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Case Study #23 — Fastener Failure Workshop

Plastic Injection Molding
 

Industry:

Plastics
Injection Molding Machinery

Problem:

A manufacturer of custom-made injection molding machines developed a new model to mold plastic objects as large as a household door. The injection head that heats and liquefies the plastic was held together with large metric socket cap screws. On the first product test run, the bolts broke when the machine reached full operating temperature. When this happened, molten plastic, heated to 600°F (316°C), was sprayed across the room.

Injuries:

Three technicians had been observing the test. Fortunately for them, they were positioned where no one was injured.

Damages:

A portion of the machine's tooling was heavily damaged, but repairable. The area surrounding the machine appeared as if a giant-sized "Silly String" toy had been shot about at random. Cleanup after the plastic cooled and solidified wasn't difficult. The biggest issue presented was finding out why this dramatic failure occurred.

Observations:

The fasteners breaking were M36 x 90 mm, Socket Cap Screws, Property Class 12.9. These were used in a circular pattern of twelve screws surrounding the injection head. All of the screws were broken. With the mess created by the molten plastic, not much could be seen of the threaded end of the fasteners. Scattered around the floor were the heads. Manufacturer's identification marks were present, but so faint that they couldn't be read. The fracture surfaces had a very craggy appearance.

Investigation:

The fact that all the screws broke at the same time was unusual. We needed laboratory testing to sort out this situation. First, core hardness tests showed that the products were normal. A check of the surface carbon showed it was also well balanced. A Scanning Electron Microscope (SEM) view of the fracture surfaces showed very little break. This is a normal appearance for a fastener with hydrogen embrittlement (HE) or stress corrosion cracking (SCC). However, this failure happened much too quick, and there wasn't any corrosion. A chemical analysis of the steel turned up the normal elements for medium carbon alloy steel. However, there was also the unexpected element of bismuth.

Conclusion:

Bismuth is an element that can be added to steel to improve machinability. However, it's clearly prohibited for the use in a socket cap screw. Selenium, tellurium, and lead are also not permitted as these elements have a relatively low melting temperature. In the case of bismuth, it melts at 520°F (271.3°C). As bismuth was dispersed throughout the steel's microstructure, when the operating temperature reached the bismuth's melting point, it was trapped. Then the bismuth expanded at an extremely fast rate, and coupled with the fact that the screws had been tightened and under great stress, this made the screws blow apart.

Recommendations:

Large diameter socket screws are made from bar stock that is often used for many other types of non-fastener products. Steel suppliers have little knowledge of what their material will make, or where it ultimately will be used. For safety, it's very important to work with a fastener manufacturer that has tight controls on its materials and processes.

 

As appeared in Fastener Technology International magazine.

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