What are vinyl windows made of?

If I drill into the vinyl, what is under there? What is the core of the sash made of? One of my windows will need to be partway open to accommodate a portable air conditioner. In order to lock the window (as is, someone could slide either sash freely), I want to drill a hole through the lower sash face and into the upper sash face to take a security bolt. The bolt head would be recessed and requires a "key" to remove. As an alternative to this, I could make vertical stops and screw them into the jambs.

Is it solid in there?

 

Most are hollow chambers. I would suggest you make stops and screw to the jambs. The holes will be less obvious when removed.

 

Thank you for the fast response! It's probably just as well, and maybe easier too. I can still countersink the stops to make the bolt heads hard to get to, especially the one on the outside, and paint the stops white to look good.

 

Instead of any holes why not use a metal rod in the upper sash section to hold it in place.

 

Hi Norm - thanks! That works fine on the inside sash, but the outside sash needs something that is hard to remove. If someone cut the screen, the prop or stop could simply be pulled out, and the sash could be lowered. That's why I'm thinking about putting a security bolt in that piece. The air conditioner piece that fits in the window is only about 8" tall, so the sashes could be moved quite a bit to gain access.

 

OK I understand. I just don't like the idea of putting a hole into the window itself. You may beak the seal and weaken the frame and also allow moisture to seep into the framing.
How about screwing a block of wood or a metal stop into the side of the window frame instead, preventing the sash from moving? Use the same antitheft type bolt that needs the key in order to remove it. If a break in takes place with that system, there is little doubt anything else will stop a break in at that point.

 

OK - I did it. A block cut for each sash, with the outside sash piece screwed into the frame on the side. The inner piece isn't screwed since by the time you get to that from the outside, you would already have needed to break something. I used 3/4" x 1 1/2" PVC trim board thinking it would look nicer. This was the first time I've used that and I liked it for this project. Not having to prime and paint made up for the price difference. My wife is happy so all is good. It actually looks like part of the window, and I figured it so that the pieces can stay in place even when the AC is removed for winter, and even when cleaning the tilt-in windows.
Thanks guys, for the help. It's great to have such fast and good support!