Vacuum Casting: Practical Design Guide

The technology quickly creates detailed parts by drawing liquid material into the mold under vacuum pressure. The process is generally as follows:

 

1. Create a precise master mold: This is often done with the help of 3D printing (e.g. SLA, SLS) or CNC machining techniques. The master needs to be able to withstand a certain temperature, usually 40°C, and these manufacturing techniques produce high-resolution products with a naturally smooth surface.

 

2. Silicone mold making: Liquid silicone is poured into a casting box with a master mold and cured under vacuum. The silicone perfectly encapsulates the master mold. After curing, the mold is cut, the master mold is removed, and a pouring and venting system is added.

 

3. Perform vacuum casting: The mold is placed in the vacuum casting machine and the mixed polyurethane resin is injected into the mold cavity and cured under vacuum to prevent air bubbles from mixing in. After curing is complete, the final part is removed and the mold can generally be reused about 30 times, which is suitable for small to medium scale production. 

 

Vacuum casting has significant advantages, such as the ability to manufacture parts with complex geometries and delicate details, which can achieve fine features that are difficult or costly to achieve with traditional manufacturing processes, such as thin walls and inverted buckles; shorter turnaround times compared to traditional methods, such as injection molding, which do not require complex tooling and have high design flexibility; and high flexibility in material selection and part design, which allows the use of a wide range of materials and meets the needs of different products. However, it also has certain limitations, such as limited mold life, generally used 20 - 25 times; relatively slow cycle time, not suitable for high-volume production; sensitive to temperature, need to carefully control the process .

 

 

 

In vacuum molding, the mold release angle is the key to ensure that the part can be smoothly removed from the mold. If the demolding angle is too small, the part may stick to the mold during demolding, resulting in damage to the part or damage to the mold; while too large an angle may affect the dimensional accuracy and appearance of the part. Generally, a release angle of 0.5° - 2° is recommended, depending on the complexity and geometry of the part. For example, for simple shape, smooth surface parts, the mold release angle can take a smaller value; for parts with complex inverted or internal structure, it is necessary to appropriately increase the mold release angle.

 

 

Tolerance is the range of the actual and design size of the parts allowed deviation, affecting the assembly accuracy and product performance. Vacuum casting tolerance control is strict, linear size tolerance is usually ± 0.1 - ± 0.3mm, the design should be based on the use of the requirements set tolerance, high precision parts such as assembly holes, shaft tolerance should be small, the appearance of the part can be relaxed. It should be noted that the tolerance may be different in different directions, and designers should understand the casting process characteristics to prevent quality problems caused by improper tolerances.

 

 

1. Importance of Uniform Wall Thickness: Uniform wall thickness is critical to the success of polyurethane vacuum casting. Uneven wall thicknesses result in varying cooling and curing rates, which may cause part distortion, shrinkage marks, or internal stress concentrations that affect part quality and performance. For example, when wall thickness varies greatly, thicker parts cool slowly and shrink, while thinner parts cool faster and cure first, causing stresses to build up at the junction of thickness and thinness, distorting or cracking the part.

 

2. Recommended Wall Thickness Range: Generally speaking, the minimum wall thickness recommended for small and medium-sized parts is 0.75 - 1.5mm, while the wall thickness of large-sized parts needs to be increased in order to withstand greater external forces and ensure structural strength. The optimum wall thickness depends on the material properties, functional requirements and production process. Rigid materials can reduce the wall thickness, the need to withstand impact or pressure parts need to increase the thickness.

 

 

1. Buckling Methods: Buckling is a reverse geometric feature on parts in polyurethane vacuum casting that requires special design techniques to handle. Common methods include the use of detachable mold components, designing movable cores or slides, and extracting these parts after curing before demolding. For example, internally inverted parts can be demolded with removable cores. The flexibility of silicone molds can also be used to handle inverted buckles, for cases where the inverted buckles are shallow and there is little resistance to release.

 

2. Overhanging structure design points: Overhanging structure refers to the unsupported part of the overhanging part, the design needs to consider the casting stability. By adding support structure to prevent deformation or collapse, support can be removed after demolding. Pay attention to the position and layout of the support to avoid affecting the appearance and function of the part. Adjust the thickness and angle of the overhanging part, e.g. increase the thickness to improve the strength, reduce the angle to reduce the risk of deformation.

