Nozzle Drool

The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

Injection molding. High moisture concentrations in thermoplastics that are injection molded can result in nozzle drool, foamy melt, lower melt viscosity, flow lines and silver streaks in the gate area, rough or scaly part surface, voids in the part, poor dimensional control, and reduced mechanical properties. 

Control nozzle drool by lowering nozzle temperature not with positive shutoff nozzles. 

Nozzle designs with positive shut-off devices have been successfully used. The gas must be free to escape through the nozzle. Material freeze off in the nozzle or malfunction of the positive shut-off device could develop pressure to cause blow-back of the material through the feed zone and hopper or create hazardous conditions. In such cases, conventional free flow and reverse taper type fitted with a heater band for temperature control of the nozzle prevents nozzle drool or freeze off of the material and is used and in nylon processing. Sprue cutter is associated with the nozzle to help the process. A nozzle forms a seal between the injection system and the mold. 

Nozzle drool Excessive nozzle temperature Reduce heating... 

PBT resins are very fluid in the melt and sometimes drooling from the injection nozzle can be a problem in machines that do not have melt decompression. A simple free-flow nozzle can be used satisfactorily to minimize drooling if its temperature can be adjusted by a separate heater band. PBT does not drool as badly as nylons. 

A slight retracting of the screw, usually no more than 5-7 mm as the mold opens to suck-back any resin that might have drooled out of the nozzle after the sprue was pulled. Correct nozzle type and temperature craitrol can eliminate a need for this step, even when using very fluid resins. 

No matter what type of screw is used, it is usual to employ a back-flow valve on the tip of the screw and to prevent drooling from the nozzle it is usual to employ decompression (suck-back) or to fit a valve on the machine nozzle. 

General purpose screws can be used, for example, those with an L D ratio of about 18 1 and with a feed zone of 0.5L, compression zone 0.3L, a metering zone of 0.2L and a compression ratio of about 2 1. The screw is usually fitted with a non-return valve but valves are not essential on the nozzle unless high back pressures are involved. Good results (for example, to prevent drooling) have been obtained by the use of mechanically or hydraulically operated types. 

Tlie use of a shut-off nozzle is not recommended due to high pressure losses and the risk of degradation the use of a shut-off nozzle also results in an increase in injection cylinder pressure and, an increase in water absorption. An open nozzle, together with decompression, is suggested cellulosics will not drool if properly dried and then processed at the correct temperature. 

Injection molding, to have higher output rates, corresponds to the heating zones on the barrel. For close tolerance in molding, the nozzle temperature must be precisely controlled with separate temperature controller to prevent freeze-off or drool. During processing, the hopper area should be with separate temperature controller, in order to not affect the ability of the sensor in the feed heating zone to control temperature. 

Screw length to diameter should be important to have uniform melt at higher output and also for uniform melt temperatures. Improved performance comes from radius between the flights and root diameter of the screw. It should be equal to or greater than the flight depth in the respective zones. Screw decompression (screw suck back) and reduction in nozzle temperature can be used to control drooling into the mold. However, it is necessary to identify and correct the cause of drooling. 

Some of the causes of drooling are too high melt temperature, wet resin, long holdup time, polymer degradation, poor nozzle temperature, high back pressure, and poor nozzle design. 

To prevent drool and nozzle freeze-off, good nozzle temperature should be maintained. The heater band is provided and should be as far forward as possible on the nozzle and should not touch the platen. Nozzle can also be heated by heat pipes. Nozzle designs have been successfully used, with the radius of the nozzle tip being less than the radius of the sprue bushing. 

---