If there is even the slightest suspicion that a manifold has flashed, the processor should put a few pellets of a red resin or contrasting colorant into the melt stream (in the nozzle tip), then shoot the mold two to four times before it’s pulled to aid in locating the source of the leak.
While pulling a mold suspected of a manifold leak would seem like a waste of production time to many, the eight to 10 hours it might take to do this could save several days if you catch the manifold in the initial stages (less than one-third full) of total encapsulation.
But it takes an experienced processor to notice these signs and request that the mold be pulled for a manifold inspection based solely on suspicion. Many will just let it go until it becomes obvious that the toolroom has a problem. We will assume as much and elaborate on the above five steps.
On valve gate molds with valve stems accessible from the back of the clamp plate, it is possible to remove the valve stems cold with a small slap hammer, but this practice could cause the head of the stems to break off, so it is safer in most cases to warm up the nozzles and, if possible, the manifold to about 250 degrees to allow the pins to be removed with less impact.
As with all tooling, make sure the valve stems are numbered (use a carbide cutter in a hand-held grinder), a home location preferably somewhere on the head or stem, and place them in a wooden or plastic base drilled with an appropriate number of marked holes to keep them upright and separated.
Remove the backing plate (usually the “A” side clamp plate) to gain access to the backside of the manifold. Here you might see your first clue. Look for the path of the red resin. Manifold and nozzle seals that fail will allow the plastic to flow from that point outboard and follow the path of least resistance.
The plastic will flow through wire channels in a laminar method, just as it does through a runner, so the resin around the initial point of failure will be more burned (degraded). This means the freshest looking flow front is usually furthest away from the source of the leak, and has been cooking the shortest amount of time.
Notice the direction of the shot waves of resin that ripple away from the leak source paying particular attention to these flow fronts and the condition of the plastic. Is it burned (first place to leak) or fresh (last place to fill)?
Before attacking the plastic with a ham-mer and chisel, look hard at the source
of the leak (here is where the red resin really helps) and try to confirm by examining the seal point in this area looking for a resin film between the tooling that would indicate separation. If an area looks promising, remove the plastic and check things out before continuing on.
A faulty seal between a nozzle and the manifold is easier to detect since the plastic rod formed at the junction should be smooth with no plastic washer attached to it.
At this point you will either discover the source/root cause of the leak, or you will find nothing and proceed on with more melting/chipping/cleaning.
Make good use of the heat gun (a Master Appliance Heat Gun #HG-751-B works great) and your brass chisel. The heat gun will soften much of the plastic in the channels to a point that you can more easily remove it from around the wiring, while carefully working your way down the channels.
If nothing is found during initial plastic removal, then remove enough plastic to separate the manifold/nozzle/electric box assembly from manifold plate. This will allow the plate to be cleaned in the ultrasonic while the nozzles and manifold are being inspected and cleaned by hand. Crank up the ultrasonic temperature to 200 degrees and tilt the plate forward in the tank, so the resin/weepage will liquefy and run out of the counter bores and channels.
While the plates clean in the tank, you can now remove the nozzles from the manifold, carefully, one at a time looking for signs of seal separation and feeling/watching for loose bolts. If gate tip edges are critical, wrap the tips in masking tape to protect them from accidental dings
After all nozzles have been removed from the manifold, set them aside (still connected to the electrical box) and clean the manifold with a brass putty knife and ending with a fine/medium coarse Norton stone.
A Hot Runner System is used to maintain a molten flow of plastic from the molding machine nozzle to the gate of a plastic injection mold.
In general, the system is composed of three main parts; the sprue bush, the manifold block, and one or more hot nozzles. Why need hot runner system for plastics industry? Please check advantages of hot runner system.
The following is a list of common problems and answers for hot runner systems.
1. The part is not filling
Cause: Melt temperature too low, injection pressure too low, gate too small, nozzle too small, mold too cold, exit from machine nozzle too small, nozzle blocked.
Remedy: Raise nozzle and manifold temperature, raise injection pressure, enlarge gate, raise mold temperature, fit larger nozzle, enlarge hole in machine nozzle, clear blockage.
2. Nozzle drooling
Cause: Insufficient suck back, Melt temperature too high, gate too big, insufficient gate cooling, incorrect Nozzle type selected.
Remedy: Increase suck back, lower nozzle and/or mold temperature, reduce gate diameter, increase gate cooling, contact your Nozzle supplier for correct nozzle selection.
3. Nozzle not working
Cause: Heater failure, Thermocouple failure, Nozzle blockage, Incorrect allowance for expansion of nozzle.
Remedy: Check/replace heater, check/replace thermocouple, remove clean nozzle, re-machine nozzle cavity.
4. Poor colour change
Cause: Incorrect colour change procedure, wrong type of nozzle.
Remedy: See guide for correct colour change below, Contact your supplier for correct nozzle selection.
Recommended Procedure for Colour Change
Increase mold temperature by 25 ℃
Increase manifold and nozzle temperature by 30 ℃
Retract molding machine nozzle
Purge the molding machine as per your standard practice using a purging agent
Re-start normal cycle – 6 shots
Lower manifold and nozzle temperature 20 ℃ – 1 shot Minimum
Lower manifold and nozzle temperature 10 ℃ – 1 shot Minimum
Lower mold temperature 25 ℃
Check the next moulded parts for colour consistency & quality, and if required repeat
Steps 1-9
New colour is now ready
5. Excessive flash on part
Cause: Too high an injection pressure, temperature too high, poor shut off face flatness. Insufficient clamp pressure on molding machine, tool plates flexing.
Remedy: reduce injection/pack, lower nozzle/manifold/mold temp, increase machine clamp force, change tool.
6. Burn marks/streaks on part or near gate
Cause: Not enough venting in tool, injection speed too high, gate profile incorrect, material not dry.
Remedy: Add more venting, lower injection speed, increase “J” dimension on gate profile, dry material.
7. Excessive tip wear in nozzles when using plastics with high glass fill content
Cause: Tip material too soft for application.
Remedy: Change to Carbide tips, such as MASTIP.
8. Gate vestige too large
Cause: Gate too large, incorrect nozzle selection, gate profile machined incorrectly.
Remedy: Fit bush/sprue nut to reduce gate, Contact hot runner system supplier for correct nozzle selection, check gate
machining profile.
9. Gate freezing off too soon, or during cycle
Cause: Melt too cold, gate too small for material being used, excessive cooling around gate, too much contact between nozzle and mold, gate profile incorrect or incorrect type.
Remedy: Raise nozzle temperature, raise mold temperature around gate, check machining of nozzle cavity and make sure contact is at a minimum, check machining of gate profile and change if needed.
10. Flow lines on large flat part
Cause: Incorrect nozzle type
Remedy: Change nozzle type
11. Bloom on part opposite gate
Cause: Mold too cold, melt too cold, cold slug in part.
Remedy: Raise mold temperature, raise melt temperature, use MOT nozzle.
12. Cold slug in part
Cause: Wrong nozzle selection, head of the nozzle too cold.
Remedy: Contact your hot runner suppier or nozzle supplier for correct nozzle selection, machine cold slug trap opposite gate, ensure contact area on nozzle head is minimum.
13. Intermittent blockage caused by cold slug, tip fails by trying to extrude through nut
Cause: Too much head loss through nozzle head.
Remedy: Reduce head contact to a minimum, Sit head in thermally insulated material.
14. Plastic sticking to front of bush nut or sprue nut
Cause: Not enough contact between nut and mold to dissipate heat.
Remedy: Change nut type with increased contact area to dissipate heat from nut.