 

 

1. Dimensional Requirements: Tabs are used for mounting screws and other connectors, and their dimensions are designed to meet specific requirements. The minimum height and diameter is usually 1mm to ensure strength. Wall thickness should not exceed 60% of the nominal wall thickness of the part to reduce shrinkage marks in the curing process to ensure surface quality.

 

2. Location layout: The layout of the tabs needs to take into account the assembly requirements and structure, to ensure that it is easy for the connectors to be installed and operated, and to avoid setting them in high stress areas to prevent damage. For example, in the design of electronic enclosures, the tabs should correspond to the circuit board mounting holes for easy fixing; in the design of mechanical parts, the tabs should be far away from stress-concentrated areas, such as edges and corners.

 

 

1. Enhance strength and reduce warpage: reinforcement is used to enhance the strength and rigidity of the parts and reduce warpage deformation during molding. Reasonable layout of reinforcement can be dispersed external forces, enhance the load-bearing capacity. For example, crisscrossing reinforcement in large flat parts can significantly improve bending strength and prevent warping during use.

 

2. Design parameters: The thickness of reinforcement is recommended to be 0.5 - 0.7 times the wall thickness to ensure strength and avoid shrinkage. The height should be adapted to the structural and strength requirements and should not be too high as it may affect the appearance and mold release. Spacing is usually 2 - 3 times the height to balance strength and cost. Reinforcement should be oriented in the same direction as the force, e.g. along the direction of tension in tensile force parts and perpendicular to the bending axis in bending force parts.

 

 

1. Realizability: Vacuum casting can accurately reproduce fine relief detail, which is particularly suitable for parts with decorative or functional surface textures. Whether it is a small text, complex patterns or delicate texture, can be designed through the master mold on the part of the perfect display. For example, artistic decorations, brand logos for electronic product housings, etc. The high precision of vacuum casting can meet the high demand for details.

 

2. Design considerations: When designing relief, its depth, width and edge transition need to be considered. The depth should not be too deep, so as not to affect the resin filling and cause lack of material. The relief width and edge transition should be reasonable to prevent damage to the mold when demolding. The depth is generally controlled at 0.1 - 0.5mm, adjusted according to the size of the part and material properties. Soft materials should reduce the depth, large-sized parts can be appropriately increased to maintain the visual effect. Avoid sharp angles and narrow recesses to reduce casting difficulty and risk of mold damage. When making the master mold, high-precision machining process such as laser engraving and EDM should be used to ensure the surface finish and precision of the master mold.
 

 

Polyurethane vacuum casting can use a variety of materials, each with its unique performance characteristics for different application scenarios.

 

1. Class ABS resin: it has good comprehensive performance, such as high strength, hardness and toughness, good surface gloss, easy processing and post-treatment. It is suitable for making parts with high requirements on appearance and mechanical properties, such as electronic product shells, toys and models.

 

2. Polyethylene and polypropylene resins: excellent chemical resistance, weather resistance and low density, while having good flexibility and processing performance. Commonly used in the production of chemical resistance, outdoor use or weight requirements of the parts, such as chemical containers, outdoor furniture, automotive interior parts.

 

3. Class polycarbonate resin: high transparency, high strength, high heat resistance and good dimensional stability, excellent impact resistance. Suitable for the production of optical properties and mechanical properties are very high requirements of the parts, such as transparent lampshades, protective masks, electronic display screen shell.

 

4. Acrylic: extremely high transparency, good surface gloss, easy to dye and processing, with good weather resistance. Commonly used in the production of products that require high transparency and beautiful appearance, such as display racks, decorative items, advertising signs.

 

5. Silicone resin: It has excellent high temperature resistance, low temperature resistance, electric insulation and chemical stability, and also has good flexibility and mold release performance. Commonly used in the production of parts that need to be used in extreme temperature environments or have special requirements for electrical properties, such as high-temperature seals, electronic potting materials, aerospace parts and so on.

 

6. Silicone rubber: excellent flexibility, elasticity and aging resistance, non-toxic and harmless to the human body. Widely used in the production of products that require soft touch, good elasticity and biocompatibility, such as medical supplies, kitchen supplies, cell phone protective covers, etc.

