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Post by eyesinthesky on Dec 11, 2015 12:40:10 GMT -5
I poured my first mold with the Plat 55 and was surprised how long it took to degas the silicone. Are there any helpful suggestions for degassing? Does the shape of the container used to degas significantly make a difference? i.e using a large diameter cup to make the thickness of the Plat 55 thinner vs. a smaller diameter cup with thicker amount of the Silicone. Is it better to draw the vacuum and leave it or draw the vacuum and release it multiple times?
FYI I was mixing 120 ml in about a quart size cup and after about 30 minutes of degassing (of the 45-60 minutes working time) there were still bubbles on the surface that had not collapsed. Vacuum was about 21 inches, the max my pump can due at this elevation. Any suggestion would be helpful. Thanks,
Doug
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Post by mike on Dec 14, 2015 9:46:59 GMT -5
Thanks for all the info. Biggest issues sounds like it was a lack of vacuum. 29" of Hg is the magic number. At very high elevations, it is impossible to get there but to give you an example 29" of Hg in Kalamazoo Michigan (approx. 700' above sea level) is similar to 26" of Hg in Denver or other similar elevations. 26" of Hg will still vacuum the silicone very efficiently and quickly but it will not in Kalamzoo. It would take hours. So I'm not saying it has to be a distinct number however if we are not applying enough vacuum it will rise but simply never "break" or collapse which lets you know you've removed 98% of all the air you introduced during mixing.
I like to use a flatter, more open container when degassing. This accomplishes 2 things ... makes the thickness of the liquid rubber more shallow making the distance for the air bubbles to travel to the top smaller AND gives you more surface area for the bubbles to expand, evacuate, and pop. Tall, skinny, or narrow containers are the opposite ... bubbles have to travel further as well as they will stack on top of each other which makes the expansion much higher.
Not sure what type of pump you have but I'd recommend a minimum of 5 cfm pump. 6 cfm is what we use and previously distributed. I think there is a company Best Value Pumps that makes/distributes pumps and chambers and seem to be the most economic that I've seen.
Mike
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Post by eyesinthesky on Dec 14, 2015 11:51:08 GMT -5
I am using a Gast oil-less vacuum pump and I am at the elevation of Denver. I think I'll go to the Plat 40 and try that. It has 1/10th the Viscosity of the Plat 55. I chose the Plat 55 because I wanted to install screws into the mold as demonstrated in one of your videos and it worked great. Will that technique work in the slightly softer Plat 40?
Doug
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Post by mike on Dec 14, 2015 13:03:35 GMT -5
Yes, it should work fine. The more length you give the silicone to hold on to ... the better.
Mike
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Post by eyesinthesky on Dec 24, 2015 18:08:02 GMT -5
I went ahead and bought a new pump and chamber from Best Value Vacs last night and they shipped it out today, Christmas Eve. After examining my molds, they are fine, just a little learning curve with the Plat 55 I guess. But I did want a better pump and they have a good selection and so far excellent customer service. I am getting use to more mold making techniques but it does require that learning curve. Merry Christmas! |
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Post by mike on Dec 28, 2015 9:22:47 GMT -5
There is definitely a learning curve but please let us know how we can help shorten it. I think you will be very happy with a bigger/better pump. As far as bubbles, they will typically never come into play unless they are on the surface of your mold. Sometimes brushing the first layer on the surface of your master is a good way to make sure you've "wet" the surface out and made sure you have a layer of silicone against your part. This does not allow air to hold on to the surface of your part and allows any air to then float up and away from your master giving you a perfect mold. Pouring perfectly degassed Plat 55 will also help in your success. Glad its coming together for you. Please keep us posted.
Hope you had a great Christmas as well.
Mike
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Post by eyesinthesky on Jan 6, 2016 11:51:17 GMT -5
Here is an update, and of course follow up questions. The new pump and chamber work extraordinary well. At my elevation I can draw about 26 inches. At that level, the Silicone raises and falls as expected in a timely manner (what a difference a few inches make). I'm using both Plat 55 but also Plat 25. However, I have noticed (specifically here with the Plat 25) after the doming and subsequent collapse, after another minute or two the surface is nearly completely smooth except for a very few bubbles. There seems to be out gassing of the Silicone for a long time (at least 10 to 20 minutes or more). It is very small and I assume it is some form of gases produced due to the chemical reaction between the base and catalyst. Is this correct? If so, when should one stop degassing and remove it from the chamber? Is a minute or so after the collapse of the dome usually sufficient?
On perhaps a related note, the data sheets for the Plat Silicone states a full cure schedule of 7 days. Can you elaborate what this means? Should I not use the mold for the entire time? Is this when the chemical reactions occurring in the Silicone have completed? Thanks for your help,
Doug
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