Top Design Tips for Urethane Vacuum Cast Parts
Urethane Vacuum casting is a renowned versatile production process. It entails the use of a silicone mold and a master pattern to created final plastic parts. We place the master pattern inside a sealed box during the urethane casting process, cover it with liquid silicone and then cure it. After the silicone sets, we cut them in half and release the master pattern as the cast urethane mold is now ready for use. To produce custom cast urethane parts, we pour urethane casting resin into the mold. The polyurethane material often mimics the physical features of the plastic we use in injection molding. Generally, the final part needs little to no post-processing.
Below Is The Top Designing Tips For Urethane Vacuum Cast Parts
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Ensuring Even Wall Thickness
Many engineers opt for urethane vacuum casting over injection molding since it offers a greater variation in wall thickness. However, it is still vital to ensure the mold design has uniform walls. The uniform walls lower the risk of deformation during curing. It also mitigates other design issues and ensures the mold fills up. Our experts opt for a minimum wall thickness of 1 mm, but there are instances when walls may be as thin as 0.5mm in small parts. For larger parts, we increase the parameter in proportion to the size of the part to ensure the wall can offer adequate support.
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Account for Shrinkage
Shrinkage is likely to occur when two walls of different thickness intersect in a urethane mold design. Sincere thinner walls solidify at a quicker rate than thicker ones, the point where they attach to the nominal wall will shrink. This results in a sunken area in the nominal wall. Our experts ensure the rib thickness falls between 50-60% of the wall it attaches to prevent sunken areas and minimize shrinkage.
However, there some urethane cast parts that shrink due to other reasons. The final dimensions of a urethane part change depending on the accuracy of the mold and pattern. It also depends on the urethane casting material and part geometry. A shrink rate of 0.15% is generally acceptable. Engineers must, however, revisit their mold design if it surpasses this limit.
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Consider Undercuts and Drafts
Normally, undercut and drafts are not an issue in urethane casting as liquid silicone can take the shape of any mold. However, when using urethane casting to create a prototype, it must closely resemble the final product. This may involve incorporating undercuts and drafts. Long zero-draft features come with a low risk of part breakage during part extraction from the mold. Therefore, a small angle may be useful for larger production runs.
The Important Design Points For Employ the Use of Ribs
Adding ribs to the mold design of urethane vacuum cast parts helps increase the part’s strength and rigidity. This is without adding the thickness. There are a few general guidelines our experts adhere to when using ribs.
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First Point
The longer ribs offer greater reinforcement. Casting a long rib may, however, make the part tasking to mold. Therefore, the rib must not exceed a height of about three times its thickness. Our experts use several short ribs instead of a single long one to increase the stiffness of the part. This helps us avoid the height issue entirely.
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Second Point
the width of the rib at the part intersection must be 40% to 60% of the part thickness. If you intend o include a fillet radius in the design, it must be at least 25% of the part thickness. The feature aid to make the rib very strong. Thirdly, our experts prefer designing a rib that lowers the possibility of sinking. Draft angles for each side must lie between 0.25 and 0.5 degrees while the draft angle must be equal on each side. We add an extra 1.0-degree draft for every 0.025 on textured surfaces. In addition, we set consecutive ribs at least two times the thickness away from each other to equally distribute the part’s weight across all the ribs.
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Third Point
The rib orientation determines where and how the part experiences stress. Hence, engineers must be careful to arrange the ribs such that it optimizes the part’s bending stiffness. If we place the ribs wrongly, it will not positively impact the part’s strength. Notably, one must take care of sinking and intersections and sinking when dealing with ribs. A heavy rib can cause a part to dip at the point where it intersects with a part. To avoid this, our experts use fillets to ease the stress on the part’s surfaces and lower the sink on the part’s opposite side. The fillet’s radius must be at least a quarter of the part’s thickness. Further coring of the part lowers the amount of used material and aids to maintain a uniform wall thickness throughout the part.
In summary, urethane vacuum casting is an ideal rapid prototyping method that is less costly than hard tooling. The above tips aid us to get the most out of the production process and optimize mold design. Contact us now if you have any inquiries.
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