Five Common Problems And Countermeasures in Plastic Granulation Processing

Black spot

★ The quality of the raw materials themselves is poor, and there are too many black spots;

★ Local overheating or excessive shearing of the screw may cause material carbonization to worsen;

★ The head pressure is too high, there is too much reflux material, and the carbonization of the material is aggravated;

★ The service life of the machine is relatively long, the gap between the screw and the barrel increases, and the adhesion of carbonized substances to the barrel wall increases;

★ If the natural exhaust port and vacuum exhaust port are not cleaned for a long time, the accumulation of carbonized substances will increase;

★ Other impurities mixed in due to external environment or human factors;

★ The mouth mold is not cleaned thoroughly;

★ The discharge port is not smooth enough, and materials may accumulate for a long time and be gradually carbonized;

★ Some threaded components are damaged, resulting in increased carbonization of materials at dead corners.

 

Broken bar

1. Causes of external impurities

★ It should be checked whether the dead corners of the equipment in the mixing and discharging processes are cleaned and whether there are impurities mixed in;

★ Try to add as little crushed material as possible or manually screen the crushed material to remove impurities;

★ Increase the number of filter mesh and sheets;

★ Try to cover any holes (solid or mesh) that may have debris falling. 

2. Causes of internal impurities

Mainly due to severe carbonization caused by processes or equipment, the carbonized material is carried out into the material strip, causing breakage under traction.

3. Poor plasticization of materials

★ The extrusion temperature is too low or the screw shear is too weak, the material is not fully plasticized, resulting in material lumps and breakage under the traction force.

★ Low melting point additives (including EBS or PETS, etc.) in the formula system can cause poor plasticization and bar breakage under the premise of weak screw shear, increased gap between screw and barrel, and weak shear.

4. Differences or changes in raw material properties

★ Blending components have significant differences in flowability at the same temperature, and due to mismatched flowability or incomplete compatibility (including physical entanglement and chemical reactions), this is theoretically called "phase separation". Phase separation generally does not occur during blend extrusion, but more often during injection molding. However, if the MFR difference is too large, it may lead to breakage under the premise of relatively weak screw shear.

★ Change in viscosity of blending components: For the same material, if MFR decreases, hardness, rigidity, and notch increase, it is possible that the molecular weight of the batch of materials may be slightly larger than before, resulting in increased viscosity. Under the original processing temperature and process, poor plasticization can be caused. At this time, increasing the extrusion temperature or reducing the main screw speed can solve the problem.

5. Poor exhaust

★ When the processing temperature is too high or the screw is locally sheared too strongly or the screw is locally overheated, it causes the decomposition of certain flame retardants and other additives, releasing gas. The vacuum does not extract the gas in a timely manner, and the gas is trapped in the material bar, causing the bar to break under the traction force.

★ The material is severely affected by moisture, and the processing water vapor is not timely discharged through natural exhaust and vacuum, resulting in being trapped in the material strip and causing it to break under the traction force.

★ Natural or vacuum exhaust is not smooth (including blockage, air leakage, high gasket), causing air (or steam) to be trapped in the material strip and causing strip breakage under traction.

6. Material rigidity too high, water too cold or too much water, traction mismatch

The rigidity of the material is too high, the water temperature is too low, and there is too much excess water. The discharge of the machine head is very soft, but the excess water immediately becomes very hard, causing breakage under the action of mismatched traction. This phenomenon often occurs in materials with very fast crystallization speed or high rigidity, such as PBT or PET fiber addition, PC fiber addition, AS fiber addition, ABS fiber addition, etc., especially when conducting experiments on small machines. At this time, increasing the water temperature and reducing the water flow rate can help maintain a certain degree of softness of the material entering the pelletizer, which can solve the problem.

 

Hollow

1. Poor exhaust

Poor exhaust (or vapor), caused by natural or vacuum exhaust (which may result in excessive moisture content in the material itself, decomposition of flame retardants and other additives, vacuum blockage, small or leaking vacuum, or high vacuum gasket placement), can result in the presence of gas (vapor) in the particles, forming a hollow space.

2. Poor plasticization

★The processing temperature is too low, the material is not fully plasticized, and the light ones (small holes) cause hollow particles, while the heavy ones (large holes) cause broken bars.

★Too many low melting point additives (including EBS or PETS) can lead to poor plasticization and the formation of voids when the gap between the screw and the barrel increases or the screw shear is weak.

★ Water temperature too low: When the cooling water temperature is too low, the material contracts when encountering water, causing shrinkage holes, such as in PP products - this phenomenon is mainly aimed at crystalline plastics; In general, crystalline plastics (such as PP, PA, PBT, etc.) should use low water temperature, while non crystalline plastics (such as ABS, PC/ABS, HIPS, etc.) should use high water temperature.

 

Venting

1. Natural exhaust outlet discharge

★ If the feeding speed does not match the main engine speed, appropriately reduce the feeding speed or increase the main engine speed;

★ The temperature in the area from the feeding section to the natural exhaust port is too low, and the material is not plasticized, resulting in the extrusion of raw materials by the screw;

★ The temperature near the natural exhaust port is too high, causing a serious decrease in material viscosity. The screw in this section slips, and the material cannot be transported to the front section in a timely manner. Under the subsequent extrusion of the material flow, it causes clinker to emerge;

★ The position of the natural exhaust port of the screw does not match the position of the natural exhaust port of the barrel, resulting in material leakage;

★ Without the installation of reverse conveying components or reverse meshing blocks, the pressure of the natural exhaust outlet screw groove cannot be reduced, which may cause material leakage under the subsequent squeezing effect of the material flow.

2. Vacuum discharge

★ The vacuum pumping force is too large, which sucks the material into the vacuum pipeline, causing material leakage;

★ The vacuum position of the screw is not equipped with reverse conveying components or reverse meshing blocks, which cannot reduce the pressure of the screw groove in the vacuum section, causing material leakage under the vacuum pumping effect;

★ The temperature in the vacuum section is too high, causing a serious decrease in material viscosity. The screw in this section slips, and the material cannot be transported to the front section in a timely manner, resulting in material leakage under the action of vacuum pumping force;

★ The extrusion processing temperature is too low, the material is not plasticized, or additives such as flame retardants are not fully dispersed in the resin, resulting in material leakage under the action of vacuum suction;

★ The screw combination is also reasonable, the position is also matched, and the temperature, host and feeding speed are also matched. On the premise of low vacuum gasket, under the action of material flow extrusion and vacuum suction, material leakage occurs;

★ Excessive head pressure (reasons include: blocked die, too many filter screens, low head temperature, etc.), resulting in increased reflux and material leakage under the action of vacuum extraction.

 

Bridging

1. Hopper bridging

★ Too many fillers, moisture absorption, and agglomeration can increase the friction between the mixture and the hopper wall. Adding liquid external lubricants such as "white oil" can reduce the friction between the mixture and the hopper wall, as well as between the mixture;

★ Mixture agglomeration (including high-temperature agglomeration and agglomeration caused by excessive addition of liquid additives), reducing the drying temperature or high mixing time, reducing the amount of liquid additives added, and adding powdered resin or additives that have an absorption effect on "oily" substances (such as high adhesive powder, AS powder, PP powder, etc.) can solve the problem.

2. Cutting and bridging

The main reason is that the melting point of some processing aids is too low, and the processing temperatures in Zone 1 and Zone 2 are too high. The materials soften in the feeding silo and adhere to the equipment wall, causing difficulties in subsequent feeding. Reducing the processing temperatures in Zone 1 and Zone 2 appropriately can solve this problem.

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