When discussing topics within the realm of Material Science, invariably the term ‘strength’ always comes up. How strong is a material? How does it compare to another material? Strength can be subjective as it has many different properties within the term. Tensile strength and compressive strength are two areas that are discussed with plastic injection molding as the parts molded typically have some function that requires use without breaking or fatigue. Do you have a new project coming up that needs a certain level of strength, but aren’t sure what to ask your molding partner? Let’s talk through tensile strength and how it can help narrow down your resin choices.
What is Tensile Strength?
All materials will stretch when being pulled in opposite directions. Each material will have plastic deformation which will allow it to return to its original shape. At a certain transition point, that deformation leaves the plastic range and permanently deforms. It will not return to its original shape. The ‘tensile strength’ is that critical value where the material transitions from plastic to permanent deformation. Brittle materials may have a low value and they will fracture quickly without stretching. Elastic materials can stretch an incredible amount without breaking, which may give them a high tensile strength value. It is a fine balance between being elastic and brittle, stretching a little or a lot, and deforming permanently without returning to the original shape.
What are the Most Used Plastics for High Tensile Strength?
Tensile strength is one of the most critical mechanical properties of plastic injection molded parts, especially those that are used as structural members. As you select a resin for your next project, you’ll want to know which of the 1,000s of resins available have the highest tensile strength.
Nylon
Nylon is a high performing thermoplastic with a tensile strength of 12,400 psi (85.4 MPa). It is often chosen as a competitor to rubber, latex, and silk depending on the specific application. It has a higher melting point of 232 °C and can be found in clothing, rubber, tires for automotive and aerospace applications, threads, ropes, auto parts, and plenty of more uses. Nylon has good impact strength, fatigue and electrical resistance, and can withstand environments using fuel and oil. Nylon has low dimensional stability and won’t stand up to most mineral based acids and solvents, so choose it carefully for your next project.
Polyphenylene Sulfide (PPS)
Polyphenylene Sulfide has a slightly higher tensile strength than Nylon coming in at 12,500 psi (86 MPa) and a higher melting point of 280 °C (536 F). PPS a semi-crystalline thermoplastic that withstands higher temperature and has good dimensional stability in most environments. It has good electrical insulation and chemical resistance properties too. You can often find PPS being used in automotive brake and fuel systems, in surgical instruments for medical settings, as guide plates in industrial applications, and in bushings and pumps for chemical applications.
Polyetheretherketone (PEEK)
Polyetheretherketone is a common engineered thermoplastic with a tensile strength of 14,000 psi (96.5 MPa). It is chosen because it is both chemical and water-resistant, plus it can survive in high temperature settings up to 250 °C. PEEK shows good dimensional stability and has great insulation properties. It can be found in bearings and washers in automotive sectors, with implants and instruments in the medical field, in fiber optics within the aerospace industry, and telecommunications for phone jacks and circuit boards.
Polyetherimide (PEI)
Polyetherimide has an even higher tensile strength of 15,200 psi (105 MPa) compared to the previous choices. It is another engineered thermoplastic that has material properties comparable to PEEK but has a lower impact strength threshold. PEI is often chosen because it can withstand higher temperatures than many other resins and keep excellent dimensional stability at those higher temperatures. It weathers well and survives direct sunlight, plus can withstand some solvents before showing signs of degradation. One of the main drawbacks to choosing PEI is the higher cost compared to other materials in similar settings. PEI can do well in particular settings, but you may find a comparable material for a lower cost if return on investment is a criteria for your new project. PEI can be found in multiple applications within automotive, used as electrical connectors and motor parts, and in non-implant prosthetic applications.
Polyamide-imide (PAI)
PAI has the highest tensile strength of the five most used resins we’re discussing. It shows a tensile strength of 21,000 psi (145 MPa), which is nearly 75% higher than Nylon. PAI is a perfect choice for high temperature and high strength applications which exceed where others fail. Even at high temperatures and highly loaded applications, PAI shows excellent dimensional stability and friction resistance. It’s a common choice in the aerospace industry with its high capabilities, but it can also bring a higher cost to your budget too.
Conclusion
Tensile strength is one of many factors that should be discussed at the beginning of your next project. Many resins have excellent tensile strength, and some can have strength beyond steel at greater than 100,000 psi. Thy can also be 75% lighter than steel or 60% lighter than Titanium. Finding the correct material to meet your needs, whether that is strength, budget, or somewhere in-between, is a complicated process. At SEA-LECT Plastics we pride ourselves in tool and die manufacturing that allows us to deliver high quality products free from defects. You should be including discussion points on the function, environment, quantity, and recycling of your product to ensure the best resin is chosen for your product. SEA-LECT Plastics has an elite team that produces world-class prototypes and products, and we have decades of experience with plastic injection molding operations and creating tooling for perfect parts. We can offer support to determine what type of mold you need, what resin to choose, and how to best invest in your future. Give us a call at (425) 339-0288 or email us at mattp@sealectplastics.com. We’ll help to determine the best manufacturing for your next project.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.