Printing Molds on the Stratasys J750
Here we are, part 3 of 3 in this blog series! In my previous posts, I covered the steps in modeling the custom screwdriver molds using SOLIDWORKS CAD. In this post, I explain the steps to prepare the models for 3D printing.
The mold cavity models were saved as STL files in SOLIDWORKS so they could be processed for 3D printing. The printing technologies available at GoEngineer are Stratasys FDM and Polyjet machines.
Fused deposition modeling (FDM) builds plastic parts layer by layer with a strand of melted plastic. FDM printing leaves ridges on each layer. This creates a surface the injection molded part sticks to, making it difficult to remove from the mold cavity.
PolyJet technology utilizes a UV-cured liquid resin that prints in much finer layers. This creates a smooth surface finish, thus making it the better choice for 3D printed mold cavities.
I printed both sets of cavities on the Stratasys J750 with Vero Blue material and used the glossy setting to get the smoothest mold cavity finish. No other post-printing modifications were needed, so the mold cavities were ready for the molding press.
Injection Molding the Parts
Let’s look at the process used for injection molding the screwdrivers.
The benchtop injection press has a heated barrel with a temperature controller. This sets the plastic melt temperature to the material-recommended processing temperature. The raw plastic material (in pellet form) is poured into the barrel and melted. The mold cavities are placed in the vice below the injection point with the inlet hole aligned. The melted plastic is injected into the mold cavities by pulling smoothly and steadily on the lever arm until the cavity is full. The mold is removed from the vice and is cooled for several minutes before the part is removed.
Molding Handle Blanks
Molding the handle blanks went well. The metal shaft insert shut off perfectly and the parts filled consistently after a couple of temperature adjustments. There was very little flash at the parting line, indicating that the shut-off surfaces fit together well. I molded several handle blanks first, knowing I would have to purge the press to change to the TPE material.
The overmolding process requires that the injection molded handle blank be placed into the mold cavity of the grip molds. The TPE Rubber material is injected over the parts to the handle to form the grip. The molded handle blanks fit perfectly, which meant the shrinkage factor used to scale up the mold cavity was accurate. I made a couple of melt-temperature adjustments to the TPE material to get the parts to fill consistently.
Now I have my very own set of custom GoEngineer branded screwdrivers. More importantly, I gained knowledge about utilizing 3D printed mold cavities for injection molding prototype parts. During this process, I also learned that by using SOLIDWORKS Plastics Simulation, during the design phase, I was able to accurately predict the material flow and filling behaviors. This allowed me to make geometric revisions to the part before creating the mold cavities. This helped me save time and materials by eliminating rework.
If missed part 1 and 2: