(57) Salivation Injection Molding Process Defect Handling Manual (65 types)

1. What is Salivation (Appearance)
Refers to the phenomenon of resin leaking from the nozzle of the molding machine.
Generally, the resin at the nozzle tip of an injection molding machine is not fully cured. When the internal pressure of the barrel is high or the resin viscosity is low, the melted resin will leak out.
Salivation occurs when the resin viscosity is low or the internal pressure of the molding machine barrel is high.
2. Causes of salivation
(2-1) Low resin viscosity
Most injection molding machines use open nozzles and are conditioned to prevent outflow. However, if the molecular weight decreases due to decomposition, or if the set temperature of the barrel is set high, the resin will flow out due to the reduced viscosity.
(2-2) High internal pressure in the barrel
There are two reasons for applying a certain pressure to the resin in the barrel: one is the expansion of the gas, and the other is the back pressure during metering.
The reason for gas expansion is the decomposition gas of the resin and the moisture in the particles. After they vaporize and expand, the pressure that has nowhere to go flows to the front end of the nozzle, forming saliva.
On the other hand, in terms of back pressure, it is necessary to apply a certain amount of back pressure because it is needed to prevent air from getting involved in metering. However, if applied excessively, the resin will be compressed into a compressed state, resulting in increased internal pressure and salivation.
In addition, air can also expand due to heating, so if a large amount of air is involved during metering, salivation will become more severe.
3. Countermeasures for Salivation
(3-1) Increase resin viscosity
Set the barrel temperature, especially the nozzle temperature, slightly lower. Setting it too low can affect fluidity and produce cold materials (mixed with insufficiently melted resin), which can lead to poor appearance. Therefore, it is recommended to use the minimum limit within the recommended temperature range. In addition, changing the material grade to a higher viscosity material is also an effective method.
(3-2) Reducing internal pressure
First, reduce the screw speed and back pressure. However, if the back pressure is 0, it is easy to get involved in the gas and may cause other molding defects. Therefore, a minimum back pressure of about 0.2 Mpa should also be applied. The screw speed should be set at about 100 rpm.
In the sense of preventing gas mixing, the following effective measures can be taken: (1) Strengthen drying, and (2) Do not raise the temperature of the barrel too high. In addition, increasing the amount of plastic drawn is also effective. Extrusion can cause the screw to recede, thereby reducing resin overall. This creates a gap at the front to gain time before salivation occurs.
(3-3) Change nozzle shape
Due to the simplicity of specifications and cost considerations, most molding machines use open nozzles. For materials with serious salivation, using closed nozzles is also a solution. In addition, the smaller the aperture of the nozzle, the more difficult it is to form salivation. Many molding machine manufacturers have nozzles with different apertures and shapes. The price is mostly below 100000 yen, which is worth buying. However, due to the low fluidity of the material, the premise is that there should be a certain degree of leeway for molding conditions