 

7. TPU (thermoplastic polyurethane): combining the high elasticity of rubber and the high strength and easy processing of plastic, with excellent abrasion resistance, oil resistance, water resistance and mold resistance. It is suitable for making parts that require high abrasion resistance, high elasticity and good processing performance, such as sports shoe soles, industrial rollers, seals and so on.

 

8. Epoxy resin: high strength, high adhesion, good chemical resistance and electrical insulation, curing shrinkage is small. Commonly used in the production of high strength and bonding performance requirements of the parts, such as electronic components encapsulation, mold manufacturing, composite materials.

 

9. polyurethane foam: lightweight, heat insulation, sound absorption, cushioning and other characteristics, can be adjusted through the formula to obtain different density and hardness of the foam material. Commonly used in the production of thermal insulation materials, sound-absorbing materials, packaging materials, cushions, etc.

 

When choosing materials, you need to base on the environment where the parts are used, function, appearance and cost. For example, if you work in a high temperature environment, choose high temperature resistant silicone or polycarbonate resin; if you consider flexibility and elasticity, choose silicone rubber or TPU is the most appropriate; if you consider the cost, choose the class of ABS or polyethylene resin that meets the performance.

 

 

As an example, we recently manufactured a vacuum casting project for an electronic product casing that required good exterior texture, a certain level of strength and dimensional accuracy, as well as the ability to accommodate and provide good protection for the electronic components inside.

 

1. Design optimization: During the design process, the design guidelines mentioned above were followed. The housing is designed with a uniform wall thickness of 1.2mm to ensure uniform cooling and curing during the casting process, effectively avoiding deformation and shrink marks. For the mounting screws, the height and diameter of the tabs are designed to be 1.5mm, and the wall thickness of the tabs is controlled to be 0.7mm, which not only ensures the strength of the tabs, but also reduces the risk of shrink marks. In order to enhance the overall strength of the shell, in the reasonable arrangement of the internal reinforcement, the thickness of the reinforcement is 0.8mm, the height of the shell according to the structure to determine the spacing between the reinforcement of 5mm, and the direction of the shell may be subjected to the direction of the external force is consistent with the shell, effectively improving the rigidity of the shell. In addition, the surface of the shell is designed with some embossed brand logos and decorative textures, the depth of the embossment is controlled at 0.2mm, and through the high-precision master mold production, it successfully realizes the fine embossed effect, which enhances the appearance quality of the product.

 

2. Material selection: according to the use of electronic products shell environment and performance requirements, the choice of class ABS resin. This material has good strength and hardness, and can provide reliable protection for the internal electronic components; good surface gloss, in line with the requirements of electronic products on the appearance of texture; at the same time, easy to process and post-processing, to meet the project's production process requirements.

 

3. Result Feedback: By adopting optimized design and suitable materials, the electronic product shell is smoothly molded in the process of polyurethane vacuum casting, and the quality of the product meets the expected goal. The surface of the parts is smooth, without obvious defects, and the dimensional accuracy is controlled within ±0.2mm, which meets the assembly requirements. The strength and rigidity of the shell have also been effectively ensured, after simulated use test, can withstand a certain degree of external impact without damage. The appearance of the product is highly recognized by the customer, with clear relief details, enhancing the brand image of the product. This case fully demonstrates the importance and effectiveness of following the design guidelines for polyurethane vacuum casting and choosing materials wisely in actual projects.

 

Conclusion

 

Vacuum casting, as an important manufacturing technology, provides an efficient and flexible solution for rapid prototyping and low-volume production. By following the design guidelines and practical tips explained in this paper, including the reasonable design of release angles, tolerances, wall thicknesses, inverted and overhanging structures, bosses, reinforcements, and relief details, as well as choosing the right materials according to the part requirements, the quality and performance of polyurethane vacuum casting parts can be effectively improved, the production cost reduced, and the production cycle shortened. In the actual project, designers should fully consider the function, appearance, use of the product environment and other factors, and integrate these design points into the product design, so as to give full play to the advantages of polyurethane vacuum casting, and realize the successful development and production of products.