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[sheerline]

Just a quick one this, I noticed one or two folks have, in the past mentioned that when cutting and drilling acrylic (perspex) that the material melts as they cut it. The main reason for this is that they probably have the wrong type of plastic as there are two types. One is  known as 'extruded' the other, 'cast' and it is this type you should use if you wish to obtain clean cuts.
Using this stuff is a joy compared to the extruded type as it cuts cleanly, producing small granular particles of material and can be applied to the belt sander to produce good clean edges. It also drills beautifully and clean holes are produced without melting although I recommend the application of plenty of paraffin if going deeply into a block. Use a moderate speed for drilling and don't allow the drill bit to get hot, if necessary, repeatedly pull out of the hole and cool the drill bit with paraffin before re- entering.
The same can be said for tapping holes, use only cast perspex and apply plenty of paraffin as this will prevent the tap from sticking. Good clean tapped holes are easily produced with this method.
For all I know, someone may have already covered this subject so if I have repeated a topic, please excuse this form of repetition.
Chris

[banjo]

If we are unfortunate to have the "wrong" stuff...do you have any hints for cutting or drilling that?

[sheerline]

Hi Banjo, my knowledge of working this extruded stuff extends only to my own experience and is not based on any words of wisdom from experts but when I have used it, it needs lots of coolant which isn't practical for  home workshop machinery like a bandsaw for instance. If cutting by hand, use a course blade and take it slow, again, plenty of paraffin or some kind of lubricant for the blade to prevent sticking. This produces a rough looking cut which is difficult to finish  on the belt sander as the stuff heats up and melts so you have to finish edges by hand........ a real pain! , Fine tooth saw blades simply clog up so if you must use this type of blade, use lots of coolant and take it really slow.
You can spot the difference between the two types when you peel off the protective coating, especially if you hold both up side by side, the extruded plastic has a slightly blue-ish hue to it whereas the cast material looks perfectly clear.
Not always easy to spot unless you are used to handling and looking at it frequently but with experience, I believe I can almost feel the difference and if I knock both types off the edge of something hard, I would swear the extruded type exhibits a duller sound than cast material.
I avoid the stuff like the plague and only ever specify 'cast' when buying.

[andrewh]

Gents,

From a distance it appears that we are actually discussing at least 2 different materials, not one type in 2 forms!

As Sheerline says acrylic (perspex) does come in cast and extruded form.  Blocks and stubby shapes are usually cast, sheet and long shapes are usually extruded. 
Both are the same material (polymethlymethacrylate) and are very nearly in the same physical state.  As an engineer I know to take care of cast acrylic in pressure- retaining duties or where it takes many stress cycles, as it will eventually show internal cracking resulting from the casting internal stresses.

Both forms of acrylic SHOULD machine identically in all respects apart from the slight difference that extruded may have a "grain" (anisotropy) because of the process.

I was motivated to write because of the mention of a bluish tinge and melting in machining!
All perspex is a thermoset material - it cannot melt, and never to my knowledge takes a bluish colour (it used to discolour to yellowish brown - look at an old aircraft dome)

So the extruded sheet is clearly some other plastic - it could be an acrlyic, but not a thermoset one. 

The blueish colour sounds to me enormously like PVC, which is, indeed a thermoplastic with a lowish melting point and characteristic blue tinge, and would have to be machined, like all thermoplastics,  to avoid melting.

So please try melting a piece of the stuff - if it goes soft it isn't perspex or any of that family.  If it is PVC it will burn very smokily (but most plastics do) and produce lots of Chlorine and hydrochloric acid - so don't burn it much.

If my guess-from-a-distance is right, and it is PVC or similar, then razor sharp tools, low cutting angles, slow speeds and lots of cooling are all in order - the cutting fluid would be working as a coolant rather than mainly as a lubricant.  Compressed air would also work as a coolant, and is less messy.

