It invariably pays to consult tecni-form early in the process of designing the item which you want to be rotomoulded
Basic principles in designing articles for rotational moulding
- Knowledge at the start of a project of the basic design criteria for rotational moulding will save time and effort.
- Tecni-Form are always pleased to discuss your project, free of charge.
- It is best to get Tecni-Form involved as early in the development as possible.
It invariably pays to consult Tecni-Form early in the process of designing the item which you want to be rotomoulded. A collaborative design effort is much more likely to achieve a successful outcome without wasted time or cost. Some of the design criteria which are important in rotomoulding may well be different from those that suit other methods of manufacture.
Tecni-Form can work from a physical model, drawings or CAD data (from almost any program). Tecni-Form convert that information into a durable moulding tool which produces exactly what you require.
Rotomoulding is capable of manufacturing sophisticated hollow shapes with such features as:
- Complex mould split lines
- Male or female moulded threads
- Moulded-in inserts
- Side cored undercuts
- Zero draft (taper for extraction of the piece)
- A wall section from under 1.5mm to over 40mm
- Multilayer wall sections which can be made from several different polymers
- A large range of surface finishes: texturised (by mechanical application or photo-etching), smooth, polished – or a combination.
Remember also that a hollow shape can be cut in half after moulding to create two parts. Cutouts, holes, slots and the like may be created; and assemblies can be built up of mouldings and any type of bought-in component.
Some general design guidelines will be helpful:
- In rotomoulding, the raw polymer is tumbled inside a heated mould: it penetrates into surface shapes only under the influence of gravity; therefore these shapes must allow moulding material to flow easily
- Sharp internal corners and edges should be avoided as they create stress points and locally reduce the wall section
- Sharp external corners and edges, whilst structurally satisfactory, may not fill correctly and therefore not be aesthetically acceptable
- Do not design single wall sections formed onto other single wall sections; they can be injection moulded but not rotationally moulded.
Some design guidelines specific to 3-D CAD:
- Test the accuracy of CAD data transfer before starting work on a live project. Tecni-Form have the facility to validate and translate almost all CAD formats, but ‘fine tuning’ may be needed
- You only need to provide data that defines the outside of the moulding; internal detail is not required (the model does not need to be shelled)
- Generally, it is best to omit draft angles other than natural drafting of the part due to styling. This is because draft will be affected by the position of the split line, the tooling method and the de-moulding technique – one or more of which may not be known
- Do not apply shrinkage or casting contraction to your 3-D model
- Do not omit features from your CAD model because you think that they will be added to the tool later, model everything you want
- You will need to provide a ‘key feature’ drawing showing various requirements like tolerances, surface finishes, and colour specification. Tecni-Form can give help on this.
A Guide for the use of 3-D CAD data to directly manufacture tooling for Rotational Moulding is available in the Design Library.
Supporting product design
- Tecni-Form can do more than design mould tools and make rotational mouldings: they can also contribute significantly to the design of your product
- Tecni-Form can cover any or all of the product design stages, from creating visuals to building prototypes
At this moment, your product may still be conceptual, a sketch on paper, a CAD file or a physical model. Tecni-Form have the capability and experience to take the project forward. Involving Tecni-Form at the earliest stage ensures that the correct mould configuration and manufacturing variables are addressed.
Tecni-Form can provide any of the following services:
- Styling concepts with output as drawings or renderings
- Engineering design
- Manufacture of aesthetic, functional or fully representative prototypes
- Fully detailed production drawings, critical feature drawings and assembly drawings
- Supplier liaison to ensure selection of the optimum manufacturing process
- Optimisation of the design for the selected process
- Validation of prototype and initial production runs.
Design work is carried out in 3-D CAD. Depending on a customer’s preference the following CAD systems can be used:
- Catia V5 & V6
- PTC Creo
- Siemens NX
Tecni-Form handle projects of all sizes. They use both their in-house CAD capacity (SolidWorks) as well as partner industries and design consultancies.
Reliable data translation is a concern often voiced by customers, however Tecni-Form have a record of seamless data transfer to and from most CAD systems. For example, in one project where the parts were initially styled in Alias and engineered in Pro/Engineer, Tecni-Form implemented late changes to the CAD models and produced the critical feature drawings in-house. Both the drawings (as DWG files) and the 3-D models (as Parasolid files) read with 100% integrity into Solid Edge, the customer’s CAD system.
Reverse engineering can be used to assist design. The term ‘reverse engineering’ is used here not in the sense of dismantling something to see how it works but in the following, more technical, sense.
Laser scanning of 3 dimensional objects allows tooling to be produced from physical models, and products to be validated against the original CAD data. Data ranging from a simple point cloud to fully surfaced models can be produced depending on a customer’s requirements.
Cost factors to consider
Rotational moulding tools are much less expensive to make than other manufacturing methods and there is very little material waste, so it is inherently a cost-effective process. But consideration of the following can cut costs further:
- Design from the outset for manufacture by the rotational moulding process
- Avoid unnecessary complexity – it is more expensive to mould
- Material: always choose the lowest cost polymer that will do the job.
- Texture: Product surface finish is generated by the mould surface. For example, shot peening produces texture at low cost, whereas photo-etching, whilst more expensive, produces a more sophisticated surface finish, such as leather grain
- Colour: can influence the perceived quality of a product – self-coloured mouldings are lower in cost and durable, however painting may be needed to achieve levels of gloss or exact colour matching to other painted parts
- Mould design: design for the minimum number of parts of the tool; careful selection of split lines can be critical in achieving this
- Ensure the design is right before committing to tooling.
A cost-efficient process
It may be that the form and appearance of a proposed component is constrained by its application. For example, an air duct has to convey a given airflow and fit into a defined space. But there is usually scope for variations and compromises which might have a significant effect on cost.
Consider another example: suppose that the required moulding is complex and requires several chambers and ports. It possibly has to fit behind a cover, avoid items such as cabling and itself be fixed to defined pieces of structure. Some of it may show, requiring correct colour and texture. Such a design is likely to affect the order of assembly of the truck, bus, aircraft or whatever is in question.
The designer then has numerous options in how he specifies the component or would it be better designed as more than one component? It is nearly always an advantage to discuss these with Tecni-Form with a view to saving cost. Tecni-Form can help you choose how to build up the assembly; to devise shapes which are easy to mould; and to select appropriate materials. Contact the Tecni-Form design department for ideas which will show you a shortcut to an economical solution.