An injection molder developed at the Michigan Tech Open Sustainability Technology Lab used to make tensile and compression test samples for mechanical testing.
This injection molder is a successful proof of concept model that produces satisfactory tensile and compression test samples out of plastic pellets. The pellets can be mixed with other particulates, such as sand, that allows for the creation and mechanical testing of different composite materials. This early prototype performs its job, but an easier-to-use design is in the works. Below is the procedure used to create tensile bars, and when a newer model is developed, a build procedure will also be included.
To turn the injection molder on, plug it in and the temperature display will show the target and actual temperatures.
There is no need to wait for the heating element to reach the target temperature before filling with material. Use a funnel, and then use a thin rod to push all the plastic into the middle of the tube. Note that the material may not compress until it has melted a little bit.
While the plastic is heating prepare the tensile mold.
Step 1: Place the tensile outline on the aluminum sheet and spray a small amount of cooking oil into the mold. Spread evenly around the sides with a paper towel.
Put the aluminum top on the mold.
Top it all off with the circular, threaded plate. Spray some more cooking oil into the threads of this part of the mold.
Tighten the mold together using a drill. Wait until the plastic in the molder in completely melted. Test its consistency using a small diameter rod.
Once the plastic is fully melted, remove the cap off on the hot end using heat resistant gloves or a wrench. When the cap is removed, material will likely ooze out. This oozy gunk must be removed with pliers before the mold can be screwed onto the molder. It is vital that all this material is removed, otherwise the end of the plastic will cool too much and create a hard cap that will prevent the plastic from being molded. A way to prevent too much material from sliding out of the bottom is to press the small rod into the melted material from the top and slowly pull it up. This should be done before the cap is removed from the bottom. The melted plastic should come up with the rod a small ways, which will prevent the material from oozing too much from the bottom.
When the material has been cleared from the hot end, screw on the tensile mold. Clamp vise grips to either end of the mold to prevent blow-out. Wait until the mold itself is hot to the touch before inserting the push stick, and injection molding the tensile bar. You may need to put your whole weight down on the wooden handle of the push stick, as the molding process has proved to be difficult. If the push stick is pushed down but does not actually inject plastic, it will stop prematurely and this is likely because the plastic at the end of the molder cooled down too much. The mold will need to be removed, and the hard plastic cap will need to be pulled off.
After injection molding, the end of the push stick may have plastic caked around it. Remove this as soon as you pull the push stick out of the molder when the plastic is still molten. Otherwise the plastic hardens, and a heat gun is required to reheat the plastic.
Remove the vise grips and unscrew the mold from the molder while wearing heat resistant gloves. Use a drill to unscrew all the bolts from mold and remove the bottom aluminum plate from the mold. If the injection process was successful, the entire mold should be full. If the plastic did not make it all the way to the ends of the molds, not enough pressure was used during the molding process.
The top of the mold will be stuck onto the rest of the mold because of a hardened plastic slug left over from the injection. Because the top of the mold is threaded, it can be removed by holding onto the circular part of the mold with a heat resistant glove and spinning the tensile part of the mold. If twisting the mold apart isn't easy, apply more cooking oil to the threads for the next tensile bar.
Since the plastic slug has had time to harden, a heat gun must be used to reheat the plastic to remove it from the mold. After heating, pliers should easily be able to pull the slug away from the top plate of the mold.
If enough cooking oil was sprayed into the mold and it was spread evenly along the sides of the mold, the tensile bar should come out of the mold decently smoothly. Carefully bend the border of the mold away from the tensile bar to loosen the connection, and then press the tensile bar out of the mold with your fingers while wearing gloves. At this point, you should have a rough tensile bar with excess plastic where the molding occurred. Use wire cutters or a sander to remove this excess plastic. Be careful no to remove too much material.