Summary

Using the differing melting properties of ABS and PLA, you can use the functions of your 3D printer to make rudimentary injection molded parts.

For more information:
http://www.thingiverse.com/thing:83805

For even more information with complete instructions:
http://www.instructables.com/id/Use-Your-3D-Printer-As-An-Injection-Molder/

Instructions

This is one of those "I wonder if it can be done" exercises you can do in about an hour. This is only a proof-of-concept experiment that opens up the possibility of further development if anyone is interested.

The part I made is a true molded piece having no deposited layers.

The vent sprue I drilled in the front of the upper mold half didn't work as PLA was too thick to move through (you can see the tiny nub in the photos). When the die was filled, the plastic simply began to pile up around the extruder head, outside the die,.

The other issue I had was the extruder head was hot enough to deform the upper ABS die half. This didn't effect the part, but it made removing it from the mold difficult. The heat allowed the ABS mold material and the PLA part material to be mixed, making extraction impossible without wrecking the upper half of the mold.

Here's how I made this:

Print the stl file in ABS. When the print is complete, break off the upper half (the one with the opening) and place it on top of the lower half, which should still be adhered to the build plate.

Disengage the driver motors and move the PLA print head (or the same print head you used to built the die on a single extruder machine) Load PLA material into the extruder and raise the plate until the extruder head couples with the hollow on the top of the die. Turn the load function on. The PLA will flow, filling the cavity. When its full, the PLA will begin to squeeze out from around the extruder nozzle. That's when you should shut the extruder off.

Let the plastic cool completely before cracking the die open with a screw driver. If all went well, the part will be sitting inside the die, looking like a pearl.

Have fun, experiment and let us know if you make something really cool.

Injection Printing - Injection Molding with your 3D printer