Best Material and Tolerances for Plastic Part

These general requirements don't disqualify any plastic materials.

OK, so, now can anybody identify a specific plastic, which will give me all the other things I asked about?

I can identify 20 plastics that will do what you want.

You have yet to provide any specific properties that cannot be satisfied by any number of materials, including wood or steel.

Sorry.

OK. Lets try this again.

I am looking for a specific name, or a formulation to buy.

Do all twenty of those plastics have the qualities of:

1) The SMALLEST minimum tolerance for the desired coefficient of friction, and what EXACTLY is that dimension for each of the 20 plastics. I need to know what tolerances to draw up the plans, but I cannot draw up the plans until I know what tolerances to use.

2) Are all twenty the MOST durable, and not slip with a MEASURABLE minimum of pressure applied to cinch them down?

Or, are all these properties exactly the same for them all, which might explain why you are being so purposefully vague?

If so, please just name them, and I will go away.

Quite frankly, I suspect that there is one, or even a couple with a better collection of these properties, than all the others. But, you have not even given me one useful answer, in fact if I could give BAD ANSWER points you would already have 2.

Anyone else out there with some "useful information"?

Nylon is a good general engineering plastic which is self lubricating.
The problem with what you describe is it's all a tad vague, what are the sizes and forces involved, how is it clamped etc.
Nylon is easy to machine too and I'd suggest machining up a prototype mechanism to test.
it needs clever design to minimise the effects of molding tolerances and swelling of the plastic with time/moisture absorbtion.
You say it needs to slide frquently yet be clamped permanently, that seems contradictory.
If the sliding is a one off then you could go to glass filled nylon which is much stronger and more stable, but as you can imagine the surface has highe coefficient of friction.
When in doubt experiment!
Del

(I like Lyn's idea of wood)

If I could give BAD QUESTION points, you'd have a few too .

1. How is this part going to be made - molded, machined from sheet, rod, etc? Mass-produced or one-off?

2. What environment - temperature, wet, etc?

3. Strength/deformation requirements?

4. Cost and appearance considerations?

If it's going to be machined from sheet or rod, for dry ambient conditions, and has low strength requirements, I'll throw out high density polyethylene for your consideration. It's readily available and about as inexpensive as you can get, too.

Really? What difference does it make HOW it is made (other than finding the cheapest way to produce)? It is the final 'cold' finished part, which needs to be 'in-tolerance' once assembled. If I have it molded, I just need to know how much it will shrink to be 'in-tolerence' once cold.

So, far nylon is sounding like the way to go, as it is durable, and less likely to have things stick to it and foul the works should various things be accidentally spilled, or dripped on it. This device is to be used in a normally dry, room temperature environ, will not be subjected to any significant forces, or deformation. Probably, the worst thing it will have to endure is being tossed into the tool box at the end of a hard work day. It's purpose is as a highly specialized measurement device. To expound upon any futher detail would require taking this 'off-line', and signing non-disclosure agreements.

Quite frankly, I thought I went into quite a bit of detail as to what I was searching for and was not vague AT ALL, as to asking about materials, and their properties. I even thought that the answers to my questions might already reside in a textbook, materials spec manual somewhere, which someone might be able to refer me to.

(This does not apply to all, and only to degrees for others) And, quite frankly, nylon was already on my mind when I asked my questions; but, I was allowing for the possability that there just might be a material out there, with which I was unfamiliar, that might just be better. So, I put it out there, expecting information from professionals, instead I get smart ass grief. What's the deal?

IF, I had just named a particular material, and asked what the minimum tolerance was for making parts that fit together and be able move when not cinched down, and not move when cinched, would I have gotten any straighter answers? What exactly did I do here to merit the Crap?

What difference does it make HOW it is made.
The fact that you ask this question show you have little understanding of molding plastics.
There will be different stresses (and resultant changes of dimension over time) in a part machined from a block where internal stresses may be released as material is removed and shrinkages in a molded part where the plastic flows under pressure.
Simply changing the pressure temperature or cycle time of an injection molding machine can effect the dimensions of the part produced.
After molding there will be dimensional changes.
Use Nylon66 talk to people in and injection molding shop. Make some parts and experiment.

I have tried to be helpful, but you have not responded to my posts, so I'm out.
There is no point asking a question if you don't listen to the answer.
Del
(unsubscribes)

Quite frankly, you gave no usable information, at all!

This statement proves that you are in the wrong business, "Probably, the worst thing it will have to endure is being tossed into the tool box at the end of a hard work day. It's purpose is as a highly specialized measurement device."

Plastics will not make a good, "highly specialized measurement device" if it,"is being tossed into the tool box at the end of a hard work day."

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If you only knew what you were talking about, you would change your tune once you saw the finished device, and how simple it is. I will give you an honerable mention when I do my informercial, as being head of the 'they said it couldn't be done crowd.'

