Posted on Oct. 8th, 2018 | By Mandy, Project Manager
The main use of vacuum casting is in the production of prototypes where design requirement are complex and air entrapments is a major concern. This casting technique involves the use of vacuum to push liquid onto flexible molds. It makes the process possible by two-component polyurethanes and silicone molds; they also use vacuum casting for fast production of high standard prototypes or any other product for end-use.
The History of Vacuum Casting
Vacuum casting technique using silicone molds was first discovered in Europe, around 1960s in the technical universities of Dresden and Cottbus in the former German Democratic Republic. This technique involved the combination of conservative and well-recognized technologies. Flexible molds like rubber were also in use by restorers and sculptors to make sculptures without damaging them.
The Japanese later bought the technique in around the 1970s where its major application was on industrial manufacturing of automotive parts. Several years later, the technology was reintroduced in Europe and it uses spread all over the continent. Now in most European states, the technology is wildly used for research and development of the industrial capability of delivering on mass production and good quality.
In 1980, came the introduction of thermosetting plastics to the vacuum casting process. These plastics could perfectly mimic the properties and present the use of materials in mass production. Since then, more improvement has been achieved and the modern use of CAD have permitted vacuum casting to be done faster, cheaply and more accurately.
The Process of Vacuum Casting
Vacuum casting is a dependable and flexible process, capable of accommodating the development of complex designs. It's frequently used by companies for producing prototypes and other end-user product. For an excellent outcome in the casting process, it requires a high-quality master prototypical pattern.
Casting models are mostly created through stereolithographic, a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a layer-by-layer fashion using photopolymerization. It’s important for the master model to be of high quality and perfectly designed, it will only transfer any flaws in the model to the final product or prototype. Using of high-end urethane casting service makes sure the model master in the picture is as it is in the first step towards a fine final product.
Once the model is well designed and got, it's inserted inside a two-part silicone mold and subjected to some very high temperatures. It cures after which the model and silicone mold is opened in a symmetric way. It creates a hallow area within the model exactly like the master model. The mold is then placed well inside a vacuum chamber hence the name vacuum casting.
While inside the vacuum chamber, they fill the required amount of material into the mold. The commonly used material for vacuum casting is urethane. Urethane casting is perfect for replicating design models, matching the needed characteristics for industrial grade material like glass- filled nylon, polycarbonate, and high-density polyethylene.
Upon filling the mold with urethane, so-called urethane molding, it’s sealed. This is done using a high- intensity vacuum, to ensure they leave no air molecule inside. After the casting process, they open the mold, they can paint the prototype or produce as per the recommendation.
How to Make Air Free Silicone Mold
The biggest challenge facing silicone mold maker‘s is not the air bubbles in the silicone rubber but rather the air bubble form at the surface of the casting that makes the mold unusable. There are many methods that can ensure air-free silicone mold.
1. Exceptional Pouring Methods
The surface where silicone mold and casting materials meet is vital. Therefore, special care is needed to ensure no bubbles form while pouring the silicone rubber into the mold and model. It is wise to pour the rubber in small quantity in the mold box and tilt the pouring angle to make sure the rubber flows to the whole surface. This way, they wet the surface of the model with a fine layer of rubber and pups out any air bubble with a paintbrush.
2. Apply Compressed Air to the Silicone Molds
An air compressor is a wonderful way of popping out any air bubbles. After pouring the first round of rubber into the mold box, use the air compressor to blow the material over the model to wet the surface, the first layer of the film should be bubble free.
3. Use of Vacuum to remove Air from Silicone Molds
Bubbles come because of the air getting trapped during the mixing process. Stirring the mixture in slow steady motion will help but traces of air will still be in the mix. However, they will be removed with the help of the vacuum chamber.
4. Use of Pressure to Force Air into the Solution
This technique works on the principle of forcing air into a pressure vessel with the molding rubber mixture, the air pressure presses on the material squeezing out air bubbles. The procedure is easy, get a pressure vessel that fits a mold box, check if the setting time of the rubber mold is set and know how long it will take.
Pour the mixture into the mold box inside the model, seal the pressure vessel and set it to 60- 100psi. let the mold making material settle whereas the vessel is under pressure, this technique works really well in a rubber mold making.
