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Planning a Rotationally Molded Part |
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High quality molded parts come from high quality molds.
Those are a result of high quality planning. |
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You should consider the manufacturing aspect of your part from the very beginningt. Part design affects mold design and mold design affects the quality and cost of your finished part.. Here are some things to keep in mind when designing a rotationally molded part. |
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- Keep the corners and edges rounded. The larger the radius of the corner the better it will mold. Square corners and edges can be difficult to completely fill. They also build up extra plastic. The extra plastic along the corners can be good for strength but can cause warpage since the thick plastic areas will cool at a different rate than the thinner areas.
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- Try to limit the depth of valleys or channels. A rule of thumb to use is to keep the width to depth ratio at no greater than 1 to 4. If the width of a channel is 1" then the depth should not be more than 4 inches. The resin you select may also have an impact on the width and depth of your surface features since some resins flow more easily than others.
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- You part will shrink as it cools. Different resins have different shrinkage rates but all will experience shrinkage. Depending on the mold design, the shrinkage may create warpage.
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- Keep the design as simple as possible. Ideally you'll be able to have a two part mold. This is the simplest and least costly approach. Every feature that requires an additional part to the mold will increase the cost of the mold as well as the time and cost of running the mold in production.
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- Plan on vent holes. Every rotationally molded part will have at least one vent hole. During the molding process the air inside the mold expands and contract as it heats and cools. A vent is required or the mold could actually explode. As a result of the need for a vent, there will be a vent hole in the finished part. These can be patched or used for a fitting if desired.
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