Coolant and PolyPropylene?

Why do so many people machine (mill) PolyPropylene (PP) dry with an airblast? Is there some issue with common flood coolants? What about a fog buster coolant mist air blast? Anything else I should know?

It's only 10 small parts but I'm not too worried about cleaning plastic chips out of the system. Some 3d surfacing required, tolerances are all +/- 0.010"

Almost all plastics are hygroscopic and will absorb the coolant to some degree making dimensional accuracy problematic.

Polypro is not hygroscopic, but it's soft and gummy and a PITA to deburr.

Suspect people using air blast are just trying to get the heat out with the chip, or blast the chip away from the tool so it doesn't wrap around and melt. Coolant on PP is okay to use, spray mist is probably better.

Plastics like Nylons are hygroscopic and will absorb coolant, so air may be more appropriate if there are tight tolerance features.

Normal plastic machining techniques appply- low RPM, high chipload, razor sharp tools. Leave enough material for a light finish pass to minimize the burrs.

steve-l said:

Almost all plastics are hygroscopic and will absorb the coolant to some degree making dimensional accuracy problematic.

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I'm not 100% sure about that.

I think the problem is that even in industry that people think of "Plastic" as one class of "material", whereas there is a vertiable zoo of polymers and copolymers with different formulations and associated processes that can have dramatically different physical, electrical and chemical properties.

Hygroscopic VS Non-Hygroscopic Resins

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Plastics Technology

^^^ Useful link

Hygroscopic vs. non Hygroscopic resins...

Hygroscopic Resins

E.g. Nylon, ABS, Acrylic, Polyurethane, Polycarbonate, PET, PBT,...

^^^ These will absorb water vapor and if in a low relative humidity environment will give up their absorbed water vapor causing dimensional changes as internally absorbed water migrates to the surface.

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Non-Hygroscopic Resins

E.g. Polyethylene, Polypropylene, Polystyrene, PVC

These polymers are not completely hydrophobic (like a wax) but only adsorb water vapor to their surface.

Polypropylene in particular is used in medical applications for vials and storing liquids , same in the food industry.

Polypropylene - Wikipedia

Interesting all the variants of polypropylene including more crystalline formulations.

Hard to say what with all the different components used in coolnats for machining what further effects that might have on various polymers.

guest said:

Polypro is not hygroscopic, but it's soft and gummy and a PITA to deburr.

Suspect people using air blast are just trying to get the heat out with the chip, or blast the chip away from the tool so it doesn't wrap around and melt. Coolant on PP is okay to use, spray mist is probably better.

Plastics like Nylons are hygroscopic and will absorb coolant, so air may be more appropriate if there are tight tolerance features.

Normal plastic machining techniques appply- low RPM, high chipload, razor sharp tools. Leave enough material for a light finish pass to minimize the burrs.

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Beat ne too it by 5 mins lol

Heat / thermal deformation is a big deal.

meowkat said:

Why do so many people machine (mill) PolyPropylene (PP) dry with an airblast? Is there some issue with common flood coolants? What about a fog buster coolant mist air blast? Anything else I should know?

It's only 10 small parts but I'm not too worried about cleaning plastic chips out of the system. Some 3d surfacing required, tolerances are all +/- 0.010"

Click to expand...

I'm not convinced that a fine mist would have less penetration into a hygroscopic-ish material VS flood coolant ?

Depending on the fineness of the mist / vapor it could have increased penetration into the material over just plain immersion.

But in a material that is not absorbing water / water vapor in the first place ? Does it make any difference anyway ?

Given that the material is an insulator for HEAT, will coolant remove that (from mechanical cutting action / heat build up ?)

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For some polymeric materials and testing, cooling the material does make it more brittle, that's a technique used to measure mechanical penetration of a polymeric material (more accurately); for example with surgical gloves etc. they slow the velocity of various implements down impacting on sheets of substantially cooled polymers under test. When HIV became a thing the medical industry had to devise all kinds of new tests for how easy it was to cut or puncture various materials. It's interesting that the cooled materials behave very predictably to more accurately model penetration failures (at room temperature and beyond) ... But has a similar dynamic for "cutting" conditions / "Machining".

A bit like cryogenic machining whether some of these liquid nitrogen fed machines would be the ultimate for machining precision polymers ? 'cuz everybody has TSLN (Thru-spindle Liquid Nitrogen lol ;-) .. ). I don't know how you dial up and dial down cooling without polymers just shattering instantly ?

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I think with OP's relatively easy tolerances might be good to do some testing ? (coolant vs mist vs air )… for carrying heat away + measures of dimensional changes due to molecular interstitially absorbed water ?

I agree with Janolic's sentiment that thermal deformation is going to be more of a factor than dimensional "swelling" of the material from absorbing coolant ? Maybe continuous air also provides some cooling ? As mentioned in OP's first post ?

Also depends on the physical "Matrix" of the material too.

Oils in the coolant "bind" to polypropylene. Polypropylene is one of the major components used for oil absorption mats and rolls. Not sure if there are significant property affects to polypropylene when the surface is oiled.

The only difference between air and coolant when machining polypropylene would be you could double your output with coolant, same with nearly all plastics including nylon. Polypropylene will not be effected by any trace oil in the coolant and does not absorb water, as in you don't have to dry it before injection molding. Coolant is a nonissue even with nylon, unless you are going to keep it wet for several hours.

cameraman, extruded plastics we can buy to machine with are pretty standard stuff. If you want to get much selection you need to buy it by the bag in pellet form.

Polyethelene is good for attracting oil too. When I machine it I don't have to skim the tramp oil out of my coolant, it gets removed with the chips, and I have a box way mill that pumps a lot of oil into the coolant.

Thanks for the replies. It seems that plastic deforms quite a bit when machining :/

meowkat said:

Thanks for the replies. It seems that plastic deforms quite a bit when machining :/

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Not if you use sharp tools and correct work holding. Machining metal doesn't teach you much about machining plastic, it is usually quite different. Using tools designed to machine plastic will work much better than tools designed to machine aluminum.

DavidScott said:

......Using tools designed to machine plastic will work much better than tools designed to machine aluminum.

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Especially true when cutting polypropylene and polyethylene. When you are cutting ABS, acetal, polycarbonate and so on they are much more tolerant of aluminum specific or general purpose cutting tools so long as they are sharp.

meowkat said:

Thanks for the replies. It seems that plastic deforms quite a bit when machining :/

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Depends on the plastic. I've had Extruded 1" thick HDPE (AKA Polywarpolene) potato chip 1/2" over 20" diameter. PEEK, Acetal, Ultem
have been nice to machine and seem to hold shape well in my limited experience.

Vancbiker said:

Especially true when cutting polypropylene and polyethylene. When you are cutting ABS, acetal, polycarbonate and so on they are much more tolerant of aluminum specific or general purpose cutting tools so long as they are sharp.

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True but I liken it to cutting aluminum with tools designed for steel. Sure it works but until you try the right tools you don't know what your missing.