The degree at which a nozzle needs to be cleaned is dependent upon the clearance around the nozzle body and the cavity insert along with the sealing/stacking surfaces of the components.
The entire nozzle does not need to be spotless with all traces of plastic removed to be reinstalled and seated properly. This can be accomplished a number of ways, depending upon the type of nozzle and how it is heated.
All that is necessary is to:
Clean the nozzle housing front and back, and to remove enough plastic around the nozzle body so they can easily enter the back of the cavity without interference.
Plastic around the front tip must obviously be removed if it stacks and seals against the back of the cavity gate insert.
Clean the back of the nozzle seats and run them over a medium Norton stone to make sure they are flat and smooth. If a depression is noted then this must be measured to ensure the depth of the depression is no greater than .001 or a slow leak could occur here (see Photos).
Cartridge Heated Nozzles
If the nozzle has an internal cartridge heater that isn’t stuck, simply pull it out (make sure it is numbered) and use a soft wire wheel to clean off whatever plastic is on the body, taking care not to damage or reconfigure the tip.
Some technicians like to burn the excess plastic off nozzle bodies with a torch, before disassembly and while the manifold plate is standing up on the bench. If this is your preference it is advised to use a controller (versus a torch) to heat up the nozzles just enough (usually around 250 to 300 degrees) to easily remove the plastic, which makes this operation safer and without stinking up the shop.
If the heater cartridge is stuck, then you can either drill it out (thus scraping the heater) or leave it alone and clean off the nozzle body with a brass pick/scraper. You also can clamp it up in a vise (soft jaws) and use a wire wheel in an air drill.
Standard cartridge heaters are typically .005 smaller than the nozzle bore, but if not cleaned out periodically will stick hard. If you do have to drill it out, use the next standard size smaller bit, taking care not to scar up the bore walls. Resist the temptation to perform this operation by eye at the bench with a hand drill, instead use a lathe to do it right.
The objective is to drill out all but the outer skin of the heater allowing you to slip a prick punch behind the skin, bend it in far enough to grasp it with needle nose pliers, hose it down with WD-40 and work it out. A time-consuming task that is best prevented through regular cleanings and the application of an anti-seize product like a release and transfer fluid before installing heaters.
Cast-In Heated Nozzles
These are more of a pain to clean because of the attached heater/thermocouple wires that should not be removed for normal maintenance. They also require great care not to poke holes in the embedded heaters or to damage the leads and (ceramic) connectors. Never use a torch on these types of nozzles and only clean what is absolutely necessary to reinstall the nozzles.
Banded Heater Nozzles
These are great simply because the heaters and thermocouple wires are usually easily removed, making cleaning the body much easier via ultrasonics. Any time a component can be cleaned in an ultrasonic tank, the process will be faster. The only problem usually stems from removing the thermocouple.
Some brands require you to straighten out the tip before you can slip it through the nozzle, which can stress crack and ruin the thermocouple. If this is the case, and the manifold is extremely time-consuming to work on, then spend the money and replace all of the thermocouples so as not to risk creating an opportunity for them to fail—negating all of your labor hours.
Hot runner systems require extra care in the injection molding environment. Additionally, they often represent a large portion of the overall time and effort required for a changeover or cleaning.
Some high-cavitation hot runner molds are designed with straight channels and 90 degree turns that make cleaning difficult. Any dead spot or low flow area is a potential sticking point for the purging compound itself, and color often hangs up at the nozzle tips. The relatively small clearance of the hot runner gate also presents a challenge for cleaning.
Purging Tips For Cleaning Injection Molding Hot Runner Systems
Check the gate clearance and use a purging compound grade that clears the tightest restriction. For instance, a glass-filled CPC is not recommended for purging most hot runner systems due to gate clearance restrictions and the potential to damage nozzle tips, as well as the possibility of fiber agglomeration at the gate.
Also, an effective CPC will remove carbonized resin and colorants, which may break off and block gates if their clearances are too small. Before purging hot runners, purge the screw and barrel to ensure that any contamination in the barrel doesn’t get into the hot runners.
Use a purging compound grade that is compatible with the processing resin being molded. This ensures less residue is left behind.
Typically, an open-mold or closed-mold method of purging can be used for hot runner cleaning. An open-mold method is best for cleaning lower cavitation manifolds, and higher cavitation molds are better cleaned via closed-mold purging.
For extremely difficult hot runner purges, try a chemical purging compound. The expansion into dead areas helps speed up the purging process and often prevents the need to send the hot runner back to the manufacturer for cleaning.
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.
The gentle and repeatable cleaning of injection molding nozzles, screw tips, static mixers, etc. is an important factor for the injection molder as this ensures a long lifetime for these key production tools. Depending on the type of polymer and design of the injection molding nozzle short cleaning times of only one to four hours can be achieved. Polymers are removed in a single step; the cleaned components can be quickly integrated again directly into the production process. Inorganic remnants are removed using a select after-treatment method (compressed air, glass shot-peening, etc.).
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