Appropriate use of plastics extrusions

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Plastics have many advantages over more traditional materials, for example low density, good insulation and corrosion resistance, price and so on, but unwise or uninformed use of plastics in substitution applications has sometimes led to the word “plastic” having derogatory implications. It is important to realise the limiting factors with plastic design in order to maximise the considerable advantages of designing with plastics.

The limiting factors

Temperature

Thermoplastics, by their very nature, have an upper limit on temperature above which softening occurs and functioning is impaired. In considering temperature it is important to consider not only the operating conditions but also the production, storage and transportation conditions. For example, products destined for inside use only may spend some time in a container on the dockside in the sunny Middle East and the temperatures reached may be quite astronomical. The limiting long term service temperature for most commercial plastics is between 600C and 1000C, and above the 1000C boundary the options for plastic selection rapidly diminish. Above 1500C the only applicable materials are the engineering plastics such as polysulphones, polymides and polyetheretherketones (PEEK). The cost effects of using these materials are severe, from £5000 - £15,000 per tonne.

The correct specification of maximum long-term service temperature thus has an enormous effect on the price of the finished article. It is not enough, however, to just specify a temperature; consideration must also be given to the following factors:

For many applications these factors may be critical and successful economic production requires careful consideration and possibly investigation of temperature at the design concept stage. If in doubt Tangram Technology, the material supplier or the potential processor should be contacted for expert advice at an early stage.

Mechanical properties

Compared to metals, plastics generally tend to have low absolute mechanical properties at room temperature in terms of tensile strength, impact strength, Young's Modulus and hardness. If, however, the comparison is to be made on the basis of cost per unit of tensile strength then the choice is not as clear cut and depends on the materials being compared. This means that a plastics article may be made thicker than a metallic article to give the same functional response for a given cost.

The real problem emerges when the Young's Modulus (E) or stiffness is an important factor. Plastics have typical E value of 1.1 to 14 GN/m2, compared with metal values of 70GN/m2 and upwards. In this the variation is so great that increasing the thickness of the article is not a realistic option because the cost per unit stiffness for plastics is approximately 10 times that of metals. If stiffness is a requirement then this must be provided by shape factors such as good design for stiffness, rather than simply relying on the material characteristics. The available engineering plastics provide excellent mechanical properties, but the high costs involved make sensible design and value engineering procedures at an early stage even more crucial.

As visco-elastic materials, thermoplastics experience creep or flow phenomena, which means that while a load below the yield stress (however this is defined) may be acceptable in the short term it may be unacceptable in the long term. This is due to the possibility of creep of the material resulting in loss of function. As with all mechanical properties and responses of plastics, creep is highly temperature dependent and becomes more of a problem at elevated temperatures.

An important point to note with the response of a plastic to any mechanical stress is the concept of strain rate temperature equivalence. This means that a plastic's response to higher operational strain rates is similar to its response to lower operating temperatures and vice versa.

Chemical resistance

Most plastics have excellent chemical resistance and this is largely complementary to metals, meaning that what attacks metals will not generally attack plastics and vice versa. As a general rule plastics provide excellent resistance to water, salt solutions, weak acids and bases, but are affected by solvents and oxidising agents.

Tolerances

Extrusion as a process can produce excellent repeatability but the tolerances achievable are not as fine as for machined or other similarly produced items. It is important that critical areas be marked and the producers' advice on achievable tolerances be sought. Requests for +/- 0.01 mm are not generally realistic. If a guide is required, the BS 3734 (Table 2 Class E2) should be used but specific instances may require variations to this. The addition of more tolerances almost invariably increases the cost of manufacturing the product.

Volume demand

In important factor in the choice of any production method is the volume demand. In general, extrusion equipment and die costs are high and depending on the profile considered the minimum economic quantity is approximately 300 -10,000 m. While 200,000 cable holders of 10 mm long may appear a large order, the total extrusion length required is only approximately 2200 mm (allowing for the saw cut) and injection moulding may well be cheaper (and allow for inclusion of other features such as a screw hole). In many cases there will also be a minimum order quantity to cover the costs and time of setting up production. In this context a total demand of 1000 m in call-offs of 100 m is almost certainly uneconomical for the producer.

The applications

The limiting factors given above may appear daunting in some instances but one has only to examine the amount of extruded profile in daily use to see that good design can provide economical and useful products in all areas of activity. Some examples of applications using various good design ideas are given below:-

Window systems:

Apart from the fact that PVC-U windows show the ability of extrusions to be fabricated (in this case welded) to form 3D structures these also show a way to circumvent the low stiffness of plastic profiles. The outer frame of a window is generally fixed to the window aperture and thus no problems with stiffness arise. The opening lights must bear the load of the glass and above certain sizes are not sufficiently stiff. In these cases the profile is stiffened by the insertion of rolled steel or extruded aluminium sections and the composite profile provides excellent stiffness. In this instance it is important to provide segregation of the components to remove the possibility of corrosion of the reinforcement. Temperature also affects window design but in this case it is the thermal expansion of the profile which must be considered, and the window frame must be made approximately 10 mm smaller than the aperture to allow for the expansion of the frame due to temperature changes (more allowance should be made for brown or dark coloured frames as the temperature reached is higher and hence the expansion is also higher).

Hot and cold water piping:

New plastics now allow applications in both central heating systems (underfloor using pumped hot water) and in hot and cold water distribution, both in houses and for district heating schemes. The most common plastics used are polybutylene, chlorinated PVC and crosslinked XPE. Performance varies with individual dimensions but the general rule is that at 800C flow temperature a life of 25 years is obtained at 6.3 bar operating pressure. This is far in excess of the normal UK operating pressure for unpressurised or vented systems and actual service life should be more than 50 years. The temperature and load effects in this instance are not short term and detailed design calculation and testing are necessary to avoid any concerns with creep or service malfunctions. This example also illustrates the resistance of plastics to common chemicals and the pipes are not affected by any normal corrosion inhibitors present in the water. One of the normal advantages of plastics, electrical insulation, is in this case a potential problem, as the cold water system cannot be used as an electrical earth although this is also being affected by the use of the PE pipes in water distribution in any case.

Electrical profiles:

The use of plastics in electrical and electronics applications is almost endless and the advantage of electrical insulation by plastics means that without plastics the modern electrical/electronics industry would not exist. Extrusions are seen in lighting tracks (where high temperatures sometimes require the use of PPO), trunking, cable management and, most importantly, in wire coating.

Plastics are an extremely valuable material in design but the best plastic in the world cannot save a design concept made without considering the inherent limitations of the material. If you want to design products in plastic and need assistance then please contact Tangram Technology Ltd. today!

 

Last edited: 11/03/10

© Tangram Technology Ltd. 1998

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