I couldn't find any threads on this in the forums, I have a horizontal panel saw and I thought I was somehow feeding my stock wrong or something was off with the panel saw.
Is there something I can do for this not to happen? and why does the first piece that's up against the guide stay straight and not the rest?

search, "melamine stress"

I seriously doubt the piece that is cut against the fence is staying straight, your probably just not noticing the movement. My solution is outlined in the link that I provided.

"I always rip oversize, then re-rip to straighten one edge and finally rip to final width."

I hope this helps.
Mark

When I rip melamine sheets, 9 out of ten will move (pinch the blade). Rough size everything. Then re-trim while clamped down on the slider. Then place that now straight edge against two stops and rip to finish size. Takes a bit longer, but allows a perfectly straight and parallel ripped panel.

WHen you cross cut, you get perfectly sized and square panels. If you try and rip without the secondary straightening cut, you run the risk of slightly miss sized finished panels. If you use dowel or groove/tenon construction you will have slightly missalignment when going to assembly.

Is also known as "banana cut".

The sheet is produced under tremendous heat and pressure. The tension that is in the sheet near the edges sort of works its way through the board angst from just sitting around. The tension in the middle needs to be cut once to help it relax.

Sometimes this tension release manifests parabolically like a cereal bowl.

This problem is much more common with MDF.

Cutting oversize then recut to final size will usually fix the problem.

The above post about the tension is correct. As the material sits around in unheated/non climate controlled environments, the exposed outer edges pick up moisture and try to expand in their length.

When you make that initial cut, the edges can then expand, that is grow in length, and thus you get the banana shape.

As said above, you need to oversize the panel slightly, then trim to final size.

I have found that I do not have the problem with parts cut on a router. I am not sure why, maybe someone has that answer?

Maybe.....

1. Your cutting smaller parts so the error is much less significant and not easily seen?

2. You've never checked them with a straight edge, because there was no reason to, because you didn't have assembly problems?

Mark

On a router you normally have a 1/2" edge trim that helps relieve the tension and you are likely cutting shorter parts rather than full rips.
Some panel saws have automatic programs you can call to take care of the problem. We have a Schelling, it will select the rip nearest the center of the panel, raise the blade into the panel and drop it at the end so that it leaves both ends attached. It will then revise the program and finish ripping the parts such that that center rip has trims. All automatic!

Thanks for all youre answers.
I thought I was alone with this problem, seems to really be the cheaper melamine panels, because when I got some more expensive ones from another company I didnt have that problem.
Apparanty everyone around here has cnc's because my rep told me I am the only one that has complained about this.

If I only had a dollar for every time I've heard that one.