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. For machines that are compatible with prewound bobbins, we've notice that nearly all machines can use the cardboard-sided type (some prefer to remove the cardboard sides) but some machines do not like the plastic-sided types. Plastic-sided bobbins can be too slick for some machines and the lack of friction may cause the bobbin to continue spinning once your foot comes off the pedal (also known as backlash). This does not happen with the cardboard-sided bobbins because the cardboard sides aren't as slick as the plastic-sided bobbins.
It doesn't matter whether you take the cardboard sides off or keep them on. Some machines have an automatic low-bobbin sensor. This sensor will not work if the cardboard sides are kept on.