Last month, we discussed the general issues of an organized and disciplined process of troubleshooting and correcting mold defects.
Click Image to Enlarge
Correct length of the valve pin is necessary to ensure proper preload on the gate insert. When in doubt, use this direct method to determine the proper pin length.
This month, we look at the specific issues of disassembling and troubleshooting hot runners—long gate vestiges, in particular.
Hot-runner maintenance is tedious work that requires focus, patience and a deft hand when removing and handling tooling. It takes discipline to stop and investigate if you feel excessive resistance when removing or installing valve pins, heaters, manifolds, and related tooling. Knowing what excessive resistance feels like, particularly in hot-runner maintenance, is an acquired skill that separates an apprentice from a journeyman.
Referential measurements must be taken periodically during assembly to ensure that probes, spacers, and related tooling are at the correct height and the manifold is resting flat on all probes. Careful handling of tooling and attention to detail is necessary to keep manifolds functioning properly and without leakage. A minor mistake can be costly.
An improperly seated or torqued probe or manifold, pinched wire, ding, or burr can cause the system to leak. Some systems rely on heat expansion to seal, so they must be allowed to “heat-soak” for 20 to 40 min before start-up. If shot prematurely, the entire manifold and related wiring will become encapsulated in a plastic “cocoon” that usually goes undiscovered until heaters begin to short out, or until someone notices smoke or plastic oozing from strange places on the mold.
Removing this plastic shell is time-consuming and expensive, and there’s a limited choice of cleaning methods. Most common is to soften the plastic with a heat gun and then chip away at it with a soft (brass or aluminum) chisel and a hammer. You can also send it out to be melted off in an oven, where high temperatures can ruin wires, heaters, and thermocouples and cost you thousands in labor and replacement parts. For this reason, hot-runner maintenance is usually only performed by the most experienced and skilled shop employees.
Keep in mind that every manifold will have its own quirks and procedures or sequence of events that should be documented in the maintenance manual and followed diligently by all repair technicians. The following instructions will work on many different styles of hot-runner manifolds in use today. Before you can begin to troubleshoot long-gate defects, you have to disassemble the hot-runner system.
- Heat up the probes to 200F to soften the plastic and allow the valve pins to be removed easier. With the hot half standing vertically, use a slide hammer or puller to remove the valve pins and pistons. If using a slide hammer, hold it steady during disassembly, as it is easy to slightly bow the valve pins and leave you wondering how to blame it on the process.
- As you remove the valve pins, pistons and any related tooling, verify that all are numbered (engraved) according to mold position—and if not, do so now.
- Insert the valve pins into a cleaning board marked for positions and drilled to size to keep them separated, accessible, and safe. Shut off the probe heats when you are finished.
- Probe tips should be covered with sections of rubber hose or tape to safeguard delicate tip configurations.
- Heater wires and thermocouples should be labeled for position before removing any components.
- If you can’t unbolt or remove the electric control box from the mold, then it will be necessary to unhook each heater and thermocouple wire individually from the control box connectors. I have seen many instances where wires are cut and spliced back together, but I don’t recommend it.
- Lay the mold down horizontally on its face, on parallels if necessary to protect the probes, with the manifold facing up, and remove the manifold bolts. This position allows for more control during removal of the manifold with an overhead hoist and makes it easier to see what’s going on underneath as you lift it out of the frame. If the probes need to be tapped out along with the manifold, you will have to do this with the plate standing in the vertical position.
- It is usually necessary to use a pry-bar to help persuade the manifold to lift out evenly. Work carefully—most manifolds are located for exact alignment with hardened dowels, which are easily broken if put in a bind. (Even if the probes are not bolted directly to the manifold, they will usually stay attached to the manifold since the plastic between them has cooled.)
- Carefully raise the manifold out of the frame while watching closely for heater and thermocouple wires that are in the way or restricting removal, and set the system off to the side.
- Once the manifold and probes are out of the mold plates, immerse the nasty, residue-encrusted plates in the ultrasonic cleaning tank and let the magic happen while you troubleshoot the long-gate issues (see below).
TROUBLESHOOTING LONG GATES
Review past long-gate defects for repeated occurrence in particular positions. Note what corrective action was taken to ensure you don’t repeat an ineffective repair. In this example, we assume that all long-gate defects are new to the position. Long-gate issues with a tapered shut-off are usually due to the following 10 probable causes. The first six are mold issues, and the last four are process-related issues that should be checked at the press before the mold is pulled for repairs.
Problem 1: Worn or hobbed taper of the valve pin or gate shut-off area.
- Examine the fit and condition of the two under a microscope and interchange new tooling with old to determine what area is worn. Sometimes it is necessary to grind or lap the valve pin with 400-grit lapping compound for a tight fit at the gate.
- Verify that the valve-pin alignment mechanism (usually a bushing) inside the probe is not worn, which will cause premature valve-pin wear.
Problem 2: Worn piston cup o-rings.
- Examine the o-rings for flat or abraded spots. When installing new o-rings, the groove should be filled with high-temperature grease before you install the o-ring. Pack the squeeze-out around the outside of the o-ring to ensure long life. Replace these o-rings every 750,000 to 1 million cycles and verify the correct o-ring size and material.
Problem 3: Excess resin weepage inside piston cup that limits forward travel of valve pin.
- Make sure all weep channels are open.
- Discuss with processing technicians the importance of not letting the mold sit idle in the press at processing temperatures. Keeping plastic moving through the manifold greatly reduces burning that creates excess weepage and off-gassing.
- Measure the wear on the body of the valve pin as it relates to the manifold guide bushing. More than 0.001-in. clearance in some cases can let plastic leak past this seal.
Problem 4: Blocked air-feed lines through plate.
- Clean the lines. Regular immersion in an ultrasonic tank will keep them clean.
Problem 5: Defective probe heater or thermocouple.
- Perform a continuity check with a voltmeter at the amphenol on all heaters and be sure to do a ground-fault test on the thermocouples.
- Use a pyrometer to verify probe temperature.
- Hook the manifold up to controller, ramp up heater temperatures in 50F increments, and look for fluctuations in the read-outs.
Problem 6: Incorrect stack height of valve pin to gate insert.
- Most hot-runner manufacturers use mathematical formulas to calculate theoretical thermal expansion of valve pins and related tooling and plates. Length of the valve pin relates to preload on the gate insert: No preload will cause long gates; too much will fracture the gate. If the correct length of the valve pin is in question, and specific print dimensions are not available or trustworthy, use a more direct method, as shown in the accompanying diagram. Valve-pin to gate preload usually runs from 0.002 to 0.005 in., depending on the resin and how thick (and brittle) the steel is surrounding the tapered shut-off of the gate.
Problem 7: Insufficient air pressure to close valve pins.
- Use an air-pressure gage to verify correct pressure on both sides of the valve that controls valve-pin actuation. Tapered valve-pin shut-offs usually require at least 100 psi of line pressure.
Problem 8: Insufficient cooling (water) on gate inserts.
- Verify that water channels are open and clean.
- Verify correct water temperature and flow and that an excessive amount of jumpers is not being used at the press.
Problem 9: Incorrect process parameters.
- Verify process parameters and compare with historical settings.
Problem 10: Incorrect timing of valve-pin closing.
- Valve-pin closing is usually actuated by the injection hold or pack time. This can sometimes be verified visually at the press by dry-cycling the mold.
About the Author
Steven Johnson is the maintenance systems manager for Progressive Components and has his own business, MoldTrax in Ashland, Ohio. He can be reached at email@example.com or (419) 289-0281.