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Vacuum Casting Design Guide: Practical Tips for Success

vacuum casting design guide

Published:  March 1, 2024         Reading Time:  About 3 minutes

Vacuum casting is an efficient manufacturing method that creates low-volume parts with qualities close to injection molding. This technology delivers high-quality and detailed prototypes and creates a small batch of products within a short lead time. Applying vacuum casting ensures parts have excellent mechanical properties and diverse end-use applications.

Understanding the vacuum casting design guide is key to the success of your project. To fully take advantage of this excellent technique, your 3D-printed design must follow expert geometric recommendations. This is why you must thoughtfully approach the design process to prevent errors and ensure optimal quality.

This guide is a complete overview of the considerations for the vacuum casting process. We will show you essential vacuum casting guidelines for project success, from basics to advanced practices. But before then, let’s briefly discuss the process itself.

What Is Vacuum Casting?

Vacuum casting, Urethane casting, or Polyurethane casting uses silicone molds to produce plastic and rubber components. This technique is conducted under a vacuum. It replicates injection molding to create intricate parts in polyurethane resins and cast nylon.

Part and silicone mold after Vacuum Casting of ABS like resins

In vacuum casting, liquid materials are drawn into a mold under vacuum pressure, yielding detailed components quickly. It starts with crafting a precise master model, often through 3D printing or a CNC machine. The model is then encased in silicone to form a mold placed in a vacuum chamber.

After the silicone sets, the master model is removed, leaving a pristine silicone mold. This mold is used in a vacuum casting machine, where the casting material fills it, capturing all design intricacies. After curing, the final part is extracted, and the mold can be reused for around 30 parts, making it suitable for small to medium production runs.

Its vacuum environment ensures high-quality casting without bubbles or surface imperfections. This process is widely used in rapid prototyping for pre-series testing and small-batch production.

Design Considerations for Vacuum Casting

As we mentioned earlier, following the vacuum casting design guide is essential for the success of the process. This section will show you the design basics for vacuum casting and expert tips for the best results.

Draft Angle

In vacuum forming, the draft angle is crucial in facilitating the easy removal of the plastic part from the mold without distortion or breakage. Without it, the plastic might adhere to the mold, making removal difficult or impossible. The product could even deform or break during extraction.

vacuum casting design guide draft angle

Ideally, you should design your part with a draft angle of at least 1 to 2 degrees. However, the draft angle’s size varies depending on the material, product shape and complexity, and mold size. Materials prone to deformation or products with intricate shapes may require a larger draft angle. On the other hand, simpler shapes or more rigid materials may suffice with a smaller angle. Designing the product with an appropriate draft angle is essential to ensure effortless removal from the mold without compromising the final product’s integrity.


A practical tip is maintaining standard tolerances according to ISO 2768 Coarse (C) for vacuum casting. If specific features are not required, you apply standard tolerances. While tighter tolerances are possible, we recommend them only for critical features. Tolerance is either +/- 0.5mm or +/- 0.1mm per 30mm, whichever is larger.

Wall Thickness

Vacuum casting allows for designing thin and thick wall sections within the same part without affecting the final component, offering greater design freedom. The walls of the vacuum-formed product should strike a balance; they should be thick enough for strength and durability yet not so thick that they cool slowly and become brittle. Wall thickness significantly impacts the strength and durability of the final product.

vacuum casting design guide wall thickness

Thicker walls are generally stronger and more resistant to deformation but may take longer to heat and form, requiring larger and costlier molds. Conversely, thinner walls are easier to heat and form, requiring smaller and less expensive molds, though they may be more prone to deformation and less robust.

Typically, we advise a minimum wall thickness ranging from 1mm to 1.5mm for small to medium-sized parts, adjusting for larger parts as necessary. However, the ideal thickness can vary based on the requirements of your project and the chosen material.

Incorporating Undercuts and Overhangs

Undercuts and overhangs are design elements that are either recessed or protruded over a space, complicating the molding and casting processes. However, vacuum casting can effectively manage these challenging features compared to other production techniques.

vacuum casting design undercut

When planning to incorporate undercuts or overhangs in vacuum casting designs, it’s essential to recognize their influence on mold complexity, potentially necessitating extra adjustments or considerations. Consulting with a manufacturing expert is advisable to guarantee that these intricate details are executed accurately and efficiently. At RapidDirect, we are equipped and eager to assist in this process.


