screws for plastic

Re: screws for plastic

John K Jordan

The screws I use for plastics look a lot like sheet metal screws but the threads are subtly different. You can search Amazon for screws for plastic to see.

Some of the screws are self tapping so they go nicely into a drilled hole.

I get most of my screws for plastics by disassembling things made of plastic before throwing them away.

Sheet metal screws will also usually work in many plastics if you are careful in sizing the pilot hole. Some plastics can be brittle. I'd cut a tapping slot or two in the end with a metal-cutting disk on a Dremel. (sometimes I do this with a small triangular file.)

What kind of plastic and what is the application? I built something from Lexan yesterday (polycarbonate) but I decided to use glue instead of screws.

JKJ

Re: screws for plastic

Larry

I'm build a MPCNC and looking for a way to hold the drag chain .

I prefer to drill and tap

Dick Coers

So many plastics are so brittle, I prefer to drill and tap. Put the tap in a cordless drill, just takes seconds. I hate it when I've made the part, and then crack it when trying to assemble it with a screw. Same goes for drill bits. Best money ever spent are twist drills ground for plastic. They scrape more than cut. Drill brittle acrylic like butter and never any splitting. My second favorite story about plastic; polycarbonate is so soft, you can put it in a box brake and bend it over a 90 degree angle, cold. That's the reason it's the bullet proof plastic, the bullet just gets stuck in the soft plastic. You can also hand plane polycarbonate, it's that soft!

all plastic is not create equal

Bill Tindall, E.Tn.

Nylon taps and threads like metal. Some don't. Some will take self taping screws without breaking; some don't. In any case the tapering shape of wood screws is not suitable for most plastic.

Re: screws for plastic

Gary Smyth

Typically I use a metal straight (not tapered) pan head tapping screws with a hole two wire drill sizes larger than the screw. Occasionally a nylon washer to re duce my effort of over tightening the screw. I also place no closer than one inch from a edge. If I have to get closer to the edge because of space, I look to some other alternative.

Hey JKJ, What adhesive for polycarbonate

Keith Newton

It's been a long time since I worked with any polycarbonate, and was thinking at that time there was't any adhesive that really worked on it. I did a quick search and found several that made me suspicious about how well they actually worked.

A month or two ago while dropping off some scrap metal at the recycle yard, there was some equipment that had just been unloaded which had some nice big .45" thick polycarbonate sheets around it for safety shield. I ask if they wanted me to remove it for them, and they said go ahead. It would surely make some nice router bases or templates, and more if it can be glued satisfactorily.

PS Drop me a mailing address and I'll send you that piece of tool steel that we talked about a while back. I never found the other stuff I was part of an auction lot it came with.

Re: Hey JKJ, What adhesive for polycarbonate

John K Jordan

I bought a tube of this: Weld-On 16:

https://www.amazon.com/gp/product/B003HNFLMY

I think most people use a thinner Weld-On since it sets up quicker, but I think the 16 has a higher strength. It does bond the two pieces together quickly but recommends 24 hours for full strength. I tried bending a glued joint and it didn't break.

Here is a description of the Weld-On glues for plexiglass and lexan

http://www.eplastics.com/plexiglass_glue

http://www.eplastics.com/lexan-polycarbonate-glue

I'm using some pieces of polycarbonate that the ClearVue cyclone people sent to me about 5 years ago. If I remember correctly they said to glue it with the IPS Weld-On glue.

Our local metals recycling place sometimes has plastics at scrap prices but I haven't seen any cast acrylic or lexan yet. I did find some big sheets of HDPE 1" thick that I've been using for a variety of things. And lots of aluminum and steel, a few pieces of brass.

I'll send you a note. Thanks!

JKJ

Adhesives and polycarbonate

Bill Tindall, E.Tn.

While this stuff is tough enough to use for bullet proof glass it has a property that can easily destroy it. When exposed to some solvents it crazes and if under stress it deconstructs spectacularly.

Experiment- Bend a compact disk to put the bend under stress. Add a drop of acetone to the bend. Wear safety glasses. It shatters into lots of pieces that fly about.

The solvent in a "wrong" glue will have the same effect.

Re: Adhesives and polycarbonate

John K Jordan

>>>When exposed to some solvents it crazes and if under stress it deconstructs spectacularly.

That will make an interesting experiment, thanks! I'll try it on a thicker piece too. It will be interesting to see the effect of the wet vs surface crazing from dried solvant. I'll try it with the polycarbonate Weld-On 16 glue too as well as the acetone. I don't see acetone listed on the tube or the MSDS.

My current use for lexan is in a non-stress situation; I used it because I had it on hand. I made a 3-sided box to fit below my bandsaw table to cover the lower guides, designed to allow air to flow sideways across the guides. I've been using a "temporary" one made from cardboard for years and it has worked well, preventing the bit of sawdust spray from below the table. With the plastic box I can more easily keep an eye on things. I use magnets to hold it in place.

BTW, I love the dust collection in this 18" Rikon. I never have a buildup in the cabinet regardless of what and how much I am cutting. Sometimes there is a teaspoon or two of dust in the corners and sometimes not even that. I use three 4" hoses from a 6" duct.

JKJ

Re: Adhesives and polycarbonate

Larry Clinton At Frankfort, (Central) Indiana

You are correct! Several years ago when I was an Engineer over several assembly lines, I had an issue with some acrylic clear shields used on pneumatic assembly machines cracking fairly often and having to be replaced. I had researched polycarbonate (Lexan) and decided to fabricate some and install them on the more troublesome machines. They were easier to fabricate as the material bent 90 deg in a brake and drilled / tapped easily.

After a few weeks usage the shields cracked around every screw mounting and anywhere else where they were stressed. I found the cause was that we had a "lube station" about 50 ~ 75 foot away where we dipped lubricated small parts in a screened bucket and allowed to drain by hanging the bucket above the tank. The lubricant was Aeroshell 7, thinned with 1,1,1-trichloroethane.

I found the Lexan was ultra sensitive to any chlorinated solvent

and would literally disintegrate when stressed and exposed to these. Had some egg on my face when I had to have all new acrylic shields made and the Lexan ones replaced!

We also had a major customer that required we ultrasonic stake a Lexan shaft extension to a TV control we produced. (They made the Lexan material). We recommended using Nylon as it was easier to mold and assemble but they insisted. A few months after the units were installed and in the field, they had a major issue and recall due to the extensions failing and the knobs falling off.

It was determined the issue was that the TV's owners were using a household cleaner to clean the sets which contained a chlorinated solvent. The company immediately (and quietly) switched to a nylon part!

Re: Hey JKJ, What adhesive for polycarbonate

Barry Irby

Years ago, my son made a fish tank. Actually it was some sort of bio filter for a large tank but the filter itself must have held 40 gallons of water and had all kinds of baffles and fittings. I don't know what plastic sheet he used or what glue, but it worked perfectly. As I recall he would cut and assemble the pieces and hold them in place with blue tape. Then he would seal the joints with a glue bottle that had a tiny syringe attached as an applicator. The tiniest amount of very thin glue would wick into the joints. The joints had to be very closely fitted. That tank and the internal contraption held up for years.

What I am saying is, It can be done. I can ask him what plastic and glue he used if you need it. Might try perusing sites about building fish tanks.

Re: Adhesives and polycarbonate

John K Jordan

Fascinating! The things they don't teach you in school...

Well actually they do teach it......

Bill Tindall, E.Tn.

.....but it would be in a graduate level polymer physics class.

My daughter, the polymer chemist, told me of another instance. A company was developing a competing product to Lexan. It appeared to be a winner. Water bottles were molded from it and they looked great and no BPA component that some fret over. Then it was discovered when the bottle was heated, for example hot water, that anywhere there was a finger print the oil from the finger print caused crazing. A perfect rendition of the finger print was visible as tiny cracks (crazing). She thought this phenomena could be the key clue in a murder mystery novel.