The Troubleshooter, Part 13: Acrylic breakage

This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers inthe molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

Acrylic problems encompass everything from voids in thick sections to part failure in thin to thick transitions, plus the usual assortment of black specks and white streaks.

This month I want to talk about just one of those problems, one in which the part failed due to restricted flow and cracking in the thick to thin transition. The part looks a lot like a bracket that mounts on a bathroom tile or window glass with a suction cup on one side and a horseshoe-shaped set of arms sticking out at a 90¡ angle on the other side. The arms appear to be intended to hold something like a rain gauge or an outdoor thermometer (see Figure 1).

The part was gated at the end of each arm, which is a little bit unusual as it is a fairly small part. Due to its wall thickness, the part could be filled from a single gate location if so desired. I probably would have gated it at the common point where the arms attach to the body of the bracket, thus filling the main part of the bracket out first, then letting the arms fill out evenly. In this manner, the air is pushed ahead of the plastic flow front and out the perimeter venting that works so well for acrylics. But that isn't what we have been asked to work on this time.

Visual Inspection

When troubleshooting acrylic, I usually do a visual check first. In this case, I could see just a hint of a flow line inside each of the gate areas, which are located at the ends of each of the horseshoe-shaped arms. The flow lines were telling me the gates are somewhat restricted for the flow properties of the material. Either we needed to open the gates or change to an easier flow acrylic. After checking with the molder, I found he was already using an easy-flow grade of acrylic.

If I have a choice, I always open the flow path to correct gate blush or jetting problems rather than use an easier flowing material, because I don't like to give up the properties of the stiffer flow materials. The only time I like easy-flow grades is when I'm trying to speed cycles up or trying to fill a thin-wall part, which is not the case here. I much prefer making the trip to the toolroom and offering the toolmakers an opportunity to help out.

Before visiting the toolmakers it is a good idea to check the runner diameters and sprue bushing size as well as parting line vents, to get your ducks in a row, so you can get everything taken care of on one trip.

As you might expect, the runners and sprue were undersized, which is not unusual for amorphous materials. It seems as if I am always trying to explain to molders and toolmakers alike why the O diameter or small end of the sprue needs to be the largest dimension in the flow path. It's so the nozzle orifice can be big enough to provide enough material to the runner system so as not to lose pressure in the runner during the injection portion of the cycle. Just think of it as a fluid delivery system and it will make sense.

Also, if the nozzle orifice is too small, as is the case when the sprue O diameter is undersized, you will find the molding technicians running higher heats on their nozzle controllers trying to keep the nozzle from freezing off. But this only seems to cause stringing, drooling, or brown streaks at the nozzle when they "heat spike" the nozzle temperatures.

A better solution is to open the sprue O diameter and nozzle orifice so the barrel and nozzle heats can be brought down in order to get the very optically clear parts that our customers seem to like so much. When you get the heats just right, the acrylic actually gets a glossy look instead of the crystal styrene kind of dull or scratchy look that I see so often.

Not only that, but when the heats are being run on the lower end of the material manufacturers' recommendations, the cycle can usually be speeded up some significant amount.

Fix and Optimize

I would optimize this four-cavity mold by sizing the sub runners at .250 inch, the main runner would be .300 inch, the sprue O diameter would be .350 inch and the nozzle orifice would be .325 inch. The nozzle would be a full taper style.

The sub gates, which were .050 inch, would be opened up to .090 inch and the tunnel cross sections would be increased to keep from getting shear when using normal/medium to medium/fast injection speeds.

I would vent the runner at the sprue puller, on the end of the main runners and again at the end of each of the sub runners. The runner vents should be .003 inch deep (I like to feel the flash at the end of the runners), then go out .060 inch from the parting line and drop into a .040-inch deep channel to atmosphere. The width of each runner vent should be the same as the diameter of the runner and the vent lips need to be polished to a mirror finish to make them self-cleaning.

Part vents can be either individual or perimeter-type vents. In either case, the vent depth for parting line vents would be .002 inch with a .040-inch land to the .040-inch-deep channel to atmosphere. Again, as with the runner vents, be sure the vent lips are polished to make them self-cleaning.

The material is an easy-flow grade of acrylic, which is the customer's choice. I like it because I can drop my barrel heats now that the flow path is opened, and cycle like a bandit. The downside to the easy-flow acrylic is that it scratches fairly easily.

The only other problem I saw on the part was in the cored out area of the bracket itself where the thick to thin wall transitions are. The part designer forgot to put a note on the print for the toolmaker to radius the cored out area where the walls come together. Just a .030-inch radius would do.

With the tool optimized we put it back in the machine and started it running in cycle. We dropped the barrel and nozzle heats 70 deg F across the board and adjusted the pressures until the parts filled and packed as we wanted.

The cycle had been 40 seconds before, and we were able to speed that up to a 30-second cycle. The parts didn't break at the base of the curved arms anymore and the visual look was much better than we had previously. We were now getting that glossy look on the surface of the part.

I might mention that we also raised the mold temps slightly, from 75F to 110F, which also helped in molding parts with less molded-in stress, and the higher mold temps did not hurt our optimized cycle.So what did we tell the toolmaker who just shook his head when we told him to put in the larger size sprue bushing? Nothing, absolutely nothing at all. We just presented him with the better looking parts that didn't break any more, and we let him look at the face of the stop watch with the faster cycle time on the dial. Slowly but surely, the toolmakers are getting the message.