Plastics History And Applications

Plastics, History, Applications and Stresses By S Hornsey VSR (Africa) cc with appreciation to Main Industries for the supply of technical specifications

Plastic materials display properties that are unique when compared to other materials and have contributed greatly to quality of our everyday life. Plastics, properly applied, will perform functions at a cost that other materials cannot match. Many natural plastics exist, such as shellac, rubber, asphalt and cellulose; however, it is man’s ability to synthetically create a broad range of materials demonstrating various useful properties that have so enhanced our lives. Plastics are used in our clothing, housing, automobiles, aircraft, packaging, electronics, signs, recreation items and medical implants to name but a few of their many applications. The synthetic plastic industry started in 1909 with the development of a phenol formaldehyde plastic (Bakelite) by Dr. L.H. Backland. The phenolic materials are even today important engineering plastics. The development of additional materials continued and the industry really began to blossom in the late 1930’s. The chemistry for nylons, urethanes, and fluorocarbon (Teflon®) plastics were developed; the production of cellulose acetate, melamine and styrene molding compounds began; and production of commercial equipment to perform the molding and vacuum forming processes began. Acrylic sheet was widely used in aircraft windows and canopies during World War II. A transparent polyester resin (CR-39), vinylidene chloride film (Saran), polyethylene and silicone resins were also developed. The first polyethylene bottles and cellulose acetate toothpaste tubes were manufactured during this time period. The post war era saw production of vinyl resins started the use of vinyl films, molded automotive acrylic taillights and backlighted signs introduced and the first etched circuit boards developed. The injection molding process entered commercial production. Due to the newness of the materials, the properties and behavior of the plastic materials were not well understood. Many products were introduced that failed, creating a negative impression about plastics in the public’s mind. Chemists continued the development of materials, such as ABS, acetals, polyvinyl fluoride, ionomers, and polycarbonate. The injection molding, thermoforming, extrusion, transfer molding and casting processes were all improved. This allowed the industry to provide an even greater number of cost-effective products suitable for many more demanding engineering applications. As the material composition become more complex, then so do its inherent problems. Fabricators and machinists are encountering similar problems with plastics as often encountered on traditional materials such as carbon steels and non-ferrous materials. Many of these problems, an example being dimensional instability are linked to high concentrations of residual stress and uneven material strains.

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Mechanical Properties and Stresses in Plastics Design A designer or engineer will often use design equations that work with metals while a part is being designed. Most metals behave like springs; that is the force generated by the spring is proportional to its length. A plot of the force as a function of length is a straight line.

When materials actually work this way, they it is called “Linear” behavior. This allows the performance of metals and other materials that work like a spring to be quite accurately calculated. A problem occurs when the designer tries to apply these same equations directly to plastics. Plastics are “non-linear”.

Stress High concentrations of stress within certain parts of a component can have disastrous results. If the loads can be predicted and the part shape is know then the designer can estimate the worst load per unit of cross-sectional area within the part. Load per unit is often referred to as Stress.

VSR has been proven to reduce residual stresses in metal components by up to 87%.

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Strain The measurement of how much the part bends or changes size under load compared to the original dimension or shape is called “strain”. Strain applies to small changes in size; VSR has the ability to redistribute surface strains by up to 100% within a few minutes of treatment.

Strain = (Final Length – Original Length) / Original Length = Change in length or Deformation / Original length If the change in size is in millimeters and the original dimension was in millimeters, then the units for strain are mm per mm. Stress, Strain and Modulus are related to each other by the following equation. The modulus or stiffness of a material can be determined when the material is loaded in different ways, such as tension, compression, shear, flexural (bending) or torsion (twisting). They will be called Tensile Modulus, also known as plain Modulus, Flexural Modulus, Torsional Modulus, etc.

Modulus = Stress / Strain Or Modulus = Load / change in shape when loaded. (Stiffness). The stress / strain equation is the equation used by designers to predict how a part will distort or change shape when loaded. Predicting the stress and strain within an actual part can become very complex. Fortunately, reducing the stress and redistributing the strains can be very easy with the correct application of VSR.

