CASE


04

2023

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12

Reaction Injection Molding


Reaction injection molding is often referred to simply as 'RIM'. The process involves injecting a two-part polyurethane resin (PUR) into a mold at low pressure. Relatively low tooling costs make it ideal for prototyping and low-volume production. It is suitable for making parts up to 2.5 m (8.2 ft) long.

Reaction Injection Molding

Reaction injection molding is often referred to simply as 'RIM'. The process involves injecting a two-part polyurethane resin (PUR) into a mold at low pressure. Relatively low tooling costs make it ideal for prototyping and low-volume production. It is suitable for making parts up to 2.5 m (8.2 ft) long.

Related processes include:

Reinforced Reaction Injection Molding (RRIM)

Alternative and competing processes include:

CNC Machining

Injection Molding

Vacuum Casting

 

What is Reaction Injection Molding?

The two ingredients that react to form polyurethane resin (PUR), polyol and isocyanate, are fed into the mixing head where they are combined at high pressure. There are two principal prepolymer systems: toluene diisocyanate based (TDI) and diphenylmethane diisocyanate based (MDI).The predetermined quantities of liquid chemicals are dispensed into the mold at a low pressure(one to two bar).As they are mixed they begin to go through a chemical exothermic reaction. The part is fully cured in about 30 minutes and is demolded.

PUR is a thermosetting resin and the reaction is one way. Therefore, once it has cured, the resin cannot be reused or recycled.

For high strength and more demanding applications the resin can be reinforced with glass fibres, or mica,for example. This is known as reinforced reaction injection molding.

QUALITY Even though this is a low-pressure process, the liquid PUR reproduces fine surface textures and details very well. The surface finish is determined by the quality of the mold and spray painting.

TYPICAL APPLICATIONS Some typical applications include prototypes and low-volume production runs of car bumpers,automotive interior panels, housing for large medical devises and vending machines.

COST AND SPEED Tooling costs are low to moderate. They are considerably less than when tooling for injection molding due to reduced pressure and temperature. Cycle time is quite rapid

(15-30 minutes). Labour costs are low to moderate and automated processes reduce labour costs significantly. Prototyping and lowvolume production require more labour input.

MATERIALS PUR is the most suitable material because it is available in a range of densities, colours and hardnesses. It can be very soft and flexible or rigid.

ENVIRONMENTAL IMPACTS PUR is a thermosetting material and so cannot be directly recycled. The isocyanates that are off gassed during the reaction are harmful and known to cause asthma. The polyol/MDI system produces fewer isocyanates than the TDI method.

Automotive panel Parts that are finished by spray painting can be masked to highlight design details such as logos and patterns. The level of sheen on the coating is categorized as matt (also known as egg shel), semi-gloss, satin (also known as silk) and gloss. High gloss, intense and colourful finishes are produced by a combination of meticulous surface preparation, basecoat and topcoat.

Complex and intricate details RIM is used to produce parts similar to injection molding. Indeed, it is often used to make prototypes of parts that will be injection molded in the long term because the tooling costs are considerably cheaper, but the unit costs are much higher.Details such as snap fìts, threaded inserts and vents can be molded into the part with minimal cost implications.

Prototyping For prototypes and very low volumes

RIM molds can be produced by CNC machining blocks of rigid PUR tooling resin. This technique is relatively inexpensive and is a very rapid way of producing a functional prototype suitable for application and testing.

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