Vacuum casting is a crucial process in the production of plastic parts. Due to its effectiveness and speed, it is a useful technique for producing small quantities of polyurethane parts. This method is very useful for producing product prototypes as it allows manufacturers to assess the market before starting mass production.
The purpose of this article is to answer the question: What is vacuum casting? We will explore all aspects of the urethane and silicone casting process, covering its application, the materials used, and the benefits and limitations.
What is vacuum casting or urethane casting?
Vacuum casting or urethane casting is a process in which urethane resin is poured into a space between two silicone mold halves. Using a vacuum chamber to extract air from the mold during the vacuum casting technique helps ensure the object takes the correct shape.
This vacuum casting technique is often used to produce rubber and plastic items. Because urethane molding can be faster and more cost-effective than other techniques such as injection molding, it is often used in prototyping projects or small-batch production. The process produces high-quality, bubble-free castings with an impeccable surface and smooth texture because it is carried out under vacuum.
The main advantage of vacuum casting is therefore its high precision and repeatability, making it an ideal option for low-volume plastic parts that require precise dimensions. It also allows the creation of more complex designs. However, it is important to note that vacuum casting is not suitable for all applications, especially for heat and pressure sensitive materials.
Vacuum casting process: step by step
The vacuum casting operation requires a mold with a cavity in the shape of the part, just like traditional injection molding. Here we explain the vacuum casting process step by step.
Step 1: Create a 3D model of the part
Firstly, you need to create a high-quality master model with excellent dimensional accuracy. To obtain the proper structure and surface quality, this master model is usually created through 3D printing or CNC machining. Final vacuum casting keeps every little feature exactly as it is. Therefore, producing a perfect master model is crucial.
Step 2: Create the silicone mold
The next step is to build the silicone mold using the master plan created in Step 1. How to build the silicone mold:
- Hang the master model in the casting box and equip it with casting cores, inserts and openings. Also attach risers to the component to allow air to escape from the silicone mold.
- Then pump silicone under vacuum into the casting box and around the master model, capturing every detail.
- Let the silicone cure in the oven at 40°C for 8-16 hours. The exact time depends on the size of the silicone mold.
- Once the silicone rubber is cured and dry, remove the case and risers. Then, carefully split the mold with a knife to expose the negative mold cavity of the part.
To ensure a defect-free and non-sticky mold surface, use the correct release agent.
Step 3: Mix the polyurethane resin
Before using polyurethane resins for vacuum casting, you must first heat them to a temperature of 40°C before mixing. Then mix a two-component casting resin, add the desired color and pour the mixture into the bowl of the machine. Then place the pouring openings into the mixing and pouring bowls and reinsert the mold.
Mix casting resins and color pigments and allow to ventilate under vacuum for 50-60 seconds during the automatic pouring process. To avoid voids and air pockets, pour the resin into the mold under vacuum. The vacuum ensures shapeless bubbles and prevents flow resistance due to air pockets in the tool. Gravity feeding is ideal for filling the mold.
Step 4: Remove the casting from the mold
After pouring the resin, the final steps include oven curing to improve the item's material properties, trimming the edges of the flash material, and another polishing cycle with different sandpaper as needed.
As the number of castings increases, the silicone mold will gradually lose its shape if these limits are exceeded, which leads to limitations in dimensional accuracy.
Guidelines for Effective Design in Vacuum Casting
These guidelines will help you make significant changes to features during the design phase and create accurate vacuum-cast products.
Wall thickness
Vacuum casting allows us to process different wall thicknesses in a single part. To ensure structural integrity and achieve optimal results, it is best to keep the wall thickness constant. We recommend a thickness of 1.5mm.
Tolerances
Tolerance design ensures your parts fit together and meet required standards. There is a 0.15% shrinkage when casting in a silicone mold. Therefore, it is important to take this shrinkage into account when manufacturing the parts.
Ribs
Particularly for large flat sections, the addition of ribs increases strength and reduces deformation. Also note that the ribs must be thinner than the adjacent walls so that they do not shrink or sink. The rib thickness should ideally be at least 60% of the nominal wall thickness.
Bosses
Projections are often used in the construction of cabinets or panels. They increase strength at the attachment points. They must be at least 1 mm in height and diameter. It is also important to ensure that the wall thickness does not exceed 60% of the nominal thickness of the part to reduce the risk of sinking.
Undercuts
Unlike processes that require hard tools, undercuts in vacuum casting do not require inserts and can be designed freely.
Embossed details
To ensure clarity and visibility, embossed or recessed texts and logos must have a minimum depth or height of 1 mm and a minimum width of 1 mm. To improve readability, you should also maintain 1 mm spacing between characters.
Various materials for silicone molding and vacuum casting
Silicone molding and vacuum casting use different materials, each selected based on the properties required for the final product.
Silicone Mold Materials
Foundry silicones are divided into two categories: tin and platinum hardening silicones. Tin hardening silicones, also called condensation hardening silicones, have a shrinkage of 1 to 4 percent. Although model manufacturers make every effort to consider shrinkage rate, for high tolerance applications it is preferable to use higher quality platinum cure silicone or high temperature vulcanization (HTV) rubber.
Additionally, a Shore hardness of 40-50 ensures a durable, easy-to-demould shape, even for parts that would get stuck in a rigid mold due to undercutting. The use of translucent silicone is also recommended as it will reveal air pockets, bubbles, contaminants and other internal elements of the mold.
Vacuum Casting Materials
There are numerous vacuum casting materials, each with a unique combination of rigidity, elasticity, toughness and flexibility.
