Common problems and solutions of plastic granulation processing

black spots

1. The quality of the raw materials themselves is poor, and there are many black spots;
2. The screw is overheated locally or the shear is too strong, resulting in aggravation of the carbonization of the material;
3. The pressure of the machine head is too large, there is too much reflux material, and the carbonization of the material is aggravated;
4. The service life of the machine is long, the gap between the screw and the barrel increases, and the adhesion of carbonide on the barrel wall increases;
5. If the natural exhaust port and vacuum exhaust port are not cleaned for a long time, the accumulated carbonization increases;
6. Some of the original thread parts are damaged, resulting in the carbonization of the material at the dead corner;
7. The mold mouth is not cleaned up;
8. The discharge port is not smooth enough, and materials may accumulate for a long time and are gradually carbonized;
9. Other impurities mixed in by the external environment or man-made;

Broken strips

External impurity causes

1. Check whether the dead corners of the equipment in each link of mixing and discharging are cleaned up and whether there are impurities mixed in;
2. Add crushed material as little as possible or manually screen the crushed material to remove impurities;
3. Increase the number of mesh and sheets of the filter;
4. Try to cover holes (solid cover or net cover) that may fall from debris.

Internal impurity causes

It is mainly caused by the process or equipment caused by serious carbonization, and the carbonized matter is brought out into the strip, causing the broken strip under the action of traction.

Poor plasticization of materials

The extrusion temperature is low or the screw shear is too weak, the material is not fully plasticized, the pimple appears, and the broken strip is caused under the action of traction force, and the low melting point additives (including EBS or PETS, etc.) in the formula system will also cause poor plasticization and break the strip under the premise of weak screw shear or increased gap between the screw and the barrel and weak shear.

Differences or changes in the physical properties of raw materials

1. The fluidity difference between blended components at the same temperature is too large, due to the mismatch or incomplete compatibility of fluidity (including physical entanglement and chemical reactions), theoretically speaking, this is called “phase separation”. “Phase separation” generally does not occur in blending extrusion, and more often occurs in the injection molding process, but if the MFR phase difference is too large, there may be broken strips under the premise that the screw is relatively weak shear.
2. Change of viscosity of blended components: For the same material, if the MFR decreases, the hardness, rigidity and notch become larger, it is possible that the molecular weight of the batch is larger than before, resulting in a larger viscosity, resulting in poor plasticization under the original processing temperature and process, at this time increasing the extrusion temperature or reducing the screw speed of the host can be solved.

Poor exhaust

1. The processing temperature is too high or the local shear of the screw is too strong or the screw is locally overheated, resulting in the decomposition of some flame retardants and other additives, releasing gas, the vacuum does not pump the gas out in time, the gas is trapped in the strip, and the strip is broken under the action of traction.
2. The material is seriously damp, and the processed water vapor is not removed by natural exhaust and vacuum in time, trapped in the material strip, and caused by the broken strip under the action of traction.
3. Natural exhaust or vacuum exhaust is not smooth (including blockage, air leakage, gasket is too high, etc.), causing gas (or steam) trapped in the material strip, causing the strip to break under the action of traction.

The material is too rigid, the water is too cold or too much water, and the traction does not match

The material rigidity is too large, the water temperature is too low, there is too much water, the discharge of the machine head is very soft, and the water immediately becomes very hard, under the action of the traction force mismatch, resulting in a broken strip. This phenomenon often appears in PBT or PET+ fiber, PC+ fiber, AS+ fiber, ABS+ fiber, etc. Very fast crystallization speed or very rigid materials, especially when the small machine is more serious, at this time increase the water temperature, reduce the amount of water, so that the strips entering the pelletizer maintain a certain softness, can be solved.

Hollowing

Poor exhaust

Due to natural exhaust or poor vacuum exhaust (the material itself may be too heavy in water vapor, it may also be decomposed by additives such as flame retardants, and there may be vacuum blockage or vacuum is too small or leakage or vacuum gasket pad is too high), resulting in the presence of gas in the particles, forming a hollow.

Poor plasticization

1. The processing temperature is low, the material is not completely plasticized, the light (small hole) causes the particle hollow, and the heavy (large hole) causes the broken strip.
2. There are too many low melting point additives (including EBS or PETS), which cause poor plasticization and hollow under the premise that the gap between the screw and the barrel increases, or the screw shear is weak.
3. The water temperature is too low: the cooling water temperature is too low, and the material shrinks when exposed to water, resulting in shrinkage holes, such as PP products – such phenomena are mainly for crystalline plastics; Under normal circumstances, crystalline plastics (such as PP, PA, PBT, etc.) should use low water temperature, and amorphous plastics (such as ABS, PC/ABS, HIPS, etc.) should use high water temperature.