The technology of Vacuum Casting or Silicone moulding has been known for a long time and very interestingly becomes a part of the prototype industry when we need to make short series of plastic pieces, mostly from 15 to 200 units.
What is Vacuum Casting / Silicone Moulding and How does it work?
Vacuum casting is specially used for small production series or to duplicate prototypes and models in a short space of time. Starting from a master part made by 3D printing, a silicone mould is crafted which later gets injected with a polyurethane into a vacuum chamber. These silicone moulds faithfully reproduce a series of prototypes with all the minor details of the master prototype. It is essential to have a prototype made in 3D technology with a good surface finish. Hence, SLA 3d printing technology is used for making the master part.
The plastics cast in the silicone mould are polyurethanes with very different characteristics depending on the application (resistant to Tª, elastic, different hardnesses, colours, transparent, and more). Thanks to the possibility of casting 12-15 pieces in each cavity, the cost of making a short prototype series is significantly less than making the parts via high precision 3d printing technologies like SLA or SLS.
Unlike 3d printing, this technology allows to create short run production in wide range of materials to give functional characteristics similar to PMMA, PC or ABS, Nylon, food grade material, rubber or gummy materials such as Polyurethane (PU) among others. You can also provide colour to the injected parts or even make them transparent and the tolerance can be controlled up to 100 mm / ± 0.20 mm.
The process of manufacturing the silicone mould by Vacuum Casting
Before making silicone moulds, you need a prototype produced by SLA which can be used as a master mould. Then you have to make the box in which the silicone will be poured to make the mould:
#1 Confirmation of the face of separation: There are three key points to take into account in the manufacturing process and demoulding, which are the visual and hidden surface, the mounting surface and the complexity of the moulding. Other factors should be taken into consideration, for example, the control of bubbles during injection, the size and shape of the silicone mould, the amount of silicone to be used, the shape of the masterpiece, and more. It is usually easy to control the separation surface, as for example in some covers, drawers-structures. However, certain other pieces are difficult to separate.
#2 Adjusting the separation line: The adjustment of the separation line means cutting the silicone mould to the edge of the masterpiece so that it is easy to separate. If the pieces have holes, small holes must be left so that the air bubbles can leave the box when empty.
#3 Sprue adjustment: An essential step for the final casting. Generally, the sprue column is fixed on the non-visible surface of the part. For some simple and/or symmetrical pieces, the sprue column should fit in the central part of the piece, so that the liquid resin will run equally in the mould when injected, and the bubbles can be easily controlled. Meanwhile, other factors must be controlled, for example, the treatment of the injection piece, the appearance and assembly, the construction of moulds, and more. For some very small or thin components, the sprue column should be set on the edge or the side face.
#4 Construction of the mould structure: A wooden or aluminium box is made according to the dimension of the masterpiece. Then the pattern piece to which the filling column has previously been stuck is hung, leaving a distance of at least 25 mm between the pattern piece and the walls of the mould. Once the whole structure is assembled, it is necessary to check if it is stable or if it has losses, then put the box in a flat place and pour the silicone into it.
#5 Liquid silicone poured: Calculate the necessary weight of silicone, pour it into a bucket and add the catalyst according to the proportion in the silicone. The mixture is placed in the vacuum machine and slowly poured into the box containing the masterpiece. It is essential that the silicone does not touch the piece before the level of it reaches its bottom because it can break the structure of the mould.
Then the box is placed in the vacuum machine to remove any bubbles that may have silicone; this is especially important in parts that have a large surface and has no holes through it. At the end of the aspiration, the box is taken to an oven to cure at about 40ºC for about 8-12 hours, depending on the size of the masterpiece.
#6 Separate the mould: The curing mould is taken and with a blade the saw-shaped mould is cut to facilitate the later union of the two parts thereof, avoiding the deflection. Once cut, the pattern is removed. Sometimes it is necessary to make some drains so that the air bubbles can come out when doing the vacuum. Finally, put the mould in the oven and preheat to about 70ºC.
#7 Vacuum models: The pre-heated mould is removed from the oven, cleaned with an air gun and sprayed with a release agent to make it easier to separate the pieces once injected.
In general, if the resin when injected erodes the mould, or if the structure of the piece is simple and easy to demould it is not necessary to use the release agent. As far as possible it is convenient to avoid the use of these agents, on the one hand, because they are difficult to clean and on the other hand because in moulds made for parts with a lot of shine leaves marks. Then, the mould is assembled, the resin is weighed, and placed in the vacuum casting machine for 5 to 10 minutes.
Next, the machine is operated so that the two components of the resin are mixed for about 1 minute and then cast into the mould until the cavity thereof is filled. The timing and rhythm of degassing have a critical relationship with the generation and disappearance of bubbles. In the end, put the mould that has been filled in the oven to solidify the resin below 70º C.
#8 Solidification: The curing time is different depending on the material; normally, it needs between 40 and 90 minutes. After curing, the mould is removed, the injection piece is opened and removed, and then the gap and the pouring channel are cleaned.
Application of Vacuum Casting / Silicone Moulding
Vacuum Casting is generally used in industries covering automotive, defence, sporting goods, medical devices and consumer products.
Vacuum casting is best suited for the pre-launch or testing phase of your product as it saves both time and cost implementation. Also, if you are looking for concept models or prototypes for certification or aesthetics validation, vacuum casting is the best-suited procedure for you.
Advantages of Vacuum Casting
#1 Unlike other 3D printing techniques vacuum casting, makes it possible to obtain high precision and finely detailed parts, 100% faithful to the original prototype. Even in the scenario of some of the most complicated geometries, vacuum casting produces high-quality results, with accuracy comparable to the final product.
#2 With the use of resins, vacuum casting allows you to select the materials to be used in production, with the right choice of hardness. It also gives you the option to select among prototype with crystal type transparency or painted finishes.
#3 The use of silicone moulding method or silicone moulds reduces the overall production cost as compared to the steel or aluminium moulds. Additionally, you can reach the highest scale of economies with the mass production of around 12 parts in one go. Hence, it is more valuable than any 3D printing methods.
Limitations of Vacuum Casting
#1 The silicone moulds used to create prototypes with vacuum casting technique has a shorter lifespan as compared to injection moulds
#2 Due to the fact that this method requires an original model to create its silicone mould, an initial investment is needed, in contrast to any 3D printing methods. However, this investment eventually becomes a benefit thanks to the economies of scale of production.