Plastics are a critical building material across most industries. Because plastics can be used in everything from decorative pieces to mission-critical components, engineers must be able to gauge the mechanical strength and suitability of the materials used. The tensile strength of plastic and other mechanical properties play a critical role in determining which materials are appropriate for a particular application.
Tensile Strength of Plastic and Product Design
A drive for lighter, stronger products, using fewer materials, and being more eco-friendly, has driven many companies to look at new materials or methods for manufacturing their products. But changing to new material isn’t so easy, and an engineer must understand how the polymer will react under specific conditions. Rushed material selection can lead to product failure or worse. Proper planning, a thorough understanding of plastic materials, and prototyping are required. Start by defining the application’s mechanical, thermal, environmental, electrical, and chemical requirements. Then evaluate polymers against these requirements.
Below we discuss some of the mechanical properties that may be important to your application. Tensile strength and other mechanical properties of polymers allow engineers to understand the strength properties that are critical for many products.
What are the Mechanical Properties of Plastic?
The mechanical properties of plastic affect how it reacts under different types of loads and conditions. Tensile strength at yield and elongation at break are two common metrics for material performance. But choosing the material for your product goes beyond this since deformation is not your desired outcome.
Material data sheets are a starting point for choosing appropriate plastics – they shouldn’t be used for determining your polymer or product design. These sheets are provided by the polymer supplier and describe the properties of a specific material grade. However, depending solely on data sheets may not get you the results you expect. The performance results are based on testing to recognized industry standards conducted under specific conditions (e.g., temperature, humidity, machine, speed, pretreatments), which may vastly differ from the conditions your product will experience. You must also consider material compatibility and how they will react with other materials used. For example, some adhesives may cause certain plastics to become brittle. There may also be regulations that should be considered for what materials are acceptable for food or medical devices.
With the understanding that there is more to selecting a plastic than looking at the data sheets, we will look at a few of the mechanical properties that can be found on them. While there is no standard for data sheets, tensile strength, modulus of elasticity, and elongation are important mechanical properties to consider when designing a product.
Tensile Strength of Plastic
Tensile strength is the material’s ability to resist deformation when opposing loads (known as tensile loads) are applied. A tensile test is performed by holding the plastic sample between two clamps on a hydraulic press. One of the clamps pulls the plastic until it breaks.
There are two types of measurements — ultimate and yield strength. Ultimate strength is the maximum stress the material can withstand before fracturing. Yield strength is the stress at which material goes from elastic to plastic deformation. Elastic deformation is when the material returns to its original state after being put under stress. With plastic deformation, it does not and results in permanent, irreversible damage. Yield strength is the common criterion engineers use to define failure since permanent part deformation happens.
Modulus of Elasticity
A tensile test is also used to calculate the modulus of elasticity. Elasticity, as mentioned, indicates how the material restores to its original shape after being stretched. Also called tensile modulus or Young’s modulus, the modulus of elasticity is a measure of the stiffness of an elastic material. It is a ratio of stress (force per unit area) along an axis to strain (ratio of elongation or compression over initial length) along that axis. It is the force at a specific elongation (e.g.100, percent). This only applies to temporary deformation under stress. A higher modulus of elasticity means the material is stiffer.
Elongation at Break
Elongation at break measures how much bending and material can withstand without breaking and measures the polymer’s ductility. A tensile test measures the force required to break a specimen. The elongation at break is the ratio between the increased length and the initial length after breakage. Rigid plastics and those with fillers may have a value under five percent, whereas some elastomers may have a value of several hundred percent.
Elongation at Yield
Elongation at yield is also determined through the tensile test. It determines the stress-strain changes (difference in length from the original) up to the point of yield stress. Ultimate elongation is the percentage of change in length from original to rupture.
The Support You Need to Find the Perfect Polymer
Finding the right polymer to meet your many needs is a complicated process. At SEA-LECT Plastics, we will work with you to understand your product’s function and environment, as well as your quantity and budget, to help you choose the best resin for your project. Our design and engineering team has the experience to guide you through material selection and design for manufacturability so your injection molding process goes smoothly. Contact us to get started.