3D Printed Parts: Printed Part Accuracy

Regardless of the type of additive manufacturing technology being used for the printing of plastic parts; fused deposition modeling (FDM), stereolithography (SLA) or digital light processing (DLP), part accuracy is one of the main considerations when deciding what type of printer to purchase. The accuracy one can expect from the final printed part is dependent upon a number of issues. This article will discuss the various aspects of the printing process that impact part accuracy and include:

• Machine Accuracy
• Minimum Feature Size
• Print Artifacts
• Plastic Shrinkage
• Post Processing

Machine Accuracy

The typical 3D printer specifications will specify two parameters for their printers; x/y positioning accuracy, many times referred to as resolution, and the z layer height. For a FDM machine, these values define what the printer is theoretically capable of in the positioning of the print head. For example, the Mark Two 3D printer x/y resolution is 0.05mm or 50 microns. While this provides part of the answer to accuracy, because we do not live in an ideal world, it becomes important to know what the variation of this value is; 0.05 ± ?????. This variation represents the repeatability of the positioning system and applies to all 3 axis of motion. Unfortunately repeatability is not normally called out in 3D printer specifications.
This variation in positioning accuracy is an inherent aspect of the machine design and includes:

• Printer Construction
• Positioning System
• Type of Motors
• Motion Axis

• Printer Construction

  Take a look at virtually any machine tool; mill, lathe, etc. whether manual or CNC, and a commonality among them all is how massive the frames are; lots of cast iron and steel. This is done for strength, to keep things rigid and to absorb vibrations.

  When evaluating 3D printers, it is important to select a printer that has a rigid frame. Any amount of twist or deflection in the frame while printing will directly impact accuracy.

• Positioning System

  Equally important to the frame is the design of the positioning system and what kind of backlash can be expected. What quality of linear slides are being used? Are screws being used for positioning or toothed belts. If screws are used, are they lead screws or ball screws. Ball screws are more expensive but have much less backlash. If a toothed belt is being used, how tight must it be?

• Motor Type

  Motors used for positioning can be either stepper motors or servo motors with stepper motors being the most common. Stepper motors are designed to rotate a certain number of degrees per step; usually 1.8°, are relatively inexpensive and make use of an open-loop controller. The problem with stepper motors is when the controller sends a motor the signals to step a given number of times it receives no feedback that the motor actually performed the steps. Because the motors are being driven incrementally any missed steps by a motor can quickly buildup and affect the positioning accuracy. Systems that typically make use of stepper motors will many times incorporate a homing capability into the design. This allows the positioning to be reset while operating to remove any accumulated errors.

  Servo motors are similar to stepper motors but make use of a closed-loop controller. The motor sends its actual position back to the controller which compares it to the expected position. This comparison generates an error signal to drive the motor to where it is supposed to be. Once this error signal drops to zero, the motor stops. Because of this close-loop aspect, servo motors are used on high end CNC machines. Unfortunately they are substantially more expensive then stepper motors which accounts for the use of stepper motors in the typical 3D printer.

• Motion Axis

  The final aspect of machine accuracy is the number of axis that are being driven, keeping in mind each axis will have a potential positioning inaccuracy as a result of the earlier described factors. FDM printers as well as many SLA printers will have 3 axis of motion and therefore 3 potential positioning errors.

  DLP printers on the other hand only have a single axis of motion; up and down. This single fact explains the extremely fine surface quality that can be achieved with DLP technology. Because each layer is being exposed by a projected image there is absolutely no motion in the x/y plane of the projected image. Assuming the optics are held securely the pixels on one image should absolutely match the location of the pixels on the next and so on. The only source for variability is the movement of the build platform. However linear slides can be constructed to extremely tight tolerances to reduce variability in the x/y movement. Likewise, ball screws offer minimal backlash for the z positioning. Assuming the build platform raises and then lowers for each new layer, any backlash will be removed by the downward pressure of the build platform yielding consistent z heights.

Minimum Feature Size

Coupled with part accuracy is the minimum feature size a 3D printer is capable of producing. This is a combination of the positioning accuracy of the printer and most importantly, the size of material being extruded by FDM machines, laser spot size on SLA machines and pixel size on DLP machines.

