Get the Screws Turning Part 2 of 3 – Injection Molds

by Jeff Lendermon

Exploring the technology of 3D-printed injection molds

My previous post ended with a successful SOLIDWORKS Plastics flow simulation. Read that post here. As I wrote that post I wondered: How can I get a decent estimate of pressure produced with a hand-press injection molder?

The injection pressure required to fill the handle portion of the mold is 205 Psi. The pressure needed to fill the grip is 1,290 Psi.

I created an assembly by modeling the main components of the injector using SOLIDWORKS CAD.

I needed:

  • Lever Arm
  • Plunger
  • Head Unit
  • Lever Bracket

After completing the assembly I created a static analysis to measure the output force on the face of the plunger.

The setup steps:

  • Define the three moving joints by using pin connectors.
  • Restrain the head unit.
  • Apply force to the end of the lever arm.

(I assumed I could physically apply a 50 lb force)

  • Mesh and run the study.

I completed the study and measured the resulting force on the face of the plunger. It was 618 lbs.

Next, I calculated the resulting pressure using this force:

Pressure = Force/Area.

The diameter of the plunger is .75 in. and the area is 0.4415 sq. in. The resulting pressure is 1,400 Psi, enough to fill the cavities.

This was great news. Now,­ I would not have to find a gorilla to pull the lever for me. Next, is creating and printing the molds and injection molding the parts.

Creating the Molds in SOLIDWORKS

In this series I covered the following:

  • Designing a custom screwdriver using SOLIDWORKS CAD
  • Running SOLIDWORKS Plastic injection molding analysis to verify the injection process
  • Utilizing SOLIDWORKS Simulation to verify pressure capability of the desktop injection molding press. The next step is designing the CAD models for the injection molds.

Satisfied with the flow results from SOLIDWORKS Plastics, I modeled the mold cavities for the handle and grip molds.

I used the cavity feature within an assembly to make the simple mold geometry. Here is how to use the cavity feature in SOLIDWORKS, edit the part in the context of the assembly and then choose Insert>Molds>Cavity. Select the design components, in this case, the bodies of the screwdriver. Another option is scaling up the cavity to account for plastic shrinkage.

The locations of the vents in the molds were determined by the air trap analysis results from SOLIDWORKS Plastics.

The GoEngineer logo that shows through the screwdriver grip is created by the grip mold cavity shutting off on the molded handle face. The creation of this detail tests the accuracy of the printed mold cavities. If this area does not shut off completely the logo will not show through. Too much interference could hold the mold cavities apart and cause them to flash at the parting line.

The handle mold is scaled up to account for shrinkage of the HDPE plastic. The molded part needs to fit inside of the grip mold. It also needs enough clearance so the mold parting lines will shut off. With too much clearance the TPE material flashes over the areas of the handle that are to remain exposed. Finally, the grip mold is then designed utilizing the geometry of the finished screwdriver without scale. The finished mold designs were saved as STL files for 3D printing.

If missed part 1 and 3:

Get the Screws Turning Part 1 of 3

Get the Screws Turning: Part 3 of 3 Exploring the technology of 3D printed injection molds

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