Pipe vs tube for injector barrel

I’m having some trouble finding a good pipe/tube for our injector barrel. I’d appreciate guidance from anyone who has built one especially if you have sourced in US.

The issue I am having is that the steel pipe I can find all is welded. I have found galvanized and aluminum pipe that is seamless, but would those work? As I understand, galvanized steel does not weld well unless you grind off the outer layer.

I have found 1-1/4″ OD, 1″ ID steel tubing or 1-1/2″ OD, 1-1/4″ ID steel tubing, but these do not have the NPT threads on them. It is nice in the standard design to be able to use standard NPT fittings, so I was hoping to go that way. And, another question I have is if I were to use the 1-1/4″ OD, 1″ ID steel tubing, then I presume that a 1″ diameter steel rod would be too snug to properly slide for the injector. From what I have been able to find, most recommend about 1mm difference between the tube/pipe ID and the rod OD.

Please share what has worked for you.


Update. I ended up purchasing 1.25″ OD, 1.00″ ID SS pipe from a local metal wholesaler. Initially I didn’t see there was a seam but once we cut it back at our shop it did end up that there was a minor seam. We tried filing it down a bit but that did not work, but hen we tried milling down a very small flat onto the 1″ shaft and that seemed to work. We took off about 0.02″ which seemed to be enough and I don’t expect the melted plastic will leak into it. We’ll keep you posted how it goes.

I would like to suggest looking for hollow piston rods. Those tubes are used in every Hydraulic machine and ALL of them are seamless.

onlinemetals has a great variety of seamless tubes btw https://www.onlinemetals.com/merchant.cfm?id=18&step=2&top_cat=1

Contact me if you want an email with full details because this pp forum does not accept my full post for some crazy reason.

.003 thous. is industry recommended tolerance for fit of 1″ screw into barrel, Florida site states .020 thou. is end of life sloppy, but most sch.40 pipe is like 50 to 60 thou. over to being with. I tried to post my links and tables but it is not showing up on pp forum.

Having to post this message in 3 or more parts since the PP site refuses to take allmy text in one shot:

Hey guys:

Just to let you know that by using standard sch. 40 pipe to build extruders – – your build is already WAY OVER recommended tolerances for screw to barrel.
1″ sch. 40 pipe ranges from 50 thou. to as much as 62 thou. over 1″ i.d.
Anything more and you are wasting a lot of electricity / efficiency because much of the molten polymer is not moving forward, but instead slipping over the screw flights backwards.

Oh, and by the way, if you research deeply about DOM pipe it still has a welded seam, you just cannot see it because of the drawing and polishing process, but X ray analysis can see it, so that is where a burst would likely occur.

The best is cold draw seamless as Peter-musk mentions, true seamless without a weld, but good luck finding a few thou. over an inch i.d. to create a nice, tight extruder barrel.

Hi @andyn
We ended up running short on time to complete our aluminum molds so we rather created silicone molds and used the injector to extrude some liquid plastic (we used PS) and then hand pressed the plastic into the molds. We later found that we could also use a toaster oven to heat up the plastic to make it liquid and similarly press it into the molds and get some pretty good keychains. Here are some pictures of the keychains we created. And, here is a video of us making a keychain: https://www.facebook.com/preciousplasticla/videos/1070415263097141/


Nice looking machines! I’d be interested to see pics of the keychains and the moulds you used to make them.

We ended up using 1″ SS pipe. It still had a welded seam, but the seam was not nearly as large as steel pipe. We did a modification to the injector plunger to make it slide better. We made 2 injectors and tried 2 different things. On the first one, we milled a 2 mm wide channel along the whole length of the plunger. It worked for a while and then jammed and we still have to figure out how to get it out. For the second one, we turned down the thickness of the entire plunger by 1 mm except for the last inch. That seemed to work like a champ. We did over 500 injections with the machine over 2 days because we were making keychains at an Adidas event for NBA All Star Weekend. I’ll post more details about our shredder and injector builds soon.

Chromoly tube for bicycle will work the same, it must be not double or triple butted, just normal with inside constant diameter, the more usual size are 1″ – 1″.1/8 – 1″.1/4 – 1″.1/2

As Peter mentioned, DOM (Drawn Over Mandrel)tubing is seamless and available in a number of sizes and wall thicknesses, this is what I am using. It is commonly used in building race car roll cages and tube frames but not all metal suppliers deal with it. It may not be the cheapest option but Online Metals sells it is small quantities and you may be able to buy small remnants from a local race shop.

I have been looking for a non-welded tube too, and it seems there is a product called cold drawn seamless tube that fits the bill. The cheapest source I could find for small quantities was race car builders – it is the stuff they make roll cages out of.

Dimensions might depend on what they have on hand, but to avoid the problem of matching steel bar to the ID of the tube. I thought it would be worth trying a heavy steel washer (whether purchased, or cut out with a hole saw) mounted on some threaded rod. 16mm diameter threaded rod is unlikely to bend with whatever force I will be applying by hand, and if it does, the tube should keep everything aligned.

Good luck

Peter Musk