Plastic types, recommendations

(Adam X) #1

I’ve got a couple projects in the works that I’d really love to make out of a plastic of some sort (instead of plywood, which is my default).

I need a thickness of .5" - .75", something that machines reasonably easily, has a moderate to high glass temperature, is pretty dense and preferably very rigid/stiff. I’m aware of most of the common materials: acetal, abs, plexi, polycarb, etc. Any others I might be missing that’d be a good fit? Or would one of the already listed be the best?

Projects on tap include a new dust-boot bracket, and a spindle/router clamp/mount.

Thanks!

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(William Adams) #2

There are a couple of references which you can refer to at: https://www.shapeoko.com/wiki/index.php/Materials#Plastics

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#3

You’ll find a lot of information in the link above, as well as at many of the plastic suppliers and sources like McMaster-Carr, Curbell, or Tap. All have selection guides that cover materials they sell, and in some cases, a broader range.

(read this with a grain of salt and do your own research, as so much depends on application and design. I have some experience here, but am not an expert. If you can tell us more about the application, you will get better information. Things such as rough design, loading, allowable deflection, etc)

Without further information about what you are doing, your “VERY rigid” requirement limits you a lot. HDPE isn’t, PVC may be fairly stiff, depending on the blend, ABS and Polystyrene again depend on the blend, but are only generally not very stiff. Polycarbonate is ok in this respect, and tough, glass filled polycarb being quite good. Nylons can be pretty much anywhere from moderate E to pretty stiff, for a thermoplastic. Acetyl (Delrin) is pretty good as an engineering material overall, and is generally about the same as nylon stiffnedd-wise.

The glass reinforced polymers are much stiffer, but have their own issues in machining. Glass reinforced nylon and Acetyl are very good, and glass reinforced polycarbonate is awesome, and even glass reinforced PTFE (teflon) isn’t bad. The problem is that you need really good dust and chip control, as the machining process will produce glass fibre dust that is very, very harmful, health-wise and to the machine, if not controlled.

You also have the thermosets. A filled castable epoxy may be a good choice if you need very stiff final product. These can be filled with reinforcing materials that reduce tendency to brittle fracture, many of which are much less of a concern than glass fibre or carbon fibre but still increase stiffness. Precast to near form,and finish machine is a good way to go. Even for a one off, it can be economical to cut a mold in something like beaver-barf, cast a blank with appropriate reinforcement, then finish machine. I don’t have enough experience here to make any real recommendations, as I tend to use these for their fill and bonding properties.

If you are looking for off-the-shelf, you might find it worth your while to get a sample kit that has a selection of materials you are interested in to do test pieces.

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(Jose Prieto) #4

True http://community.carbide3d.com/u/enl_public

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(Adam X) #5

@enl_public Thanks for this very well thought out response!

I follow through Will’s link to the wiki to this table, which was exactly what I was looking for: http://www.machinist-materials.com/comparison_table_for_plastics.htm

It would seem that short of a pretty expensive polymer, or a glass-reinforced composite, the old-standbys are all of pretty equal footing: abs, acetal, acrylic, polycarb, polyethylene. Each has their own strengths, so to speak.
I think I will end up doing as suggested and getting some small bits of various ones and seeing how they work in application.

Thanks for the input!

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#6

Follow up, as I have now had some sleep (have been trying to kick a nasty lung infection for three weeks)

THere is a LOT to selection of material, and it is complicated by the inconsistency in reported properties, even from the same supplier.

Given the above, my go-to materials tend to be Nylon (good strength, toughness, and moderate stiffness), acetyl (less strength, moderate stiffness), PVC (inexpensive, less strength yet-maybe 60% of nylon- and stiffness can be as good as the others, but often less, and lower toughness), and acrylic (inexpensive in some forms, moderately strong and stiff like acetyl, generally transparent, but low toughness and can be a bit annoying to machine)

For really stiff, strong materials without going fibre reinforced, you end up with the pricey materials like PEEK and PPS–prices in the range of US$300 for a 25mm/1"dia round 1m long vs nylon at about US$18… Prices are about the same from Curbell, McMaster, and other suppliers-- it is a competitive market. Sheet and bar goods are in similar proportion.

You might look at https://www.curbellplastics.com/Research-Solutions/Plastic-Properties

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(Adam X) #7

I’m thinking about this a bit further and wonder about a really high-ply count baltic-birch plywood instead of a plastic. Joke is I’m a woodworker first, cnc dork second and didn’t think of this…

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(William Adams) #8

If you haven’t seen this, be sure to look up:

Wood Handbook: Wood as an Engineering Material (Centennial Edition).

My concern about plywood in the applications you cite has always been that the adhesives will fail under repeated stress.

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(Adam X) #9

Only 509 pages - some lite reading :wink: Thanks for the link!

I got to wondering after seeing allll the DIY cnc machines out there that use 3/4"BB ply as structural members. Never having seen one of those machine in person, are they all just racked (get it?) with inaccuracies?

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