- General purpose resin similar to ABS;
- Nylon-like substance with glass filling for greater rigidity;
- TPE elastomeric material for use in accessories, buttons, handles, gaskets and seals;
- polycarbonate-like materials;
- Clear or translucent cast urethane (similar to PMMA);
- Heat resistant materials similar to PU or POM.
Additionally, some unusual urethane casting materials are also available, including food-grade, conductive, low-density, and fire-retardant alternatives, as well as shell, wax, epoxy, ceramic, and composite materials.
Surface Processing Options for Vacuum Casting
The molding process offers a variety of surface finish options, including
- Vacuum metallization
- Steam polishing
- Screen printing/pad printing
- painting
Vacuum casting applications in various industries
Vacuum casting technology is adaptable and has countless uses across a variety of industries. This method is an ideal option for high-quality results as it allows the manufacturing of complex and precise components. Here are vacuum casting applications used in various industries.
Auto Industry
The automotive industry benefits greatly from vacuum casting. Ensures complete isolation and protection of car components such as coils, sensors, rotors and stators. It is also often used for small quantity production and prototypes. Vacuum cast elements such as engine covers, panels and door handles.
Medical industry
The medical industry demands high-quality mass production and rapid prototyping, and vacuum casting provides both. Dental implants, medical device prototypes and custom dentures can be manufactured by vacuum casting, ensuring that dentures are produced with exact weight, size and symmetrical proportions.
aerospace industry
This technology can be used to produce precision parts for aircraft. Vacuum casting is often used to produce precise details and achieve greater precision in components such as fuel systems, air ducts, and even some aircraft exterior elements.
Consumer goods industry
The consumer goods industry uses vacuum casting to produce limited quantities of everyday items such as packaging, accessories, household appliances, baby toys and electronics. Although the final products of these items are manufactured by injection molding, vacuum casting is suitable for creating product prototypes for inspection and testing.
Important technical specifications for vacuum casting
- Material Selection: A wide variety of materials are available, including rubber, resin and plastic;
- Maximum part dimension: The dimensions of the vacuum chamber limit the size of the mold;
- Minimum wall thickness: The mold wall thickness must be at least 0.75 mm to ensure correct filling. However, the recommended wall thickness of 1.5 mm provides optimal performance;
- Surface structure: Glossy, matte, structured and polished surfaces
Advantages and limitations of the vacuum casting process
Examine these process pros and cons to determine if the vacuum casting process is suitable for your intended applications:
Benefits
- An advantage of the vacuum casting process is the ability to produce parts with intricate details. The 3D printed or CNC milled master model is used to create the silicone mold and then mirrored into the finished casting.
- Parts manufactured by vacuum casting have exceptional dimensional precision. Components manufactured using the vacuum casting process fit together perfectly.
- The basic process equipment, silicone molds, are comparatively cheaper to produce than metal molds. While silicone molds can be made for a few hundred dollars, injection molding can cost thousands of dollars.
- With vacuum casting, the delivery time is very short as the process takes less than 15 days from start to finish.
restrictions
- The silicone molding process works best with polyurethane resins and elastomers, but is not suitable for molding hard industrial materials such as metals. There is a similar special process for metal casting called vacuum casting or pressure casting.
- These silicone molds don't last long. You can produce twenty to twenty-five high-quality parts before the level of complexity and precision decreases.
- Because the surface finish depends on the post-processing of the part, it is sometimes limited to the exterior because certain internal features of the pattern are difficult to access.
Comparison of vacuum casting with other manufacturing processes
Not sure which plastic manufacturing process to use? Now let's compare vacuum casting with injection molding and 3D printing to help you make the right production decision.
Vacuum Casting vs. Vacuum Casting injection molding
Comparing vacuum casting and injection molding, injection molding produces objects by applying strong pressure to molten material in a metal mold. Although it takes longer to produce than the silicone molds used in vacuum casting, it is perfect for high-volume production.
Still, the initial cost of the tools is much higher. In contrast, vacuum casting uses silicone molds. Since there is less money involved in making molds, this provides flexibility for design changes.
Vacuum Casting vs. Vacuum Casting 3D printing
3D printing is an additive manufacturing technique that creates objects layer by layer, using a digital file as a starting point. This means that any intricate and complex geometry shapes that can be created digitally are supported. Although parts can be created relatively quickly and with great customization, creating many duplicates takes time, increasing the cost of the 3D printing service.
On the other hand, vacuum casting takes a little longer because it requires a high-quality master model. However, once a mold is created, it is faster to create multiple copies of a part with consistent surface finish and material quality.
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Concluding
An economical way to produce complex rubber and plastic parts in small quantities is vacuum casting. When testing a product during development, product teams often use vacuum-molded components. Silicone vacuum casting is a cost-effective manufacturing solution with excellent performance and offers a wide range of applications.
Common questions
How much does vacuum casting cost?
For producing small quantities (between 10 and 50), vacuum casting is an economical technique. Typically, you should budget between $200 and $1,000 for the mold, with individual units costing around $10 to $100. However, costs can vary significantly depending on the type, scope, and complexity of your project.
Can vacuum casting be used for large-scale production?
Due to the manual effort required and slower production rates compared to other technologies such as injection molding, vacuum casting is less economical for large-scale production, although it is very efficient for small to medium batch production. .
Is vacuum casting suitable for complex projects?
Yes, vacuum casting is great for complicated shapes. Complex geometries and intricate features that may be difficult or expensive to produce using traditional manufacturing techniques can be replicated using silicone molds.
