The living hinges on this styrenic display box just weren’t holding up to normal wear and tear. Tapering the drop from its 90° angle (upper left) to a 30° or 40° angle would remove the flow restrictions and improve the cosmetics.
The part I picked this month is unusual enough that I thought it would be worth sharing. It’s a box with two living hinges, being fed through a three-plate runner. The material is a rubber-modified polystyrene—probably one of the K-resin products from Chevron Phillips. (Even if it’s not K-resin, many molders still use that name because that’s what they’re used to calling a material like this.)
As often happens, the parts arrived without a note to provide me with details. Not to worry, though; the molder called about 10 minutes after I received them. I asked all the necessary questions and began my review. The problem with these parts, the molder told me, was the cycle time and cracked living hinges.
He was running the parts in a two-cavity, three-plate mold where the runner was fed by a heated sprue bushing. I guessed it was a prototype mold to work out the bugs before they built a production tool with more cavities; possibly a hot runner would emerge out of my recommendations.
The feedpoint to the runner appeared to be on the small side, but I would check it more closely later. The sucker pins were located directly over the drops feeding the part, which restricts flow, and the narrow end of the drops were designed with a 90° land instead of being tapered.
The good news was that the gate location on each part was just about perfect for flowing material through the living hinges to fill and pack the box and lid. This is really important when running a part with living hinges because knitlines in the hinge area will cause hinges to fail after 100 flexes or fewer. A good living hinge will typically flex a million or more times before it fails. However, we were dealing with a styrenic product and even with the best design, the hinges might only flex 1000 times before stress whitening or breaking occurred.
I could see stress whitening in all of the living hinge areas. Stress whitening is just one step away from the hinges cracking and breaking off the part.
So the question became, why use a styrenic product in the first place? The answer had to be for the clarity properties of the styrene, certainly not for the life of the hinge or for chemical resistance. I suspected this was a display box used for nails, screws, bolts, or some item that needed to hang in front of customers in a hardware store or home improvement center. The number of times a display box hinge would be cycled in an application like this is probably less than 50, which allowed polystyrene to be used.
Let’s review
The trapezoidal design of the main runner was OK for a three-plate runner. The feedpoint to the runner was 0.150 inch and looked to be undersized for the runner’s 0.205-inch depth, but I decided it might be OK for this application once all the suggested changes were made; and if we were still having fill and pack issues, we could then open up the feedpoint. Generally, I like to make the feedpoint for a three-plate cold runner mold 90-150% of the runner depth or diameter.
The transition of material from the main runner to each drop was slightly restricted as it turned the corner, thanks to the sucker pins in the flow path. However, I saw a collar around the top of each drop where the drop connected to the main runner; this was where the toolmaker attempted to improve material flow around the sucker pins. It would certainly help with the flow problem at this point, although it’s not the best way to handle flow restrictions.
The bottom of the drop, where the material flowed into the part, was designed with a 90° angle, and I guessed this was restricting flow. A better design would be to taper the bottom of the drop at a 30° or 40° angle to remove any restriction to flow caused by the sucker pins.
The diameter of the gate at the bottom of each drop was 0.040 inch. My rule says that for PS parts, this dimension should be roughly 75% of the wall being gated into. Since the nominal wall was 0.045 inch, we were in good shape.
The finishing touches
Following these changes, what should the cycle time be on a part like this? The calculation I use is one I came up with many years ago. For a mold fed by a heated sprue bushing, I multiply the nominal wall thickness of the part by 225 and add 1 second for each 100 tons of machine clamping force. In this case, the nominal wall of 0.045 inch times 225 plus 2 seconds (200-ton press) produces a target cycle time of 12-13 seconds.
Of course, this calculation only works if the appropriate changes are made to the mold. It also helps to have the mold waterlines and venting on the part and runners reviewed.
For this mold, runner vents would need to be 0.003 inch deep, as wide as the runner being vented, with a land of 0.060 inch, and dropped into a 0.040-inch-deep channel to atmosphere. The vent lips should be draw-polished to make them self-cleaning. Part vents need only be 0.001 inch deep for this material, 0.200 inch wide, with a land of 0.040 inch, and then dropped into a 0.040-inch-deep channel to atmosphere. The same draw-polishing requirement applies.
For the waterlines, avoid using quick disconnects and waterline jumpers on the backside of the mold.
With these adjustments, the molder could optimize the mold, eliminate rejects, and speed up the cycle time. They would also help these guys out when it came time to designing a new mold. I called the molder back, gave him my suggestions, and waited to hear from him. It was only a couple of days before I got the good news I expected: “Everything is running great. Thanks a lot!” Just what I like to hear.
The Troubleshooter's Notebook
Part/material: Polystyrene box with two living hinges. Tool: Two-cavity, three-plate cold runner fed by a heated sprue bushing. Symptoms/problem: Living hinges were cracking; cycle was too long. Solution: Relieve restriction to flow in the part by tapering the bottom of the 90° drop into a 30° or 40° angle; ensure turbulent water flow and proper venting.
About
Bob Hatch Bob Hatch is one of the leading on-the-spot problem solvers in the molding industry. Mr. Hatch spent time as the technical programs manager at Channel Prime Alliance and managed a molding operation for more than 25 years. Currently, he writes articles for Injection Molding Magazine under the pseudonym The Troubleshooter.
