Anyone know a good plastics primer? Like a DIY guide for electrical engineers? Every now and then, I get a great idea that involves making some doo- dad out of plastic.
Only problem - I don't know diddley about plastics.
I'm thinking this widget should be made of nylon, but there must be hundreds of plastics / polymers out there. Without being a chemist, how does one know which type to choose? Unfortunately, the plastic fab houses here, which number in the low single digits, are generally unhelpful.
I guess I'm hoping there's a $35 book out there (or a free Internet site) that can reveal all. Any ideas, suggestions? Thanks,
and hop around their links. I can't endorse them from actual experience yet -- we seem to be stuck at the level of punching holes in off-the-shelf enclosures -- but there's enough info there about the process and the materials that I think I'd manage to get pretty close to an acceptable result.
On the road there, there's a (currently) free 3D format conversion tool available at
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Even if you don't need to convert between ACIS and STL, it can be useful to see if a third party app can read the model you generated.
So, copy something that does something similar. Often the basic plastic type is marked on the part (recycling symbols) or can otherwise be determined (eg. flame test). Of course there are many possible additives for UV, plasticizers, flame retardancy etc. that are usually added. Engineering requirements (function, compatibility with other materials and environment, commonality with other parts, availability, cost, resulting part cost and design constraints due to material characteristics) and regulatory requirements (food safety, avoiding certain additives such as polybrominated ethers, recyclability) will determine the choice. Design with plastic parts is not much different from other types of mechanical engineering, but plastics and plastics processes have a few unique characteristics.
Nylon is very hygroscopic and shrinks a lot. Not so great where dimensions are critical, but has some other good properties.
Without being an engineer, how does one know which type of op-amp to choose? ;-)
There are polymer chemists, there are part designers, there are mold designers, there are molders, and there are manufacturers. Often each works for a separate company. Personally, I have hands-on experience in all but the first (and a course or two in the that), mostly to deal intelligently with all the different suppliers you need to make a product in the global economy.
I think your best bet is to emulate existing stuff that works well and not try to get too fancy. There's too much to learn. I have a bookshelf full, and I don't think any book was much under $100.
The SPE book _Plastic Part Design for Injection Molding_ is a decent primer if you mostly want to design parts. Should be around $100.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Some of the most important are PE (from LD- to HD- to UHMW-), polypropylene, polyester (PET, nylon, etc.), acetal, teflon, PVC, ABS, polystyrene (especially for vacuformed films), acrylic, polycarbonate, and a bunch of others. Most of these are thermoplastics, so they can be molded to shape.
Most are excellent for machining, with PE, teflon and acetal preferred for that I think. You can get big blocks of the stuff and carve whatever you want, as long as your tooling is sharp enough. Acrylic and polycarbonate are good when you want something transparent.
And there are numerous resins for cast products, especially polyester ("bondo", among others) and epoxy.
Simple enough... think of a plastic product and what it's made of, and figure how that best fits into your product. Look up some data and see if the numbers match up (strength, bending, stiffness, creep, temperature, etc.).
Polycaprolactate is an interesting one which can be molded by hand, since it has a low melting point. I'm not sure about creep, but it's fairly strong at room temperature. Can be machined / carved when cold too.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
"mpm" wrote in message
news:030e3fbd-a32a-4be9-84fc-811ae62154d7@y3g2000vbm.googlegroups.com...
> Anyone know a good plastics primer? Like a DIY guide for electrical
> engineers?
> Every now and then, I get a great idea that involves making some doo-
> dad out of plastic.
>
> Only problem - I don't know diddley about plastics.
>
> I'm thinking this widget should be made of nylon, but there must be
> hundreds of plastics / polymers out there.
> Without being a chemist, how does one know which type to choose?
> Unfortunately, the plastic fab houses here, which number in the low
> single digits, are generally unhelpful.
>
> I guess I'm hoping there's a $35 book out there (or a free Internet
> site) that can reveal all.
> Any ideas, suggestions? Thanks,
Utter nonsense. The JVC stereo remote in my hand is marked HIPS, for example. (High Impact Polystyrene).
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
You can go to McMaster Carr supply, look under raw materials - plastics, and find characteristics of the different plastics they sell.
One time I wanted to play with plastic injection molding so I cut a small mold out of aluminum, used my lathe and a drill press. For an injector I bored a piece of aluminum rod, the parts I was making were small, I used
3/8" hole & piston. My piston was some hardware store 3/8" steel rod. For plastic I cut some poly rope, the stuff that melts real good when you melt it apart so it doesn't fray. I put my mold in a vise, put some poly rope in the cylinder, heated with a propane torch, held to the mold and rammed the piston in by hand. I got the best parts after the mold warmed up a little.
