The process of designing a mold to develop a rotomolded product should entail an engineer who comprehends and completely understands the principles of the rotational molding process and also designing for rotomolding. Rotomolding features the ability to create hollow products as well as producing single wall or double wall products, depending on the design and the molds for production.
As soon as a part has been determined to be suitable for the rotational molding process, the design stage can begin further exploration in finding the best design in effort to incorporate all of the desired features and function into the product or component, while maintaining a focus on keeping the product cost conducive. Being capable of taking a product concept or idea directly into a design that can be rotationally molded requires a designer or engineer that is familiar with the rotomolding process, its limitations and strengths. A designer or developer of rotationally molded products has to acknowledge that there are no pressures or stresses involved that directly distribute the plastic materials thru the mold, as with thermoforming, injection and also blow-molding. In rotational molding, the mold rotates with a pool of liquid or powdered plastic. As the mold rotates, the molten plastic adheres with the scorching surface areas of the mold cavity to build up the preferred wall thickness. This can greatly affect the design specifications of the component being manufactured. Let's consider a couple of aspects of rotational molding layout which are generally considered as well as used throughout the rotational molding design process.
- • The ideal design for a rotationally molded part is any hollow shape where several aspects in the part are smoothly blended from one shape to the next. The smooth blending of shapes results in a final product that has increased strength and is easier to produce and as a result, more economical.
- • The successful design of any plastic product is dependent to some extent at least, on proportioning the style of the component to accommodate the particular part.
- • The angles and shapes of the part can be manipulated to add strength and the design process takes this into account.
- • Ribs or kiss off points may be added to the design of the part to add rigidity and strength.
- • Designs can also allow for foam to be added to the structure for extra strength. Foam filling parts can greatly increase rigidity and the ability to bare weight.
A number of design issues can be exposed during the rotational molding design process. With some brief reference or education on the front end, many of these issues can be prevented or designed around, preventing further design or production issues with your rotational molding project. Here is some common design issues experienced in designs for rotomolding.
Like most plastic manufacturing processes, sharp corners are a problem for rotational molding. Sharp corners should be avoided when possible when designing a product for rotational molding. Sharp edges drastically reduce the ability of the material flow during the molding process. The inability of the material to flow thru the mold properly may cause voids or areas in the sharp corners where the material did not fill in or lay down properly. Sharp corners are more likely to create stress in the product, creating an area capable of failure or fatigue.
Sharp corners in a roational molding design typically will collect more heat than other areas of the mold, thus building up thicker areas of wall thickness with thin inside corners.
Sharp corners can be avoided in most rotational molding designs thru adding a simple radius to a corner. This will generally increase the overrall quality of a rotomolding design while usually making the part more aesthetically appealing. An outside radius should be no less than .125" and should be as large as possible. The inside radius of a corner should never less than the nominal wall thickness of the product. A poorly added radius could mean a future product failure in the field.
Design criteria for draft angles are crucial to the success of rotationally molded products. Draft angles primary function are to help release the part from the mold as it cools. Geometric shapes, heavy textures and engraved features such as logos may require additional draft in comparison to simple geometric shapes such as a block or a vertical water tank. As a reference, heavy textures typically can get away with 3 degrees of draft, while undercuts and protruding male features may need as much as 5 degrees of draft.
Bridging between wall surfaces is a common issue that is often discovered in the rotational molding design review process. When a designer or engineer does not leave enough space between wall surfaces for the material to flow through properly, the material will "bridge" or "web" between the walls, impeding material flow through the mold. This restriction of material flow can lead to the creation of voids in other areas of the product.
A general rule of thumb to avoid bridging or material flow issues with your mold is to have a minimum of 5 times the wall thickness between any walls or ribs in the product.
Large or long flat surfaces are difficult to acheieve via the rotational molding process. Because rotational molding is always forming hollow structures, the parts generally have no internal structure. An engineer familiar with rotational molding will know to avoid flat walls or break up any large or long flat surfaces with some geometry to give the part some structure as well as minimize any warpage in the part. This can be achieved by adding ribs, kiss offs or even a crown to a large flat surface.
Because shrinkage, warpage, cooling and environmental factors are all variables of the rotational molding process, industry tolerances should be discussed to better understand what is acceptable for the successful function of each product. The industry standard for flatness in rotational molding is ±.20 per inch; As previously mentioned, when in combination with other variables it could further increase.
It is important that every mold or fixture for each rotational molding product is carefully designed and produced to ensure that the quality of the product meets or exceeds the customers expectations, while being able to be repeated continously in a production rotomolding environment. The desired final aethestics of each product needs to be carefully considered in the design phase in effort to capture all of the desired details and features.
Rotational Molding offers more flexibility in the design of the products and the necessary molds for rotomolding. The molds or tooling for the rotational molding process can be modified after they are produced for changes or improvements that might be made to the product or tooling. Because of the nuances of the rotomolding process, shot weights of many products can be changed by simply adding additional material to the pre-existing shot weight without modifying the molds.
If you have questions about an existing rotomolding project or may be looking to start a rotational molding project, please request a rotational molding quote for additional information from a leading rotational molding expert!