Q. What's the difference between plastic-sided and cardboard-sided prewound bobbins? I see that you sell them both and they both will fit my machine.
A. Paper or plastic seems like a rhetorical question. But for prewound bobbins, it's not. What's the difference between a plastic prewound and paper prewound? Besides the medium that the thread is wrapped around, not too much. Both types typically hold more thread than a self-wound bobbin and have a tighter wind due to a suite of high-tech machines used during the winding process.
Most machines are shipped with plastic-sided prewound bobbins or empty plastic bobbins. Plastic bobbins are uniform in shape and the thread cannot change the shape of the bobbin (the plastic keeps the thread from spilling over the top). Plastic-sided bobbins also allow machines with bobbin sensors to sense how much thread is left on the bobbin.
Paper-sided prewounds are made in a two-step process. The thread is wound around the core and then the sides are stamped together. Generally speaking, there is slightly more thread on a paper-sided prewound than a plastic-sided prewound because paper is thinner than plastic, providing more space for thread to fill the bobbin.
The reason why both paper and plastic exist is because there's two different machine styles. European-made machinery historically favors paper sides, while Asian-made machinery favors plastic bobbins. We recommend choosing a bobbin based on application over whether or not it is plastic-sided or paper-sided.