After a 100 masks the machine will need a good a clean is all. If using these machines continuous then they appreciate a bit more regular oiling and brushing out the lint underneath. The only other thing you may have to do is reset the bobbin tension after cleaning as sometimes it will get slightly knocked off if you continue to try and ram/pull the material trying to free the mess. Regular maintenance during use will keep it tip-top for ya
ps: my source - own & use a 15-91 daily and have done 100's of masks so far for donations, so i tend to work the machine to it's limits