Sure, I'll sign an NDA to provide you free advice - not.

Here's a link to some plastics selection charts: http://www.mcmaster.com/#plastics-selector-packs/=jz9u4m

Here's a link to a pack of samples of 15 different plastics you can touch and feel: http://www.mcmaster.com/#plastics-selector-packs/=jz9obe

Here's a link to a company which molds plastics: http://www.reblingplastics.com/materials.htm. You may find some of the reference material on their site useful.

Since this is a 'measurement device', I assume figures have to be molded in and/or printed on. That presents a whole new set of issues, since most plastics do not readily take ink or paint. I withdraw polyethylene from nomination. Incidentally, I cannot vouch for any of the above references.

Thank you for your productive advice. As I mentioned before, I was not trying to pump anybody for any information, which is probably not already in a textbook, or materials manual. I was just hoping someone could steer me to it. Not everything is 'Googleable', at least not yet anyway.

[p]I mentioned the Non-Disclosure, because folks were pumping me for information, that I was not at liberty to give without one. A Non-Disclosure is a staple when dealing with patentable intellectual property. Properly written it protects both parties.

ABS, Acetal, PVC, HDPE, LDPE, Nylon, PC, PC/ABS, PEEK, PEI, Kapton (PI), PMMA (acrylic), PET, PP, Polysulfone, PPE+PS, PPSU, Polystyrene, Phenolic, PTFE, UHMWPE. There are 21 listed that will work based on the OP's vague requirements. There are probably at least another 21 I could name with little effort.

You can't hold tight tolerances with plastics spring loading the part into a dovetail or V shaped groove is probably the easiest solution, you can design the spring into the plastic material, but please please please make the spring tapered and blended nicely at its root else it will snap off. That's my pet hate as I see it all the time with plastic clips and catches.
Like I said it's about clever design to suit the material and process.
Del

SWB123 The reason that you have been given vague answers is that what information you given us is vague. You dwell too much on the material and not the application. In not providing more info on the application we can not recommend one plastic over the other.

Like you want it to slide but then not slide locking it down with minimum force. What type of force is it going to see when lock down? How you going to lock it down? How much weight is on the slide? How much impacts it going to see?

There are many plastics that could "fit the bill" and other concerns aside, Nylon, ABS, and Polypropylene would work. (There are literally hundreds of grades of each) These however can be relatively pliable and some can creep if left in permanent tension.

Acrylic/Polycarbonate and blends of these can provide significantly greater rigidity, but with some potential brittleness concerns if stressed.

To increaseyour co-efficient of friction you could look at the surface finish in the areas involved so that a minor release in clamp pressure allows free movement.

It may be that what you consider "precision" equipment may be not so for some people here, so I'll suggest that you have a look at the plastic slide rules that have been on the market at various times. These have sliding/clamping elements in them and for what they are worked well.

As far as tolerancing, you will have some issues. You should specify your needed end size and leave it to the processor to achieve the outcome. Shrinkage of injection moulding depends on material type, melt temp, mould temp, injection pressure and a few other parameters. The mould maker and processor will build the mould to account for these.

What is the relative size of the parts? holding a +/-0.05mm dimension on a 5mm size is different from a 250mm piece.

i have bookmarked http://www.ides.com/info/generics (i don't remember why -- ? it might be useful ?)

i made a dumb scan for "heat+resistant+adhesives"? THEY found this http://www.calsakpolymers.com/section.asp?CategoryID=619 ???

My "go-to" plastic for jobs lacking detailed specifications is linen-melamine. I suggest you try it. If you find a problem with it, let us know what the problem is and we will recommend a better alternative.

I have to agree that this is not as simple of a request as you seem to think. Also, some of your requirements are vague.

Here are some more considerations: Do you want something with high static friction and low sliding friction? Otherwise the locking mechanism with have to hold it in place when cinched. You will also need high abrasion resistance so the sliding surfaces don't wear each other down. You might be concerned about chemical exposure (oil, grease, etc.?) since you stated you would put it in a toolbox. You stated it is to be a highly precise measuring device, so you need it to be dimensionally stable, to be accurately manufactured, and not to creep under the clamped load.

Perhaps a teflon (PTFE) material would work for you. It is extremely low friction & resistant to certain chemicals. Machinable grades will be good for prototyping to good tolerances. Again, not if you need to have anything printed on it.

Without more info I can't give any better suggestions.

Machine it out of Delrin for prototyping. When you reach large volumes of your moneymaker, mold it out of the same.

Yes, Delrin (POM) is a possibility as well. Would need to ensure it won't be in contact with acids. It also swells reversibly in some solvents and fuel components (particularly short-chain alcohols like ethanol and methanol) which might not be suitable for precise measurements.