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What type of "plastic" rod machines best?

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Peter Bell	09/01/2010 08:57:41
399 forum posts 167 photos	I like to keep a a bit of "plastic" rod in the workshop, just in case. But I have now run out of stock and while looking around e-bay see that there is a vast choice. Just for odd jobs what is best/easiest to machine from natural nylon, polypropylene etc? Peter
JasonB	09/01/2010 09:22:10
25215 forum posts 3105 photos 1 articles	You could try Acetal, easier to work than nylon. Polypropylene is not as rigid. Jason
Martin W	09/01/2010 11:16:55
940 forum posts 30 photos	Peter It depends on its final use as, I expect you know, their mechanical properties are very varied plus some are more stable when exposed to things like sunlight (UV), heat, acids etc. As Jason says Acetal, often referred to as Delrin, machines very well but is slightly less stable than Nylon when exposed long term to sunlight. Nylon has a higher melting point than delrin but produces very stringy swarf when machined. For general purpose work I tend to use delrin/acetal as it takes a nice finish and taps well is relatively strong and fairly stable. Not only that it is readily available, quite cheap and doesn't produce any nasties when being machined. In order to get a good finish on any of the plastics I found that it is really important to use VERY sharp tools. I use HSS that I have ground and then sharpened using a fine diamond hone, I don't have a dedicated tool sharpening machine. This for me works very well, so its sharp tools and don't let any heat build up when working the stuff. Hope this helps a bit Martin W Edited By Martin W on 09/01/2010 11:23:23
Peter Bell	09/01/2010 11:23:49
399 forum posts 167 photos	Thanks for your help Martin and Jason. I take your points about sharp tools and sunlight. A bit of Delrin sounds ideal for what I need so I will order some offcuts pronto. Peter
Circlip	09/01/2010 11:59:50
1723 forum posts	ABS, Polystyrene, Polypropeline What's the final use??? A lambo Countach is a great car, but absolute CR*P in the present weather conditions. http://www.directplasticsonline.co.uk/ Regards Ian.
Ian S C	09/01/2010 12:11:29
7468 forum posts 230 photos	Hey Circlip saw someone on TV tonight In Sussex with a team of Huskies!!!!!Would an Igloo make a good workshop.Ian S C
mgj	09/01/2010 12:36:11
1017 forum posts 14 photos	Trouble with Delrin is that its highly hydroscopic. Changes dimensions like a demon, so if its to fit in a bearing, or you want to make a bearing liner, make it good and slack, because it'll tighten up on you first time it gets damp! It does have an advantage - it's cheap, and deservedly so.
Jeff Dayman	09/01/2010 13:26:19
2356 forum posts 47 photos	Delrin is NOT highly hygroscopic, it has very low water absorption. However, after Delrin is machined, it does move around in size due to release of internal stresses and due to machining heat. Sharp tools and air cooling are a good idea with Delrin machining. Nylon is quite hygroscopic, perhaps this is what MGJ was thinking of. Filled materials (ie glass fibre filled, mineral filled, etc) are more stable than unfilled plastics when machined. Good luck, Jeff
Peter Bell	09/01/2010 13:36:35
399 forum posts 167 photos	Great link that---did not realise there was so much variety of material. Are the filled materials also as machinable? Not going to use it for anything critical but found nylon certainly hydroscopic when trying to fit some large premachined bushes the other week, had to warm them up presumably to dry out before they were the right size! Thanks Peter
Circlip	09/01/2010 13:54:32
1723 forum posts	Some Nylons are less hygro than others and in some applications, the absorbed water content assists the basic material. Filled materials tend to have a scruffy finish when machined from their extruded or cast shape. And before we get there, some can be solvent welded AND solvent polished. Regards Ian.
mgj	09/01/2010 14:24:43
1017 forum posts 14 photos	Well no doubt you have a different answer to the problem of tightening bearing blocks/gear change sleeves made of Delrin in racing cars in damp weather. (The plastic that is stable and non hydroscopic is of course PTFE - but it costs.) Or perhaps a better explanation as to why APDS ammunition in the 120 tank gun spun off and was so inaccurate - many years ago. Actually it was tracked to the absobtion of atmospheric water in the Delrin driving bands during storage. On firing the friction turned the water to steam - assymmetrically - within the bands, so the band fractured in an uncontrolled manner, and on shot exit the proj failed to fly true. (sometimes spectacularly so - wow was it fun to watch) There are three reasonably good examples of the effect of water absorbtion by Delrin, giving rise to significant degradation in performance (not being able to turn a steering column tends to be a touch awkward at speed) - Obviously your Delrin is different from everyone elses, and if so, I suggest people use it. As Circlip says there are various engineering nylons, but Delrin is a specific trade name. Good stuff, and it has many good points, but dimensional stability in damp/wet conditions is not one of them - which is one reason why its not much used for seals etc in the Dairy industry. Edited By meyrick griffith-jones on 09/01/2010 14:27:38 Edited By meyrick griffith-jones on 09/01/2010 14:28:55
Gordon W	09/01/2010 15:36:07
2011 forum posts	Nylon absorbs water and swells, don't know about Delrin. Knowing the bush will swell by x amount, why not allow for that in the M/Cing? My steering bushes are plastic and not tight, don't know what they are made of. Which is my point, how do we, as small scale buyers , have any real idea what we are getting ? I asked same question about HSS some time ago.
