What are the important mechanical and physical properties of Plastics?

STRESS

Load per unit area is called stress. If the loads can be predicted and the part shape is known, then the designer can estimate the worst load per unit of cross sectional area within the part. If the force is in newtons and area is in square metres, then the units for stress are newtons per square metre and 1N/m² equals 1 Pascal (Pa).

STIFFNESS (MODULUS)

The measurement of the stiffness of the material is called the “modulus” or “modulus of elasticity”. If the maximum amount of bending that can be allowed and the shape of the part are known, then the designer can often predict how stiff a material must be. The higher the modulus number the stiffer the material; conversely, the lower the number, the more flexible the material. The modulus also changes as the temperature changes. Modulus numbers are given in MPa.

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 sizes.

STRAIN        =             (Final length  – Original  length)          =         Change in length or Deformation

                                               Original length                                                        Original length  

 

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 or torsional modulus.

MODULUS  = STRESS   or in other words MODULUS = Load ÷ Change in shape when loaded (STIFFNESS)

                        STRAIN

Choose the type of modulus in the property sheet that most nearly duplicates what the customer expects the major load to be. If the load is unknown, use the lowest moduli value of the two. These numbers can be used for short term loading if the load is to be applied for only a few days at the most. The stress/strain equation is the equation used by designers to predict how a part will distort or change size and shape when loaded. Predicting the stress and strain within an actual part can become very complex. Fortunately, the material suppliers use tests that are easy to understand.

YIELD POINT

The yield point is that point when a material subjected to a load, tensile or compression gives and will no longer return to its original length or shape when the load is removed. It is a very important concept because a part is usually useless after the material has reached this point. Some materials break before reaching a yield point (e.g. some glass filled nylons or die cast aluminium).

TENSILE STRENGTH

This is the maximum strength of a material without breaking when the load is trying to pull it apart. This is the most common figure given by suppliers to report tensile properties in their advertising. Plastics may demonstrate tensile strengths from 4.8MPa to 345MPa.

ELONGATION

Elongation is always associated with tensile strength because it is the increase in the original length at fracture and expressed as a percentage.

COMPRESSIVE STRENGTH

Compressive strength is the maximum strength of a material without breaking when the material is loaded. This term becomes less meaningful with some of the softer materials such as P.T.F.E. does not fracture. Consequently, the compressive strength continues to increase as the sample is deforming more and more. A meaningful compressive strength would be the maximum force required to deform a material prior to reaching yield point.

SHEAR STRENGTH

Shear strength is the strength of a material when the material is loaded with the surfaces being pulled in opposite directions. Some examples of items that experience shear loading are the nail holding a picture on the wall, the cleats of an athlete’s shoe and tyre tread as a car accelerates or brakes.

FLEXURAL STRENGTH

Flexural strength is 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 (squeezed together), while the bottom of the beam is in tension (stretched). Somewhere in between there is a place with no stress and this is called the neutral plane. Skis, fishing poles, pole vault poles and diving boards are examples of parts needing high flexural strength.

TORSIONAL STRENGTH

Torsional strength is the strength of a material when a shape is subject to a twisting load. Examples of parts that require a high torsional strength are screws and the drive shaft on a motor vehicle.

POISSON’S RATIO

Sometimes a designer will need a value for Poisson’s ratio. This ratio occurs in some of the more complex stress / strain equations. It is simply a way of saying how much the material necks down or gets thinner in the middle when it is stretched.

CREEP

When large weights are hung on bars of different materials, all will experience some initial and immediate deformation or stretching when the 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 metal bar will return to its original shape. The length of a plastic bar will continue to slowly increase as long as the load is applied. This “creep” increases as the load and/ or temperature are increased. Some thermoplastics like nylons will creep more when they have been softened because of the presence of moisture.

FATIGUE STRENGTH

Plastics, as well as other materials subjected to cyclic loading, will fail at stress levels well below their tensile or compressive strength. The combination of tension and compression is the most severe condition. This information will be presented in S-N curves or tables which stand for Stress – Number of cycles. A part will survive more cycles if the stress is reduced. The stress can be reduced by reducing the deflection and / or decreasing the thickness of the part. Some examples of cyclic loading are a motor valve spring or a washing  machine agitator.

IMPACT STRENGTH

Impact strength is the ability to withstand a suddenly applied load. Toughness is usually used to describe the material’s ability to withstand an impact or sudden deformation without breaking. No single test has yet been devised that can predict the impact behavior of a plastic material under the variety of conditions to which a part may be subjected. Many materials display reduced impact strength as the temperature is lowered. Thermosets and reinforced thermoplastics may change less with changes in temperature. Some of the impact tests commonly used in supplier literature are as follows: – Izod Test (a swinging pendulum is suddenly impacted on the material; Tensile impact Test; Gardner Impact Test, Brittleness Temperature Test.

NOTCH SENSITIVITY

Some plastic materials have exceptional impact performance and very good load carrying capacity. However, the performance of a material can be greatly reduced by having sharp corners on the part either from the design or from machining operations. The Izod impact strength of a tough material like polycarbonate is reduced from 20 to 2 as the radius (R) of the notch is reduced from 0.5mm R to 0.05mm R. The sharp corners not only reduce the impact resistance of a part, but also allow for a stress concentration to occur and encourage the premature failure of a load carrying part. Minimising sharp corners may make the machining operation but it may be critical to the part’s success. Edges of sheet (such as acrylic and polycarbonate) being used in impact applications like glazing must be finished to be free of sharp notches.

MELT INDEX

Melt index determines the internal flow rate of a plastic through a die at a given temperature and load. In more common terms this test will tell you how fast the plastic will flow when heated and affects how the material will process, fill a mould or flow through a die. Done according to ISO 1133, a sample of plastic is charged into a heated cylinder within the melt index apparatus. A weighted piston is used to push the plastic through the cylinder and through a die at the end. The temperature at which a sample is tested is predetermined by the standard for each given polymer.. After the cylinder is charged and the weighted piston is in place, the piston is blocked and the plastic is heated for 6 – 8 minutes. At that point the block is removed, and the weighted piston forces the molten plastic through the die. A sample is collected for a specific time interval and the extrudate is massed. The melt flow is determined by the following equation;

                                   Grams of extrudate              x 10 = melt flow ( g/10 min)

                                      Time in minutes

Melt flow is affected by the molecular structure of a polymer. The more complex the molecule, the melt flow will tend to be lower. If a plastic has polymer molecules of approximately the same length, the melt flow will tend to be higher. Melt index is also affected by fillers and reinforcements in a plastic. Crystallinity will affect the melt flow of a material. The range of melt flows can be from 0.5 g /10 min to as much as 25 g /10 min.

SPECIFIC GRAVITY

For solids and liquids, specific gravity is the ratio of the density of a material to the density of water at 4⁰C which is taken as 1.00 (since 1cm³ of water weighs 1 g & 1 m³ weighs 1000kg or 1 tonne). If the size of the part is known, specific gravity can be used to determine the weight of that part in a variety of materials.

WATER ABSORPTION

Water absorption is the degree of penetration of water into the inner structure of another material. A common measure for the degree of absorption is the percentage swell, which measures the change in the surface area of a material. Plastics such as polyethylene have extremely low water absorption, whereas nylon has a relatively high rate of water absorption.

GLASS TRANSITION TEMPERATURE

Glass transition temperature is the temperature above which an amorphous polymer is soft and rubbery. Below this temperature, an amorphous polymer is hard, brittle and glassy.