Is a special circular saw blade needed to cut plastic?

Question

We purchased two portable air conditioners for our bedrooms. It include a plastic window kit that can be fitted for the window. I.e., this:

I need to trim off about 6 inches, which requires a simple straight cut. Doing it by hand is challenging -- both physically and to get it straight. (I took that approach years ago, with a different unit, in my apartment.)

This time I have a circular saw and realized that would be a lot better and cleaner. However, I'm not sure if I need a special blade? Currently I have 18- and 24-Tooth Saw Blades.

Answer 1

Back in the day when I used to put up vinyl siding, I found that the best way to cut it with a circular saw was with a cheap 140 tooth plywood blade installed backwards (to give the teeth a negative rake and prevent chipping):

For really hard plastics, I've used a diamond abrasive blade before and that did a great job (these are incredibly expensive if we're talking about a single cut though). I would imagine an abrasive blade for cutting metal would do just as well though:

You should be able to pick up either of these for less than $5.

Wear good safety glasses and do a trial cut first with whatever option you go with - plastic will chip long before it will bind the blade in most cases, and these can be both sharp and high velocity.

Answer 2

I have a 7 1/4 inch circular blade which is specifically for laminates and plastics. When cutting flooring, I wasn't so impressed with its performance at first but when I tried standard crosscut and ripping blades I was then very impressed. The standard wood working blades got gummed up and melted or fractured the laminate while lugging and jamming the saw motor.

Looking online now, the only blade I see which looks close is this (at $30):

Answer 3

Heat is also an enemy - the faster the blade, the more the plastic can melt and distort.

I've successfully used a nibbler to cut thinner plastics accurately without deforming them, and repeated cuts with a sharp craft knife and a steel ruler clamped down to act as a fence/guide.

Another workable answer is to use a hot-knife to melt your way through the plastic. Again using a metal ruler as a guide. The advantage here is that the sides of the cut will get a little thicker, providing some additional strength and more surface for sealing against the window frame.