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Injection Molding Problems and Solutions
The injection molding process is basically a straightforward process involving injecting molten material into a defined shape and letting it cool until it solidifies. However, resolving a problem in
injection molding can make this simple process more complex to understand.
The injection molding process is influenced by three factors: pressure, temperature, time. Any changes in one leg of the triangle affects the others. For example, an increase in cooling time can
affect the temperature of the part or the temperature of material as it sits in the barrel of the machine. Also, a change in the melt temperature of the material can effect the pressure needed to fill the cavity of mold. In this section we review common problem found in injection molding as it pertains to the cause and to how to resolve these problems.
8.1Nozzle 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. Tab. 8.1 lists causes and solutions for nozzle drool.
Tab8.1 Nozzle drool
| Causes | Solutions |
1. The presence of moisture causes a reduction in the molecular weight of the material and promotes drooling as well | 1. Drying the material according to recommended times and temperatures is needed to prevent drooling in this case |
2. 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 | 2. 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 |
| 3. Location of the nozzle heater too close to the nozzle orifice can cause drool | 3. 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 |
| 4. High back pressures can create drooling due to excessive working of the material, increasing the melt temperature | 4. 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 |
| 5. In the case of hot runner molds, high manifold, gate, or hot tip settings can cause drooling to occur away from the nozzle in the tool | 5. Lowering the temperatures in one or all of these areas will decrease the chance of drool |
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Tuofang company is specialized in designing and manufacturing Plastic Injection.
No. 345 Xinjiang Road, Xinqian Street of Huangyan, Taizhou City, 318020, Zhejiang Province, China
