How Accurate Is Vacuum Casting

The use of vacuum casting in producing high-quality components is nothing new. From automotive dashboards and aerospace parts to medical devices and consumer goods, the applications of vacuum casting are endless. This technique is the go-to casting operation because it offers design flexibility and gets your products to market on time. 

However, how accurate is vacuum casting? While vacuum casting produces accurate and repeatable components, the level of accuracy depends on many factors. This article will explain how vacuum casting works, the factors that affect the accuracy of vacuum casting, and tips to get the best results. 

What is Vacuum Casting and How it Works

Vacuum casting, also known as urethane casting, is a manufacturing technique that creates high-quality plastic and metal components. It involves casting molten material in a mold under a vacuum to produce detailed parts. The vacuum casting process begins with the creation of the master model. This model represents what you want the final part to look like. 

After that, you build the silicone mold from the master model. After obtaining the mold, place it in a vacuum chamber free from air. Prepare the casting material and pour it inside the silicon mold under vacuum. The final step in the vacuum casting process involves curing the part in an oven at a predetermined temperature and time. Vacuum casting is popular because one can produce detailed parts with precision. Additionally, it has low tooling costs and short lead time. 

Key Factors that Affect Accuracy of Vacuum Casting

Vacuum casting is an accurate process as it produces parts exactly to specifications. The casted components are dimensionally accurate and have consistent quality. However, some factors affect the level of precision of vacuum casting. Some of these factors include; 

1. Quality of the Master Model

The quality of the master model has a direct connection with the accuracy of the final part. There are different techniques one can use to create the master model. It is important to use a process that produces accurate structures. These include CNC machining and 3D printing. This is because any inaccuracies will get transferred to the casted part. The master model material is also important. It must remain stable and be able to withstand the curing process. 

2. Mold Quality

The quality of the silicone mold is key in vacuum casting. The part takes after the mold. For this reason, you must create the silicone mold with precaution. When carrying out this casting process, it is important to keep in mind the shrinkage that will occur during curing. It would be best if you minimized the shrinkage to increase the accuracy of the components. Furthermore, there is a limit to the number of times you can reuse a silicone mold. This is because these molds degrade with time. 

3. Material Shrinkage

In vacuum casting, material shrinkage is inevitable. The part shrinks at a rate between 0.5 to 1%. While this may seem negligible, it matters in high-tolerance applications. Some factors can potentiate material shrinkage. This includes curing too fast or at a very high temperature, which leads to stress build-up and warping. The casting material you use and part design also matter. Thin-walled parts made with thermoplastics will likely shrink. 

4. Casting Parameters

The casting parameters and conditions will also determine the accuracy of the final part. The vacuum pressure must be sufficient to remove all air and bubbles. This will ensure that the molten material fills the silicone mold. The way you mix the resin is also important. They must be properly mixed and degassed to ensure uniform curing. Furthermore, pouring the resin too fast will lead to turbulence and inaccuracies. 

5. Part Complexity

Another factor that affects accuracy in vacuum casting is the geometric intricacies of the part. Generally, the more features on a component, the lesser the accuracy. The reasons behind this are not far-fetched. Highly detailed features are more labor-intensive and harder to cast. You must properly optimize the mold design, which can introduce errors. Furthermore, it becomes more challenging with parts with internal features like channels and cavities. The accuracy of these features is easily affected by trapped air. 

6. Part Size

The size of the part influences how smoothly the casting operation would go. Extremely small and large parts are more challenging to handle. Small parts usually have thin walls and intricate features. This increases the tendency of dimensional inaccuracy. Additionally, the molds used for small parts tend to wear out faster, which decreases the level of precision. Big and large parts also come with challenges. They are more difficult to work on. There can be cases of uneven curing because of the large mold and casting material used.

7. Operator Expertise 

Your skill and expertise in vacuum casting will determine the accuracy level of the parts produced. Even with everything set up correctly, it is useless if the operator’s experience is not top notch. A skilled operator will handle the material properly and determine if additives and pigments should be added. A reliable operator will prepare the mold and adequately manage the vacuum cycle. All these will lead to accurate and consistent results. 

Best Practices for Vacuum Casting

When producing a part using vacuum casting, you should ensure quality control at every stage. The following are some vacuum casting tips and best practices to get accurate and precise components. 

1. Prioritize Design for Manufacturability (DFM) Analysis

After creating a digital design for the final part, you should carry out a DFM analysis. This will help simplify the features and make them more castable. Optimizing the part’s design entails ensuring uniform wall thickness, avoiding deep cuts and getting rid of complex overhangs. DFM analysis in vacuum casting also involves incorporating draft angles and venting channels for easy degassing.

2. Properly Prepare the Mold

Proper mold preparation makes all the difference in vacuum casting. It would help if you designed the molds with parting lines to ensure resin flow and prevent misalignment. Furthermore, you should apply the right mold release agents during vacuum casting. This will prevent damage and ensure it accurately produces parts when used multiple times. 

3. Material Selection and Handling

The casting material you use is also important. Carefully select the resin material you are using. Ensure it has the desired mechanical strength to handle the rigorous process of vacuum casting. The resin should also come with a low shrinkage rate. If the material that is the best for your project lacks some important properties, consider using additives. These additives, such as glass and carbon fibers, improve the appearance and performance of the casted part. 

4. Optimize the Vacuum Casting Conditions 

Another way to get accurate and defect-free parts is to optimize the vacuum casting process. You should use the right vacuum pressure and timing to ensure all the trapped air is removed from the mold. If possible, preheat the silicone mold before introducing the molten material to prevent thermal shock and damage. Additionally, properly cure the resin to make sure it solidifies with minimal shrinkage. 

Conclusion

Vacuum casting is a popular manufacturing technique. It produces accurate and precise components. However, some factors directly influence the level of accuracy you get. These include the quality of the master model, silicone mold, and material shrinkage rate. The part’s size, casting conditions, and skill level are also important factors. To get the best results, analyze the designs and optimize them to ease casting. Additionally, you should properly prepare the mold and select the right material to produce parts without defects. 

Nieka Ranises