A review of design requirements for crystalline and amorphous plastic materials sets the stage for healing a troubled hot runner part.
When I’m looking for a subject to write about, I like to pick one that addresses the problems encountered in the daily business of molding and tooling.
This month, when I spotted a box that had a runner and several parts in it, it struck me that this was a perfect subject for this month’s article. The parts were used in subassemblies for the automotive industry and are typical of parts currently used in many different applications.
The material in these parts was crystalline—an unfilled nylon. I picked these parts and runners because I wanted to start my analysis by making a point about crystalline materials and how they differ from amorphous materials when it comes to sizing runners and gates.
Common crystalline materials include Polyethylene, Polypropylene, Nylon, PBT Polyester, and Acetal; common amorphous materials include Polystyrene, ABS, ASA, Acrylic, SAN, and Polycarbonate. In molding parts made of crystalline materials, runner diameters and gates should always be smaller than parts made of amorphous materials. Crystalline materials are mostly milky white in color. They’re much more chemically resistant and shrink more than amorphous materials. Adding fillers to either family of materials reduces shrinkage and increases stiffness, but watch out when you mold a nylon with a filler in it, such as 33% glass-filled nylon 6/6. The glass-filled nylon is very rigid when molded, but after the parts sit around for a few weeks and the nylon sucks up moisture, the part’s stiffness will reduce to half of what it was right out of the mold. Other materials do not have issues with moisture like nylon does.
Amorphous materials are more easily attacked by chemicals. For instance, many household cleaning solvents can smudge or cause cracking or crazing on the surface of a part molded of an amorphous material. Some amorphous materials—for example, ABS—are slightly more resistant to cleaners due to the impact modifiers in the material. Chemicals can attack polycarbonate, and the same goes for polystyrenes. One of the selling points of amorphous materials is that their natural color is transparent instead of milky white—a necessary quality in many applications.
An undersized feedpoint started the problems with this automotive part, but the short shots and breakage resulted from very thin walls (0.040 inch) feeding into a thick post (0.135 inch).
Feedpoint issues
The molder’s note that came with the parts I mentioned above said short shots and breakage on the sidewalls were his biggest problems. I looked at his examples of good and bad parts and then examined the cold runner. It certainly was a cold runner but when I looked at the feedpoint, I could tell this was a hot runner mold feeding into a cold runner.
The biggest problem with the addition of the hot runner to our review was that I didn’t have a print to look at, so I couldn’t see what the flow tube or bore diameter was, I had no idea whether or not the molding machine nozzle had been opened up to match the flow tube diameter, and I had no way of knowing if the hot tip gates were tapered down to no more than a 0.002-inch-long land. So many unknowns to work with! All I could do was call the molder and get the toolroom supervisor on the line to see if he could put the pieces of this puzzle together for me.
After the phone discussion I had more to go on and started my cold runner, hot runner system, and part review. I could see the feedpoint to the cold runner was not big enough to fill and pack the parts through the subgates attached to the cold runners. I guessed that, with the feedpoint undersized, molded parts were either over- or underpacked, depending on the heats, speeds, pressures, and cycle times being tried. Sometimes the parts stuck in the mold and cracked during ejection and sometimes the sidewalls would run short.
The feedpoint to the cold runner measured about 0.080 inch and each drop had to properly pressurize four runners, each approximately 0.180 inch in diameter. If this were a normal cold runner with a cold sprue, I would recommend a sprue O-diameter of 0.250 inch to get the material to flow easily through the runner and gates to fill and pack the cavities.
But this wasn’t a true cold sprue and we couldn’t go as big as 0.250 inch without stringing and drooling nylon out of the hot tip gates. Faced with something that wasn’t working and no option for starting over, it made sense to recommend enlarging the 0.080-inch feedpoint into each of the cold runners to 0.100 inch, for starters. If that didn’t fix the problem, it could be opened up to .110 inch, and so on. The increases should stop when the gates started to string or drool. Also, the backpressure on the screw should be set at a minimum level.
Next, I looked at the subgates. They were each 0.050 inch in diameter and fed into a 0.065-inch-thick rib attached to a 0.070-inch-thick wall. The subgates were close to the correct size so I didn’t recommend any changes.
The problem with the breaking sidewalls became apparent when I measured the part. A 0.070-inch-thick wall fed into a 0.030- to 0.040-inch-thick wall, and this skinny wall then attached to a .135-inch-thick post. If you don’t know by now, I’ll say it again: Material that flows from thick to thin to thick will give you nothing but problems—loss of packing pressure control, shrinkage, and stress. So, the thickness of the sidewalls needed to be uniform, which would correct some of the over- and underpacking problems. Increasing the diameter of the feedpoint to the cold runners would take care of the rest of the overpack and shortage problems.
Troubleshooting in the dark
The biggest problem here was that I didn’t know what the processing conditions had been or anything else that might have been tried, such as pressure settings, mold temperatures, injection speed, and so forth. Most molders can be really creative when sampling a mold. They run the barrel heats and mold temperatures up and down, and they usually slow down the cycle to keep the parts from warping, mostly due to the higher barrel heats they tried. Long cycles and higher barrel heats are a recipe for material degradation due to a longer material residence time in the barrel.
All I could do here was speculate on why the feedpoint to the runner was too small (I usually blame these small feedpoints and small gates on the mold designer or moldmaker trying to help out the molders). I guessed that the inconsistent filling of the part sidewalls would be greatly improved by opening up the feedpoint to the cold runner and increasing the wall thickness of the part’s skinny wall sections. Also, the GP molding machine nozzle, hot runner sprue bushing, and flow tube needed to be the same size; I recommended 3?8 inch in diameter.
I e-mailed the suggestions to the molder and waited for his reply. It came almost a month later and it was a good report.
The Troubleshooter's Notebook
Part / Material: Unfilled nylon part for automotive application. Tool: Four-cavity hot runner/cold runner combination.
Symptoms / problem: Short shots and breakage on sidewalls due to nonuniform wall thickness. Solution: Enlarge feedpoint to the cold runner; make wall thickness uniform; ensure the machine nozzle and hot runner sprue bushing diameters match at 3/8 inch .
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.