Martin W	09/01/2010 15:51:14
940 forum posts 30 photos	Peter As I said in my initial post, this has been confirmed by other postings, it really depends on the final use. Yes delrin/acetal is hygroscopic and according to specs will absorb and or lose some water from or to the atmosphere. This will change its size slightly and if very close running tolerances are required then it could cause interference as Meyrick suggests. Meyrick I would love to have seen he expressions peoples faces when the shells didn't go as predicted. I can still remember the consternation caused when Rapier GtA missiles misbehaved in the early days, especially when there was an aircraft in the air towing the target. However as a general engineering plastic it is not too bad and as described in the specification doc has, and I quote, "The homo-polymer has higher mechanical strength, stiffness, hardness and creep resistance as well as a lower expansion rate and often presents a better wear resistance." and before the big guns launch a broadside this is a direct extract from the product data sheet. So for general work requiring a reasonable finish where a small amount of dimensional instability can be tolerated it is fine. But it is horses for courses. You don't use copper to make a lathe bed nor lead for a spring and the same applies to plastics!! Yes filled plastics can be machined but then the problem arises as to the filler material. Some of these can be very abrasive and have the potential to blunt tools and deposit abrasive material on machinery. I know that when these materials are used in injection moulding equipment they wear the dies much quicker than unfilled plastics due to the abrasive nature of the filler. As they say the choice is yours and whatever you choose have fun after all its just a hobby and should remain just that. All the best to all Martin W Edited By Martin W on 09/01/2010 16:01:27
Jeff Dayman	09/01/2010 15:53:42
2356 forum posts 47 photos	Posted by meyrick griffith-jones on 09/01/2010 14:24:43: As Circlip says there are various engineering nylons, but Delrin is a specific trade name. Good stuff, and it has many good points, but dimensional stability in damp/wet conditions is not one of them - which is one reason why its not much used for seals etc in the Dairy industry. end of quote Delrin is NOT a nylon, it is Acetal resin, or PolyOxyMethylene polymer (POM). Nylons are in the PolyAmide family of polymer resins (PA). Perhaps you should stick to dispensing advice on items in your area of expertise, which clearly is not plastics. Just FYI I have no expertise on gun ammunition and can offer nothing about plastics on that topic. If you go onto www.dupont.com you can find a wealth of information on Delrin resins. www.boedeker.com has a wealth of information about plastics for machining and also grades reccommended for different applications. Cheers Jeff
Versaboss	09/01/2010 16:49:45
512 forum posts 77 photos	Gents, I think you have forgotten another important type of plastic, PET (aka PETP or Mylar). As I have a little stock of that stuff and also of Delrin, I can say that they are quite similar in the mechanical properties and machinability. About PET it is said that its water absorption is very low. Maybe there is some confusion or mix-up here??? Acetal resins (and Nylon too) are also available oil-impregnated for bearings. Greetings, Hansrudolf
Gone Away	09/01/2010 17:26:47
829 forum posts 1 photos	Posted by meyrick griffith-jones on 09/01/2010 12:36:11: Trouble with Delrin is that its highly hydroscopic. Changes dimensions like a demon, so if its to fit in a bearing, or you want to make a bearing liner, make it good and slack, because it'll tighten up on you first time it gets damp! I don't know where you got that from, Meyrick but anyone who's worked in on spacecraft engineering would disagree with you. Nylon is extremely hygroscopic and as soon as it gets into a vacuum, it dumps the water and goes "twang". It's use has been banned by NASA since the 60's Delrin is quite good in that respect and is often used instead of nylon on spacecraft. Delrin has its own Achilles' Heel however in that it tends to build up internal stresses during machining which can later relieve themselves causing distortion. This is addressed in the aerospace industry by a stress relieving treatment prior to a final light machining. I'm retired now and it's been a while and I don't recall the exact details but I do remember that an oil bath is involved. I don't know if the process could be adapted for home use. It's possibly this effect that you have noticed with bearing liners although in my experience it's generally parts that are very asymmetric that show the worst effects.
