The Troubleshooter, Part 104: Suction cups can be difficult to mold

A cosmetic touch-up is all that’s needed in this well-designed part.

This month I received a suction cup in a package along with a note. The note said to call right away. I recognized the name and company as someone I worked with several years ago, before I went into business for myself.

I looked the suction cup over and saw many flow lines on the underside and what looked to be an undersized gate. The material was a soft, flexible material such as an easy-flow low-density polyethylene (LDPE), a thermoplastic elastomer (TPE), a flexible polyvinylchloride (PVC), or possibly a polyurethane (PUR).

This suction cup had a handle designed into it on the top so I guessed the application to be something to do with lifting plate glass when fixing windows—possibly a way to lift replacement windshields for auto repair.

I called the name on the note and found out I did not know the person who sent me this part and that the company had been sold a couple of times since I last worked with them. We discussed a couple of things and they agreed to send me parts for evaluation and a written report.

Determining the material

What’s interesting here is that the part functioned quite well; they just didn’t like the flow lines and blush. Blush around the gate is always due to a restricted flow condition at the gate. The injection speed could be slowed down but often the part ends up with sink near the end-of-fill areas or where ribs intersect with the part walls. In this case, sink would appear somewhere in the handle area and in the curvature of the suction cup design.

The mold or material temperatures could be increased, both of which would probably help the cosmetics around the gate; however, the downside with either of these is usually slower cycle times.

I asked my contact what material was used in this part and he did not know. I told him to try and find out and I would see if I could figure it out on my own.

The first thing I did was put the part in a tub of water I have in the corner of my office. The part floated, so I knew right away it was PE or PP or an alloy of PE, propylene, or rubber of some kind. At least we ruled out flexible PVC.

Next, I burned a corner of the part. The flame was yellow and had a fragrance of melting candle wax, indicating polyethylene. That was close enough for me. I could proceed as if the material being molded was an easy-flow PE.

Dimensional correction

The wall on this approximately 5-inch-diameter part was 0.145 inch thick for 1.75 inches from the gate, and then reduced down to 0.040 inch at the outside. The handle on the front was generally 0.275 inch thick with a 0.300-inch center post. All in all, it was a pretty good design.

I also could see a couple of projections on the top of the part that were 0.200 inch in diameter and 0.250 inch tall. Unless these guys had come up with something new, these projections are used to release the vacuum by prying the projections up with a pair of pliers.

Next, I looked at the feedpoint to the underside of this cup. I could see a 0.530-inch-diameter recess around the gate that had a 0.340-inch inside diameter. The problem was that it was recessed 0.025 inch into the molded part. This alone was a restriction to flow that needed to be addressed, but I could see the sprue O-diameter was no more than 0.125 inch. Since the sprue O-diameter for feeding a part like this should be closer to 1.5 times the wall thickness (0.145 inch), it should have been 0.218 inch.

It would have been better to use a heated sprue bushing or several drops out of a hot runner mold to feed these gates, but maybe the number of parts needed per year was not enough to justify the additional expense of hot runner equipment.

Using a recessed sprue bushing to permit a larger sprue O-diameter and increasing the nozzle orifice diameter could be enough to produce decent-looking parts out of this mold. This would enable the molder to eliminate the flow lines and blush around the gate area. At the very least, it would open up the processing window and allow the molding technicians to make a much better-looking part.

This was a case where the changes we made would help with cosmetic problems but would not let us reduce material or mold temperatures to speed up the cycle.

I called the molder, told him I was e-mailing my report along with an invoice, and asked him to let me know how everything turned out. I also reminded him that he could go to the free section of the IDES.com website and look up other materials that he might want to sample for this application.

After a couple of weeks I hadn’t heard anything from the molder but I did get my check. I’ll follow up with him in a couple more weeks to make sure the issue was resolved.