Some additional terms used to describe material behavior. Yield Point The yield point is that point when a material subjected to a load, tensile, compressive, etc. gives (yields) and will no longer return to its original length or shape when the load is removed. Some materials fail before reaching yield for example certain cast irons and die cast aluminum’s. Tensile Strength The maximum strength of a material without breaking when the load is trying to pull it apart. This is the system often used by material suppliers to list tensile properties in their sales literature. Unlike thermal stress relieving VSR will not reduce the tensile strength of certain materials

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Elongation is always associated with tensile strength because it is the increase in the original length at fracture and is expressed as a percentage. For an example the HDPE pipe as tested in the previous tests has an elongation factor of >600% whilst at yield it is only 16%. It will therefore undergo considerable deformation before fracture takes place. Compressive Strength The maximum strength of a material without breaking when the material is loaded. This term becomes less meaningful with some of the softer materials. Teflon®, for example, does not fracture. Compressive strength would be the maximum force required to deform a material prior to reaching the yield point.

Flexural Strength The strength of a material when a beam of the material is subjected to bending. The material in the top of the beam is in compression, while the bottom of the beam is in tension. Somewhere in between the stretching and squeezing there is a place with no stress and it is called the neutral plane. A simple beam supported at each end and loaded in the middle is used to determine the flexural modulus given in properties tables. Stress relieving normally increases the flexural strength of materials

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Poisson’s Ratio This ratio occurs in some of the more complex stress/strain ratio equations; it is generally a simple way of saying how much the material necks down or gets thinner in the middle when it is stretched. Its value is most often between .3 to .4 for plastic materials.

Torsional Strength The strength of a material when a shape is subjected to a twisting load. The drive shaft on a vehicle is a typical example of a component requiring high torsional strength The use of VSR does not reduce the torsional strength of a component.

Creep All materials will experience some initial and immediate deformation or stretching when a load is first applied. As long as the yield point has not been exceeded, a metal sample which acts like a spring will not stretch any more regardless of how long the weight is left on. When the weight is removed, the component will return to its original shape. The length of a “thermoplastic” bar will continue to slowly increase as long as the load is applied. This is called creep. The amount of creep increases as the load and / or temperature is increased. If the weight (stress) is left on over a period of time, the amount of bending or elongation continues to increase and the value for the modulus will decrease with time. This decreasing modulus that is a function of time (and even temperature) is called “creep modulus” or “apparent modulus” . 5

This is the modulus that the designer should be using to more accurately predict the behavior of the plastic materials. This information is normally available from the supplier or manufacturer. Following is the formula for calculation of creep.

Apparent Modulus= .

Stress . Initial Strain + Creep Strain

=.

Stress Total Strain

.

Creep is affected by: Load (stress) Temperature Period of time that load is applied Since the stress is kept constant, the equation becomes: Apparent modulus x Total strain = Constant (stress) If the strain increases then the apparent modulus must come down. As the strain increases with time and temperature, the apparent modulus decreases with time and temperature This means that the strain is held constant and the decrease in the load (stress) is measured over time. The equation then becomes: Apparent Modulus / Stress = Constant Strain This calculation can be critical in components where dimensional stability is the main criteria, as is proven by the above formulas plastics do succumb to the ageing or weathering effect as suspected on the VSR test of an eight year old pipe.

Vibratory Stress Relieving has been proven if correctly applied to give 100% dimensional stability to components.

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Conclusions •

This paper has been a brief attempt at explaining the history and many applications of plastics in today’s environment. Similar problems as are encountered with the design and manufacture of steel components are often encountered using plastics, the most common of which appears to be dimensional stability and creep rupture. Both are commonly associated with high stress concentrations within the material.

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As the creep ratios are affected by time and temperature with plastics this is an important point to bear in mind as with pipelines containing either water or gas as they are under a constant stress (internal pressure). It therefore stands to reason that careful consideration must be given to material selection and the possible stress relieving of pipes.

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As described within the paper many of these problems can be overcome by using careful material selection, good design and good engineering practice during manufacture and. It must be born in mind that no form of stress relieving is a replacement for poor design and bad engineering practice during manufacture.

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As the tests carried out have clearly demonstrated stress levels in plastics can be reduced by the application of VSR, and material stability is restored.

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An interesting aspect of the tests, and it is proven by the attached stress / strain / creep formulas is that plastics appear to suffer from the affect of material ageing.

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It would however be recommended to carry out more detailed tests in a more controlled environment before recommending VSR as a method of stress relieving.

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Although VSR has a sixty-year proven track record on conventional materials, its applications on plastics and similar materials are yet to be exploited. Owing to the huge savings in component downtime, VSR offers considerable advantages if proven successful in this unique field.

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July 1999

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