RIM Polyurethane is ideally suited to the housing of scientific instruments, in fact any product that requires a bespoke high quality enclosure, but which is manufactured in volumes of fewer than 2,000 units per annum for example. RIM PU is durable, stable and able to take loads in compression. It can be moulded as a solid or as a foam structure and is reasonably tolerant of varying wall sections, although as with most casting and moulding processes, wall sections should be as consistent as possible to avoid uneven exo-therming and resultant sinkage and distortion.
High quality finishes can be achieved with in-mould coating, especially UV stable decorative finishes and soft touch surfaces. Enclosures which are large and bulky can be moulded in foam materials to reduce weight without sacrificing strength and stiffness. PU RIM can be used to provide very large mouldings, in fact the bigger the enclosure the greater the justification for RIM as opposed to regular injection moulding, as the tool cost savings increase as function of the size.
These are typically a single colour applied to the tool surface prior to the introduction of the RIM material. The colour coat is chemically bonded to the PU and in general we will use the same pigment in the PU material to ensure a good depth of colour and ensure this remains even if the part is subject to surface wear. The surface texture of the finished moulding will be determined by the tool surface and so it’s possible to achieve complete uniformity of finish across any, and all batches from a given tool.
RIM PU is not UV stable and if parts are left in daylight for long periods they will start to discolour. This effect can be seen on all but those parts with very dark pigments. UV stable coatings largely prevent this discoloration – although they will not prevent parts subject to direct sunlight from being affected (please contact us to enquire). We always recommend that parts or surfaces that will be subjected to prolonged exposure to light should be pigmented in the bottom third of the greyscale above black.
Soft-touch and in-mould gaskets
Of course in-mould coating can also provide significant thicknesses as the layers are applied, and this characteristic can be exploited to provide soft touch and energy absorbing surfaces. It can also be used to create integral gaskets and glands directly on the surface of the parts. Sealing beads can be created as a continuous extension of a surface that might be mated to another for example. Enclosures can be made to seal at a given surface with a raised bead that could be radial or perpendicular to a face. Glands can extend across a wide area or be restricted to a close proximity annulus for example.
Benefits of Rapitypes Reaction Injection Moulding (RIM moulding)
RIM PU is the result of a reaction between two liquid components isocyanate and polyol as they are injected into a closed mould at low pressure. The two components are mixed together as they flow into the mould as a low-viscosity liquid and the reaction, which increases the temperature and the pressure at the surface of the tool, forms the polyurethane part in the tool cavity.