Cause analysis and solutions for black spots and breakage of waste plastic strip granulating.

Recycled materials are often mixed with various impurities, inconsistent components, containing different kinds of auxiliaries and so on because of different sources of raw materials. Many problems will arise in the process, such as products with black spots, granulation easy to break bars, granules with bubbles, cutting strip tail wire. How to solve these problems that plagued the recycling industry for a long time? Today we will give you a detailed analysis.

Black spot problem

Black spot problem is the most common problem in recycling production, for light-colored products, because of the existence of black spots, affecting its use. We do not discuss the problem of black spots of raw materials themselves, in addition to the raw materials themselves, the general causes of product black spots are as follows.

1. Part of Screw overheating and shearing too strong

Overheating and shearing of the screw will cause excessive local temperature of the material, which will lead to carbonization of the material, and the carbide will be brought into the material strip to form a black spot.

2. The extruder head pressure is too high (including blockage, filter screen, low head temperature).

If the pressure of the head is high, the material reflux of the head will be aggravated. If it is serious, it will cause carbonization of the material of the head and form black spots.

3. The machine is old, and the gap between screw and sleeve increases.

The gap between the screw and the sleeve will be enlarged when the machine is used for a long time, and a large number of materials will remain in the gap for a long time. Carbonization will occur and black spots will be formed.

4. Natural exhaust port and vacuum vent are not cleaned for a long time.

In the production process, the exhaust hole pair gradually forms the material accumulation, the accumulated material will be slowly carbonized, in the subsequent extrusion granulation is taken to the material bar, forming a black spot.

5. Die (including outlet and internal dead angle) is not clean.

If the outlet is not smooth enough, there are shallow grooves and pits, as well as some of the original thread damage (notch angle, wear and so on to form a dead angle) will cause material accumulation and then carbonization, forming black spots.

6. The external environment or human caused impurities mixed.

Attention should be paid to the production environment and operation procedures to avoid mixing foreign materials into raw materials and finished products.

Strip broken problem

 1. Filter reasons

Too much mesh or large quantity of material will cause resistance to material transportation and cause breakage. At this time, the main engine should be lowered or the feed speed should be increased.

2. The material contains impurities.

External impurities: Check whether there are impurities in the dead corner of the mixing and discharging equipment, and add a filter screen to remove the impurities mixed in the material.

Internal impurities: too high die pressure (including die blockage, too many filters, too low head temperature, etc.), will cause increased material reflux and increase material carbonization, extruder partial overheating, screw shear too strong will also cause material carbonization caused by broken bar.

3 . Long service life of machine

Extruder service life, screw and barrel gap will increase due to wear and tear; vacuum or natural exhaust outlets (including gaskets and dead corners here) for a long time is not clear; die (here including the outlet and the head internal dead corner) is not cleaned up; these can cause material carbonization or cause material to stay for a long time and lead to The factors of carbonization will lead to the occurrence of granulation and broken bars.

4 . The time interval for replacing the filter is too long.

If the filter screen is replaced too long, the material residue is cooled and solidified to block the filter screen, it will also cause broken bars.

5. Bad  plasticization of materials

If the extrusion temperature is too low or the screw shear is too weak, the materials and low melting point additives are not fully plasticized, and the broken bars will occur at the plasticized materials when the extrusion granulation is carried out.

6. Reasons for physical properties of raw materials

When the fluidity of the blend components is different at the same temperature, the viscosity of the two components is different, resulting in broken bars, which can be solved by increasing the processing temperature and the main engine speed.

7. The material bar is difficult to steam or exhaust.

Natural exhaust or vacuum exhaust not smooth (including blockage, leakage, gasket too high), resulting in gas (or steam) trapped in the material, under the action of traction, in the gas accumulation will cause broken.

Because of the complex components of the recycled materials, there will be a variety of additives in the products, sometimes there will be decomposition of additives, and often there will be decomposition and degradation of additives to release gas, if these gases are not discharged in time, wrapped in the material during extrusion is also easy to cause broken bars.

The material is seriously damped. If the water vapor can not be discharged in time during processing, the vapor body will be trapped in the material bar, and the broken bar will also be caused when granulating.

8. Material is rigid, water-cooled or over draught, and traction mismatch.

Material rigidity is too big, water temperature is too low, too much water, traction will be due to stress concentration, material slice fracture.

Material just out of the head is very soft, for rigid material, water immediately becomes very hard, material from soft to hard process, if the traction is not appropriate, will cause broken bars - this phenomenon often occurs in PBT or PET plus glass fiber, PC, PS, AS plus glass fiber, ABS plus glass fiber crystallization speed is very fast or very rigid. When the material is processed, especially when the experiment is done on a small machine, the problem can be solved by raising the water temperature, reducing the excess water and keeping the sliver into the granulator moderately soft.