Chapter 2

Plastics that show their strength

Choosing the right plastics material to use for a technical part is best done in successive steps.

A sequence of requirements, considered one after the other, will narrow down the possibilities and finally lead to the right material for the application.

Plastics parts for technical applications usually have to meet a whole range of requirements. They include minimum values for strength, stiffness and thermal deformation temperature; often, impact resistance, notched or unnotched, creep resistance and chemical resistance also play a role.

Commodity plastics sometimes offer one or other of the required properties. But in most cases, only one of the engineering plastics will meet the whole range.

There are no simple rules for choosing the most suitable resin. Rather, it is a question of understanding the salient characteristics of the engineering plastics.

From this starting point, the selection can be whittled down, step-by-step, until the right type is pin-pointed. The typical properties of some important engineering plastics can be summarised as follows:

Summary of typical values for tensile strength, tensile modulus and
maximum temperature resistance for short exposures

• Acetal resins, also known as polyoxymethylene (POM), with trademarks such as Delrin®, show high values for strength, stiffness and surface hardness, which in some respects gives them metal-like properties. One reason for this is their high crystallinity, which also ensures good resistance to fatigue and abrasion.
• Nylons, polyamides (PA) with trademarks such as Zytel®, are available in various sub-types (for example, PA6 and PA66), offering a broad range of properties. Generally, they offer a good balance of high strength and toughness, and they withstand aggressive environments. In addition, they have good damping behaviour, high abrasion resistance and a low friction co-efficient; these features often allow them to go on working when lubrication breaks down. Their good flow characteristics mean nylons are easy and economical to process.
• In thermoplastic polyesters, including polyethylene terephthalate (PET), with trademarks including Rynite®, and polybutylene terephthalate (PBT), with trademarks such as Crastin®, outstanding electrical characteristics (insulating properties, arc resistance) complement their good mechanical properties. In plastic gear drives, where especially low friction and good abrasion-resistance are called for, the practice is often to make one cogwheel of POM, the facing one of PBT.
• Polycarbonate (PC) is the most important amorphous engineering plastic material. It is as clear as glass, its toughness is very high, and its strength and hardness are good. Thanks to its glass transition temperature of 130°C, these good mechanical properties can be used in the -150 to +135°C temperature range. Moulded PC parts can be worked mechanically and polished.

Determining the choice

Conveyor belt made of Delrin® has good friction and abrasion-resistant properties, low creep and high dimensional stability

The integrated hinge is flexible but the clip is rigid when closed. Zytel® nylon meets these apparently contradictory requirements and its thermal deformation temperature is sufficient for the 120°C temperatures met in the motor compartment

Its high surface hardness has made Delrin® acetal a favourite for plastics gear drives.

To choose the right plastic resin for any given part, the requirements for that part should be arranged in order of importance.

Conveyor belt – robustness and dimensional stability

In accordance with this guideline, high strength and impact resistance – combined with very good abrasion resistance and low friction – are the most important qualities for sections of a conveyor belt. Smooth operation also requires good elastic behaviour with low elongation and high resilience, low creep and good dimensional stability irrespective of temperature and humidity conditions. These were the main requirements which steered the decision towards using POM, which also resists lubricants and chemicals that may be transported on the belt. The type of Delrin® chosen ensures silent operation of the belt and its good flow properties mean it can be injection moulded economically.

Hose clip – a multi-functional part in the motor compartment

A hose clip used in a car’s motor compartment serves as a good example of function integration with plastics. This single part integrates three functions —hooking the clip into its mounting, acting as a hinge and locking the clip to hold the hose. It takes a plastic material, in combination with good design, to combine the flexibility needed for a hinge with the stiffness needed for the closed clip. The material of choice for this application is Zytel® nylon. The need for a material with a thermal deformation temperature, which can withstand up to 120°C near the motor as well as fuels, lubricants and cleaning agents, also played a decisive role.

Gear drives – that need no lubrication

Choosing the right material for plastics gear drives is relatively simple. In this application, POM has proved its worth many times over. Its high surface hardness, ensuring very low friction and abrasion – and its combination of high strength and flex-fatigue resistance, with good elastic behaviour are the decisive factors here. Its high dimensional stability, low noise generation and lightness are further advantages. Suitable combinations, such as Delrin® 100 with Delrin® 100AL, make unlubricated gear drives possible.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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