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The role of plastic in the design and manufacturing of automotive vehicles has never been more essential. Stringent regulations and changing consumer habit have driven and continue to drive demand for more affordable, lightweight and fuel-efficient vehicles.

Fuel efficiency has become one of the most important features in automotive vehicle design due to the rising fuel prices and stricter environmental regulations across the world. And while some materials may benefit from recent changes within the automotive industry, others will find themselves on the losing side.

Currently, there are approximately 30,000 parts in any single vehicle, of which anywhere between one third and a half of these parts are made of plastic. There are thousands of different plastics in the world, and dozens of basic plastics and polymers are used to make an automobile. However, more than 70% of the plastics used in automobiles come from just four different polymers:

·       Polypropylene

·       Polyurethane

·       Polyamides

·       PVC

Plastic has become one of the key materials required for the structure, performance and safety of automobiles in recent years, with growth in plastic consumption being driven by light weighting trends for fuel efficiency and consequently lower greenhouse gas emissions. The high absorption properties of plastics also allow the vehicle to meet stricter safety standards while the use of engineering plastics allows for minimisation of the mass of parts used in vehicles as they offer more design freedom compared to metals.

However, when opting for the use of these ‘softer’ materials, the assembly method must be considered. The structure of a vehicle will still be made of metals however and bolting plastic parts to metal components can cause an obvious issue of plastic substrate relaxation as well as stress cracking of the plastic material depending on the hardness. This is where compression limiters come into the equation.

What Does a Compression Limiter Do?

Put simply, the purpose of a compression limiter is to provide and maintain joint integrities of assemblies which contain plastic component.

Compression limiters are designed to protect the plastic components of an assembly from the compressive loads generated as the assembly forces are applied during the tightening of bolts.

Designed to be slightly shorter than the dimensions of the plastic they surround, compression limiters allow a small amount of compression in the plastic substrate – usually from 3% to 5% – until the metal parts are mated.

Any further clamp forces applied will be absorbed by the limiter without inducing stress in the plastic substrate which and therefore, avoid stress cracking of that material.

Design of a Compression Limiter

There are some crucial design elements to factor into the manufacturing and production of compression limiters; the mating faces of the bolt flange or washer will be contiguous with the plastic substrate and the face of the compression limiter when the load is applied. In turn, this will prevent the clamp force being degraded by the relaxation of the plastic.

The proof load of the compression limiter should be at least equal, if not greater than bolt alignment.

It is important to note than compression limiters can only be used when the materials used in the mating components are strong enough to take the clamp loads. The clamp load of the bolt is transferred to the mating component through the compression limiter.

Through this, it can be established if the material of the mating component is strong enough to withstand the required clamp force generated by the torqueing of the bolt by dividing the clamp load applied to the limiter by the cross-sectional areas of the part.

If the calculated stress exceeds the yield strength of the material, permanent deformation will be induced, resulting in loss of clamp load.

The selection of the bolt should be such that the maximum torque applied does not exceed 75% of the bolt proof load, giving a safety limit. Different grades and classes of bolt are available for selection as a result.

The clamp load can be calculated by using the ‘nominal diameter’, the co-efficient of friction and the torque applied.

Once these calculations are worked out, the compression limiter can function in its designated purpose.