This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Bob Hatch is technical programs manager for resin distributor Channel Prime Alliance. Before his present assignment, Bob managed a molding operation for 25 years. You can reach him at bob.hatch@channelpa.com.

Restricted flow getting you down? Maybe it’s the material.

An undersized sprue is a common source of trouble, but not always, as demonstrated with the following defective part.

I received a package that contained just the sprue, runners, and a small section of the part in which the sprue, runners, and gates were located. The note said the part was a large, double-sided piece that looked like two boat trailer fenders joined together by the gate area. The material was an 8-melt copolymer polypropylene that was colored by the molder at the machine using a color concentrate blender.

Right away I suspected the sprue O-diameter was too small to push the large volume of material needed to fill and pack a part of this size. Second, I could see a blister on the side of the sprue, which indicated to me that the resin pellets might have a little bit of surface moisture on them. Third, I noticed a lot of flash around the opening by the two short main runners and by the sprue bushing feed point—basically, the volume center of the part.

The problem, according to the note, was warped parts, sinks, and poor surface cosmetics. All of these defects made sense, considering the restriction to flow.

Flow Path Sizing A-OK

I started with the sprue O-diameter. Its diameter should have been 1.5 times the part’s nominal wall thickness, which was .140 inch. Multiplying .140 by 1.5 yields .210 inch—the same dimension as this part’s sprue O-diameter. No problem there.

How about the nozzle orifice? It was .175 inch—a little small, but passable for polypropylene, even if it was an 8-melt. So, the flow path through the machine nozzle and the sprue appeared to be OK.

Next, I checked the area where the sprue fed into two short runners. The runners were full round and both measured .300 inch in diameter. Since the runners were just short connectors, the diameter vs. the length wouldn’t create a problem.

I moved on to the gates. The piece contained two edge gates, each responsible for filling and packing one-half of this really big part. Also, I found two hot runner drops feeding into a couple of subgates. I guessed that these drops either worked with the two other edge gates, put in as a precaution, or were originally used before the switch to the cold runner edge gates when the hot tip gates did not work.

If the latter was the case, then the hot tip gates did not perform to their expectations because 1) the hot tip gate diameters were too small for the thick walls of this part; 2) they were not tapered from the drop to the surface of the part; and 3) the volume of material needed to flow through these gates would not have been adequate to fill and pack the parts.

I see this a lot. Molders often prematurely give up on new technological advances and jump right back into their former comfort zone.

Finally, I checked the wall into which the edge gates entered. The nominal wall was .140 inch, as I mentioned, but the gate area wall was .135 inch. It’s not enough to be a problem but it had me going for a minute.

Reduce Viscosity

Now what to do? The flow path checked out and I couldn’t put my finger on something that would be the magic action to correct all of the molder’s complaints. I decided that a change in material would get the job done. The 8-melt polypropylene they were using would have to be replaced with a higher-melt-flow product. I looked through a material database and found a 20-melt product that still retained the impact, tensile, and stiffness of the 8-melt material.

The new material would provide better flow to fill and pack the part like the customer wanted. When I encounter warp on a polyethylene or polypropylene part, I find that going to an easier-flow material gets rid of the warp or the tendency of a part to buckle halfway between the gate and end-of-fill areas. The 20-melt material would also let them reduce the barrel melt temperatures a bit, say some 20 deg F, which in turn would give them the opportunity to speed up the cycle a few seconds.

At this point I was ready to throw in the towel, so I e-mailed my thoughts to the molder. Within a couple of hours I had my response. They wanted to know where to get a sample of the 20-melt copolymer polypropylene that I had suggested. Fortunately, we were able to get them the processing information they needed and a sample shipped out that same day.

A few days later I heard from the molder, who said they had already run the 20-melt and although they were not getting perfect parts yet, the initial results were extremely good—good enough for me to close the file on this problem.

But what about the low level of surface moisture on the sprue that caused the blistering? I chose to ignore it because this time of year we see a lot of material brought to molding machines from a cold warehouse, and the condensation on the pellets as the material heats up in a nice, warm molding shop doesn’t usually concern us. We know the surface moisture on the pellets goes away with time, or we can run it through a material dryer at 130°F.

How about the sprue being too small to handle the volume of material? Well, I was wrong on that point.

Last but not least is what we could do about the flash around the base of the sprue and the runners. I wasn’t really concerned about this. With the easier-flow material, the part should fill and pack with less pressure, which is no doubt causing the flash. Otherwise, we could put better support in the center of the mold or slow down the injection speed to stop the flash.

I would not be surprised if the molder came back to me wanting recommendations on venting. I could see the need for additional vents, but I didn’t want to completely overhaul the mold, just fix their problem. I can work on those other problems later.