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FDM

Printing tips

Tolerances

It is impossible to get microns of tolerances. Also, the greater the precision, the longer the print.

Always give tolerances to parts which must be interlocked (holes, contact surfaces). E.g. for the Prusa printer, an expansion of 0.3 [mm] generally gives tight locked mechanisms, 0.5 [mm] lightly locked mechanisms and 0.7 [mm] and over gives enough room for parts to slide easily into each others.

Reinforced junctions

Sharp edges change for contacting materials are a point of failures in 3D prints. As possible, add either a chamfer or a fillet to such surfaces to strengthen the connection:

Chamfer and fillet
Figure 1: Chamfer and fillet

  • In green : no extra support
  • In purple : fillet
  • In orange : 45° chamfer

Slicing direction

The orientation the part is printed in as has a big impact on its strength. It depends mostly on the forces direction.

The idea is for a layer to resist more to increase its "2D" size (i.e. the slice as cut by the slicer). It may require support material which may interfere with the print precision, but is all-in-all a good practice for mechanical parts:

Slicing rotation
Figure 2: Slicing rotation

  • If forces are radial like guides for an upper part, the area of the "nipples" are much smaller on the left and may break easily.
  • The 90° degree rotated part on the right has much bigger surfaces but requires support all along the object.
  • The middle one has great base supports and bigger slices areas, plus the print once put flat distributes the radial forces over multiple heights.

Nozzle size

  • The nozzle size installed on the machine must meet the one set in the slicer. Otherwise, under or over-extrusion happens.
  • The standard nozzle size is 0.4 [mm] and is a good compromise between speed and small details.
  • For big parts or those without details, nozzle up to 1 [mm] may be installed to speed up the print. Bigger nozzles up to 2 [mm] may require a fine-tuned printer with a special heat insert to cope with the increasing flowrate.
  • Smaller nozzles down to 0.1 [mm] exist and should be used on small parts only.

  • To print 18 times the previous shown part the times are:

    Nozzle size Print time
    0.2 [mm] nozzle, 0.1 [mm] layer height 14 h 17 mins
    0.4 [mm] nozzle, 0.3 [mm] layer height 5 h 37 mins
    1.0 [mm] nozzle, 0.9 [mm] layer height 2 h 6 mins

Layer height

  • The layer height affects the details, the strength and the print time.
  • The bigger it is, the lower the print time and the details are.
  • Bigger heights tend to create stronger parts due to the extra extruded material per layer.
  • As a rule of thumb, the layer height should be \(\frac{1}{4} * D_{nozzle} <= h_{layer} <= \frac{3}{4} * D_{nozzle}\)

Infill percentage and patterns

  • To lower the print time, one can play with the infill percentage.
  • 100% infill may not create the strongest parts due to the straight pattern used.
  • Time impact is negligible for small parts which contain nearly no infill (i.e. whose walls are \(2 * D_{nozzle}\) or less).

Many patterns exist depending on the usage:

  • Models and figurines: 0-15% infill, rectilinear/grids/lines patterns
  • Small forces: 20-50% infill, grid/triangles patterns
  • Functional parts: 50% + infill, cubic/honeycomb/gyroid patterns
  • Flexible parts: infill depends on the usage, concentric/crosse patterns

Difficult materials

Some plastics like ABS are hard to print without warping to happen. They require a heated bed, a closed case and slower printing speeds.

  • PLA or PETG should be used as default material.
  • ABS, PC and Nylon must be reserved for higher constraints parts.

Warning

Some materials like PLA, ABS and Nylon emit harmful fumes. Only print in well ventilated areas and prefer an enclosed box for the printer.

Screw holes

Screws can be fastened to 3D prints thanks to various methods. In order of resistance of the screw thread:

\(Tapped Hole / Printed Thread < Threaded Inserts < Nut\)

  • A tapped hole (or directly printed thread) offers sufficient resistance to close cases and maintain two parts together with no force. The thread will easily break with axial forces.

  • The threaded inserts are unique to plastic parts. They are metal thread with external "lips" to melt and maintain them into the hole. An iron solder is generally used to heat the insert gently and push it in.

  • Using nuts is the most reliable but consumes the most space.

ABS smoothing

ABS can be smoothed easily with the help of an acetone vapor chamber.

Warning

Use the proper eye, hands and breath protections. Acetone is toxic.

PLA smoothing

PLA parts can be smoothed with the use of specific epoxy resins.

Warning

Use the proper eye, hands and breath protections.