One of the drawbacks of additive manufacturing (AM) or 3D printing (3DP) is the quality of the surface finish, writes Tim Plunkett.
Building parts layer by layer will naturally result in visible layering or ‘stair stepping’ as it is often known. Let’s take a look at three options to improve the surface finish of your additive manufacturing AM components.
Layering is a result of three variables: the curvature of the part’s geometry, the orientation of the part with respect to the build chamber, and the layer thickness.
Out of those three, changing the part’s geometry so it can be built by AM is the least likely option. There are, however, plenty of possibilities with respect to the other two.
Parts with flat surfaces will generally build free of layering if they are orientated parallel or normal to one or more of the build chambers’ Cartesian planes. If the part is curved in only one plane (such as a hollow extrusion profile or aerofoil) then it is best built with the curved profile lying in the XY plane, normal to the z-axis. In this way each layer is a constant profile and there is no transition across the chamber in the x and y directions. More importantly there is no stair stepping through the wall thickness.
Parts with doubly curved surfaces are the most challenging and there is no simple way to make the visible layering disappear. Some consideration of build orientation can assist, but there are always going to be areas of the geometry where the surfaces flatten out and large sideways steps between each layer are inevitable. Surface finish in these areas will therefore be compromised.
If part orientation cannot help, then reducing layer thickness is arguably one of the best ways to improve surface finish. If you can put four layers into the space where you once only had one, then the results will be noticeably improved. Modern AM machines are a lot more sophisticated than their forebears and many can be specifically calibrated to build at layer thickness measured in microns instead of millimetres. Building parts in 16 or 25 micron layers (that’s 0.016mm and 0.025mm) is now possible. If surface finish straight off the machine is important to you then these options are really worth considering.
As an alternative, you could consider post operations such as dressing the part by hand, sand blasting and machining. These are the ultimate way to get a top quality finish, but the processes require skill and are labour intensive and therefore expensive.
Post operation options also depend on the part’s material - plastics are easier to finish than metals. If your AM parts are to be painted or chrome plated, then there is really no alternative to these post operations at present. This is why we recommend getting the best ‘off the machine’ finish you can. If you let us know which surfaces are cosmetically more important we will ensure they are given the best finish possible.
If surface finish is really important to you it may also be worth considering CNC machining instead; but as you may have guessed this has another set of pros and cons that may conflict with your ultimate requirements. The more we know about what you are looking to achieve, the more able we are to recommend the best solution to suit your needs.
Tim Plunkett is the founder of Plunkett Associates, Gloucester, UK.
