The Troubleshooter: Care for hot runner systems

This article continues The Troubleshooter’s Rules of Thumb series. 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.

So far we have worked our way through the five basic elements of troubleshooting an injection molding problem: nozzle, sprue, runner, gate, and vents. By optimizing these five pieces of the puzzle we can correct a high percentage of our molding problems. How high? Probably 80 to 90 percent at least. I have molders telling me all the time that by correcting problems in these five areas they have corrected 100 percent of their molding problems. But I know we have more to work on than just the problems in these five areas.

For example, connected with the nozzle section of my review procedure are the problems most commonly associated with heated sprue bushings and hot runner systems. The problems are the same for both. First of all, the molding machine nozzle must be drilled out or opened to match the flow tube diameter of the heated sprue bushing or hot runner manifold. The flow tube diameter is usually .375 inch for semicrystalline materials such as PE and PP, but somewhat bigger for the amorphous materials like ABS, acrylic, and PC or variations of these. I generally see a .500-inch-diameter flow tube used for amorphous materials.

Think about the hot runner system or heated sprue bushing as an extension of the molding machine barrel. This additional flow path is just an extension of the fluid delivery system that transfers material from the barrel to the mold cavities without degrading or changing its melt condition.

Setup: Hot vs. Cold
Typically the first problem is this: Many molding shops allow the setup people to determine the nozzles appropriate for the molds they are hanging in the molding machines. With two- or three-plate cold runner tools, where the sprue pulls and drops with each shot, they are used to installing nozzles that have an orifice slightly smaller than the hole in the sprue bushing, so they tend to do the same for a heated sprue bushing. This is wrong, of course. In this case, the orifice in the nozzle needs to be exactly the same size as the hole in the heated sprue or hot runner bushing (see Figure 1).

We typically open up the diameter of a general purpose nozzle to .375 or .500 inch, which eliminates the nozzle shear points and dead areas in the material’s flow path. It also eliminates pressure losses caused by going from a .250-inch nozzle orifice to a .500-inch flow tube diameter in the hot manifold. Maybe the biggest benefit of this procedure is being able to change colors much more quickly.

It is easy to spot this problem by looking for heat spikes in the processing conditions. If the nozzle isn’t matched to the flow tube size you will see setpoints for the nozzle that are some 50 deg F higher than the barrel front zone heats. I am talking about temperature setpoints here, because it is impossible to get a melt temperature reading from the nozzle setpoint. The melt temperature of the material coming out of the nozzle is a combination of all the setpoints throughout the barrel and nozzle.

When a heated manifold system is sized properly, the barrel front zone, nozzle, and manifold all run at the same temperature. If you have temperature control at the gates, the gates should be set 10 deg F lower than the manifold heats. The gate temperature control can actually be set quite a bit lower than the manifold heats if needed since the gate heats are basically used only to eliminate stringing or drooling problems. If the heats are set too high in any of these flow path areas, it becomes very difficult to stop the stringing and drooling problems at the gate or orifice end of the manifold. If you have to run the gate temperatures higher than the manifold temperature it just means that you still have a problem with the gate diameter or land dimension. This is why it is so important to open the flow path to the correct diameters all the way through the nozzle, manifold, and gate sections.

Gate Sizing and Design
This brings us to problem number two: gate sizing and the relationship of the diameter to the angle of the taper. The gate diameter is based on the part’s thickness and flow requirements. The gate should also be tapered where it transitions to the surface of the part. The right diameter and the proper taper is the final key to having a smooth-running heated sprue bushing or hot runner system.

The gate diameter should be one-half of the part wall thickness when you are running PE and PP, and two-thirds of the wall thickness when running acetal, ABS, acrylic, SAN, and HIPS. When you run PC and other stiff-flowing, high-temperature polymers you need a gate diameter equal to 90 percent of the part wall thickness. I commonly see a hot system that is set up for PE or PP where the molder is trying to run PC or PES or some other high-heat, stiff-flowing material through it. In these cases, he can’t even get the part to fill out with all the pressure he can crank out of the molding machine.

The recommendation for the taper of the gate varies from supplier to supplier but it is usually somewhere around a 30° angle with a gate land of approximately .002 inch. This small gate land seems pretty skimpy to most toolmakers but once they realize this design will not cause the gate area to crack or break out, they are all for it. Most hot tip gate designs refer to this .002-inch gate land with a “sharp” notation on the drawing (see Figure 2).

The basic reason for doing all this work to the nozzle, sprue bushing, and gate area or orifice in a heated sprue bushing is to prevent excessive processing heat from causing molding problems such as part warpage and longer cycles. It also allows the conductive heat from the heater bands coupled with the shear heat from the screw RPM and backpressure settings to stabilize at a lower uniform heat than you might have been using in the past.

Many molding problems associated with heated sprue bushings or hot runner systems, such as hard-to-fill parts, flashing of mold vents, material additives plating out on the core of the mold, and others are connected to improper sizing of the material flow path.

Equally as important as sizing the flow path is following the correct design according to the gate type selected. Technical gates (Figure 2) use a recessed area at the gate to hide any vestige caused by the larger-diameter gate designs. They also need a transition dimple below the gate to maintain a uniform wall thickness. Technical gates typically have larger diameters than cosmetic gates.

Cosmetic gates do not require recessed areas or transition dimples because the gate diameters are usually smaller and the taper at the gate eliminates gate vestige.

Gate Materials
In addition, if you are running glass-filled materials through your hot runner system, you need to consider inserting the gate area with machinable carbide or possibly welding the gate area with a compatible hard weld product to guard against abrasive wear. If you are going to run acetal or PVC you probably need to look at stainless steel for the gate area to guard against corrosion problems.

Two things cause wear at the gate: 1) turbulent flow of the polymer material caused by undersized gates and 2) a fast injection speed. Therefore, we need to specify long-wearing alloys or stainless steel in this location when drawing up the mold prints. The ironic twist here is that the gates wear to a certain point and then the wear stops. The point at which they stop wearing turns out to be just the right size of gate for the material being used and the injection speed needed to get the part to fill and pack just right.

So then what do we do? Because the gates look ragged or rough on the surface of the part, we send the mold to the toolroom and have the gates welded up and resized back to the original print dimensions. Then we have to start over again trying to get the part to fill and pack with what is again an undersized gate. Sending the mold to the toolroom for cleaning up the gate area is a good idea, but don’t resize the gate to the original dimensions. Just clean up the gate area, give it a good draw polish, and send it back to the molding department.

This sounds pretty easy to do, but it must not be, because I have molders telling me all the time that they hate hot runners or heated sprue bushings. If they can just get the toolroom to make the changes I have outlined here, they would change their minds. They are easy to run and generate less regrind than cold runners. In addition, cycle times can usually be reduced by a few seconds since you’re not waiting for a big, thick sprue to set up.