Common problems and solutions in waste plastic granulating cutting and vacuum pumping

Common problems and solutions in waste plastic granulating cutting and vacuum pumping

1.      Particle adhesion problem

A series of granules that are adhered to each other are usually referred to as double or granular chains.

The reason may be that the temperature of processing water is too high or the velocity of water is too low.

During processing, several technological problems may cause this phenomenon alone or jointly. For example, the processing water is too hot is one of the reasons for the particles, in this case, the water temperature should be lowered to give the surface of the particles enough to quench; in addition, the flow rate is too low is also a cause of the particles connected, it will lead to particle cutting chamber speed slowdown, and then particle agglomeration. In addition, if the hole distance of the die is too close, the expansion of the die outlet will cause particle contact during the machining process. The solution is to replace the existing die with a large space and a small number of holes.

 2. Trailing problem

The so-called trailing is that the edges of the particles are somewhat prominent, just like the shape of a broom tail. The reason is that the cutting device has not been able to carry out crisp cutting.

The solution for the wire cutting line is to replace the hob and the bottom cutter to provide a new and sharp cutting edge, or to redefine the distance between the machines according to the manufacturer's manual values. For underwater grain cutters, the templates and blades need to be inspected to ensure that there are no notches, as notches and grooves often cause tailing.

3.Powder problem

For many crystalline materials, such as general purpose polystyrene, pelleting often produces scrap, which will add a lot of trouble to the downstream use. This problem can be generally solved by adjusting the process parameters of the equipment. When the extruded material enters the cutting tool, the temperature of the line should be slightly lower or close to the Vicat softening point to ensure that the material is as hot as possible, so as to avoid rupture.

For a particular polymer, choosing a hob with an appropriate cutting angle also has a significant effect in reducing the end of the material. For unfilled polymers, Stellite or tool steel hobs should be used as much as possible and the edges of hobs and base knives should be kept sharp to avoid breaking the polymer. For subsequent equipment after granulating, pressure or vacuum equipment should be avoided.

For underwater pellet line, it is necessary to ensure that enough knife pressure against die surface is maintained during the processing, and the residence time after pellet cutting is properly adjusted to ensure that the particles are hot when entering the dryer.

4. Material Strip drift problem

Material strip drift is a phenomenon of wire bunching to one side on the feeding platform. It will lead to the deterioration of material quality, the existence of slender bars and processing disorders. If the cutting plane of the pellet cutter is not parallel to the extruder die, then the wire will appear to the left or right side of the trend of crowding, eventually leading to the bar drift. In addition, the gap between the lower feed roller and the scraper is not constant, and the diameter of the lower feed roller is not consistent.

 5. Granules hollow problem

1、 Bad exhaust

Exhaust (or steam) is not smooth: due to natural exhaust or vacuum exhaust is not smooth (may be the material itself water vapor is too heavy, may also be flame retardants and other additives decomposition, and may be too small vacuum or vacuum leak or vacuum gasket cushion too high reasons), resulting in the existence of gas (steam) particles, forming hollow materials.

 2、Poor plasticization

Processing temperature is low, the material is not completely plasticized, light (small holes) caused by hollow particles, heavy (large holes) caused by broken bars.

Too many low melting point auxiliaries (including EBS or PETS) can form hollow materials due to poor plasticization of materials when the screw shear is weak.

3、The water temperature is too low

Cooling water temperature is too low, material shrinkage in water, resulting in shrinkage holes, such as PP products - this phenomenon is mainly for crystalline plastics; in general, crystalline plastics (such as PP, PA, PBT, etc.) should be used low water temperature, non-crystalline plastics (such as ABS, PC / ABS, HIPS, etc.) using high water temperature.

6. The problem of material grain flat

The problem of flat grain formation indicates shrinkage voids, which is caused by improper rebar shrinkage.

Shrinkage porosity and hollow pellets indicate that the tempering of the strips is not appropriate. When the shrinkage gap is slight, it may be just a small hole in the end face of the particle, but when it is serious, it may produce hollow particles. This phenomenon occurs when the core temperature of the bar is close to the melting state, while the cooling water temperature is low, the outer layer of the bar is cooled sharply, resulting in a hard shell, leaving the heat in the material. The core. In addition, the strip does not have enough soaking time in air or water, resulting in the core of the wire can not transfer heat to the surface of the wire, which can not be a good cross-section cooling will also cause this phenomenon.

Due to trapped volatiles in the melt, shrinkage voids also occur in the particles produced by underwater granulation. An effective preventive measure is to check the vacuum holes in the extruder.

7. Natural vent and vacuum outlet

Natural vent material comes out:

The feeding speed is not matched with the main engine speed, and the speed of the main material is reduced or the speed of the main engine is increased.

The temperature from the feeding section to the natural exhaust port is too low, and the material is not plasticized. Under the action of screw extrusion, the raw material comes out.

The temperature near the natural exhaust port is too high, and the material viscosity is seriously reduced. The screw of this section slips, and the material can not be transported to the front section in time. Under the action of subsequent material flow extrusion, the clinker comes out.

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 the emergence of raw materials. Generally, there is no reverse conveying element or reverse meshing block, which can not reduce the pressure of natural vent groove, resulting in the emergence of raw materials under the action of subsequent material flow extrusion.

Raw materials come out of the vacuum port.

The vacuum suction is too large to suck the material into the vacuum pipeline, causing the raw materials to come out.

The screw vacuum position has no reverse conveying element or reverse meshing block, so the screw groove pressure in the vacuum section can not be reduced. Under the action of vacuum pumping, the raw material comes out.

The temperature of vacuum section is too high, and the viscosity of material is seriously decreased. The screw of this section slips, and the material can not be transported to the front section in time. Under the action of vacuum pumping, the raw material comes out.

The extrusion temperature is too low, the materials are not plasticized or flame retardants and other additives are not fully dispersed in the resin, resulting in the emergence of raw materials under the action of vacuum pumping.

The screw assembly is also reasonable, the position is also matched, the temperature and the main engine and feed speed are matched under the premise of low vacuum gasket, in the material flow extrusion and vacuum pumping force, resulting in the emergence of raw materials.

Head pressure is too high (including: die blockage, too many filters, head temperature is too low, etc.), resulting in increased reflux, under the action of vacuum pumping, resulting in the emergence of raw materials.

8. The material of hopper is stacked to form a bridge.

Too many fillers, moisture absorption, agglomeration, resulting in increased friction between mixture and hopper wall, adding "white oil" and other liquid lubricants, reducing the friction between mixture and hopper wall and mixture, can solve this problem.

Mixture agglomeration (including high temperature agglomeration and liquid additives added too much and agglomeration), reducing drying temperature or high mixing time, reducing the amount of liquid additives, adding powdered resins or additives (such as high rubber powder, AS powder, PP powder, etc.) which have absorption effect on "oily" materials, can solve this problem.

Sometimes the problem of bridging can also occur at the discharge port, which is mainly due to the low melting point of some processing aids, the high processing temperature in the first and second zones, the softening of materials at the bottom of the feeding bin, and the adhesion to the equipment wall, resulting in difficulties in subsequent feeding, and the proper reduction of the processing temperature in the first and second zones.