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Rotational moulding is a process whereby a hollow mould is filled with a powder resin and then rotated bi-axially in an oven until the resin coats the inside of the mould and cures. The mould is then cooled and the part removed.
There are many advantages to this process. A primary advantage over other processes is size; some of the largest ovens now incorporate a 5.5m swing. Entire boat hulls and automobile bodies are being rotomoulded in one piece. On the small part end, many small PVC parts such as ear syringes and face masks are rotationally moulded.
Another advantage is unrestricted design. Complex geometry is easy to incorporate into a one-piece product design. The application of solid modeling and low cost tools enables the moulder to produce virtually any design. Rotational moulding is a very cost effective method of redesigning four or five thermoformed or sheet metal pieces into one hollow plastic product.
Rotationally moulded products are recognized for their strength and durability. In this no-stress process, material tends to collect in corners and ribs making these areas thicker and thus stronger. The material does not thin out in corners as with other processes.
Finally, ease in prototyping. One of the reasons that Rotational moulding is a cost effective alternative to other processes is the low cost of the tooling used in this process. Since Rotational moulding is a stress free process, tools are primarily a thin hollow shell and can be produced from aluminium, stainless steel, mild steel and nickel. The product forms on the inside of the mould therefore tools do not require internal cores. Tooling can be produced quickly and relatively inexpensively to other processes.
Process Overview
Plastic is introduced to a mould in powder form up to the mass required for the required product (Stage 1).
The mould is then closed and passed into an oven chamber. The mould is then heated externally to a temperature typically between 220°C and 400°C and is rotated around both vertical and horizontal axes (Stage2).
As the mould rotates, the inner surface passes through the mass of powder at the bottom of the mould. As the mould heats up, the powder begins to melt and adhere to the inner surface of the mould.
The mould continues to rotate in the presence of heat and more plastic melts and builds up to produce an even layer over the surface of the mould.
The mould is then withdrawn from the oven whilst still rotating and moved into a cooling chamber (Stage 3). Cool air is directed at the mould and in some cases a water mist is used to cool the mould.
When the plastic inside the mould has solidified, the mould can be removed from the cooling chamber.
The plastic component is then removed from the mould (Stage 4) and allowed to finish the cooling process unrestricted by the mould.
The plastic is formed without pressure or centrifugal force and as such has no moulded in stresses. The cycle length can vary from five minutes to one hour depending on the material used, the wall thickness and the machinery involved.
There are four different types of tooling most commonly used in Rotational moulding - Fabricated Sheet Steel, Stainless Steel, Cast Aluminium and CNC Machined Aluminium,. Each type of tooling has it’s own benefits and product design parameters.
Crossfield Excalibur Ltd specialises in Sheet Steel and Stainless Steel Tooling.
Sheet Steel moulds are one of the moulds most commonly used tools in the Rotational moulding process.
Sheet Steel Moulds
Sheet Steel tools are usually fabricated from 2-3mm thick CR4.
The advantages of sheet steel tooling are usually cost – sheet steel tools are in general much cheaper than cast aluminium and CNC aluminium tooling.
The main advantages of Sheet Steel Tooling are listed as Follows:-
1. Cost Effective
2. Can produce much larger tools than cast aluminium
3. Can be easily modified, repaired or adjusted
4. Lighter tool weight putting less stress on your machinery
5. Quicker to manufacture than Cast Aluminium
6. No risk of porosity
Single Cavity Moulds - Single cavity moulds produce one part from each tool.
Back to Back Moulds - Also known as "Double impression", usually produce one hollow part which can be cut into two products.
Multi-Cavity Moulds - can produce multiple hollow parts.
When designing your product keep in mind how the mould will open and close around the product. The least expensive, least maintenance mould has two pieces. If your product will require multiple piece moulds, you are increasing the chance of wear and flashing due to the operator handling and additional parting lines required. The toy industry has mastered the concept of 2-piece mould/product design, resulting in low cost tools used in high production.
Flats and dowels may require maintenance as dowels loosen. A rule of thumb for dowel spacing on the parting line is every 6-8 inches.
One of the biggest maintenance problems that moulders experience is part line deterioration and flashing, the following steps will help in reducing this problem.
Avoid parting line on sharp corners or along knife edges, always add a radius.