Bosses must be at least 1mm tall and have a minimum diameter of 1mm. The wall thickness for bosses should not exceed 60 percent of the nominal thickness to reduce sinking. Bosses should be at least 1mm tall and wide. The bottom radius should not exceed 25% of the wall thickness, and the boss walls should be less than or equal to 60% to avoid shrinkage.


vacuum casting design ribs

Adding ribs to large, flat areas enhances strength and minimizes warping. Rib thickness should be less than the wall thickness to reduce shrinkage and sinking effects. Our experts advise keeping rib thickness below 60 percent of the nominal wall thickness and limiting rib height to less than three times its thickness.

Embossed Details

Design text and logos that are either recessed or embossed with a minimum depth/height of 1mm and a width of 1mm to ensure visibility. Leave a 1mm gap between letters for optimal results. Include draft angles on the embossed details to facilitate easy release from the mold. Besides, avoid complex undercuts that can complicate the demolding process.

Materials for Vacuum Casting

plastic vacuum casted parts

Matching the desired properties of a preferred material is often challenging when designing for vacuum casting. However, the versatility of this manufacturing process allows for the use of a wide variety of materials, expanding the selection options available. The casting materials replicate the dimensions and specifications provided by the engineering team, particularly concerning thermal and physical properties. Common materials utilized in this casting technique include rubbers, resins, and thermoplastics.

ABS-like Resins

ABS-like resins replicate the properties of ABS plastic, providing a combination of strength and flexibility ideal for various applications. With excellent surface detail resembling injection-molded parts, ABS-like resins come in flame-retardant, food-safe, and UV-stable options, making them versatile for different requirements. Moreover, they offer a cost-effective solution for producing prototypes or small series of end-use parts. Key benefits include good mechanical properties, fine surface finish, and exceptional detail representation. Manufacturers often use them in consumer electronics, automotive components, and toys.

Polyethylene-like and Polypropylene-like Resins

polypropylene like resins

Polyethylene-like and polypropylene-like resins mimic the properties of polyethylene and polypropylene, providing flexibility, durability, elasticity, and toughness. These resins offer excellent surface detail, an injection-molded appearance, toughness, and lightweight characteristics. These polyurethanes enable the cost-effective production of high-quality prototypes or small series of end-use parts.

Polyethylene-like resins offer high flexibility, excellent impact resistance, and good chemical resistance. Thus, they are suitable for packaging components, containers, and flexible parts. They also exhibit durability, toughness, and excellent fatigue and chemical resistance. They are ideal for producing snap fits and automotive parts, providing reliable performance in demanding environments.

Polycarbonate-like Resins

These resins are engineered to replicate the properties of polycarbonate, offering high strength and clarity. These resins boast high mechanical strength, transparency, and heat resistance, making them ideal for various applications. Polycarbonate-like polyurethanes are suitable for producing eyewear components, light covers, and other transparent parts requiring strength.


Acrylic resins used in vacuum casting emulate the characteristics of PMMA (polymethyl methacrylate). These resins offer high clarity, transparency, and UV resistance, making them suitable for various applications. Acrylic resins are commonly employed in vacuum casting processes to produce lighting fixtures, lenses, and transparent components. Their ability to replicate the properties of PMMA ensures the creation of visually appealing and durable end-use parts.

Silicone Resin

silicone resin for vacuum casting

Silicone resins are characterized by their low molecular weight and branched-chain structure. They offer excellent heat resistance and high performance, surpassing traditional rubber materials. With thermal stability allowing resistance of up to 500 degrees Celsius for short periods, silicone resins are valuable in electrical insulation and as release agents. These properties make them indispensable in industries requiring materials capable of withstanding high temperatures while maintaining reliability and performance.

In vacuum casting, silicone resins are essential release agents for thermoplastic or elastomeric materials like polyurethane. Using silicone resins enhances the efficiency and quality of the vacuum casting process, ensuring smooth demolding and precise replication of intricate details in the final products.

Silicone Rubber

Silicone rubber utilized in vacuum casting offers outstanding properties as a durable and resilient elastomer with a unique molecular structure. Noteworthy properties of silicone rubber include good flow, transparency, shrink-free curing at ambient temperature, long-term stability of mechanical properties post-curing, and resistance to casting resin.

