Hello, I am designing a 3d printer for school, but we are debating whether to 3d print the carriages or cast with aluminum… Any advice?
Print head designs vary, but the lulzbot TAZ even has the extruder in the head. The cost of printing a carriage is low, so you may as well prototype it that way, and then move to another material if required.
Have a look at some existing printers and see what has been done.
Hey guys,
as with many questions like this in engineering, the simplest/nastiest answer is: Depends!
What is your printer design supposed to do? Speed? Print Quality? Size? Compatibility? Production volume? Cost? What are your main goals for your design? If you look at printers that are made to print fast, like the Ultimakers and Leapfrogs, their print heads only consist of the hotend(s). So that’s a hint that weight is definitely a concern in that case. Print quality sort of plays into the same corner, where a lighter (and stiffer) carriage is going to reduce artifacts from high speed movements in the print (think F=m*a). Both the material choice and design are going to matter here.
I would also advise you to look into the costs of the methods you are looking at. For a one-off printer, printed parts are pretty hard to beat in terms of production costs. Casting by definition already involves more material, labour, expertise/experimentation/design effort to get a dimensionally accurate and good looking result. If you are looking into cast parts, you probably also have the facilities available to machine parts? How would that fit your design?
If that doesn’t satisfy you, you can also always ask yourself; why is the weight not a factor? Playing devil’s advocate sometimes works well to get out of a stalled situation like this.
My personal opinion is, that casting is a pretty terrible choice for making printer parts. I would tend towards printing the parts in a carbon-fibre filled engineering plastic (XTCF-20 for instance), the carbon providing for extra stiffness, or looking into bent sheetmetal or (CNC-, depending on your volume) machined parts.
Sincerely,
Jonas
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Moving mass is always a negative causing lower acceleration and more vibrations, so unless you have a reason for a heavy material, stick with the lightest solution. Biggest reasons to add weight with metal to the carriage are to 1) be more heat tolerant to avoid softening or warping near the hot end or in a heated chamber 2) to add high thermal conductivity for cooling 3) be stronger to handle higher accelerations and heavier extruder(s). If you don’t need to add the weight for a really good reason, then don’t do it.
My advice for a school project however is go with the simplest solution that will get the job done. You’ll always have unexpected problems that will eat up your time later and you can always revisit the fabrication method if you get done early. 
Wouldn’t a heavier head damp some vibrations?
Not in the case of a printer. In some situations a heavier mass with a flexible interface (think of a block of cement on rubber pucks) can protect a target (like optics sitting on the cement) from vibrations (like a truck driving down the street outside). In this case the vibration travels through the ground, into the rubber, then they try and move the cement block but can’t overcome the momentum of that large mass, so the vibration die out as some heat in the rubber and never make it to the optics.
In the case of a printer, you want the head and the object to never vibrate relative to each other, but the head is directly part of the system. So when the head tries to turn an corner it has to pull on something to make that turn causing all those flexible points (belts, rods, plastic, bearings, frame, etc) to flex and vibrate before they get to the build plate. The heavier the head the harder the pull for the same turn and speed and the bigger the wiggles. At best, this causes the nozzle to wiggle relative to the object being printed introducing some tiny defects that show up as a visible (but not one most can feel) ripple pattern near the corners. At worst it causes the steppers to not keep up and miss steps or the printer to bend or crack or break.
