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 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.
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.
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
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.
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/
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.
Having to post this message in 3 or more parts since the PP site refuses to take allmy text in one shot:
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.
.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.
Contact me if you want an email with full details because this pp forum does not accept my full post for some crazy reason.
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
I’d Like to know if there is any consensus about how much gap there should be between plunger and tube?
@marcosinnovate you mentioned removing 1mm from entire bar, except which end?
I am having trouble getting the tube to stay straight after welding (last one was done very slowly, only points, let cool down between each one, but still suffers some bending), so a looser plunger may be easier, but how much looser before plastic will start to come back round and eventually clog it..?
An opinion from as many people as possible I think would be valuable, as I have not managed to find anyone who says more than here (a very tight tolerance is required, but maybe not so much if the author has 1mm taken off all round..?)
I didn’t do the standard build. I used a 20mm thick piston that is a close fit and smaller diameter push rod. If your tubes are bent but still round, something similar should work.
@s2019 even thou you didn’t do the standard build, how much space do you have between tube and piston..?
Trying to get some ball park figures to try and narrow down on the ideal measurements,… when is too much and plastic leaks, when is not enough and the machine blocks up, etc… whether people have tried making a thinner “body” to the piston, with a wider end (like the author here), and whether this works, etc..
as many opinions as possible would be great, everyone please add the dimensions of your machine!!
I don’t know about ideal dimensions. My machine needed to be more compact. It uses a rack/gear drive for much higher leverage. This let me use a larger piston diameter (about 41mm). I don’t remember the clearance but basically it was for a sliding fit. The piston and cylinder are aluminum. You can see in the picture that you will get some leakage of plastic past the piston. This has not been an issue. By making the piston and push rod two separate pieces, if is easier to experiment.
@s2019, 41 mm piston ? I take this as invitation to try this out 🙂 I’ve been always afraid to go bigger than 30mm due to possible melting problems at the core but ok, let’s try.
@andymid, yes slide fit is ok. We machine barrel and plunger with exactly 0.15 (total) clearance otherwise problems occur exactly as you describe. Normally it has to be also 2 different materials to avoid plunger and barrel starting to bite on each other – at least in ‘pro’ machines 🙂
@s2019 interesting size! definitely will consider larger size if I can get my hands on some gearing later on… aluminium tube too..! how thick is the tube? must get a lot of pressure with that setup, I’d imagine the tube pretty thick?
@plastichub Cool! My latest try is 0.25 clearance, and if s2019 says plastic leakage is not a problem, then hopefully this will be so..
I have mild steel tube and stainless steel plunger – I DID have problems biting while preparing a previous setup like this – with much less clearance, and as a last step to try and get everything moving smoothly, I was trying an old millers trick of using a paste abrasive (which they apparently used to finish cylinder clearances), very fine, and was going really well as the plunger went in turning and turning, seemed to be loosening up the slight friction that I had, until suddenly it bit and a small flake of metal seemed to have gathered enough size to complete trap the plunger… took me a couple of hours to get it out! Then there as several scratch marks inside where the flake of metal had rasped the inside until could get the plunger out… I filed down what I could… Thereafter, there was still a small amount of tube that hadn’t been abraded, so I tried a smaller section of plunger, also seemed to go well until only a couple of cms from other end, and then it also trapped – this time needed a 10ton press to remove this small part, and the interior of the tube was now totally unservable..
Any ideas on this final finishing I’d appreciate, as I don’t have the tools to get inside the tube very far, and making the clearance too large I think is a bad idea… that’s why I’d like to know everyones clearance as I’m sure I was going too tight the first time
Tube is 4 mm thick. I went with the larger diameter to get more swept volume (about 200cc) in a compact, desktop unit. I think the standard PP design is sized at around 3:1 leverage. The Arbor press I used is spec at 20:1 and I upgraded the lever arm for maybe another 50% so I’m close to 30:1. With the added benefit of a bit of body mass, I can drive a piston that size. I don’t think the PP design could use that diameter effectively.
The concern about melting at the core is valid. My cycle time is slower and I do a number of pre-compressions. Mine is not intended to be a production machine so this is acceptable.
I think when I do another upgrade, I’ll look into the variable piston diameter approach @andyn used in one of his machines. This would give very high pressures when needed for small molds with small passages.
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