Sorry for the brain dump - trying to be helpful

andrew

[sheerline]

Hi Andrew, thanks for your input on this one, I don't purport to be an expert on this subject but have been working with Perspex for a while now and can only impart my own personal experience on the subject. When I contact my suppliers, they know I only  ever buy cast material.. usually sheet form. If I want thick blocks.. say half inch or more, they cut it to size for me. They know I have to drill the blocks so they don't get the order wrong as they themselves have had to cut it so therefore know in advance if the are going to supply the wrong stuff. However, when ordering sheet, they have on occasion indeed got it wrong and inadvertantly sent out extruded. This becomes apparent to me as soon as it arrives because A: I immediately notice the slight blue tinge (not always a reliable method of identification) and B: the moment I apply it to the belt sander, the edge melts with the friction.  I have also used Polycarbonate on occasion and  found this material behaves in a similar fashion to extruded acrylic when worked.

[andrewh]

Sheerline,

I know that you make beautiful see-thru creatiions in perspex - and , as you say the cast material is lovely and crisp to work.  So is extruded perspex, completely indistinguishable.

There may well be an acrylic sheet which is a thermoplastic - but is isn't related to perspex and the other polymethlymethacrilates except by a bit of the name

Sanding thermoplastic sheet can be done, but same requirements as for machining - radically sharp, open  abrasive, lots of cooling (put it in the freezer first) slow speed and stay below softening point.

I'm not helping much.  I think that we are near to each other - I'm  near Lutterworth.  If I can be of any assistance with identification or specification drop me a PM.

andrew

[sheerline]

Hi andrew, perhaps there is a little confusion here created by the names given to the types of material we are talking about. When I refer to extruded Pesrpex it is only based on the name of the material given me by my suppliers and perhaps it actually is a different substance and not really Perspex. All I can say is that whatever it is .. I avoid it because of it properties. For the sake of ease of working we are certainly agreed that cast Perspex has to be the stuff to ask for.
Regards.... Chris

[andrewh]

Chris,

The more I have looked at this the less clear I have become! 
Except in one thing  -  which is that you are right and your original post laid things out well and clearly

Forgive me jumping in and possibly muddying the waters,

andrew

[sheerline]

Can't have this Andrew, not two of us at it... thats usually my trick! :-\

[StarLocAdhesives/FiveStar]

There are many types of `perspex` type materials even in the perspex brand PMMA, copolymerised with toughening monomers such as butyl methacrylate etc, some with rubber type tougheneing, in the basic PMMA form (polymethyl methacrylate) they can be softened with heat and melted down, thermformed etc, even though many class them chemicaly as thermoset, many/most are actualy thermoplastics easily softened by heat and even used as a basis for a lot of heat activated adhesives

The 2 types normaly found in PMMA based `perspex` are cast and extruded, the cast and extruded are not chemicaly identical even though both very similar and both PMMA, some arent PMMA but still have similar properties

Cast acrylic is formed by the pouring of a liquid monomer/curing agent into a mould, normaly shets of glass, ths is of a very high molecular weight so is not good for thermoforming, good for drilling as it doesnt melt, the properties can be varied with additives but also by the temperature of cure and cure initiators

Extruded acrylic is what it says, its squirted out into sheets, its a lower molecular weight polymer, still PMMA but ust cured diffreently, the type of cure of a PMMA polymer can cause great differences to the mw of the polymer formed, again they can be varied with additives copolymers etc but also by the temperature of cure and cure initiators

In practice its normal to not use a pure PMMA for either, using an additive comonomer to crosslink it, depending on the degree of crosslinking double bonds in the comonomer that crosslinks with the main monomer, the polymer will have different properties to the thermoplasticity,rigidity,chemical resistance etc adding about 3% of a glycol dimethacrylate can make it non thermoplastic, chemical resistant and more rigid

I always find to drill perspex, i use plasticine around the drill to form an area that can be filled  with water, i drill through the water using it as a coolant and lubricant for the drill

[sheerline]

Phew SLA, thats some chemical breakdown explanation there and I don't pretend to understand it all as I'm no chemist but from what you have said, cast acrylic has a higher molecular weight than extruded so presumably that means it is of higher density? If this is the case, it would perhaps add some kind of credence to the comment I made in the earlier post regarding the different sound produced when knocking two identical size pieces of each against a hard object ie, the extruded plastic emmits a slightly duller sound.
This is all very academic of course but for the benefit of modellers using this material I suppose we are still agreed, cast perspex + coolant/ lubricant= happy modeller!