Five Common Problems And Solutions in Plastic Granulation

01 Black Dots

★ The quality of the raw materials themselves is poor, with many black spots;

★ Local overheating or excessive shearing of the screw can 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 on 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 mold mouth is not cleaned thoroughly;

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

Part of the threaded components are damaged, resulting in increased carbonization of materials at dead corners.

02 Broken Bar

(1) External impurity reasons

★ Check whether the dead corners of the equipment at each stage of mixing and discharging are clean and whether there are impurities mixed in;

★ Try to add less crushed materials or manually screen the crushed materials to remove impurities;

★ Increase the number of filter mesh and sheets;

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

(2) Internal impurity reasons

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 material plasticization

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

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

(4) Differences or changes in raw material properties

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

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

(5) Poor exhaust

★ If the processing temperature is too high, or the local shear of the screw is too strong, or the screw is locally overheated, it may cause the decomposition of certain flame retardants and other additives, releasing gas. If the vacuum does not extract the gas in time, the gas will be trapped in the material strip, causing it 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, leakage, and 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 material rigidity 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. Under the action of mismatched traction force, it causes breakage of the bar. This phenomenon often occurs in materials with very fast crystallization speed or high rigidity, such as PBT or PET fiber, PC fiber, AS fiber, ABS fiber, etc. Especially when conducting experiments on small machines, it is more serious. At this time, increasing the water temperature and reducing the water flow rate can help maintain a certain degree of flexibility of the material entering the pelletizer, which can be solved.

03 Hollow

(1) Poor exhaust

Poor exhaust (or vapor) can result in the presence of gas (or vapor) in particles due to poor natural or vacuum exhaust (which may be due to the material's own moisture being too heavy, decomposition of flame retardants and other additives, vacuum blockage, vacuum being too small or leaking, or vacuum gaskets being too high), leading to the formation of hollow spaces.

(2) Poor plasticization

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

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

★ Too low water temperature: When the cooling water temperature is too low, the material shrinks when it comes into contact with water, causing shrinkage holes. For example, when making PP products, this phenomenon mainly targets crystalline plastics; In general, crystalline plastics (such as PP, PA, PBT, etc.) should use low water temperature, while amorphous plastics (such as ABS, PC/ABS, HIPS, etc.) should use high water temperature.

04 Material Overflow

(1) Natural exhaust outlet discharge

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

★The temperature in the area from the feeding section to the natural exhaust port is too low, causing the material to not plasticize and resulting in raw material extrusion under the action of 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 squeezing effect of the material flow, it causes the emergence of clinker;

★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 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 subsequent material flow compression.

(2) Vacuum discharge

★ The vacuum suction force is too large, which sucks the material into the vacuum pipeline and causes 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 suction 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 vacuum pumping force;

★ The screw combination is also reasonable, the position is matched, and the temperature, as well as the main engine and feeding speed are matched. However, the vacuum gasket is too low, which causes material leakage under the squeezing of material flow and vacuum suction force;

★ Excessive head pressure (reasons include: clogged mouth mold, too many filter screens, too low head temperature, etc.) causes an increase in reflux, and under the action of vacuum suction, it leads to material overflow.

05 Bridge Construction

(1) Hopper bridging

★Too much filler, 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, which can solve the problem;

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

(2) Cutting and bridging

★Mainly due to the low melting point of certain processing aids, the processing temperatures in Zone 1 and Zone 2 are too high, causing the materials to soften in the feeding bin and adhere to the equipment wall, resulting in difficulties in subsequent feeding. Lowering the processing temperatures in Zone 1 and Zone 2 appropriately can solve this problem.

NANJING HAISI is professional manufacturer of Plastic Granulation, we supply Twin Screw Extruder, Single Screw Extruder, Plastic Recycling Extruder, Two Stage Extruder, Lab Scale Machinery, Mixing and Feeding Machine, Cooling and Pelletizing Machine, Crushing Machine and so on.