5. How to Make a Perfect Master Pattern or Design
There two methods of getting a perfect model or design for your vacuum casting. One way to consider is CNC technology. This can provide parts of the highest accuracy, placing tight tolerance such as +/-0.05mm. But it takes longer to process, unlike SLA. The other way of getting a master pattern is the SLA. It’s faster particularly on complex design or patterns, it’s also easily combined in all internal 3D CAD input features.
6. Coloring Cast Parts of the Vacuum Cast
Vacuum casting saves time that would be used in post-processing, this is achieved by offering a range of color on different prototypes to a client color demand. For instance, it can alter silicone to produce parts with colors like red, yellow and blue.
However, it’s good to note that the colorful cast may not resemble the international Pantone and RAL color in case a customer requires a specific one. The best way is to paint the finished product manually.
Silicone Mold vs. Injection Mold
1. Financing Cost
Silicone is cheaper in terms of cost compared to the injection mold. The reason for this difference in cost, is the nature of injection mold, being built on steel or aluminum parts. The cost if silicone mold can be from hundreds to thousands while the injection one will range from thousands to tens of thousands.
2. Tooling Lead Time
Vacuum casting has a shorter lead time than the injection molding machine. A conventional injection mold will take a 4–8 weeks, to be manufactured. Unlike the silicone mold that would only need a 1-2 week to be fully complete and all the complex parts finished in the short time.
3. Types of Material Usage
The material used on both molds range from opaque to transparent; the durometers are an array, from 30A-90D to UL 94-VO. Below is a list of other material types that are used.
This type has similar properties to engineering production of plastics ABS. Also has a well balanced physical property like superior dimensional strength, its beige or black color, flexural modulus 1830 MPA, and hardness shore D 84.
This material type has a high-temperature resistance, a good impact, and flexural resistance. They require these features in the application of thermoplastic.
Has clear optical properties, similarly to acrylic of engineering production. Such as high transparency, good UV resistance, high reproduction accuracy, and easy polishing.
Just like engineering production of PC, its properties are optically clear, good flexural impact resistance, and good mechanical properties for easy coloring
Pros and Cons of Vacuum Casting over CNC Machine
Vacuum casting produces all complex parts or design in one piece while the CNC machine will need to split the production in sizes and structure.
Silicone rubber shrinks after curing which is normal for its nature. This affects the molecule density of the silicone liquid, once they cure the silicone rubber. The product shrinks as the silicone rubber does. The shrink rate of high-quality silicone rubber is 1%-3%, this phenomenal affects the tolerance making the CNC technology a better option.
Advantages and Disadvantages of Vacuum Casting over Injection Mold
- Can create big parts of up to 48” x 96”
- Takes less time for production to be complete.
- Cost of the mold is relative cheaper since they are built on MDF, high-density foam, and epoxy
- The casting process is efficient for pattern or design that requires a lot of repetition.
- They can add Additional details since the defects or dints on the silicone mold can be transferred to prototype or product.
- It’s hard to achieve a consistent thickness on the prototype because some sides of the silicon mold are deep and are a problem to reach.
- The cost per piece of the vacuum foaming is non-competitive compared to other automated processes where the quantity of production is more.
- Finishing cost of a prototype may be costly and labor intensive since they can use only one material at a time.
- Higher production rates hence more output.
- They can add Insert and filler in the mold for additional strength.
- Has close tolerance on small intricate parts.
- Uses different material at the same time using Co-injection molding.
- Low past production work since the ejected parts are usually in a finished state.
- Minimal waste since the scrap from the product can be collected and reused.
- The machine can run on full automation, lowering the cost per part, unlike the vacuum process.
- Financial cost for a startup is high.
- Requires a great deal of technical know-how and engineering time.
- Take a long time to fabricate tooling, making the time taken for the parts to reach the market, become a major setback.
Application of Vacuum Casting Technique
Vacuum casting is the preferred method in the modern production of prototypes in most manufacturing industries. Here are some markets that use technology is widely used.
- Production of prosthetic and medical devices.
- Automotive parts industries.
- Aerospace manufacturing.
- In function testing.
- Home decoration equipment.
- Rapid plastic prototyping.
The machines that are used in the vacuum casting process, do all rapid duplication of plastics parts using the silicone mold. These machines have a vacuum chamber that helps in sucking out air traces inside the mold. The depressurizing chamber creates a vacuum cavity necessary to avoid any air bubbles interfering with the production of prototypes or product.
In most modern vacuum casting, they control the duplication process using an industrial grade CAD software.