Can using Clay plus pressure cause distorted Silicone Mold?

[RobFayette]

Hi,
I first want to thank RA for letting me join your forum. I was reading the post about "Best Resin for Pressure Casting" and was quite impressed my the knowledge of the people. That kind of knowledge seems hard to find from what I have experienced so far. I have been making knobs and other small parts for old cars for the last 3 years.

I am now making a mold for a steering wheel that is 18" in diameter. I am planning to cover half of the steering wheel with clay and then pour the first half of the mold using silicone rubber. I intend to degass the silicone and pressurize the pour while it is curing. My question is will the pressure tend to somehow distort the finished mold because of the clay? Someone told me that I might be better off to not pressurize the pour while it is curing. He said you don't need to pressurize because the bubbles in the silicone will all rise to the top of the mold and not cause a problem. I was going to insert a picture of the mold but I don't know how to do it. Do you use the "Img" button and if so how? Thanks for your help. Rob

[mangozac]

Hi Rob,

First up here's a nice little guide explaining how to post pictures on a forum: . The same instruction apply to Resin Addict. It's not as simple as say attaching pictures in an email, but once you're familiar with it it's easy enough

Regarding your question, I think the clay should be fine. Your friend is right that the bubbles should rise to the top, however this is highly dependent on the silicone product you are using - many are not runny enough to allow the small bubbles to escape. While degassing before pouring should take care of most of the bubbles, there's still the chance of some forming on the walls of the mould as you pour and they will need pressure (or perhaps otherwise a lot of vibration) to remove.

The important thing is that when you place the clay you need to make sure you don't create any air pockets within it. Otherwise these will be crushed when you place it under pressure and that will cause your mould distortion.

One last thing: make sure the clay you are using is compatible with your silicone. This generally means using a sulphur free clay.

[RobFayette]

Thanks for your reply Mangozac. I watched the Video on uploading images to the forum and signed up on photobucket. The image I want to upload to here initially is on my other computer so I'll do that tomorrow.
As far as the clay issue, I'm pretty sure I have some air pockets in the clay under the steering wheel rim so now I'm in a quandry as how to proceed. Do I not use pressure and hope all of the bubbles rise or do I pressurize and hope that the distortion doesnt cause a problem. It seems like to me that since the clay is used in the 1st silicone pour which forms one half of the completed mold if there is some distortion it will be in the 1st half of the mold. Then when the second half is poured it will be up against the distortion in the first half so the two halves will end up fitting together perfectly and the distortion won't matter. Is my logic illogical? It's really cool to be able to get an experienced opinion. Thanks. Rob

[Hiflt4]

I would suggest plasticine, You can get it a hobby or art stores under a number of brands I perfer the solid color bricks. you can even heat it up to a liquid state in a pot on the stove on low and pour it into your frame for a nice smooth and level surface then let it cool a bit to solidify before pressing originals into it. I use this clay while pressure casting and vacuume degassing and it works great, tho I usually pop it into the fridge/freezer for a few min before i pour to make it nice and firm. Also its easy to remove from your parts, alittle minireal spirits and a toothbrush.

I do highly reccomend degassing it before pouring and then pressure casting it. But a good high psi pressure cast should do you just fine.

And even at 80 psi I have never had any deformaty with plasticine at room tempature. and its cheap and reuseable.

[RobFayette]

Thanks for your info Hifit4. I had no idea you could melt plasticene and pour it. That could come in quite handy.

If this works heres is a pic of the steering wheel in the mold that I am concerned about the clay becoming distorted under pressure. Rob

[Hiflt4]

Depending on the weight you may need to place more solid items for it to rest on *kind of like pilons for a dock. Also make sure thoese bolts are glued down/together. Otherwise your mold may seep into/under them.

[RobFayette]

Here's what the finished mold looks like. I hope to pour the 1st half tomorrow.

[Hiflt4]

Im interested to see how the mold come out.

[mangozac]

Wow that is some hard core mould making!

Melting the clay/plastecine is a good trick. That's how they make the parts for claymation movies - melt the clay and then pour it into moulds to make up the characters' body parts.

I'm a little confused as to what you're aiming for though. Are you planning to reproduce the steering wheel in silicone or resin? It looks like you're planning to pour silicone over the existing wheel to make a mould of it?

