Materials & Technical

Component design considerations for Reaction Injection Moulding
When designing a component to be moulded using the RIM process, the following criteria must be considered.

• The type of polyurethane selected will depend on the physical properties required for the component. Foams can be used where thick sections are required to achieve rigidity, and can accommodate large wall thickness variations. Solid polyurethanes are similar to injection moulding materials, and more suited to thinner wall sections.
• The component must release from the mould. Therefore, a draft angle of at least 1.5 degrees must be designed into the component, and there must be no re-entrant angles. Multi part tooling and loose inserts in the mould can be accommodated, but will increase moulding time and component cost.
• Wall thickness for rigid self-skinning PU foam mouldings should not be below 6mm, in order to avoid the material becoming brittle. Maximum thicknesses of around 35mm can require an extended cure time, increasing cost.
• Wall thickness for solid PU materials should ideally be 3 – 4mm, with a minimum thickness of 1.5mm and a maximum of 15mm.
• Material flow considerations are vitally important for both foam and solid mouldings.
• During the RIM process, a light pressure builds inside the mould. To enable this pressure to release, and for air to expel, all bosses and up-stands must be connected by webs to the split line and at least one part of the split line must be at the very top of the mould. The mould can be tilted to assist this. With a component comprising both thick and thin sections, it is important that the material flows through the thin section first, to avoid air traps.
• Optimum surface finish is achieved on the lower face of the moulding. Where possible, components should be designed to have exterior faces at the bottom of the mould and the split line above.
• Inserts can be moulded in with a high degree of accuracy and repeatability, and can dramatically reduce assembly costs at later production stages. Inserts can be used either for fastening or reinforcement purposes.
• Sharp corners can trap air bubbles during the moulding processes, therefore components should be designed with a minimum of a 1mm radius on any corners or edges if possible.

PPL Marine Products Reaction Injection Moulding (RIM)

PPL Marine Products has been a leader in the polyurethane RIM process since the late 1960s. With a vast experience and knowledge of the RIM process and a broad and varied customer and product base, PPL Crompton Urethanes continues to provide high quality RIM polyurethane products backed up by unrivalled technical expertise.

The RIM process differs from conventional injection moulding in that the plastic is produced by injecting liquid chemicals into a mould where they react to form the plastic, rather than by melting solid granules and forcing them into a mould at high pressure. The advantage of the RIM process over injection moulding is that the pressure and temperature required is much lower. This results in lower tooling costs, and the ability to mould large components.

The process is ideal for large components where the cost of tooling for injection moulding could be prohibitive, and for smaller components required in limited quantity where the amortised cost of the mould becomes a significant factor.

There are a wide range of RIM materials available, both as solid plastics and as foams. These materials are mostly based on polyurethane chemistry, with properties ranging from tough, flexible materials to rigid, structural materials.

PPL Marine Products has developed a range of composite materials using polyurethane RIM with fibre or steel reinforcement. Such reinforcements allow polyurethane RIM materials to be used for highly stressed components in environments ranging from 4 deg Kelvin (-269°C) up to 80°C.

With several decades of experience in this field, PPL Marine Products is able to assist customers with all design aspects of components in order to take maximum benefit from the materials available and produce the ideal manufacturing solution.

Moulds for Reaction Injection Moulding

Due to the relatively low pressure and temperature involved in the RIM process, moulds can be made from either aluminium or epoxy resin. Which of these materials is used for each application depends largely on the required life of the mould and the complexity of the component to be moulded.

For production of 10,000 mouldings or more, aluminium moulds are cost effective, and cost considerably less than injection moulding tools. Aluminium moulds should usually be used where a component has fine detail.

PPL Marine Products has developed moulds made from epoxy resin to cater for lower production numbers than aluminium tooling. PPL Marine Products ’s in-house epoxy resin mould manufacturing capabilities ensure that epoxy resin tooling is cost effective, with a shorter lead-time than aluminium tooling.

Rigid Solid Polyurethane Foam

Description

Solid Polyurethane RIM materials are similar in appearance and physical properties to conventional injection moulding materials such as ABS, but due to the low temperature and pressure of the moulding process, can be produced using relatively inexpensive moulds.

Dimensional Properties

Solid PU RIM materials are suitable for thinner wall thicknesses than Rigid Self-skinning PU foam. The typical wall thickness of components moulded with this material is around 3-4mm, with a minimum thickness of 1.5mm and a maximum thickness of 15mm.

Colour and finish

Solid PU RIM materials can be coloured through either pigmentation or painting. Specially formulated paints have been developed to chemically bond to the surface of the PU, creating an extremely durable finish.

Safety

Fire retarding agents are incorporated into the chemical formulation to achieve UL 94 V0 and 5VA ratings.

Rigid Solid PU Properties

Rigid Self-skinning Polyurethane

Foam Description

Self-skinning polyurethane (PU) foams are produced by mixing two liquid chemicals and injecting a controlled quantity of the mixture into a mould. The heat developed by the reaction causes a non-CFC blowing agent, present in one of the chemicals, to expand the mixture and fill the mould cavity. The foam structure collapses in contact with the mould surface forming a dense skin, and leaving a fine cell foam core.

Dimensional Properties

Self skinning PU foam allows the designer freedom to design components with both thick and thin sections, and to incorporate ribs on the back face, with minimal risk of sink marks showing on the surface. Sectional thicknesses can range from around 5mm to around 35mm.

Colour and finish

Solid PU RIM materials can be coloured through either pigmentation or painting. Specially formulated paints have been developed to chemically bond to the surface of the PU, creating an extremely durable finish.

Safety

Fire retarding agents are incorporated into the chemical formulation to achieve UL 94 V0 and 5VA ratings.

Rigid Self Skinning PU Properties