Question:
What are the Factors to be considered and influencing accuracy in FDM and LOM process. Discuss them.
Answer:
The accuracy of parts produced by FDM and LOM processes is influenced by a combination of machine, material, and process-related factors. While both are additive manufacturing methods, the specific factors that impact their final accuracy differ significantly due to their unique working principles.
Factors Influencing Accuracy in FDM
FDM's accuracy is heavily tied to the precision of the melted filament's deposition. Key factors include:
Thermal Effects: This is the biggest challenge for FDM. Warping and shrinkage occur as the extruded, hot plastic cools down at different rates. If a part's base cools faster than the top, it can lift from the build plate, destroying accuracy. The extrusion temperature and build plate temperature must be carefully calibrated to minimize these stresses.
Layer Thickness: A smaller layer thickness results in a finer resolution and better accuracy, especially on curved surfaces and angled features, because the "stair-stepping" effect is reduced. However, a thinner layer increases the print time.
Print Speed and Extrusion Rate: The speed at which the nozzle moves and the rate at which it extrudes material must be perfectly synchronized. If the speed is too high or the extrusion rate too low, it can lead to under-extrusion, leaving gaps and weak spots. Conversely, over-extrusion can cause bumps and dimensional inaccuracies.
Nozzle Diameter: The nozzle's diameter dictates the width of the extruded line. A smaller diameter allows for printing finer details and more precise features, improving overall accuracy, but it also increases print time.
Machine Calibration and Rigidity: The mechanical stability of the printer is crucial. The gantry system's ability to move precisely along the X, Y, and Z axes without wobbling or vibration directly impacts dimensional accuracy. Proper calibration of the stepper motors and belts is essential.
Factors Influencing Accuracy in LOM
LOM's accuracy is determined by the precision of the cutting and lamination processes. Key factors include:
Cutting Method and Precision: The accuracy of the final part is highly dependent on the precision of the laser or blade used to cut each layer's outline. The laser's kerf (width of the cut) can reduce the dimensional accuracy of features, especially small ones.
Layer Thickness and Uniformity: The thickness of the sheets of material used and their consistency are critical. Any variation in sheet thickness can cause cumulative errors in the Z-axis, leading to an incorrect final height.
Bonding and Lamination Pressure: The pressure applied by the heated roller to bond the layers can affect the final part's thickness and flatness. Inconsistent pressure or temperature can lead to weak bonds and delamination, which negatively impacts structural integrity and accuracy.
Decubing Process: LOM requires the user to manually remove the cross-hatched, non-part material. This "decubing" is a subtractive step where human error can easily damage delicate features or surfaces, compromising the part's accuracy.
Moisture and Environmental Conditions: Since LOM often uses paper, the moisture content of the material and the humidity of the environment can cause the final part to swell or shrink after manufacturing, leading to dimensional inaccuracies over time.