3D Vector’s polyurethane casting service provides an efficient way to manufacture small batches of plastic parts at a significantly lower cost than injection molding. This process delivers high-quality prototypes with excellent surface finishes and serves as a bridge to mass production through molding.

Many product designers are unfamiliar with polyurethane casting, which requires a different design approach—both for creating the master pattern and for the plastic castings themselves. Unlike injection molding, polyurethane casting involves unique design considerations that must be understood before proceeding with CAD modeling.

This article outlines key aspects of designing polyurethane castings, covering design features, standard tolerances, and the differences between polyurethane casting and injection molding.

What is Polyurethane Casting?

Also known as vacuum casting, polyurethane casting is a prototyping and low-volume manufacturing technique used to produce batches of up to 30 units. It is sometimes referred to as urethane casting.

The polyurethane casting process consists of three primary steps:

  1. Creating the Master Pattern: A positive master pattern is made using CNC machining or SLA 3D printing. Alternatively, an existing solid part that can withstand temperatures up to 40°C during silicone curing can serve as a pattern.
  2. Mold Making: A casting box is filled halfway with liquid silicone, embedding the pattern. Once cured at low heat for 16 hours, the first half of the mold is complete. The process is repeated to create the second half. The master pattern is then removed for future use.
  3. Casting: The flexible silicone molds are filled with polyurethane casting resin, solidifying to replicate the master pattern. Each silicone mold can be reused approximately 25 times before replacement is necessary.

Polyurethane casting is a cost-effective method because silicone molds are significantly cheaper to produce than metal tooling. Although they have a limited lifespan, they are ideal for prototyping small plastic parts.

While polyurethane-cast parts may not match the strength of fully functional components, they offer advantages such as smooth surface finishes, customizable colors and transparencies, and affordability for low-volume production.

Creating the Master Pattern

The first step in polyurethane casting design is developing the master pattern. Common techniques for creating these patterns include 3D printing and CNC machining. Traditional methods, such as manual sculpting, can also be used but are generally more costly.

When using digital manufacturing processes, designers must consider constraints associated with the pattern-making method and the casting process. Here are the advantages of each technique:

Advantages of 3D Printed Master Patterns:

  • Greater geometric freedom
  • Faster production compared to CNC machining

Advantages of CNC Machined Master Patterns:

  • Tighter tolerances
  • Requires less surface finishing
  • Longer lifespan compared to 3D printed patterns

Casting polyurethane

Tolerances & Dimensions

Polyurethane cast parts follow specific tolerances and dimensions, differing from injection molding standards. If polyurethane casting is being used as a prototype stage before injection molding, it is advisable to incorporate design elements such as draft angles to facilitate the transition. 3D Vector can achieve an accuracy of ±0.3%, with a lower limit of ±0.3 mm for dimensions under 100 mm. Additionally, parts with thick walls are more susceptible to shrinkage due to temperature changes, and designers should account for a shrinkage rate of approximately +0.15%. Polyurethane cast parts can also be produced in dimensions of up to 1900 x 900 x 750 mm, with a maximum volume of 10 liters.

Wall Thickness

The minimum recommended wall thickness for polyurethane casting is 0.75 mm, but 1.5 mm or more ensures better results. While consistent wall thickness reduces the risk of deformation, polyurethane casting is more forgiving than injection molding in this regard.

Draft Angles

Unlike injection molding, polyurethane casting does not require draft angles due to the flexibility of silicone molds. However, incorporating a small draft angle of up to 5° can help extend mold lifespan and make the design more adaptable for future injection molding production.

Radii

To enhance strength and stability, sharp internal corners should be avoided. A fillet with a minimum radius of 3 mm is recommended. CNC machining is ideal for creating these features in master patterns, as CNC tools naturally produce rounded internal corners when cutting pockets.

Ribs

Ribs can be used to reinforce parts without increasing overall wall thickness. For optimal results:

  • Use multiple short ribs instead of one tall rib.
  • Maintain a rib thickness of less than 60% of the part’s wall thickness to prevent shrinkage.
  • Keep the rib height below three times the rib thickness to prevent breakage.

Bosses

Polyurethane cast parts can incorporate bosses to accommodate fasteners or threaded inserts, and these should be supported with gussets or connecting ribs to prevent deformation. To maintain structural integrity, the wall thickness of a boss should be less than 60% of the part’s wall thickness, while the base radius should be at least 25% of the part’s wall thickness to improve durability.

Threads & Holes

Threads and through holes can be incorporated into polyurethane casting, but it is generally recommended to use inserts rather than adding these features to the master pattern. Through holes are typically achieved using dowel rods placed inside the silicone mold, while threads are best added using metal threaded inserts rather than attempting to cast them directly. This approach ensures greater precision and durability while minimizing potential defects in the final part.

Joints

Polyurethane casting allows for multi-component parts with tongue-and-groove joints. To ensure proper assembly, a small reveal gap should be included in the design.

Surface Features

Logos, text, and other surface details can be added to polyurethane cast parts. CNC machining is the preferred method for incorporating these elements into master patterns due to its precision. For best readability letters and numbers should be spaced at least 1.3 mm apart.

Conclusion

Polyurethane casting is a versatile and cost-effective method for prototyping and low-volume production. With careful design considerations, it can produce high-quality plastic parts with excellent surface finishes. By following best practices for pattern creation, tolerances, and feature design, product designers can achieve optimal results.

For expert guidance and a free quote, contact 3D Vector today.