticine's have chemicals like sulfur which will inhibit the cure of some silicone rubbers. I test this by making a small bowl of Plasticine the size of my thumb then mix a small amount of rubber to fill it. And using combinations of materials not made by the same company can lead to disaster.

All very sound advice, thank you guys for posting.

How do you guys deal with pouring in cooler weather? It is winter here at this time, but day time temps are still into the mid 20's.

Do you have a warm house? I do all my work indoors, but I don't know if the stuff you use smells bad, I use Smooth-On products and they don't stink. Cold weather will definetly affect your results.