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Urethane casting is a versatile production method, and the items it creates are vital for industries ranging from automotive to film. However, satisfaction with the outcome starts with following best practices for practical design. Consider these points when evaluating your needs and resources.
1. Plan for Shrinkage
Waste reduction is one of the main advantages of urethane casting. It’s a formative manufacturing method enabling people to pour the material into a cast that’s the precise size of the desired part. Even so, shrinkage can occur during the curing process.
Ensure your casting design accommodates that reality. The part’s shrinkage occurs due to the liquid substance’s thermal expansion and the casting’s flexibility. Even so, the shrinkage is generally +/- .15%, making it much less problematic than in methods such as injection moulding.
If you notice shrinkage happening repeatedly and beyond what’s expected for urethane casting, examine the factors that often cause it. They can include the casting size or the temperatures of the mould and environment.
Consider performing a shrinkage test on a sample component when the casted part must meet tight tolerances. Refer to the urethane manufacturer’s specifications regarding anticipated size changes. The test involves pouring the urethane into a deep, rectangular mould and letting it set overnight.
It’s measured after removal and then one week later. The goal is to determine how much shrinkage occurred between demolding and curing.
2. Design the Casting to Facilitate Easy Removal
A casting design is only effective if you carefully consider how to remove it at the proper time without damaging the part. Some people use topical release agents to improve the chances of the casting coming away clean. However, they can leave a residue on the cast, requiring solvents or soaps to remove.
Fortunately, people can plan for hassle-free removal in their designs. The most common way is to rely on draft angles, which are tapers applied to the casted part’s vertical walls. They should be between 0.5 and 5 degrees, depending on the component’s dimensions and additional processing steps. Urethane casting design differs from other methods in that it does not require drafts.
However, since the drafts can significantly reduce part strain, many use them to minimize breakage and warping during removal. Urethane casting is typically an efficient, cost-effective method, but it can become more expensive and time-consuming due to preventable component breakage.
3. Talk to the Client About Specific Design Decisions
Understanding your client’s requirements and expectations before planning the design is a great way to identify possible challenges and determine the best ways to tackle them. Some customers rely on urethane casting for prototyping to show sample parts to investors or potential customers. However, people often use prototypes to show feasibility in the real world.
Knowing best practices for urethane casting design will also help you explain why certain client preferences may not lead to the expected results. For example, casting walls should be between 1.5 and 3 millimetres thick for the material to properly fill the area after pouring. Customers that suggest thinner walls should be told why that’s not advisable.
Have your client in the room when creating the master pattern with computer-aided design tools. Show them the pros and cons of each possibility. They’ll appreciate those discussions, which go beyond simply telling them that specific suggestions they have won’t work as intended. Be sure to clarify that the parts can be bigger than they might imagine. One option is to bond multiple casted parts to create larger products.
5. Plan Your Designs for Small Production Runs
Some people new to urethane casting design don’t always realize they should use this method for limited needs rather than mass production. That’s because the mould’s flexibility results in less durability.
Anticipate approximately 30 to 50 uses for each design. Factors like the component’s features and your tool size can also affect wear. Treat the reduced durability as a benefit by reflecting on what worked well with the urethane casting design and what you should improve for future efforts.
The suitability of urethane casting for small production runs is one of the major reasons people choose it for prototyping. Testing and showing a prototype to stakeholders inevitably reveals things to add, change or improve in future designs.
Treating each new design as a learning opportunity is an excellent way to foster continuous improvement while remaining open to experimentation. Knowing you may only use a design 30 times should make you feel positive about trying new things and stay upbeat if they don’t work.
6. Consider Adding Ribs
Using ribs in your design can make the resulting part stronger and less flexible without increasing thickness. However, it’s important to analyze which areas of the component will experience the most stress during use and place the ribs accordingly. Otherwise, they will have little to no effect on the resultant part strength.
The reinforcement provided by a rib corresponds to its length. However, you must avoid making ribs so long that they introduce production challenges. A best practice is that the length must not exceed three times its thickness.
Creating the rib according to the casted part’s measurements is also critical. For example, a rib used at the part’s intersection should be 40%-60% of the thickness of the entire component.
Ensure the space between adjacent ribs is at least twice the thickness of each rib.
Take the Advice of More-Experienced Peers
These six foundational tips will help you optimize urethane casting designs, even if you’ve only worked with this method for a few months. You should also keep an open mind while listening to the guidance of others in your industry who have been making urethane casting designs for longer.
Perfecting a skill always involves trial and error, and the experiences of those who have tried different options before can help you avoid their pitfalls. Be sure to return the favour once you become more well-versed in casting design and share your favourite tips with others just starting.