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dave hensley
04-13-2011, 10:56 AM
Hi all,

I am working with .125" and .25" cast acrylic. I am cutting out several smaller holes in each piece and then cutting out an outline. Think of a 5"x5" square that then has several .5"x.5" cutouts within it.

I assume that since I am removing so much material, the heat from the lasering process is causing the acrylic to bend slightly so that it comes out bowed. I am using air assist, but was wondering if there are any tricks to prevent this - either during cutting or after. I thought I might be able to heat the piece once it's done and the sandwhich it between some boards to flatten it.

Any ideas?

David Fairfield
04-13-2011, 11:18 AM
Its a real problem for me, too. Best solution I've come up with is set up the smaller pieces so they are inside a larger carrier sheet which is clamped down during lasering. Don't cut the small piece entirely out with the laser, leave little tabs holding it to the carrier. Then you can manually cut it loose. Or you can wait a while and run a separate file to cut the tabs, when the material is cool.

If a lot of surface engraving to the acrylic, sometimes it will still curl and sometimes weeks later! ugh

paul mott
04-13-2011, 11:19 AM
Sounds like you are not fully supporting the piece when it is being cut perhaps ?

Paul.

greg lindsey
04-13-2011, 11:22 AM
Dave, are you using some type of grid system, elevating the material off the table top, if not that may help. Also try cutting back on the power or increasing the speed. Please list what type of equipment you are using in your profile, this helps us to give you better advice when we know what your working with.

Dee Gallo
04-13-2011, 11:27 AM
A looong time ago, I read about someone putting it in the freezer to make it really cold before cutting... I wonder if this really works? Or helps?

dave hensley
04-13-2011, 11:56 AM
Thanks everyone!

Paul - I am cutting the pieces from 12x24 or 12x12 sheets and have those laying on my cutting table. Shouldn't that be enough support?

Greg - I assume elevating the material would allow airflow thus keeping it cooler?

David and Dee - I'll try both techniques and see how they work.

Doug Griffith
04-13-2011, 12:13 PM
How about sequencing the holes so the material has time to cool. After one is cut, jump to the farthest one away and cut that one and so on. Like using a torque wrench on a cylinder head.

Mike Null
04-13-2011, 12:28 PM
Chances are that a cutting grid would solve your problem. I suspect the the heat and trapped hot air beneath the work are causing the warpage.

You can get a grid here. and@en-gravs.com This is the email for Adam Desemone.

These are the ones I used to sell so I know they work.

dave hensley
04-13-2011, 12:35 PM
Doug - I've not heard of sequencing before. Is that a manual process or something I should be able to do with Corel or my laser? I have a VLS 3.50.

Mike - I assume you meant to provide a link? :)

Dan Hintz
04-13-2011, 12:43 PM
Dave,

Turn off vector sorting, then make sure the order of your vectors in the file is how you want them to cut (use "Move to front/back"). The tab suggestion is a good one.

Doug Griffith
04-13-2011, 12:45 PM
Doug - I've not heard of sequencing before. Is that a manual process or something I should be able to do with Corel or my laser? I have a VLS 3.50.:)

Sequencing is just my jargon for controlling the cut order and direction of vector paths. You'll have to do this manually either using layers, color mapping, or building in the correct sequence from the get go. Oh, and any optimization in the driver should be disabled.

dave hensley
04-13-2011, 12:51 PM
Thanks! That makes sense. :D

Dick Jordan
04-13-2011, 1:44 PM
The object cut order can be specified by using the "Object Manager" docker.
When you open this docker, you will see each object listed for each layer.
The cut order is from bottom to top as they are listed within each layer.
Simply drag each object up or down in this listing to establish the order you want them cut.

Scott Shepherd
04-13-2011, 2:03 PM
I'll go against the grain here and say none of that will solve your problem. Materials have stress in them. Some have more stress in them than others. Any product that is pushed through rollers or pressed will have stress on the faces of the material. It's an area where the molecules of the materials have been squished against each other (squished- a highly technical, scientific term I picked up along the way).

When it's in it's natural state, the material is fine, but as soon as you break that surface, the warping begins because you've removed a part of the material that had stress in it. The only way I know of to remove the stress is to stress relieve the material prior to cutting it. You heat the material up and all the molecules return to their normal spacing.

This is a very bad problems in materials like brass, so it's a known issues that machinists have dealt with for years.

My bet is if you stress relieved the material and then cut it, it would stay flat. However, I don't believe you'll take raw material like that and end up with a flat product.

I could be wrong, but that's my belief of the problem and solution.

Michael Kowalczyk
04-13-2011, 2:12 PM
Or you can go to www.macromonster.com (http://www.macromonster.com) and get "shelbys object & tab creator. That is of coarse if you are using CorelDraw X3 or X4. Can't tell from where I am sitting since you do not have it in your signature. I just bought it last night to have the option when I need it.

Doug, sequencing is not just your jargon. (Sorry didn't mean to bust your bubble) It is used in Artcam Pro and other CAD/CAM programs. I use it all the time so I can control the order in which the parts are cut.

Doug Griffith
04-13-2011, 4:32 PM
the warping begins because you've removed a part of the material that had stress in it.

My thought is that heat from the lasering process varies from one side of the sheet to the other. More on top than on the bottom. This uneven heating rearranges the molecules at different rates thus causing warpage. Think of shrinkage being more on one side than the other. That side will be "tighter" and cause the sheet to curl in that direction.

Annealing will help but it is not for the faint of heart. I've got a special oven for doing it and still choose not to because the annealing temperature is hot enough to soften the surface and cause transfer from the surface the plastic is supported by. Hanging plastic in the oven solves that but then it tends to warp on it's own.

Scott Shepherd
04-13-2011, 5:02 PM
I agree with you Doug, it's probably a combination of the stress in the material as well as the uneven heating from cutting it. It's really rearranging those molecules in overtime when all that's happening :)

I also wouldn't suggest you can just pop it in the oven and anneal it either. It takes controlled temperatures and many times it's annealed using a gas. Not a DIY project.

dave hensley
04-13-2011, 5:17 PM
Thanks for the additional info/suggestions! I'll try each suggestion and we'll see what works for me. :)

Rodne Gold
04-13-2011, 10:47 PM
Try using cast acrylic if you are using extruded , one of the properties of cast is that it expands equally in all directions when heated , extruded will expand only in one direction causing warping. Use high pressure with air assist and use lowish freq or ppi when cutting to limit heat put into the material.

dave hensley
04-13-2011, 10:59 PM
I've never used extruded, so this must be all heat related. I'll try different PPI settings as well at the other ideas and see what works best.

Thanks again everyone for the help! This forum is a great resource!! :)