Hot Runners & Living Hinges

By Bob Hatch

Knitlines and living hinges make poor neighbors; how can they be kept apart on a compact disk case?

I received a box the other day containing a security packet for CDs. It incorporated two of my favorite things to talk about: hot runners and living hinges.

I looked at the part and gave it my typical mental review. I could see the hot tip gate was at one end of the bottom section, as far from the living hinge area as they could get it. I could also see the living hinge had two knitlines running perpendicular to the hinge. The material appeared to be polypropylene, which is typical with these security packs.

I looked for warpage in the part but did not find any. I looked for blush at the gate and again came up empty. The part was black in color, so dirt specks were not an issue. Since I could not find any major problems, I had no choice but to call the customer.

I found out all they were concerned about were the two knitlines in the living hinge. The main issue with living hinges is that a single gate must be placed such that the flow front moves through the living hinge area in one solid front. We don’t want any knitlines on the living hinge because that’s the first place cracking will occur. I cannot tell you if the hinge will crack in these knitline locations after 10 or 1000 cycles; I only know that the first place these living hinges will crack is at the knitlines.

Another factor is temperature. If the material is a homopolymer polypropylene, knitline cracks are guaranteed in the hinge area when ambient temperature drops (like being in a car’s glove box in the winter). To be safe, the material should be a copolymer grade; its low-temperature impact is much better than homopolymer grades. The molder was using a medium-impact grade of copolymer polypropylene, which told me we had some cold-weather impact, but not a lot.

Multitudes in the valley of decision
Eliminating the material as the source of the problem, we still had to correct the knitlines issue. We had several options:

Solution #1: The best way to relocate these knitlines was to move the gate to the center of the bottom section.

Solution #2: A more subtle approach: Increase the gate diameter and the injection speed to get the flow front to push out further from the gate during injection and cross over the living hinge in one solid front.

Solution #3: Increase the mold steel temperature to promote flow and possibly achieve the same results, but polypropylene likes to be run in a cold mold, not a hotter one. The impact of polypropylene actually improves when the mold is colder, such as 50-75°F. Heating the material to 425-450°F is a better idea since PP likes hotter material temperatures and colder mold temperatures to produce good-quality parts. We didn’t really want to give up this advantage.

Solution #4: We could look at the venting issues in this mold to see if trapped air might be causing the material to backfill around the flow front, causing the knitlines. However, I did not see any dullness or roughness in the corners or areas on the part that would suggest trapped air was being compressed and causing heat buildup, burning the surface of the polypropylene. This ruled out venting issues for me.

Solution #5: As a last resort, we could use a material with a different melt flow to change the flow pattern. I usually try not to change the melt flow of the material—at least not right away. I try to fix the problem first, and then look at changing material if I can find good reasons to do so.

I looked at the options again and all I could see that would make positive changes was Solution #1 or #2: relocate the gate or increase its size.

Could the hinge area be too thin for the wall thickness? Could the hinge land be too long for the material to flow across and fill out the top section of the part? The depth was .012 inch; the ideal thickness for a living hinge is .009-.015 inch, so it could not be much better. Since this was more of a functional issue, I flexed the hinge to see if any whitish stress marks developed when I closed the lid. I did not see any stress marks, so I determined the living hinge land to be adequate. I also tried tearing the hinge by clamping the bottom section in a vise and pulling at the top section in an attempt to rip it off, but I could not get it to crack even a little bit. That was enough testing to tell me the living hinge was well designed and not an issue.

Fighting the good fight
I called the molder to discuss options #1 and #2 with him. Right off, he said the customer would not let him relocate the gate, but he might get permission to increase the diameter of the gate from 50% of the wall thickness to 75% and increase the injection speed from 1 in/sec to about 2 in/sec. This would probably be enough to push out the flow front faster and enable it to move through the hinge area in one large hemisphere-shaped mass, eliminating the knitlines once and for all.

The molder was enthused about these suggestions. He thanked me for the review and went off to visit his friendly moldmakers and discuss the issues with them. I didn’t hear back for three or four weeks, but when he called he was like a soldier returning from a victory on the battlefield. He said the moldmakers didn’t like the idea of making the gate larger and depending on the processing people to leave the injection speed alone, so they suggested using valve gates instead of just making the gate bigger.

In short, they convinced the customer to allow them to incorporate valve gates into the hot runner and relocate the gate to the middle of the bottom section. The reason the molder took so long to get back to me was because of the time it took to get the valve gates ordered, to change the mold to incorporate the valve gates in the new location, and to sample the mold.

The molder was excited because not only did he get rid of the knitlines, but he also was able to take 7 seconds off the cycle—money in the bank. He said he remembered hearing that a hot runner mold cycles faster than a two- or three-plate mold but could not remember where he heard it. I suggested that John Klees always used this point when teaching hot runner seminars and the molder agreed that he must be the source.

It was a complete win/win situation from my point of view: better-quality parts and a faster cycle time. Who would have thought?

March , 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.