Vacuum Casting

What is vacuum casting?

Silicone injection molding or vacuum casting is used for the production of small series or to duplicate prototypes and models in a short space of time. Starting from the model produced via CNC, SLA or SLS or a sculpture made by clay carving as a master mold, the mark is made on a piece of silicone. This mold is then used through vacuum casting to produce exact duplicates of the original products.

This technology is especially suitable for the manufacture of rubber prototypes or rubbery materials such as Polyurethane (PU), although additives can be added to it to give similar functional characteristics to PMMA, PC or ABS, among others. You can also color the injected parts or even make them transparent.

PS: The maximum size is 1000 mm × 800 mm × 800 mm, and the tolerance can be controlled up to 100 mm / ± 0.20 mm.

Manufacturing process of the silicone mold by vacuum casting

Before making silicone molds, we need a prototype produced via CNC, SLA, SLS, or a sculpture as a master mold.
Next, you have to make the mold box in which the silicone will be poured to make the mold:

Parting face confirmation: There are three key points to consider in the manufacturing and demolding process; these are the visual and invisible surface, the assembly and surface, and the complexity of the molding. There are also some more factors that should be considered, for instance, controlling the bubbles during injection, the size of the silicone mold, the amount of silicone used, the shape of the casting part, etc. It is easy to determine the parting face, for example in some lids or box-structures, but there are also many parts difficult to be parted.
Setting of the parting line: Setting the parting line here means cutting the silicone mold at the predetermined parting line of the master model, so the mold can be easily separated. If the parts have holes, small gaps must be left so the air bubbles go out the casting box during vacuum.
Sprue setting: This stage is also important to final casting. Usually the casting channel sprue is fixed on the non-visible surface of the component. For some simple and symmetrical parts, the casting channel sprue mutt be set in the central zone of the part, so that the liquid resin runs equally through the mold when injected, and bubbles can be easily controlled. Meanwhile, other factors must be taken into account, for instance, the treatment of the injection part, the appearance and assembly, the mold structure, etc. In case of very small or thin parts, the casting channel sprue shall be set on the edge or the side face.

Mold structure build up: A wooden or aluminum box is made according to the size of the master pattern. Next, the master pattern to which the filling pipeline has previously been attached is hung, leaving a minimum distance of 25 mm between the pattern model and the mold walls. Once the entire structure is assembled, it must be checked whether it is stable or it leaks, and then put the box in a flat place and pour the silicone into it.
Liquid silicone pouring: The necessary silicone weight is calculated, poured into a bucket and the catalyst is added according to the proportion in the silicone. The mixture is placed in the vacuum machine and slowly poured into the box containing the master pattern. It is important that the silicone does not touch the piece before the level of it reaches its bottom, because it may break the mold structure.
Then, the box is placed in the vacuum machine to remove the bubbles that the silicone may have, this is important especially in parts with a large surface with no holes through it. When the suction finished, the box is taken to an oven to be cured at about 40° degrees for about 8-12 hours, depending on the size of the master pattern.
Mold separation: The curing mold is taken and cut away from the master pattern using a blade and following a saw-tooth design to facilitate the subsequent joining of the two parts thereof, avoiding deflection. Once the mold is cut away, the pattern is removed. Sometimes it may be necessary to make some drains so that the air bubbles can escape when in the vacuum casting machine. Finally the mold is placed in the curing oven and preheated to about 70 °C.

Vacuum models: The preheated mold is removed from the oven, cleaned with an air spray gun and sprayed with a release agent to make it easier to separate the parts once injected.
Usually, if the resin injected erodes the mold, or if the structure of the part is simple and easy to demold, it is not necessary to use a demolding agent. As far as possible, it is advisable to avoid the use of these agents, on the one hand, because they are difficult to clean and, on the other hand, because they leave marks in molds made for high gloss parts. Then, the mold is assembled; the resin is weighed, and placed in the vacuum casting machine for 5 to 10 minutes. The machine is then operated so that the two resin components are mixed for approximately 1 minute, and then poured into the mold until it is filled. The timing and rhythm of degassing is critically related to the generation and elimination of bubbles. After the casting phase, the filled mold is placed in the oven to cure the resin below 70ºC.
Curing: Curing time is different depending on the material and normally it takes between 40 and 90 minutes. After curing, the mold is removed, opened and the injection piece ejected, and then the hole and the casting channel are cleaned.

PS: A silicone mold can be used up to 25 times, but the best results are guaranteed with 15-20 uses. The thickness of the duplicated part must be greater than 2.5 mm.

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