Before proceeding with the detailed design, it is-well to remember that all successful plastic products can be thought of as being surrounded by, and therefore protected by, the four basic elements that all plastic products have in common.
tooling design and construction
moulding and secondary operations
A little additional, care and attention in the design phase, of the new product development cycle will pay big dividends when that product goes into production.
While the new product is still in the preliminary design phase, some thought should be given to minimizing the secondary finishing and decorating operations that cannot be incorporated into the actual moulding process.
Rotational moulding is a relatively new plastics processing technique. A product designer who is contemplating the use of rotational moulding for the first time should proceed with caution.
One of the re-occurring problems with rotational moulding, is that many parts are designed by engineers with no prior knowledge with the process.
First time users of the rotational moulding process will enhance their chances of success by consulting with an experienced Bushmans design consultant during the preliminary design phase of a project.
Following are a few general design guidelines to keep in mind.
Minimize The Number of Mould Pieces – Flatten The Parting Line
The first recommendation, probably obvious to most design engineers is to visualize the parting line. How does the mould open and close. The lowest cost, least maintenance mould has just two pieces and a flat parting line. As you increase the number of mould pieces, inserts, pull pins and core pins or increase the irregularity of the parting line, particularly vertical drops, you increase your rotomould cost, moulding operator labour and potential for a high mould maintenance bill down the road.
Account For Shrinkage
Polyethylene is the material of choice in Australia. It is important to note that while you gain great properties such as impact strength, those properties also lend themselves to varying shrinkage. Bushmans can typically hold several critical dimensions, but not all dimensions. As the part cools in the mould, there are areas or restricted shrinkage, such as around the parting line, that make it impossible to control the part shrinkage. Therefore, when there are mating parts, you must allow for varying shrinkage.
Avoid Flat Surfaces
The one thing that polyethylene does not do is stay very flat particularly in rotationally moulded products. When cycling, the powder tends to collect in corners and radi causing these areas to have increased wall thickness. This is great for rigidity and part structure but that build up of material around a radius can also play havoc with large flat surfaces stretching, pulling and distorting them. We therefore recommend you avoid large flat areas. Design with domes, curves, contours, ribs, grids, waffle patterns anything but flat surfaces.
As the powder tumbles in the mould, it will bridge across a sharp or small radius causing pin holeing in the finished product. This is an area that you definitely need to check with Bushmans on. Wherever possible, design with large generous radi.
Avoid Designing Part Walls To Close Together
If you are producing a double wall part, be conscious of how close the part walls are to each other and make sure that there is enough room in the cavity to hold the required material. When walls are too close material will bridge, kissoff, create voids and distort outer surfaces. This is an area to discuss in detail with Bushmans but a rule of thumb to follow is leave a gap of three times your part wall thickness between the walls.