FDM machines extrude plastic through a heated print nozzle of a specific diameter. The diameter of the print nozzle therefore determines the effective detail one can expect on a part with the printer being unable to reproduce features such as raised lettering smaller than the diameter of the nozzle. A smaller diameter nozzle allows for finer detail but at the cost of print time which is the tradeoff that has to be made by the manufacturer.

SLA and DLP printers because they use light which can be focused many times smaller than the typical nozzle size on FDM printers, allow for very fine detail to be reproduced. The reduced size of the laser spot on SLA printers means a longer print time similar to longer print times on FDM printers with a smaller nozzle, but fortunately the speed of movement of the laser is usually faster than moving a FDM print head which lessens the time.

DLP printers project an entire image to expose the resin. Their native resolution is the size of each projected pixel, typically on the order of 50 microns; well suited for the typical detail found on many parts. They additionally have an advantage in that the time to print each layer is the same regardless of how much of it needs to be exposed making them capable of fairly fast overall print times especially if multiple parts can fit on the build platform.

Print Artifacts

A common issue with the final printed part is the occurrence of various artifacts on the part; these can include blobs of material or pieces of support structure stuck to the part. It is not uncommon to find blobs of material on parts made with FDM machines. The speed of the stepper motor feeding the plastic extruder nozzle must be tightly coupled with the speed the print head is moving to ensure a consistent amount of plastic is being extruded. When starting printing on a new layer the first amount of plastic from the nozzle will probably be somewhat wider than it is supposed to. Many times this can be seen on cylinders printed upright that have a slight ridge running up the surface. This ridge is where the print head typically started and stopped a motion path.

SLA and DLP printers should never have these types of print artifacts but one which all printers will usually suffer from are artifacts on the part due to support material. Printers that use a dissolvable support material should leave no marks behind on the part due to supports. However, for printers using the same material for the part and supports, the removal of them can be an issue; both in their removal and in the potential marks left behind on the part after they are removed.

Plastic Shrinkage/Warpage

Intimately well known in the area of injection molded plastic part design is the understanding of plastic shrink and being able to account for it so the final molded part matches the desired dimensional size. Through careful design of the product and the mold, highly accurate injection molded parts are easily produced.
3D FDM printing, though it shares the use of thermoform plastic with injection molding, is an entirely different process. With injection molding an entire void is filled with plastic relatively instantly compared to 3D printing, cooled and then ejected from the mold. The important item to keep in mind is that the entire part is being filled and cooled at the same time.

Because 3D printing occurs layer by layer, the cooling process operates quite a bit differently with each layer cooling and shrinking slightly as the part is being built. While the majority of cooling occurs immediately after the material is extruded onto the part, further cooling and shrinkage will continue as the build progresses with the lower portions of the part becoming cooler than the top where printing is still occurring. This temperature differential directly affects the shrink and is many times responsible for warped parts. To counter this, many FDM printers use a heated bed and build chamber. This keeps the entire part at the same temperature during the entire build to ensure shrinkage is consistent throughout the part.

The most common result of uneven cooling/shrinkage is warping of the part, many times while it is being printed. Various techniques are used to counteract this which include; use of extra material printed around the base of the part to better adhere it to the print bed, changing how much material is distributed thru the part, and letting the part cool completely before removing the part from the print bed.

Post Processing

Generally speaking, parts from a FDM machines require little post processing, at most trimming off or sanding tiny bits of plastic that may have stuck to the part, although small holes may need to be reamed out to meet the required tolerance. Parts from SLA and DLP machines generally require much more post processing that includes washing the part to remove the uncured resin followed by additional curing in a UV light box to fully cure the plastic. The photopolymers used in SLA and DLP are cured to maybe 70-80% by the exposure by the laser beam or projected image while printing and need further UV light exposure to fully cure the plastic. Additional part shrinkage could occur during this phase so dimensionally checking SLA and DLP parts should be done after the final UV light curing.

Summary

Accuracy of 3D printed parts depends upon more than simply the positioning accuracy of the printer. Numerous factors as described above influence the final accuracy one can expect from a printer. If high part accuracy is required, printing and measuring a test piece, followed by adjusting part dimensions may be required to accommodate any shrink related dimensional inaccuracies to be able to achieve the needed part accuracy.

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