Tooling Corner: Runners running amok? Try this.Tooling Corner: Runners running amok? Try this.

Editor?s note: Bill Tobin and Bob Butcher, both experienced in injection molding troubleshooting, bring their expertise to bear in specific molding challenges.

It?s always fun to critique a mold design and question the runner system. When I (Bill) ask why someone designed a multicavity tool with the runners in an H pattern, I get Generic Excuse Number 1 (?We always did it that way?) in the form of, ?It?s naturally balanced.?

Sometimes, if I can, I make a bet with my client: I bet my $100 against him or her buying my lunch. The bet is that the ?naturally balanced? runner systems aren?t really balanced. (First rule of consulting; never make a bet if you don?t already know the outcome.) This bet is usually too tempting for the molder to resist. We go to a machine running a multicavity mold. If the injection speed is slowed down and the shot is set to about 50% to 75% of normal, everything will fill ?naturally.? It will usually be a balanced fill pattern.

Just as the client is ready to take my $100, I say, ?Speed up the fill, and let?s look again.? This time, with only the fill speed changed, the short shot comes out dramatically different. Usually one or more cavities are almost completely filled while others haven?t even begun.Before computer finite element analysis simulations, or Sims, people built runner systems the way a contractor would design the water system to an apartment building; the main waterline was big, and the branches were of smaller diameter in order to maintain water pressure. This is a useful principle in hydraulics, but plastic isn?t hydraulic fluid; it is a compressible fluid.

The key to a naturally balanced runner system is to get every cavity to fill at the same rate at the same time. When the designer did a Sim on the part, the program generated the pressure required to fill the cavity. The mold builder, using a similar Sim program, now can design a runner system with gates that deliver the same pressure at the same time.

Case in point; take a typical eight-cavity mold. Under the ?naturally balanced? mentality, this would have two groups of four cavities, each serviced at the end of an H being filled by a bar coming from the sprue. Nifty. With a little diddling, the mold will do what it is supposed to. But every setup technician will complain about one or more cavities that stick. This is because the sticking cavities are the ones that filled first and are now over-packing so the rest of the cavities will fill.

Now let?s use your Sim. Lay out the mold with two rows of four cavities, a runner going down the center, and gates extending to each cavity. Anyone will tell you the gate nearest the sprue will see the highest pressures and fill first while the ones farthest away will fill last. Good. Somebody?s thinking. But with our Sim, we need only size the gates so that each cavity will see the same pressures. The gates nearest the sprues will be smaller than the ones that are farthest away. Big deal, you say. I?ve got a weird looking runner. What else did I get besides wasting time at the computer?

Go back and calculate the amount of plastic in both runner designs. Calculate the cooling time required for both runner designs. Calculate the overall cycle time. Calculate the savings.What did you get for fooling around with a runner pattern for the afternoon? Usually you get a 10 to 15% improvement in the cycle time as well as a 30 to 50% reduction in the runner weight. That means you?ve made a mold that will cycle faster and use less plastic than you quoted.?Bill Tobin

Troubleshooting Hot Runners

Difficulties have arisen with a four-cavity, valve-gated hot runner mold with an H-style manifold layout. The mold makes large polystyrene petri dishes. One cavity repeatedly sticks, causing the part to crack on ejection. Cooling and venting check out uniformly on all four cavities, but it is necessary to turn the hot runner nozzle temperature down more than 100 deg F on the problem cavity to get the mold to run. A bad thermocouple or a balance problem with the hot runner is suspected.

The diagnosis is that a bad thermocouple could indeed be the problem. This can be easily checked out by swapping wiring-zone locations. The control may also allow slaving the nozzle heater of the cavity in question to another thermocouple.

System balance is probably not a problem on a four-cavity system using an H-style manifold layout. A short shot will help evaluate balance. Since there is already uniform venting and cooling on all four cavities, carefully check out wall thickness. If the cracking part has a slightly thicker wall thickness, this is the problem. A thicker wall on one cavity will allow an over packing of the part, causing it to crack on ejection.

In a second case, an eight-cavity, hot-tip system with an XX-style manifold layout makes nylon 6/6 parts used in the automotive electronics industry. Each part weighs 4g. Generally the mold runs well. However, every once in a while, the gates freeze, causing considerable disruption and loss of an otherwise efficient process. The molder has equipped the press with a new nylon-specific screw and is closely monitoring cooling volume and temperature, but these actions still have not solved the problem.

I (Bob) recommend that the molder check the interface between the machine nozzle and the hot runner inlet. The machine nozzle orifice diameter must match the orifice diameter of the hot runner inlet. This is especially critical on highly crystalline materials such as nylon 6/6. Often, the narrow processing window required by crystalline material is difficult to maintain at this interface. This is because the end of the machine nozzle is unheated by design, and the hot runner inlet has significant heat loss due to contact with the mold base. Crystalline material can solidify and build up on the inside diameter of the hot runner inlet if the diameter of the machine nozzle is smaller than the diameter of the inlet. The cooler material will slough off intermittently, sometimes migrating to the gate areas and causing this type of freeze-off problem.

Some hot runner manufacturers offer a heated inlet that is controlled independently of the manifold. If your system is not equipped this way, I would highly suggest adding this feature. You can use a thermocouple for temperature monitoring the inlet heater but do not use the thermocouple for automatic temperature control of the inlet. Use manual control only on the inlet heater. Attempting automatic thermocouple control of the inlet almost always results in interference with the critical thermocouple control of the hot runner manifold.?Bob Butcher