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Laser Cutting Foam?


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I joined a MakerSpace a few months ago - they own a Trotec Speedy 300 laser cutter, which I'd been keen to try out on a few projects. One of the things I've been meaning to do is make customised stock boxes with foam cut-outs in customised shapes for particular bits of rolling stock, etc. 

 

The Space has quite strict rules on permitted and banned materials for safety reasons, so the foam that was recommended by other members was Plastazote, a dense closed cell polyethylene foam. I ordered some in black LD33 density.

 

I finally got around to trying to cut the foam yesterday, but had a bit of grief trying to arrive at settings that worked for cutting the foam without the bottom side melting into a sticky mess. Various combinations of settings seemed to cut cleanly 1/3, 1/2, or even 3/4 of the way through, but I didn't seem to be able to arrive at anything that cut right through without the horrible melting on the bottom. I guess if the laser gets all the way through and hits the bottom of the bed, the heat reflects back up, and creates the unwanted melting?

 

The settings I first tried that had been saved by another member for 20mm foam were 100% power, 2.3% speed, 1000Hz frequency, and 2 passes - this turned my foam into a right mess on the bottom. I muddled around based on the explanation of the settings here: https://www.troteclaser.com/en-gb/knowledge/tips-for-laser-users/definition-of-laser-parameters/ and ended up with 100% power, 14% speed, 5000Hz frequency, and 4 passes. This cut through 80-ish% of the way cleanly, which I then finished off with a scalpel. That was fine for basic shapes (cut outs for wagons), but wouldn't really work for more complex shapes around locos etc. 

 

Can anyone offer any experience on laser cutting foam, or a good strategy for finding the best settings for a laser in general?

 

Cheers

Justin

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2 hours ago, dhjgreen said:

Sacrificial layer of foam under your workpiece?

 

That does sound worth a try. I know in the forums for my cutter, some people use aluminium honeycomb as a base for this very reason - I've linked to an example.

 

https://www.amazon.com/Aluminum-Honeycomb-Grid-Core-Cell/dp/B00WF7DHJC

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2 hours ago, dhjgreen said:

Sacrificial layer of foam under your workpiece?

 

The simplest ideas are often the best! Next time I'm in I'll give it a try. 

 

The cutter does have a honeycomb type base fitted - although the fins are really quite dense, and can jiggle around within the frame rather than being fixed hexagons - quite an odd construction really. It's also got lots of dents and dings, and very charred, after being in common use for a long time, so I don't imagine it does a great job of letting heat escape anymore. I was advised to take it out and cut directly against the base only for paper and thin card, and to use the honeycomb for everything else. 

 

Is there a general approach or procedure that people use to find the best settings for materials in general though? (like, adjust one parameter first, then fine tune using another, etc?). I felt like I was juggling speed and number of passes pretty arbitrarily, and frequency was just a guess!

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Whilst I haven't tried foam yet, soft or thicker materials in my Glowforge need three or more low power passes. I've even sucessfully cut through 20mm thick birch boards by using three passes at the full thickness then three more by telling the machine that the material is the thickness minus the depth already cut. As the laser cuts through it naturally spreads a little so the cut is never perpendicular to the material. Breaking a deep cut down into multiple passes helps eliminate this.

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