Service Life of Plasticized Components of Injection Molding Machine

Brief introduction of Plasticized Components

Plasticized components includes screw, melt barrel, rubber head, rubber ring, rubber gasket.

When the injection molding machine is working, the plasticized components are like the wheels of a motor vehicle. So when the machine start working, the components will begin to subject to pressure, impact, friction, and extrusion.

Therefore, to ensure that the injection molding machine is always in good condition, it is necessary to clarify the performance of the machine. Besides, we should understand the the performance of machine and performance and quality of the plastic materials. And we should strengthen the management of the use of the injection molding machine to reduce the failure rate. Further, reduce maintenance costs and extend the service life.

Factors affecting the service life of the plasticizing system of the injection molding machine

Generally speaking, the factors affecting the service life of the plasticizing system of the injection molding machine are as follows:

1. Mechanical wear

Some modified plastics are added with minerals, glass fiber, metal powder, etc. However, these materials will cause uniform mechanical wear on the screw, three small parts and the melt barrel during the plasticization and injection processing. The nitride layer and chromium plating layer will be polished. After abrasion, the gap between the screw and the melt barrel is increased, which reduces the plasticizing effect. What’s more, it increases the leakage of the injection, reduces the injection efficiency and the processing accuracy.

Therefore, in order to reduce wear as much as possible and prolong the service life of plasticized components, we should appropriately increase the temperature during processing. And we should reduce the screw speed. Choosing chrome plating or adopting bimetal solution can more effectively prevent abrasion.

2. Mechanical fatigue and overload work

The adjusters habitually set the work at low-temperature, high-speed and high-pressure. And that gradually deteriorates the performance of plasticized components. For example, when processing PC and PA plastics, when the temperature does not meet the requirements, the viscosity of the plastic is very large. If forcing the sol action at this time, the sol pressure and the sol torque must be increased. Thus, it increases the stress fatigue of the screw.

At the same time, the viscosity of the plastic melt is very high at this time. So we should increase injection pressure and injection speed. Then, that will increase the impact and load of the three small parts and accelerate their wear and stress fracture.

3. Human factors (including operational errors or illegal operations, etc.)

1)  When metal impurities are mixed in the plastic and enter the melt barrel together, due to the extrusion effect, the screw edges, grooves, rubber rings, rubber gaskets of the screw will be worn to different degrees. Then, it results in unstable injection molding, black spots and black streaks.

2) Artificially add wrong plastic and add high-temperature plastic to the melt barrel set to low temperature. Then, it results in excessive torque of the screw during the sol, causing stress fatigue of the screw.

3) Cold start is a kind of impatient and irresponsible work attitude. When the temperature of the sol cylinder has not reached the set required temperature or has just reached, the residual material in the barrel absorbs the heat from the heating ring to make the temperature higher, while the temperature of the inner layer is still very low. Therefore, during the cold start, the screw torque is very large, causing stress fatigue on the screw. In severe cases, the screw will be twisted quickly, and the rubber head and rubber ring will be twisted. Most of the metal impurities entering the sol cylinder are brought in with the crushed material.

Therefore, we should check the breakage of the blade of the crusher frequently. Besides, we should replace the blade immediately if we find it’s worn. On the other hand, check and clean the magnets in the blanking hopper frequently. When the metal scraps adsorbed around the magnet are saturated, the adsorption force for the iron scraps on the outer layer will be weakened. Even if it is attracted, it will easily be washed away by the continuously flowing plastic and enter the sol cylinder together.

4. Correctly assemble, debug and replace parts

This aspect is also very important. If the assembly of the sol barrel is not tight enough, the screw will touch the sol barrel when the sol or glue is injected. That will cause the screw or the sol barrel to wear. Therefore, we should check the technical status of the equipment regularly. What’s more, we should pay attention to the abnormal phenomenon of the parts during processing.

5. Damage caused by improper workmanship

1) Long-term use of high back pressure sol can accelerate the wear of the three small plasticized parts. This situation generally occurs when using toner. Since the toner is difficult to disperse, we can increase the back pressure.

2) For high-viscosity plastics, fast sol can cause stress fatigue on the screw.

3) For high-temperature plastics, especially glass fiber plastics, high-speed sol should not be used.

6. Chemical corrosion

The corroded metal material is iron. Common corrosive plastics are: flame retardant plastics, acid plastics, PVC plastics, etc. After the screw, melt barrel and flange are corroded, some pits are formed on the surface. Besides, the surface is rough. And that makes the flow resistance of the melt increase when the injection molding machine works. Some materials easily adhere to the surface, causing decomposition and carbonization. Severe corrosion will increase the gap between the screw and the melt barrel. What’s more, it will increase the leakage and reduce the injection efficiency.

Whether it is a flame-retardant plastic or an acid glue, the plastic will decompose acid gas when processed at high temperature. And the plastic melt is easy to be carbonized and stick to the metal. Therefore, on one hand, the plasticized components should be made of stainless steel or surface chrome plating. On the other hand, low back pressure, low temperature and low shear processes should be used as much as possible in the production and processing to reduce plastic degradation.

In the third aspect, due to the thermal sensitivity of the above plastics, too high temperature or too long heating time will easily cause the plastic to decompose, degrade and carbonize. Therefore, the production process should avoid and reduce artificial shutdowns. If it is necessary to stop the machine, lower the temperature and close the material gate. After the melt in the sol cylinder is finished, use PP or PS material to clean the sol cylinder before stopping the machine.

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