What is hoop stress?

A wine barrel is made from wooden slats held in place by metal hoops. The metal hoops are used to hold the slats together and provide support from the pressure that pushes out radially. This radial force that pushes out is putting the metal hoops under ‘hoop’ stress. If the hoop stress exceeds the mechanical design and material properties, the barrel will burst. Now that would be a terrible waste of wine!

In order to minimize hoop stress, it is critical that all cylinders (screw bosses, insert holes etc..) are properly designed for the specific plastic used.

Assuming that all plastics materials are similar in regards to fastening design recommendations and/or not looking into all potential hoop stresses in a part are common oversights during the design phase of an injection molded product. As a result, hoop stress type failures occur more often than they should.

If a plastic is put under a load with constant force or stress it will creep over time. The larger the load, the faster the stretch/creep will occur. If not designed correctly, over a prolonged period of time, the material simply cannot sustain the load and will fail.

Material specifications and charts are created so that designs can be made to best prevent failures. With most material, especially plastics, specifications are also calculated at various temperatures since they behave differently at different temperatures. Understanding the end use of a product, its operating environment and the design principles needed for a given material is key to the products long-term success.

As discussed in the example below, the chemical resistance of a plastic will have a dramatic impact on its mechanical properties when exposed to the wrong chemicals.

Of all the types of mechanical stresses a plastic product can be exposed to, it will be the most susceptible to hoop stress. As a result, taking the time to properly identify all potential hoop stresses in a product and designing for it appropriately should always be done. Looking at the complete picture of a product to assembly is critical to its success.

In one example of a new project, two sub-assemblies were being held together by three screws that were threading into three bosses. The three screws were around a hole spaced at 120-degree intervals.  Two of the screws’ bosses were cracking on a regular basis. The plastic material was an ABS.

The obvious culprit was that the screw type or boss design was incorrect. An assessment was conducted and found both to be correct. Next, we examined the weld line strength of the bosses and processing parameters. (Virtually all holes and bosses will have a weld line. Traditionally the weld line is always considered to be the weakest point in an injection molded part.)

In reviewing the mold flow data, the weld line was not located near the crack-starting location. This indicated that it was likely not a weld line strength issue.
Further processing tests were done to vary parameters in an attempt to impact the cracking and weld line strength. Though we found we could make the weld line weaker, no processing adjustments impacted the cracking.

We requested a step-by-step review of the customer’s complete assembly line where these plastic parts were used.  It was during this review that the real culprit came to light.  Located near the two bosses was an electronics assembly. This assembly consisted of a number of PCB boards that were held together via standoffs and screws. The screws had a thread fastener applied to them to ensure that they would never come out. It was the thread fastener that caused the issue.

During the curing process, the thread locker outgassed a compound that caused a chemical reaction with the plastic. This weakened it. The two bosses that were cracking were in very close proximity of one of these screws with this thread fastener. The thread fastener that was used, was not recommended for use with ABS plastics. The thread fastener was changed and the cracking stopped.

In this case, even though the thread fastener did not even make contact with the plastic, it still had a substantial impact.

This is a good example of the value of understanding the entire process.

Please be aware that though we discuss basic screw boss design in our engineering info section, almost all materials will have specific design recommendations for their materials. It is important to follow these design guidelines for the specific material being used as they may vary from general the design principles.

For some basic information on screw boss designs please look in our part design section on our engineering info page.