Morning all,
I am in need of some stiffening rib design advice. I am not sure if I am in any violation of asking design advice here vs SW advice but I am going to ask anyway. I am molding a hopefully flat cover to a battery case. It will be molded from PC/ABS (Cycoloy). Gated in the middle bottom surface of the part. the part is 10" long x 5.5" wide x .100 thk. I am concerned about the orientation of the ribs with regards to the part. I will be running the ribs parallel to the direction of flow (length wise on the underside of the cover). I will size them to be no more than .040 thk at the base of the rib with a .5 degree draft and no taller than 2 times the width of the adjoining wall ie .2" tall. The question I am asking is should I connect the ribs running lengthwise with some interconnecting cross ribs or diagnol ribs as well? I know it is easier to add ribs later if need be by machining additional pockets in the core, but what would be a good spacing length and width wise given the parts dimensions do you think? I am attaching some pdfs for reference.
Thanks
Sid
Sid,
Do you really need the ribs? I ask because the ribs are going to make your more or less flat part much less flat. The thick section at the base of the ribs will still be shrinking when the tips of the ribs are completely solid. That will cause your part to go concave toward the text side of your part. The taller you make the ribs, the more they will cause the part to bow. More ribs will also mean more bowing. Adding cross ribs will cause your part to potato chip as it bows in both directions.
One way to minimize the bowing is to make the rib walls thicker, up to 100% of the wall thickness. Unfortunately, that leads to sink, which is usually unacceptable. Ribs that are two thirds of the wall thickness, rather than the 40% that you propose, will give you less bowing with minimal sink.
Another trick you might try would be to gate into the long side at the second screw hole. That will put your gate closer to one third of the way down the long side that is often preferred by the experienced mold designers as the point that minimizes bowing. I have no idea why this should be so, but the old guys swear by it. The screw hole will give you a pin for the incoming material to impinge on, preventing it from jetting.
The countersunk holes also make me nervous. You can sometimes get away with this, but the clamping force of the screws causes hoop stresses in the countersunk holes. If you don't tightly control the torque on the screws you are quite likely to end up cracking the part, especially on the far end where you are going to have weak knit lines across the highest stress areas.
Jerry S.