Overview
For our ME 340-2 plastic injection-molded part, we decided to make a miniature trash
can. The part is molded in two halves: the base, which is a hollow cylinder with one open
end, and the cap, which is a hollow hemisphere with an opening on one side to insert
trash (or tic-tacs). All wall thicknesses are 1/8".
A CAD rendering of the cylindrical base and the hemispherical lid.
Design (Pro-E)
When designing the trash can in Pro/E, we found that Pro/E had its advantages and
disadvantages. The advantages were the following:
1.) The procedures were very repetitive making the entire task very simple.
2.) The task manager allowed us to quickly find mistakes or make any necessary changes.
3.) The query select was very helpful when working on a very detailed part.
However, we found that Pro/E also caused us some problems and confusion. Some
of the disadvantages were the following:
1.) If after creating an assembly, you make a change to the individual component, the
program will automatically change any dependent assemblies/manufactured parts EVEN IF
YOU DO NOT SAVE IT. In some ways this could be considered convenient, but it also
causes headaches when you're not expecting it.
2.) The manufacturing process is complicated; there are many tasks to perform, allowing
more room for errors, which are hard to correct without having to start over.
3.) If you want a clear look at the dimensions, you have to go all the way back to the actual
sketch.
4.) To make changes to one screen while looking at another, simply clicking on the
minimized screen that you want will not activate the screen.
5.) If you make a change to a part and save it, the old part will not be erased unless you
"delete all old versions". If you don't do this, you will have an awful lot of copies lying around.
6.) Trying to figure out all of the parameters when manufacturing in Pro/E was tedious.
7.) When making the assembly block, we didn't like how you couldn't go from one assembly
of the block to the next without having to hit "redefine" every time.
8.) In the assembly, you should be able to modify the individual components (ex. dimensions)
instead of having to actually open the component file and repeating the monotonous
procedure to get to the actual sketch.
Overall, we think that the program would be more useful for simple models. If the model is
complex, Pro/E will cause frustration and irritability, and will consume all of your time!
Manufacturing
At left, both halves of the mold after machining and the cylindrical core.
At right, the top mold after installing both cores
Left: top mold after installing both cores, Center: the full mold complete with cores, sprue, runners - ready to go!
Right: detail of the semi-hemispherical core with protrusion for opening
The mold halves were machined on the CNC milling machine in Milos' ME lab.
The parting line placement was rather intuitive. It didn't really matter for the base cavity,
but there is only one possible place for the parting line in the top (hemisphere) mold since
the top part had to include a lip around the inside of the bottom of the hemisphere. This lip
enables the top part to snap onto the base cylinder.
The trash can/tic-tac dispenser required two cores, one for each part. The core for the
base is quite simple since it is just a cylinder to sit in the mold cavity. The core for the
hemisphere is quite a bit more complex since it must allow for the creation of a lip.
The core for the base was cut to the correct size from 1.25" bar stock and machined on a
manual lathe in the ME machine shop. A 1-degree taper was machined onto the part to
allow for easy removal of the finished plastic part.
Several steps were used to create the hemispherical core. First a 3" piece was cut from 1.5"
bar stock. The outer profile was then milled using the CNC lathe. The piece was then
put in the CNC milling machine and the top was side-milled to create the protrusion for
the hole in the hemisphere. The part was then cut to the correct length and screwed into
the mold cavity.
Runners, a sprue and gates were then machined into the mold halves and we were ready to
shoot little tic-tac dispensers/R2D2's! We are pleased with the way the parts turned out. The lid snaps
on nicely and fits pretty snugly. The major defect is some sinkage due to the large mass of
plastic near the bottom of the cylindrical bottom part. This was mainly caused by a five degree
draft on the cylindrical core, which made for a part that was easier to remove from the mold, but allowed
more plastic melt into the bottom of the base where the diameter of the core was the smallest.
Thus we experienced some minor sinkage, but it is only minor and cosmetic. They are perfectly functional
and can even be shaken without the lid falling off - a benefit because we don't want toenail clippings
or tic-tacs scattered on the floor!
We performed a precision test using the measurement device in the mechanical engineering
measurement room. The specified diameter of the large outer cavity was 1.5 inches, and the
actual dimension is 1.50266 inches. The specified maximum diameter of the hemispherical
core was 1.0 inches, and the actual dimension was 1.00301. In both cases, the actual
dimension was within 0.003 inches of the specified dimension, which we are very pleased with.
Thanks for viewing Team Can's web page. Questions? Email the group:
Jehana Ray: rayofsunshine99@yahoo.com
Rob Melville: r-melville@northwestern.edu
Matt Aldeman: m-aldeman@northwestern.edu