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Jack Liu

The Silicone Mold Vacuum Casting Process

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Vacuum casting process (English).pdf The Silicone mold Vacuum Casting Process

(Jack Liu)

Vacuum casting technic & product sales Zhongshan Torch development zones 511189226

 

Abstract:

Vacuum casting is a casting process for elastomer using a vacuum to draw the liquid material into the mold,which is one of the rapid prototype technology. This process is used when air entrapment is a problem, there are intricate details or undercuts, or if the material is fiber or wire reinforced,it is usually for small volume production.

 

Key Word:

Rapid prototype, silicone mold, Vacuum casting.

 

I chose vacuum casting job as my beginning of comprehending plant work without hesitation because of my interest and enhancing hands-on ability. Now I do not work there anymore, but the memory of those working experience often comes up. Vacuum casting is not a complicated technology at all, however, much details must be concerned if you want to generate a perfect component. Therefore, I would like to present some key point of this technic along with the whole process.

 

1 The Inspection Of Master Pattern

The quality of master pattern is the precondition for that of casting part. Firstly, compare the construction of the pattern and the drawing data to see if there’s deformation or lack. Then observe surface, confirm that whether the serial number of texture in accord with requirement on the production sheet or not if it has. The transparent and high gloss part need to be went through if it has some scratch or blur mark. Then, measure the part with caliper and compare it to the , drawing, but must be careful that don’t damage the pattern when measuring it.

 

 

 

2 Box Construction

(1) Parting Face confirmation

There are three convention of judging the parting face, that is visual and invisible surface, assembly and nonassembly surface, easy and difficult to demold. Of course there are also some more factors should be considered, for instance, the impact on controlling the bubbles, the size of the silicone mold, the usage amount of silicone, the shape of casting part and condition for the resin running, etc. Normally it is easy to determine the parting face, for example some lids, box-structures, but there are also many parts difficult to be parted, see the pictures below:

 

 

(2) Set parting line

Setting parting line here means taping the edge of the parting face of the part, so the rubber mold would be easy to split. For reducing the influence of the tape to the casting part, the tangency width of tape and the part should be less than 2 mm, but can be too small, or it will be not strong enough. For some parts with through holes, there are should be some small gaps on the closing tape, that is for the bubbles in the viscous silicone going out when poring silicone into the box and vacuuming it. Certainly, not all parts need to taped, especially for some quite complicated parts, but how to split the mold must be considered clearly, and the confidence skill to do that must be had. For the transparent and high gloss part, don’t tape it if don’t need, since it will leave a mark on the casting part.

 

(2) Sprue Setting

This step is also important to the final casting part. Normally, the sprue rod will be set at the invisible surface of the component, that means the face can not be seen after assembling the whole model, such as the inside of bottle lid, the bottom of cup and so on. For some simple construction or symmetrical part, the sprue rod should be set in the center as possible, thus the liquid resin running equally in the mold when casting, and the bubbles can be easily controlled. Meanwhile, some else factors should be considered, for instance, the treatment of casting part, the appearance and assembly, the mold construction, etc. To some very small or thin components, the sprue rod should be set on the edge or lateral face. In addition, the sprue rod takes the responsibility of holding the pattern in the box as well, so it must be stuck to pattern steadily.

 

 

(4)Build up the mold Structure

Make a box by wood board or aluminium plate according to the dimension of the pattern. Then hang the pattern which has been taped and sticked with sprue rod in the box and make it steadily. For the mold that needs some steel bars to reinforce itself, the position of the pattern and steel bars must be distributed reasonably, the steel bar must be fastened steadily, can’t be loose and touch to the pattern. The distance between the edge of pattern and inner wall of the box is 25 mm usually.

After fix up all the things, should check the structure to see if it is steady or miss some thing.

Then put the box on the flat, safe place which is suited to pour liquid silicone.

 

 

(5) Pour silicone.

Calculate the needed weight of silicone and pour the silicone into a bucket. Then add the catalyst according to certain ratio into the silicone, and put the bucket on the vacuum mix machine to mix the liquid silicone under vacuum. If the weight of silicone is small, for example below 1.8kg, we can use the vacuum casting machine to do that mixing job, but the mixing must be surely sufficient , otherwise, the mold will be partly curing incomplete.

After the mixing, pour the liquid silicone slowly into the box with which the pattern structure in. Keep the eyes always on the pouring, should not let the silicone fall on the pattern before it lift up to touch the bottom surface of the pattern, because it may break off the mold structure. Afterwards, move the box into the vacuum machine and vacuum it, this process is to suck the bubbles out of the silicone, it is especially important for pattern that has large area and no through holes, even has deep concave on the visual surface.

When finishing the vacuuming, move the box into the oven and cure it under 40 centigrade degree. Normally, it only spends 7~8 hours to finish the curing, but for some pattern that in large dimension, or has very small holes or gaps, they should be cured for about 12 hours, that makes the mold solidify enough.

 

 

(6) Split the mold

Take the cured mold out, and prune it, then use a knife to split the mold along with the tape connected to the part. If there’s no tape, split it according to the presupposed parting line. Normally, the parting face from outside wall to the edge of the tape should be like a wave, so as to prevent the deflection when assembling the mold. After finishing splitting, get rid of the tapes, take the mater pattern out. Sometime if needs, some air drain should be make, they can contain some bubbles when casting. In the end, put the mold into the oven and preheat it under 70 centigrade degree.

 

 

 

(7) Vacuum Casting

Take the mold that has been preheated out of the oven, clean it up with air gun, then spray some release agent on it if needs. Usually, if the resin which is ready to be cast just slightly erodes the silicone mold, or the structure of the part is simple and easy to de-mold, it’s no need to spray or less spray release agent on the mold, on the contrary, it needs, but don’t too much, in that case, there’s some release agent stick on the casting part, and it’s difficult to clean. Moreover, don’t spray it on the mold which is casted for high gloss part.

Afterwards, assemble the mold, weight the resin, and vacuum them in the vacuum casting machine for 5 to 10 minutes, then operate the machine to let the two components of resin mix together for about 1 minutes, then cast it into the mold, you can then exhaust after the resin full of the mold cavity. The timing and rhythm of degassing has critical relation with the generation and fading of the bubbles. In the end, put the mold that has been cast into the oven to solidify the resin under 70 centigrade degree.

 

 

(8)Solidify Forming

The curing time for different material is dissimilar, normally between 40~90 minutes. After the curing, take the mold out, open it and get the casting part, then clean up the fragment and the gate, so the finish casting part is come out.

 

Jack Liu

Vacuum Casting Technician & Project Manager

 

jack@star-prototype-china.com

 

www.star-prototype.com

 

2011-09-01

 

 

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