MATERIALS DATASHEETS OVERVIEW

When selecting a polymer for a particular application the material datasheet is one source of information on performance and suitability

By way of summary, listed below are the different properties along with a brief description of what they mean.  Before going further it should be re-emphasised that the data against different performance criteria, such as impact strength, is fairly meaningless in itself.  This is because the test is carried out on a predefined piece of material which bears little resemblance to any part that may be moulded.  Secondly, the test itself may not represent the loads and stresses likely to be experienced by the part that is to be moulded.

Material datasheets are of only limited use on their own.  The main purpose is therefore to compare different materials and different grades in order to select the one that provides the best range of properties to suit the application.

Shrinkage

Mould shrinkage is defined as the difference between the dimensions of the mould and the moulded part.  It must also be borne in mind that a plastic moulding will continue to shrink long after it has been ejected from the mould.  For this reason mould shrinkage figures are based on dimension changes 24 hours after the part has been moulded.

This figure is primarily of interest to the mould tool designer.

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plastics shrinkage

Density/Specific Gravity

Datasheets tend to show the Specific Gravity of a particular material.  This is the ratio of the weight of a given volume of a certain material to that of the same volume of water at a given temperature.

Therefore, multiplying the volume of a part in cm3 by the specific gravity will give the weight of that part in grams when moulded in the selected material.

This is important to the designer if the weight of the finished part is critical.

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pyknometer

Melt Flow

The Melt Flow Rate (MFR) or Melt Volume-Flow Rate (MVR) test measures the flow of a molten polymer under specific temperature and load conditions.

As a rule, an easy-flowing grade of material might be selected if there is a long flow length, thin walled section or a smooth surface required.

A high viscosity grade should be selected if the parts are likely to be subjected to severe mechanical stresses; this is because as a general rule high viscosity grades tend to be tougher, have improved heat resistance and overall better mechanical properties.

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Rutland Plastics mfi testing

Flexural Strength and Modulus

Flexural strength is the measure of the stiffness of a material, that is to say how well it resists bending.  The higher the number the stiffer the material.

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plastics flex test

Tensile Strength, Strain and Modulus

This gives an indication of a material’s performance under load.  The given load is defined as the stress and the amount of deformation is known as the strain.  By knowing the amount of strain a designer can have some idea of how a particular product or component will perform under working conditions.

Due to the majority of plastic materials being sensitive to temperature, the likely working conditions for the product or component need to be borne in mind when studying the performance data.

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plastic tensile test equipment

Impact Strength


plastic notched impact test piece

Vicat Softening Temperature

This is the temperature at which the plastic starts to soften rapidly.

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hdt tester

Heat Deflection Temperature (HDT)

This is a relative measure of a material’s ability to perform for a short time at elevated temperatures under load.  Therefore, the test measures the effect of temperature on stiffness.

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HDT test bar

Thermal Conductivity

This is measured to calculate the thermal insulating capacity of a plastic material.

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plastic thermal conductivity

Relative Thermal Index

This was formerly known as the Continuous Use Temperature Rating (CUTR).  The Relative Thermal Index (RTI) is the maximum service temperature at which the critical properties of a material will remain within acceptable limits over a long period of time.

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plastic test bar

Coefficient of Linear Expansion

Any material expands when heated.  This is used to calculate changes in length, breadth and thickness of a moulded part with changes in temperature.

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plastics thermal testing

Flammability

UL94 ratings are the most widely accepted standards covering flammability of plastic materials.  These are intended to indicate a material’s ability to extinguish a flame once ignited.  Several ratings can be applied depending upon the performance, and each material may receive different ratings based upon thickness and/or colour.

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plastics flame retardants

Horizontal Testing (UL94HB)

Where flammability is a safety requirement, HB materials are not normally permitted.  Generally, HB classified materials are not recommended for electrical applications except for mechanical and/or decorative purposes.

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plastics horizontal flammability testing

Vertical Testing (V-0, V-1, V-2)

V-0 is the highest performance down to V-2 which is the lowest.

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plastics vertical flammability testing

Vertical Testing (5V, 5V-A, 5V-B)

This is the most severe of all the UL tests.  5V is the highest down to 5V-B which is the lowest.

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plastics flammability testing

Limited Oxygen Index

The oxygen index represents the minimum level of oxygen in the atmosphere which can sustain flame on a thermoplastic material.

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limited oxygen index

Glow Wire

The glow wire test simulates what may occur if the material is exposed to sources of heat, such as overloaded resistors or glowing elements.

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plastics glow wire test

Needle Flame

This test simulates the effect of small flames that may result from faults within electrical equipment.

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plastics needle flame test

Hardness

There are a number of different tests to measure the relative hardness of different materials.  These tests are predominantly used as a measure for thermoplastic rubber types of plastic as a means of comparing softness and flexibility.

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Polymer Hardness scale

Rockwell Hardness

This measures the indentation resistance of a material with higher numbers reflecting harder materials.

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Plastic hardness test

Shore Hardness

These values are derived from measuring the indentation of a plastic material with a conical steel rod – type A for softer materials and type D for harder ones.  The higher the number the harder the material.

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plastics hardness test

Dielectric Strength

This reflects the electric strength of an insulating material.  It is also a measure of the maximum electric field strength a material can withstand without breaking down (failure of its insulating properties).

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Surface Resistivity

When a voltage is applied to an insulating thermoplastic, some portion of the current will flow along the surface.  Surface resistivity is a measure of the material’s ability to resist that surface current.

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Volume Resistivity

This is similar to surface resistivity except it is a measure of current leakage through the body of an insulating material.

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Dielectric Constant or Relative Permittivity

Materials being used to insulate electrical components should have a low dielectric constant.

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multi coloured plastic granules at Rutland Plastics

Dissipation Factor

A low dissipation factor is important for plastic insulators in high frequency applications such as radar equipment and microwave parts.  Smaller values mean better dielectric materials.  A high dissipation factor is important for welding capabilities.

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Arc Resistance

When an electric current travels across an insulator’s surface, this surface will become damaged over time and become conductive.  Arc resistance is a measure of the time in seconds it takes to make an insulating surface conductive.

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Comparative Tracking Index

This is defined as the maximum voltage at which no failure occurs.  CTI tests are undertaken to evaluate the safety of components carrying live parts.

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Rutland Plastics multi-coloured polymer granules

Haze

This test measures absorption, transmittance and deviation of a direct beam of light by a translucent material.  The amount of light that deviates by more than 2.5 Deg from the direction of the incident beam expressed as a percentage.  The higher the value, the more transparent the material.

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styrene moulding

Transmittance

The percentage of incident light that is able to pass through the specimen material.  The higher the value, the more transparent the material.

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Rutland Plastics acrylic moulding

Gloss

This is a measure of the ability of a material to reflect light.  The higher the value the more gloss a material has.

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plastics gloss measurement

Moisture Absorption

Plastic materials absorb water to varying degrees.  Moisture content may result in changes in dimensions and some other properties.

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plastics material drier