These attributes establish silicone rubber as a popular and dependable material for vacuum casting molds, providing geometric freedom and premium finishing options. Its high flexibility, elasticity, and temperature resistance make it suitable for seals, medical devices, gaskets, and kitchen tools. Additionally, its biocompatibility enhances its utility in medical and food-related applications, ensuring product safety and reliability.

TPU (Thermoplastic Polyurethane)

vacuum casting plastic parts

TPU-like resins are a versatile intermediary between rigid plastics and flexible rubber. They comprise soft and hard phases. Thus, these resins offer outstanding flexibility and toughness, making them well-suited for short-run production processes with minimal development periods. Polyurethane resins offer attributes such as heat resistance, elasticity, and high e-module.

Their balanced physical properties and high dimensional stability facilitate easy molding and make them particularly suitable for products requiring abrasion resistance and flame retardance. These characteristics make them ideal for gaskets, seals, flexible hoses, and footwear components, where reliability and longevity are essential.

Epoxy Resin

This resin is formed by a chemical mixture of epoxy and hardener. It exhibits exceptional durability and resistance to wear, chemicals, and weather conditions. Its ability to withstand sustained forces without deformation makes it a dependable adhesive material. With excellent bonding capabilities, versatility, and moderate temperature tolerance, epoxy resin finds widespread use in industries such as construction, manufacturing, and woodworking. Its applications range from serving as the matrix phase in composite parts to providing surface protection. Casting epoxy resins are also used for crafting various items, including sculptures, jewelry, and crafts.

Polyurethane Foam

polyurethane foam for vacuum casting

Polyurethane foams serve as core materials or bases for creating lightweight components with intricate designs. They offer excellent heat resistance, low density, the ability to manufacture superb surface structures, and flame retardance. These foams help produce insulation materials, low-weight panels, and multi-layer structures.

Polyurethane casting foams offer cost-effectiveness and efficiency in vacuum casting, boasting a shorter run time than other casting materials. PU foams are highly machinable with excellent dimensional stability and are suitable for extreme precision conditions. Moreover, they enable fast and easy demolding of the final product, enhancing the overall manufacturing process efficiency and productivity.

Bring Your Vacuum Casting Design to Life

Vacuum casting offers a cost-effective approach to producing intricate plastic and rubber components in small batches. Due to affordability and high-quality results, many product teams utilize vacuum-cast parts for function testing during product development. Thus, it is applicable across various industries.

For custom cast parts, turn to RapidDirect, the most reliable expert in vacuum casting. At RapidDirect, we pride ourselves on being your go-to expert for custom vacuum casting needs. Our team of skilled engineers and quality assurance professionals are adept at handling production needs of all scales – from low to high volumes – while adhering to the highest standards of vacuum casting design practices. State-of-the-art technology streamlines the production process, eliminating the need for hefty initial investments typically associated with high-quality manufacturing.

Our vacuum casting services are designed to offer a comprehensive solution for both prototyping and custom production part needs. We produce highly detailed elastomer parts, guaranteeing exceptional quality and swift turnaround times. Trust in RapidDirect for your next project and experience the ease of obtaining superior-quality prototypes and custom parts.

Ready to bring your design to life? Submit your CAD files to RapidDirect today for an instant quote, and embark on a journey to seamless, efficient production with us.

Try RapidDirect Now!


How accurate is vacuum casting?

The vacuum casting process yields parts with outstanding dimensional accuracy, with a precision of ±0.3% (with a lower limit of ±0.3 mm on dimensions smaller than 100 mm). Depending on the master pattern and the part’s geometry, we can achieve dimensional accuracies ranging from 0.2 to 0.4 mm.

Can vacuum casting be used for jewelry?

Many jewelers have adopted vacuum casting techniques to craft small jewelry settings. This method facilitates the creation of custom settings and mounts for freeform stones and offers protective designs for soft gem species like dioptase and opal. Additionally, jewelers can create rubber molds of original pieces and use them to produce models for copies.

What are the disadvantages of vacuum casting?

The primary disadvantage of vacuum casting is the limited lifespan of the silicone mold. They degrade over time and require replacement after numerous cycles, potentially increasing costs for higher-volume production runs. Moreover, certain materials like metals or glass-filled plastics are not easily replicated using vacuum casting, impacting process suitability. Additionally, vacuum casting might not be very suitable for very complex parts.

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