Avoid vertical parting lines.
Today’s products require multiple piece moulds, various types of inserts (stainless, alum., brass), moulded threads for caps, internal threads, caster mounting plates are moulded in product.
Numerous designs of insert holders and pull pins are available depending upon the specific application.
Various resins have different shrinkage. The moulding process can also vary these shrinkages.
Restricted areas will also vary the shrinkage to some degree.
The type of mould release or permanent coating will have varying shrinkage.
Since Rotationally moulded products are hollow, it is difficult to ensure flatness on a product. To counteract anticipated warpage, we may add a crown to the model or reinforcing ribs to the product. Process control is imperative.
Reinforcing or stiffening ribs must be designed as a hollow element similar to corrugated sheet. Good proportions for a rib are a height of 4 times the wall thickness, and a width at least five times the wall thickness. The greater the height of the rib the greater the stiffness. A rectangular vs. rounded rib will provide better stiffening. The side walls of the ribs should have draft or be tapered to avoid the part hanging up on the rib.
Kiss-off or two-closely spaced walls that mould together also provide additional strength. The kiss-off area should be 2 times the wall thickness plus .030” as a starting point, although this can vary.
However kiss-off or (wall to wall support) may cause surface deformation, in many applications kiss-off or wall to wall support may not contact each other and still achieve the support needed.
Due to nature of how the material flows and forms, sharp corners usually result in blow holes and porosity in the corners. Adding a generous radius improves the moulding of corners and distributes the stress over a broader area adding to the product’s strength.
The minimum recommended angle at the corner of a part for polyethylene and PVC is 30°, nylon is 20°, polycarbonate is 45°. Corner angles less than these amounts can result in a bridging of plastic, increased porosity and excessive shrinkage.
Incorporating draft angles perpendicular to the parting line, will reduce warpage when demoulding a product in areas where the product shrinks against/around the mould. For example, if you are moulding a donut shape. The outer diameter will shrink in free of the mould wall. The inner diameter will shrink against the mould requiring draft angles here.
Due to gasses that are expelled when the resin cures, a vent hole is required in all moulds and therefore products. The size of the vent is dependent upon the volume of the cavity. Formula to determine vent size is 1/2" i.d.. vent for every cubic yard of volume. Multiple vent holes may be required for larger, more complex parts.
For material to flow and form properly, it is recommended that the distance between the parallel walls be a minimum of five times the nominal wall thickness of the part
Uniform wall thickness products is one of the inherent benefits of the Rotational moulding process if all areas of the mould achieve uniform heating.
Basically the hotter the inside surface of the mould gets the more material will form in that area. Therefore shielded areas and deep cores will reduce the amount of heat that are gets resulting in thinner walls in those areas. Steps must be incorporated in the mould design to ensure even wall thickness.
Through the use of air amplifiers, preheating moulds, black paint, heat fins and pins, the moulder can actually control the product wall thickness to achieve different wall thickness as required by a specific product design.
Material is usually loaded into the mould cavity when the mould is open. If there is not enough room to load material with the mould open, a fill port can be added. The fill port enables the operator to load the mould when closed. If there is still not enough room for material, a drop box can be added to provide a second or additional charge of material during the cycle.
Drop Boxes are primarily used to mould multiple layers of material to provide insulation, strength, multiple colors, foaming, and/or differing internal/external part requirements.
The drop box usually mounts on the outside of the mould and holds a second charge of material. After the first charge inside the mould cavity has formed, an air actuated cylinder inside the drop box releases the second charge into the mould.
A second drop box can be added to produce products with 3 layers of material such as PE skin - foam - PE skin.
A spider or frame serves three functions:
Protection - the steel frame surrounding the mould should be designed to eliminate flexing, protect the mould and parting line.
Machine Mounting - the frame provides mounting to either a grate spider or directly to the machine arm.
Single Cavity Frames - have just one mould mounted.
Multiple Cavity Frames / Spiders - are designed for production runs and have multiple moulds mounted.
Double Stacked or Piggy Back Spiders - have two spiders built one on top of the other. Therefore a total of four spiders are mounted on each arm.
Please note: the above information is presented as a general guide only, if you have any questions or comments please contact our technical staff who will be glad to assist
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