Nozzle Drool

At the nozzle of the barrel, molten material can flow continuously, especially during part ejection. This can be noticed inside between two open mold platens. Once in contact with the cooler mold surface, this continuously flowing material cools and solidifies. As the mold closes on this cooled material, this solidified material can be caught in the closed mold and cause mold damage. This process of continuously flowing material out of nozzle is referred to as drool. Drooling can be found not only at the nozzle but can also occur at the sprue bushing, hot tip, or at the gate. Here are some causes and solutions below for Nozzle Drool.

The presence of moisture causes a reduction in the molecular weight of the material and promotes drooling as well. Solutions: Drying the material according to recommended times and temperatures is needed to prevent drooling in this case.

The excessive melt temperature of the material is another cause of drool. Barrel temperatures set too high will increase the melt temperature of the material. Solutions: Lowering the barrel temperatures starting with the middle zones, will lower the material viscosity and reduce drool. Lowering the setting of the nozzle temperature will also reduce drool.

Location of the nozzle heater too close to the nozzle orifice can cause drool. Solutions: Relocating the nozzle heater farther back on the nozzle will reduce the temperature at the tip and orifice of the screw. Nozzle shutoff valves have been used as another option to prevent material from drooling between mold halves.

High back pressures can create drooling due to excessive working of the material, increasing the melt temperature. Solutions: Lowering back pressures can eliminate or reduce drool. The use of decompression or suck back forces the melt back into the nozzle and can be used to prevent nozzle drool.
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Brittleness of plastic injection parts

Plasticinjection part brittleness is caused most often by a loss of molecular weight in the polymer material. A reduction in molecular weight results in a reduction of the mechanical properties of the material, such as tensile strength, elongation, and impact strength. Here are some causes and solutions below for Brittleness of plastic injection parts.

Brittleness from material degradation can be caused if materials are not dried sufficiently. This applies specifically to hygroscopic materials. In this case, a chemical reaction called hydrolysis takes place when materials are melted with moisture present. When hydrolysis occurs, a loss in molecular weight results in a reduction in physical properties.

Excessive use of regrind can cause part brittleness since regrind has already been exposed to heat and as a result suffers a loss in molecular weight.

Melt temperatures may be higher than recommended process temperatures. In this case, melt temperatures at the middle of the recommended melt temperature range should be used.

The molecular weight reduction is a result of material sitting in the barrel for long periods of time, referred to as residence time. The BSR should fall between 30 and 65%. If this falls well below 30%, this will indicate a residence time problem that can lead to brittleness.

High screw speeds and back pressure can overwork the material, resulting in an increase in melt temperature.

Solutions: 1.The barrel-to-shot ratio (BSR) is measurement, in terms of percentage, used in determining residence time.  2. A reduction in nozzle temperature will reduce melt temperature avoiding brittleness form occurring. 3. Reducing screw speed and lowering back pressure can prevent resin melt temperature. 4.Increasing wall thickness, rib designs, avoidance of sharp corners, and addition of radii can reduce the chance of brittle parts.
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