Monofilament thread is used in applications where a match to the fabric is difficult. It is often used in quilting, where the top may have many different fabrics that no single thread can match. While it’s versatile, monofilament has a multitude of risks that can make it infuriating.
The common, cheaper form of monofilament is made from nylon. It’s not very different from fishing line and has many of the same characteristics. One of those is memory. As monofilament is pulled off the spool during sewing it tends to retain the loops that it developed when being wound. The wiry, springy nature of the thread can lead to it looping around things that it shouldn’t. It can also get caught up in moving machine parts. At most sewing speeds that means it will quickly wind into the machine until it has bound up the parts enough to stop the machine. This is a service person’s nightmare. Typically the thread is quite fine. Being transparent makes it very difficult to see. If the machine ran a long time before stopping, some of the thread may have gotten hot enough to melt. Extracting it can mean a lot of disassembly, leading to high repair bills.
A better form of monofilament is made from polyester. It has the same transparent qualities as nylon, but it is usually more expensive. You still need to be careful with it, but it doesn’t have the memory issues and is less likely to throw troublesome loops. Another benefit of poly monofilament is heat resistance. Ironing something made with nylon monofilament can cause the thread to melt, thus removing stitches!
Most sewing machine dealers and fabric stores will carry good quality monofilament. Get polyester (Sulky, Superior) when you can, nylon if it’s not available. DO NOT get Walmart monofilament. It is basically fishing line and has a very high degree of memory and spring.
Use great care when threading your machine with monofilament thread. It will be hard to see, and it’s very easy to accidentally have it loop around something like a bobbin winder that you don’t notice until you start to sew. This type of thread is very strong, and if it is caught and unable to feed, it can easily pull the needle so it hits the plate or bobbin case. When removing it, be sure to secure the free end of thread on the spool. If it was wound on the spool at high speed, having an unsecured end can cause it to “puddle” around the spool. If the free end gets pulled into the machine, disaster will ensue!