The accuracy of the QuickCast style pattern determines the accuracy of the cast part.
A preferred policy is to implement a Tolerance Envelope for Patterns that is 50% of the required End Item Tolerance Envelope:
Blueprint Dimension = 1.000 in. (25.4 mm)
Tolerance = +/- .030 in. (0.8 mm)
Pattern Dimension (with cast metal shrink) = 1.005 in. (25.5 mm)
Tolerance = +/- .015 in. (0.4 mm)
This is insurance that if the foundry’s process variables to produce the casting are in control, you are practically guaranteed to yield a casting that is within the Blueprint Dimensional Tolerance Envelope.
Record a minimum of 2 measurements in each axis (X, Y and Z) with the actual print dimension and the applied shrink dimension. This will verify that you have included the foundry shrink factors in your pattern. One of the two measurements should be the overall dimension, as any error will accumulate and be the most obvious in the longest dimension. In addition, record all critical dimensions and supply this information to your selected foundry.
Keeping accuracy records can also help you determine the consistency of your process over time.
Accuracy of the patterns before they are produced into castings have many variables, such as equipment, material, environmental and handling.
Equipment: The X and Y axis features are limited to the size of the beam diameter, typically 0.0045 to 0.0055 in (0.115 to 0.140) diameter for QuickCast style patterns, but down to .003 in. (0.08 mm) on some machines. The Z-axis features are limited by layer thickness, typically 0.002 in (0.05 mm) for QuickCast patterns, but down to .001 in. (0.025 mm) in other build styles. Your machine condition and calibration also contribute to the pattern accuracy. Your 3D Systems Field Service Engineer is qualified to help you evaluate the condition of your machine.
Materials: Acrylate resins manufactured during the early introduction of Stereolithography are not as dimensionally stable as the newer Epoxy resins. Photocurable resins are now being developed for individual mechanical properties. As these properties differ, accuracy may also be affected. Resins that are designed to be durable and/or flexible may not achieve the same accuracy as a harder resin. Working with your 3D Systems Applications Engineer will assist you to determine the most accurate resin available based on your particular machine and applications.
Environmental: Keep the patterns in a cool dry place, away from heat and humidity. It is a good idea to keep them in a plastic bag with desiccant to prevent any water absorption.
Handling: Improper handling and stacking during post-processing (support removal and PCA unit curing) can affect squareness, flatness and induce twist in what are sometimes very thin, delicate configurations.