Anyway my parts came out quite well, I was working on a project for the plastics engineer at the Maytag plant and showed him my injection moldings. He thought they came out "darn good" considering my method. If I were going to do it better I would use temperature controls and a lever or press to get better pressure. After the success of my first part I made a simple washer mold and made some plastic washers of the size to use on the wing bolts of my R/C airplanes.
I need to get back to playing with this, building an injection molder. I have a 3 axis CNC mill that would be fun to make some molds with. I have some temperature controls, and enough of a machine shop to make something reasonably good. And if I got far enough along I have some PLC's to try to automate it.
Think of it this way... Questions to ask would be about strength, solubility, optical properties (see thru it or not), machineability, brittleness, etc. If there is some solubility allowed, then one might use solvent glues to assemble a complex shape. There are plastics that ae hot molded and cannot be dissolved or easily glued to - is that desirable or not? There is a yearly thick book Modern Plastics Encyclopedia that covers many plastics, suppliers, manufacturers, data, etc that might be a good start.
You're missing it. If I call a plastics maker to make a part for my product, I am not, nor is the plastic molder, going to be putting symbols on the parts.
What are these advanced plastics I see used in guns and other locations "carts, tripods, military instrument cases, etc. in structural or texturally centered implementations?
Then, you find someone that can give you those answers.
Seen the cap of a bottle of Bayer Asperin lately? Pretty cool.
The trigger on the new S&W M&P 9mm is PLASTIC, but *what* kind?
Plenty of plastics, resins, moldings that are 'melded' with other plastics (Rubbermaid windowed bowls).
I think the plastic that my 3D printer puts out when I proto a part is cool too!
The actual industrial process for an SPI Class 101-103 mold goes something like this:-
An engineer determines the part design requirements (including requirements for markings).
The part designer draws up the part including tolerances (and puts the marking requirements on the part drawing).
The mold designer puts creates the mold component drawings- dimensions are typically scaled for shrinkage (material and a bit process dependent) and draft angles added if the original didn't have them. One of the major functions of a mold is as a heat exchanger, and a fair bit effort will be spent on thermal design (the mold is peppered with cooling channels, internal "bubblers" and other features) for a high production mold. There are also often many thermocouples and heaters, mechanical shut-off nozzles and other complexities within the mold. Side action mechanisms and other components to handle undercuts need to be designed or specified for all but the simplest mold. Computer modelling will likely be used to assist with the mold design- finite element models including flow analysis, thermal predictions and initial process parameters using the characteristics of the specified polymer. There are not many polymer manufacturers in the world- they are generally large multinational chemical companies such as BASF.
The mold components and materials are ordered, including plates, ejection mechanism parts, and any parts required for side actions.
Skilled machinists make the mold components (using EDM machines, boring mills, CNC mills, surface grinders and other high precision equipment). An typical ordinary machine shop is really not up to this task (more like aerospace work).
Polishers do their work (much of it manual, very tedious work- one of the best I've met was a tiny Vietnamese woman). Parts are sent out for heat treatment, surface coating such as TiN, and texturing, maybe engraving of markings (or perhaps a standard insert has been specified), then the component parts are assembled by technicians.
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A test run is done, either in the mold maker's shop or elsewhere. Modifications may be necessary at this point, especially to fiddly things like gate geometry. The customer signs off on the sample parts and the mold is shipped out the door.
The mold is installed by grunts into a production machine at the custom molders or production plant. A setup technician, perhaps with the help of a process engineer, gets the mold running and determines all the molding parameters (dozens of pressures, times, temperatures, injection and flow rates, typically). Some Japanese automotive companies send their own setup and installation staff so that the custom molder can't so easily damage their expensive mold and compromise the quality of the parts.
A machine operator monitors the machine and sweeps up around it as the run is completed.
The quality folks will check the parts, perhaps with a Coordinate Measuring Machine (CMM) and using other methods according to requirements. A bit of moisture in PC, for example, can drastically reduce the strength of something (for example, a hockey helmet)-- visible by inspection (slight crazing in the surface) and can be caught by destructive testing of samples. Statistical Process Control may be used to monitor trends before the (part process) control limits are approached.
The machine is shut down, the mold is cleaned and lubricated by swarthy grunts, and it's removed with a crane or whatever and stored away to await the next run.
Of course prototype and hand-run molds can have a simplified process.. little or non-existent cooling (very slow production rate), no or simple ejection mechanism, lots of work on the parts after molding to drill side holes etc. Part cost is rather high and quality is not great, but if you only need a few hundred parts it might be acceptable.
BTW, I'm talking to the OP and anyone else who is interested in knowing, about the process.
Thanks! That's an interesting insight into the process. And somehow, after all that effort it's on sale at the 7-Eleven in a blister pack at four for 99 cents...
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