mgj	09/01/2010 17:43:24
1017 forum posts 14 photos	Jeff- I think we all know the difference between nylon and the various acetal co and homo polymers, and their engineering limitations and advantages. After all, some have been using them in critical applications for a little while. Nor even is the example resrticted to gun ammunition. It was simply a case of what can happen when a supposedly non hydroscopic material fails to live up to the makers claims. Now if you'll please stop being pedantic, and I apologise for using the term nylon loosely, we will all accept that you are absolutely right. Martin - as you can imagine, the excitement of actually firing main armament service sabot for the first time, and no one could hit a barn door, with these vastly expensive tungsten sub calibre proj's winging all over the shop - quite literally a lot of this stuff was corkscrewing all over the sky. (Suffield). The safety staff were getting a little exercised. It hadn't affected us with the practise stuff because the turnover was much greater and the bands didn't get the chance to absorb moisture. Going back awhile now but Delrin was seen as a bit of wonder material and du Pont made all sorts of wonderful claims for it. Indeed I remember the investigation because a lot of £ was riding on this, and how it was not possible for Delrin to behave like that etc etc. Well it did. The cure, rather like with my racing car bits, was to change the material. Gordon - you can allow for the change in dimension when machining - provided you are prepared for the bit to be slack in the dry and tight in the damp. Thats the problem. It swells, a little like wood does - though not to the same extent of course. To give an idea of the degree, I was just using standard H5 class reamers to provide clearance on stock sizes, and a Delrin block about 1" x1.5" would bind noticeably on that.
chris stephens	09/01/2010 18:10:47
1049 forum posts 1 photos	Hi Guys, (The following was written several posts ago, but i could not be bothered to change it in light of newer ones. Take from it whatever you want to) I think some of you are falling into the " lets call all vacuum cleaners Hoover" trap., not all plastics are Nylon, but it is all to easy to call any thermoplastic "nylon" for ease. There are numerous versions of most commonly available plastics and it is a mistake to make generalisations. Take Nylon for instance, my catalogue from Watford Plastics lists three main types, type 6, type 66 and type12, then you can have with or without glass -filled, with or without MoS, you can even have oil impregnated. Oh and don't forget the colour, which can have an effect on its application. If you follow Jeff's suggestion and go to the Dupont site, there are again a vast number of different "Delrins", all with slightly different properties and uses. Now for some sense(?), most of us will only have access to a limited number of plastics a few Nylons, maybe Delrin, HDPE, and of course Polycarbonate. I would suggest that most of us in the hobby end of the market would pick up a lump of plastic and use it for its electrical , bearing or "waterproof" properties, and not care too much about its ultimate mechanical properties. If we need a plastic for a particular use then we can look up which is the most appropriate for that job, but the original post was for a general purpose plastic, to have in store for the odd occasion when it is needed. I would think any old lump of Nylon or Delrin would do. In this case colour is most likely more important than properties! Now to back up ( well almost) what MGJ says, my catalogue says of Polyacetal (delrin). "Almost no moisture absorbtion" which I take to mean there is some, however slight. So stick that in your breech and fire it. In the ultimate, extreme application that he had a use for Delrin, any slight inconsistency in moisture content would negate millions of pounds worth of fire control computer programming! His personal experience may well be different to ours but it is valid all the same. Most of us will never need to put any material to such punishing requirements or tolerances. chriStephens
Martin W	09/01/2010 18:56:29
940 forum posts 30 photos	Hi All of the above in response to a gentle enquiry about keeping a bit of plastic on the side!! Wonderful and its what keeps forums interesting and informative. Great place to learn and get tips from various peoples experiences even if they vary due to the application materials are put to. I suppose I should have stated that lead is no good as spring at room temperature but might be fine if used in liquid nitrogen. Seriously there is a great deal of valuable information in these forums and many good folk always willing to help people like myself who, even at an advanced age well past retirement, are starting out to do what they dreamed of many years ago and that is making things out of metal that hiss, spin, look pretty (well to the beholder) with the great satisfaction of knowing that it was done to the best of ones ability and yes those two or more bits of metal are the same plus or minus the odd thou or two. So thanks to all who give their time and are prepared to share their wisdom and experience with numpties like me. Martin W
mgj	09/01/2010 19:24:44
1017 forum posts 14 photos	Sid - does it not depend on what one is using it for? If its a seal in a pipe, particularly for say hydraulic or gaseous stuff or somesuch for which it does not have an affinity, then probably it wouldn't make much difference.(whether in space or not - no idea ). Hang it out in the wet on a racing suspension and you can get a problem - I don't know enough about the presence of water in space, or the way materials are stored before use while on Terra Firma (nicely sealed in a poly bag?). But that too can make a difference Equally store it in some magazine in Canada, and then use it as a driving band and you can get terrible problems from absorbtion in a very symmetrical circular driving band. And there is no if or but or argument about that. That's what happened. The point is simple - it has, as Chris said, its uses, and its limitations, and it is as well to be aware of them.(Had a lot of trouble with Delrin locking bands on early Chieftain track tensioners BTW. Bit inclined to jam in wet conditions - until two of you just jumped on the long lever. Change to PTFE - bingo. OTOH Delrin in oil baths - worked well) In other words it all depends on what one is going to do with it. . Would I make a steam engines piston rings out of Delrin - no (despite an apparent suitability). Would I try with say PTFE- yes (not that there is much point). would i make a soft locking pad for a shaft out of it - certainly. Would I make a steam gland seal from it,- no.etc. I might also consider costs by the way. Off cuts of Delrin and many of the nylons are cheap as chips. Some of the plastics are almost hysterically expensive. PTFE for a start Edited By meyrick griffith-jones on 09/01/2010 19:36:33 Edited By meyrick griffith-jones on 09/01/2010 19:37:25

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