3D Print Casting Pattern Treatments

Safety Precautions/Loss Prevention

1. Wear proper PPE throughout duration of operation. Needed PPE includes:
   • Safety glasses
2. If spray painting, please use a well-ventilated space.

Overview

Often our foundry will use 3D printed forms to make custom patterns for projects that involve castings. When these pieces are initially printed, there is a wide variety of surface types that vary in roughness. These surfaces can create issues during molding by pulling sand out of form and causing overall less refined castings. This SOP overviews a variety of techniques that you can use to prep 3D printed forms for patterns/molding. Keep in mind that each print is different and the same technique may not work for every surface or form.

3D Printed Mold Preparation Methods

Sanding

If your print has a fine surface roughness, it can probably be treated with just sanding! Start with a coarse grit and lightly sand the surface of your piece; continue to sand the piece with finer grits until minimum grit of 220. Remember focus on surfaces that will have to “slide” against sand as the pattern is pulled out of mold. You can decrease the chance of pulling sand out of the mold if you sand in the direction the pattern will be slid out of the mold. Here is a typical sandpaper grit progression for you to follow:

1. 80 (for rougher areas)
2. 120
3. 150
4. 220

Rough and Ridged Surfaces

Only use this method if ridges on your print are deep enough to warrant needing to be “filled in” to obtain a flat surface. Follow these steps to prep these surfaces for further smoothing (sandable primer).

1. Sand the surface with 80 or 120 grit sandpaper so that the ridges are as low as they can reasonably get. You do not want to sand so much that you change the shape/angles of the form.
2. Mix a small amount of PVC piping cement with acetone (about a 1:3 ratio). Have this mixture and a small amount of only acetone on hand.
3. Use a paint brush to coat the surface in the mixture; note that it dried very quick. Immediately after applying mixture, saturate the brush in just acetone and use it to smooth the just-painted surface. Keep saturating brush as needed.
4. Repeat this process until the surface’s ridges have softened and is for the most part flat.
5. Follow the steps in the next section to complete the piece’s treatment.

Auto-Body Sandable Primer Method

Sometimes the 3D print has surfaces that are too rough to get completely smooth with only sanding. Follow these steps to get the best overall results for getting your print ready for molding.

1. Sand the print the best you can up to 150 grit.
2. Use a shop towel (blue paper towel in 106) to dust your piece. Do not use water or anything that will completely clean the surface.
3. Locate a can of auto-body sandable primer and a well-ventilated space. Follow the directions on the can to paint necessary surfaces of your print.
   • Note: Two light coats of primer are better than a single heavy one.
4. Sand your piece LIGHTLY with 400 grit sandpaper until all surfaces are smooth.
5. Wipe sanded surfaces with a damp paper towel.