A critical aspect of designing an injection molded part is maintaining a constant wall thickness throughout the part. This eliminates issues such as sink, warping, shrink, voids and a whole host of other unpleasant issues. To create a constant wall thickness a process called coring is used.
What is coring?
Coring is defined as the act of removing a core or of cutting from a central part. In the 3D printing world, coring essentially means removing material until you have achieved a uniform wall thickness (see figure 1 below.)
By examining the before and after cross sections above we can see that the material removed from the parts has achieved a constant wall thickness.
Another key aspect of coring is to ensure that the part is still able to meet the loading and bending requirements of the application. If too much material is removed, the part will be weakened. After we have removed all the material, we can then analyze the part for loading and bending. Then based on our analysis, we add walls and ribs (beam elements) to stiffen the part in the direction of bending/loading.
Consider the following flat plate:
After coring, the part has uniform wall thickness but will not be very strong when bent.
If my plate will be subject to bending in the directions shown below, I add ribs (beam elements) to oppose the bending.
If I am expecting twisting I add beams in both directions.
I’m 3D printing, why do I care about coring my parts/fixtures?
It saves time, material and money!
Coring your parts will allow you to design a part that still meets the loading/bending requirements of your application, while using the minimum amount of material.
Parts designed with constant wall stock cool much more evenly, resulting in a part with less internal stresses. This is more of an issue with FDM parts, but can also be an issue with PolyJet. Even with a heated build envelope, FDM printed parts with thick sections or large thickness changes can cool unevenly, resulting in warp.
How about an example? What is the process?
On a previous blog we looked at the machined acrylic jig, shown in Figure 6, as a candidate for 3D printing. This jig could be either FDM printed or PolyJet printed. PolyJet Rigor (RGD450) is a simulated polypropylene material that is suitable for test fixtures. Click here for more information on PolyJet Rigor.
Consider the example of the machined acrylic jig shown below (Figure 7).
Step 1: Decide on a wall thickness
This will depend on the requirements of the part and the material/process used to build it. If you are printing with FDM vs. PolyJet you will likely want slightly thicker walls because PolyJet materials are typically less robust. For the above jig printed out of RIGOR, I would use a 4mm wall thickness.
Step 2: Core the Part
Remove material until you have achieved a constant wall thickness throughout the entire part (Figure 8, below). Note, we could core out the recessed surface in the center of the part since this is a registration surface.
Step 3: Add rounds to increase shear strength of functional features.
After coring, the functional features of the part (in this case they are mainly holes) are protruding and could be subject to shearing. To resist shearing of these features we add generous rounds to the bottom of these feature where they join the base. These rounds will make a huge difference in the shear strength of these features.
Step 5: Analyze the part for loading/bending and add beam elements (ribs/walls) to resist bending.
The part below is subject to bending as shown, so beams are added in the direction of bending.
Note that these added walls did not need to be as tall as the perimeter to meet the requirements of the bending. This achieves further material reduction.
The completed cored out part looks significantly more complex than the original, but this is okay! With machining, additional features add costs. With injection molding, additional features add mold complexity. But with 3D printing, additional features are free! This further illustrates the freedom of design that 3D printing gives us.
By coring this part, we have achieved a 40% reduction in material and have produced a part that will build with far less internal stress.
In conclusion, don’t forget to core out your FDM and PolyJet printed parts/fixtures to save money and build more stable parts!