Now I know there are few posters on this board whose knowledge of thread injectors and sewing machines is superior to mine and I hope that I will be corrected if the apostasy I am about to spout is completely wrong.
The most overwhelming advice offered when some one asks about purchasing a machine is: "A metal machine is the only way to go." or some derivation of that sentiment. I would like to put an end to that misguided advice.
This advice stems from the late 60's and early 70's when Singer tried to bring the cost of sewing machines down by introducing plastic gears. It went horrible wrong. The space aged plastics of the day (or at least the plastics Singer used) did not react well to the lubricants used in the machines. The oil and grease would cause the plastic to become brittle and break easily which led to stripped out gears.
You will be hard pressed to find a modern sewing machine that uses all metal gears. Most will have a combination of nylon, plastic, and metal internals. Not only is there a cost savings with synthetic internals, there is a huge maintenance bonus. Most modern machines do not require a strict maintenance schedule or frequent lubrication. Weight, noise and other benefits are also a by product of modern materials and engineering.
"...But I am doing heavy duty sewing and metal gears will hold up better". Have you ever stripped a gear before your needle breaks? The gears are not the weakest part of the system, further more if you are doing something that will strip a "plastic gear" what is it doing to metal gear? I would rather have a destroyed gear and know where my problem is than have a slightly bent/warped gear that is throwing off my timing and making my problem impossible to troubleshoot.
Metal or Plastic? An inferior machine is an inferior machine if it is made with metal or plastic. Price adjusted for inflation of all of 40 year old machines that are still in use today would be $1200 and up if you had to buy it new. Keep that in mind when machine shopping. 50 bucks for an old straight stitch model with a proven track record is not a bad deal, and a$750 price tag on a new machine is not outrageous if you are only gonna buy one machine for the next 25 years.
Just one guys opinion, old timers and experts feel free to crucify me for my heresy.