Q. How is glass made into what is known as "milled glass" or "chopped glass"? Why are these glass fibers so strong?

Hello! Thank you for your question. Here are some definitions, an explanation of glass fiber strength, and links to some resources for further reading/viewing.

We do appreciate the opportunity to highlight the Owens-Illinois Fiberglas sales kit in the Museum's collection (ca. 1935)!

Glass Fibers: Some Definitions

The Handbook of Fillers (Fourth Edition, ChemTec Publishing, 2016, p. 264) explains:

Glass fibers are produced by two methods: milling and chopping. The milled fibers are milled using a hammer mill which results in a relatively broad (but consistent) length distribution.... The chopped fibers are produced by chopping a bundle of glass filaments to a precise length. The length of chopped fibers is substantially larger than that of the milled fibers.

According to the Engineered Materials Handbook (Desk Edition, ASM International, 1995, p. 140):

Glass fibers improve both the short-term and long-term mechanical properties of a resin. They also improve creep resistance, thermal conductivity, and heat deflection temperature, as well as the tribological properties of the base resin.

It also describes several forms of glass fibers (p. 140):

• roving (continuous strand)
• chopped strand
• woven fabrics
• continuous-strand mat
• chopped-strand mat
• milled ("continuous glass strands hammer milled into very short glass fibers," Handbook, p. 41).

Online Resources on Glass Fiber Manufacturing

• As for the manufacturing process, an informative article on the CompositesWorld (CW) website describes the making of glass fiber, step by step.
• There is another informative article on fiberglass on the How Products Are Made website.
• Searching "fiberglass," "fiberglass production," or "glass fiber production" on YouTube will return videos which describe the production process and various types of fiberglass and their applications.

Why Are Glass Fibers So Strong?

The Museum's chief scientist offered the following explanation about the strength of glass fibers:

When you drop a glass tumbler, mirror, or window pane, it is likely to break because as it falls it picks up speed, and the speed combines with the weight of the object to create a lot of force that is able to smash the glass itself. A tiny glass fiber weighs almost nothing and, when dropped, falls more like a feather or piece of thread; it has very little energy when it hits the ground. It is no more strong or weak than the other glass. But glass IS stronger than plastic. So when the fiberglass threads or fabric are coated with plastic and made into stuff like boat hulls and car bumpers, they add their strength through the plastic, making the plastic act like it's stronger.

Further Reading

• Challoner, Jack. 1001 Inventions That Changed the World. 1st ed. for the U.S. and Canada. Hauppauge, NY: Barron’s, 2009.
• Lamm, Michael. "The Fiberglass Story." American Heritage of Invention & Technology 22, no. 4 (Spring 2007): 8-16.
• Noakes, Keith. The Fibreglass Manual: A Practical Guide to the Use of Glass Reinforced Plastics. Marlborough: Crowood Press, 2003.

​Further Research

You might contact Scholes Library at Alfred University for more information about glass fiber manufacturing.

Borrowing Library Materials

If you wish to borrow copies of library items, please contact your local library. The Rakow Research Library will lend designated books from its collection and will send copies of articles requested by other libraries. Your library can request items through the OCLC WorldShare Interlibrary Loan (ILL) system or by direct request through email at ill@cmog.org. For more information, please see our ILL policies and procedures.

Please don't hesitate to contact us with your glass-related questions in the future!