The Versatility of RIM – a 3-Minute Guide
Reaction Injection Moulding (RIM) is a highly versatile process. With a range of polyurethane systems available. Products can be solid or hollow, rigid or flexible, impact-absorbing or insulating foam, or any combination of these attributes.
Apart from having such a broad spectrum of physical characteristics, RIM products can also vary widely in size, wall thickness and geometry. For product designers looking at materials and manufacturing options, therefore, RIM defies being pigeonholed.
Whilst experienced custom moulding companies, including Tecni-Form, are willing to work very closely with customers and advise at every stage of development, designers still need an overview of the process and its applications. Identifying RIM as a potential solution frequently unlocks economic advantages across prototyping, development, tooling and production.
The key advantages of Reaction Injection Moulding and RIM products give the most straightforward introduction to the technology. The major reasons for choosing a RIM approach are:
• Low-cost tooling, even for large parts
• Rapid prototyping and short development time
• Low volume economics
…producing components with:
• High quality surface finish
• Rigid or flexible
• Solid, hollow or foam filled structure
• Low residual stress
• Complex geometry
• In-mould coating options
RIM’s versatility also allows for designed-in thermal, electrical or acoustic insulation, buoyancy, fibre reinforcement and very high stiffness to weight ratio.
These are just the headline attractions of RIM, but depending on design considerations, many other benefits, such as deep-draw moulding capability, insert encapsulation and multiple wall thickness can also weigh heavily in favour of RIM.
Process in a Nutshell
In contrast to traditional injection moulding, which is mostly associated with thermoplastic materials, RIM is a thermosetting process, almost always involving the reaction of a polyol with an isocyanate to form a polyurethane. The two components are metered and then mixed before entering the mould tool as a low viscosity liquid.
Although mixing occurs at high velocity and pressure, the combined injection stream flows from the mix chamber into the mould at close to atmospheric pressure. The reaction is exothermic, accelerating the foaming process and cure in the mould tool. Demoulding can then be done within minutes, or possibly seconds, according to factors such as component size and wall thickness.
System equipment can vary. In Tecni-Form’s case a modular design approach was adopted and careful selection of the latest PU dispensing plant was made to give maximum control over the process. The new plant ensures accurate dispensing ratios through variable speed gear pump delivery, material control through balanced heated feed lines & day-tank agitation, and a variable speed dynamic mix head with additional injection capability for dosing of pigments, stabilisers, catalysts and modifiers.
By formulating polyurethane systems in-house, full control of the supply chain is achieved, while tailoring systems to best suit specific applications. An advanced mixing plant enables fast turnaround of blended systems utilising a variable high-speed shear mixer with load-cell controlled delivery of materials. Together with an efficient vacuum system and staged PLC control, it offers superior formulation repeatability and material performance control.
As with all plastic moulding processes, the design and manufacture of the mould is critical in order to maintain efficient production and product quality.
Designing for RIM
A useful starting point in RIM product design is to assess the rigidity and impact resistance required in the finished item. This step determines whether your product requires an elastomeric polyurethane system or a rigid system.
Within these two categories the system can be subdivided into solid PU (normally for thin-walled mouldings), foam PU (with a ‘sandwich’ cross-section) or a composite system (reinforced by an added filler or chopped strand material).
Rigidity– It is worth remembering that that although increasing wall section does not lead to sink marks (as in the case of injection moulding thermoplastics), it does increase cycle time, by extending the cure. However, the choice of polyurethane system influences flexural modulus, so achieving a specified product rigidity does not rely solely on wall thickness or stiffening ribs. In foamed systems, rigidity can be adjusted by altering the foam density, rather than wall thickness.
Cost Factors– This is a low pressure process, so there is no high cost, steel tooling requirement. Production tooling can be machined in aluminium. Rather than the weight of PU material in the design, it is the overall volume that has the largest bearing on cost.
Retractable pins, cores and slides, as well as undercuts, can all be part of the mould design.
Design Freedom– RIM is outstanding in terms of part-to-part dimensional stability and lends itself to creative design concepts. Wall thicknesses can be varied considerably within the same design, typically ranging from 3mm to 50mm. As for many other moulding techniques it is best to consider the direction of draw when positioning and planning the orientation of bosses, ribs, channels and slots to minimise tooling costs. A minimum draft angle of 0.5° should be allowed and increased for products with a deeper draw.