[RobFayette]

Yes Mangozac,
In the picture, I have it ready to pour Silicone around the steering wheel.Then I will cast resin steering wheels. Over that last few days I poured both halves of the Silicone Mold and today I cast the first steering wheel. I cast it as kind of a dummy steering wheel as a test to see how it was going to work out. I'm calling it a dummy steering wheel because it didnt have the steel hub or steel spokes and rim that a complete steering wheel has. Unfortunately it had lots of bubbles in it. I have some ideas how to resolve it but I'm afraid it may end up being a failure. (It won't be the first failure I've had). I will try to post some more pics soon.

[Hiflt4]

Does the mold have problems with air bubbles or does the cast have voids from air in it?

What equipment are you useing to do your casting?

[RobFayette]

The steering wheel I cast has air bubbles in it. The silicone mold looks good. I ended up degassing the silicone for the mold but I didn't pressurize the silicone while it was curing. I was worried about the clay causing distortion like I mentioned when I started this thread. My friend convinced me that I didn't really need to pressurize the silicone while it was curing, and I have come to believe that he is right.
I am using Raw Materials Suppliers brand Silicone for the mold and Smooth On 327 Resin pigmented with Smooth On So Strong pigments for the actual steering wheel. I am quickly degassing the 327 resin after mixing, but before pouring, and then pressurizing it to 50 psi after it is poured into the mold and during its curing for 2 to 4 hours. One big contributing factor to the large amount of bubbles in this first wheel that I cast I think is the fact that I ran out of resin and had to madly mix up some more to finish the pour. I didn't feel I had time to degas the 2nd batch of resin so I didn't. I am thinking that this isn't the only factor though. I am going to try to pour another wheel tomorrow and then I'll know better where I'm at as far as the bubbles go.

[Hiflt4]

are they small less than 1mm bubbles all over or are their larger sections that do not fill?

A double pour will will reduce the quality of part, but I do not think it is responsable for your bubble issues.

First thing I would try is uping the psi to 80 if your chamber is rated for it,

if not you can add a large water tight hopper to the pour location and vaccume the resin while it is in the mold before applieing pressure this will force larger bubbles out but will bubble up alot and needs to be a VERY large overflow hopper for this.

This hopper either needs to be disposable or coated in mold rubber.

Can you post a pocture of the cast?

[RobFayette]

Here are some pics of the 1st steering wheel poured and of the finished mold. I think what may have happened is that the hurried 2nd pour ,that wasnt degassed ,had a lot of air in it and even though it showed it was full by flowing up in the many vents around the perimeter of the wheel indicating that it was full after pouring, once it was under pressure the air collapsed and then there wasnt enough liquid in the mold. I am now making some troughs around the vent holes at both the perimeter of the wheel and also the vent holes at the hub of the wheel. I figure once I make the pour and see the liquid coming out of the vents then I will pour additional resin into the troughs so that there is extra resin to flow back down the vent holes into the wheel after it is pressurized. What do you think? Thanks a lot for all of your inputs from everyone who has commented.

[Hiflt4]

Ok yes i see excatly what your problem is.

The orientation you choose is great for a more advanced form of casting than what your useing, with filling it how you are *which you can not really change without remolding* the air in the wheel section has no way of escaping and is becomeing entraped. No matter how much pressure you use with they setup it will still have bubbles in thoese areas. If the cast vertically pouring down into the center spoke would allow the air to escape from the pour area but would require finishing work to smooth the gate on the finished part.

Now here are your options:
1.Fill the mold under vacuum useing a Vacuum Liquid Feed-Thru.
--This will likely require another pressure/vacuum chamber and has to be set up as such *pricy*
However you will never have a problem with air bubbles agian ever.
http://www.abbess.com/vacuum/node/124

2. raise the psi and use puddy to fill the small bubbles.
-- You can also try and vacuum the filled mold with the hopper idea above but this is far from optimal since there really isent anywhere for the air to escape too. This mold may be trash if the puddy isent a sufficent way to address it.

3.Remold with a different pour location which will fix this issue
-- I suggest pouring from the wheel center spoke 90* shift as if it was in a car pouring into the big middle spoak giveing the air the ability to move around the wheel and up the pouring gate. Even with this I reccomend useing the hopper and vacuum the resin inside the mold to minimize the air trapped in the ripple pattern of the wheel molding.

If you have qestions let me know. Its a solid mold just layed out poorly for the equipment your useing.