Issues with MDF cutting - Random burning on the back of material

[rjavier]

Guys! I've been running a 150W Laser for about three years, cutting 15mm MDF. Normally it goes fine, but from time to time we get orders to make closely-located parallel cuts that give us issues. Pictures show this best.



As you can see, it only happens sometimes. Plenty of close cuts around, 90% of them are fine. But of course, we need to get them 100% right.

We run the system with a 2HP 20 liter fully open (basically the compressor motor always running, to get the max flow).

I'm not sure what else I can look into to address this. I'm sure you guys are smarter.

Thanks!

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[diycncmill]

Hello,
is your feed rate constant during these travels?
I could imagine a decelerating feed rate to be the cause of the burn marks.
You have only encountered these issues with closely-located parallel cuts, correct?

Best regards,
Dominik

[rjavier]

Hello,
is your feed rate constant during these travels?
I could imagine a decelerating feed rate to be the cause of the burn marks.
You have only encountered these issues with closely-located parallel cuts, correct?

Best regards,
Dominik

Hey Dominik,

It is, or better said, should be. There's no messing with it in our part.
And yes, only in closely located parallel cuts.

[diycncmill]

Awkward then. I have one more idea but could be nothing:
Did you try different MDF or a different material in general?
I could imagine something inside some areas of the MDF supporting combustion.

Best regards,
Dominik

[KH0UJ]

I guess you need to re adjust your drawings, make it wider on the thin parts, as the laser fires on the thin part it will superheat to the extent it will burn itself because there`s no enough heat transfer, thin superheated material tends to be combustible that`s why it burns the material itself even if you crank up the pressure of the tip blower, it happens so quickly that there`s no cure for that aside from adjusting it, make it thick a little bit then problem solved, it cost a bit of material but at least it cuts perfectly with no burns.

[Blackened]

My experience with metal cutting and close parallel cuts. May not be the same cause or mechanism but maybe it is.

The issues I've had in the past with close parallel cuts have been solved by increased assist gas flow. I theorised that with an existing cut so close to the current path, some of the assist gas was redirected into the previous cut and there was less gas directed into the cut in progress. A larger nozzle diameter usually solved this problem but again, this is compressed nitrogen for metal cutting, and you may be limited by your compressor, rather than the aperture of the nozzle.

[rjavier]

Awkward then. I have one more idea but could be nothing:
Did you try different MDF or a different material in general?
I could imagine something inside some areas of the MDF supporting combustion.

Best regards,
Dominik

Well, that's one I can't do much about. There are other MDFs, but the one we use is the most "noble" one, or soft. We have had other 15mm MDFs which we cannot cut through (our machine runs at its highest power already). I've theorized about inconsistent points in the MDF construction that allows it to happen randomly.

I guess you need to re adjust your drawings, make it wider on the thin parts, as the laser fires on the thin part it will superheat to the extent it will burn itself because there`s no enough heat transfer, thin superheated material tends to be combustible that`s why it burns the material itself even if you crank up the pressure of the tip blower, it happens so quickly that there`s no cure for that aside from adjusting it, make it thick a little bit then problem solved, it cost a bit of material but at least it cuts perfectly with no burns.

We cannot do that. These are steel rule die boards, so we cannot adjust it. The problem still is, it only happen sometimes. So my guess could be that the MDFs quality is inconsistent across the board itself.

My experience with metal cutting and close parallel cuts. May not be the same cause or mechanism but maybe it is.

The issues I've had in the past with close parallel cuts have been solved by increased assist gas flow. I theorised that with an existing cut so close to the current path, some of the assist gas was redirected into the previous cut and there was less gas directed into the cut in progress. A larger nozzle diameter usually solved this problem but again, this is compressed nitrogen for metal cutting, and you may be limited by your compressor, rather than the aperture of the nozzle.

Not bad, but I don't know if I can do this modification, since the nozzle is factory-made.

What we are testing right now is trying to damp the back of the board (remember, this issue happens on the back of the board cut, not the front) with a slightly wet rag, and seeing if it helps avoid this issue.

Thanks in advance for all the opinions!

[Blackened]

What we are testing right now is trying to damp the back of the board (remember, this issue happens on the back of the board cut, not the front) with a slightly wet rag, and seeing if it helps avoid this issue.
Thanks in advance for all the opinions!

How did this go? I am thinking the damp areas might not cut through? But that might be preferable to burning. Another suggestion I'd make is to find some inert gas for your assist, rather than air. Nitrogen is good at reducing combustion, but you might get the same result from CO2. Perhaps you could swap gasses just for these critical parts? Not as cheap as compressed air, but then wasting time and material re-cutting unacceptable parts isn't free either.

And just to confirm, it's not burning simply because it's cutting over support structure is it?

[rjavier]

How did this go? I am thinking the damp areas might not cut through? But that might be preferable to burning. Another suggestion I'd make is to find some inert gas for your assist, rather than air. Nitrogen is good at reducing combustion, but you might get the same result from CO2. Perhaps you could swap gasses just for these critical parts? Not as cheap as compressed air, but then wasting time and material re-cutting unacceptable parts isn't free either.

And just to confirm, it's not burning simply because it's cutting over support structure is it?

Using N2 or CO2 seems like its gonna be expensive. What I wanna try is to get a more powerful compressor to test. Ours is now running at the max at 3bar, so I wanna see if 4 or 5bar could make a difference.

We have not tested damping extensively, but in small parts it actually helps. Our issue is with longer rungs, running 3 hour jobs.

And nope, no support structure (the bare minimum on the edges to hold the MDF board).

[fixtureman1]

I would try 2 layers with the parts separated cut one layer ten the other.

[rjavier]

I would try 2 layers with the parts separated cut one layer ten the other.

Not possible I'm afraid. Steel rule die boards have to be made of one solid piece, they are subject to lots of mechanical stress.

[Jerrywu]

1.re-do your beam alignment.
2.check the focal length
3.try to get a more powerful compressor to test

[Blackened]

How did you go rjavier? Did wetting the back make a noticeable difference for those longer 3h jobs? What about that bigger compressor you were going to try?

[rjavier]

How did you go rjavier? Did wetting the back make a noticeable difference for those longer 3h jobs? What about that bigger compressor you were going to try?

Hey! We are still waiting on a friend to lend us a more powerful compressor. For now what is working very well is to run the cutting job twice, at twice the speed. Same total amount of time, and actually the cuts are better looking.

But I am still sure that the MDF we use (low density) has a lot to do with it. We use low density bc its the only one our laser is able to cut, high density its a no-go. This is right now my recommendation.

We didn't do wetting for the following ones because in longer jobs, it would almost dry up. I still think the best approach is good air and two faster runs.

[Blackened]

Good to know you found a workable solution. I figured it would dry out for longer jobs yeah. I wonder if there's an additive you could use that's both non-flammable and wouldn't ruin the MDF, that would act as a more persistent wetting agent?

[rjavier]

Good to know you found a workable solution. I figured it would dry out for longer jobs yeah. I wonder if there's an additive you could use that's both non-flammable and wouldn't ruin the MDF, that would act as a more persistent wetting agent?

Maybe, but I don't think its a big issue for most. Cutting 15mm MDF with a 150W laser is barely done by anyone. If you ask most sellers they will definitely not assure you you can do it, at most 10mm. And bigger businesses that do this, they just have a big ass 300-400W laser, obviously at a massively bigger price.