There are a vast number of different plastics available for injection molding purposes, each with its own unique properties and characteristics. It’s nearly impossible to have an exact count because new plastics are constantly being developed, and plastics can be combined to take advantage of specific characteristics of each, and additives can be used, such as glass fibers, to improve strength and rigidity. If you are looking for the strongest plastic for injection molding, keep in mind that the strongest plastic for injection molding may not have all the properties your project needs, and you may find that a combination of polymers or the use of fillers or additives may provide an optimal solution.
What is Strength?
Plastics are often used to replace metals, so strength is an important characteristic. When talking about material mechanics, the strength of a plastic is its ability to withstand an applied load without deformation or failure. Because a smaller component wouldn’t withstand the same force as a larger component, it must be compared by evaluating the force per unit area, called stress. An applied load creates stresses in the material, which can cause deformation or breakage. The measure of the deformation of the material is called strain.
How the load is applied matters. The part could be pulled, pushed, or sheared, and the load can be constant or applied by a sudden impact. These are all tested using different test methods.
Mechanical testing methods are used to test these characteristics of plastic, including tensile testing, compression testing, shear testing, and impact testing. Each test method focuses on specific forces and provides valuable information about the strength of the material.
Tensile testing is one of the most common tests performed. A standardized test specimen is subjected to an axial pulling force until it breaks. The plastic sample is pulled straight with no bending or twisting. Tensile testing can be used to measure or calculate the ultimate tensile strength, tensile yield strength, modulus of elasticity (Young’s Modulus), and elongation at break. 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, meaning it won’t return to its original shape, and permanent deformation of yielding begins. The modulus of elasticity is calculated, and 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. This is a measurement of the material’s stiffness; the higher the number, the stiffer the material. Elongation at break is the ratio between the increased length and the initial length after breakage. This is a measure of the polymer’s ductility.
If your part is weight bearing, maintaining the integrity of the plastic under a compressive force is critical. In this case, compression testing can be helpful. A material with a high ultimate tensile strength may have a low compression strength, so tensile testing may not be enough. Compression testing is performed by placing a small specimen in a compression fixture or platen and applying a compressive force at a uniform, controlled rate of speed. Typically, on a spec sheet, you will see it as compression yield strength – the stress at the point of permanent yield. Ultimate compression strength is the stress required to rupture a specimen.
It should be noted that because many thermoplastics deform without breakage, compressive strength at break may not be a useful measurement. Excessive deformation under load would not be a good thing in a real-life application. Therefore, a compressive load is often given at a defined deformation point (commonly 1%, 2%, or 10%).
Shear testing is designed to apply stress so that the testing sample experiences a sliding failure along a plane parallel to the forces applied. It is an important test in applications where materials experience sliding or shearing forces, such as structural components or fasteners. Shear testing provides information about a plastic’s shear strength and deformation behavior. Shear strengths of plastics generally are not provided because they would vary by supplier and thickness. So engineers will calculate an estimated value from tensile strength using a conservative safety factor.
Many products, such as automotive parts and sporting equipment, must withstand an impact without damage. Impact testing is used to help engineers choose materials that are appropriate for the conditions they face. An impact test is used to observe how the plastic reacts when it experiences a shock loading. Impact strength or energy absorption represents the amount of energy absorbed during the impact and indicates the material’s ability to resist fracture, which is related to the brittleness of the plastic.
Strongest Plastic for Injection Molding
As you can see, the strength of plastic is complicated. Determine which characteristics are most important to your application and find one that works for your application. Additives and fillers can be used to enhance the strength. It is best to look at all properties of the thermoplastic to decide which will perform to your specifications.
One of the strongest non-filled plastics that can be injection molded is polyamide-imide (PAI), which also goes by the brand name Torlon. Its tensile strength of 21,000 psi is the highest of all plastics, and its compression strength is 37000 PSI. Its impact strength is also impressive at 2.5 ft-lb/in. However, to properly injection mold, it requires special equipment modifications and curing ovens, making it much more expensive.
Other strong unfilled thermoplastics include:
- PEEK – tensile strength: 14,000 PSI, compressive (crushing) strength: 17,000 PSI, and impact strength: 2.2 ft-lb/in
- Polyimide (PI) – Tensile strength: 16,000 PSI, compressive (crushing) strength: 35,000 PSI, and impact strength: 1.6 ft-lb/in
- Polyphenylsulfone (PPSU) – Tensile strength: 14,000 PSI, compressive (crushing) strength: 20,000 PSI, and impact strength: 1.5 ft-lb/in
- Polyetherimide (PEI) – Tensile strength: 16,000 PSI, compressive (crushing) strength: 20,000 PSI, and impact strength: 1.0 ft-lb/in
- Polyethersulfone (PES) Tensile strength: 13,000 PSI and impact strength: 1.7 ft-lb/in
Choosing the Strongest Plastic for Injection Molding: SEA-LECT Can Help
When you need to find the strongest plastic for injection molding projects, turn to the experts at SEA-LECT Plastics. They will evaluate your component design, functionality, and environmental considerations to help you choose a strong thermoplastic that won’t disappoint you. Contact us today to get started.