With the increase of people's demand and the diversification of plastic raw materials, the service life of the screw and barrel is getting shorter and shorter. In the final analysis, the screw and barrel are corroded and damaged by high-temperature resin because the screw and barrel have not been selected to make the screw and barrel, and a preventive maintenance plan has not been developed. Of course, at the beginning of construction, we used Injection Molding Screw Barrel to manufacture

Corrosive, abrasive and high-temperature resistant resins may unknowingly damage the screw and barrel. For example, certain engineering materials, thermoplastic elastomers, and biomolecules sometimes form a corrosive environment. In addition, reinforcing materials (such as glass fibers, glass balls, etc.), some fillers and additives are abrasive.

In order to avoid affecting production efficiency and product quality, processors must closely monitor the hazards that these materials may cause to equipment. In order to maintain high processing standards, it is necessary to establish a preventive maintenance plan (hereinafter referred to as "PM plan"). The screw and barrel should be inspected and measured as needed, and the screw should be pulled out to reinstall or replace, because the minimum wear will affect product quality.

The best way to protect equipment from corrosive and abrasive resins is to choose suitable materials for the screw and barrel. In order to deal with corrosion, wear and high temperature melt and wear-resistant barrel and screw manufacturing, the processing cost of polyolefin screw is about 3 to 4 times as high as that of standard polyolefin screw. This is because wear-resistant alloys are expensive and difficult to machine. But they provide a longer service life and can be repaired at longer intervals.

Wear-resistant materials

Metal is corroded by acidic liquids (or gases) that dissolve oxide coatings during processing, such as pitting corrosion on the surface of the barrel and screw, which will cause corrosion and wear. Highly corrosive polymers include polyvinyl chloride (PVC), which produces hydrochloric acid; aldehydes that produce formic acid; and fluoropolymers, which produce hydrofluoric acid; and other corrosive melts usually contain flame retardants and foam Agent. Standard cylinders made of composite materials, nitrides or tool steel will be severely damaged by fluoropolymers in a short period of time.

Usually, the resin manufacturer will provide suggestions for improving the processing equipment. DuPont pointed out that when processing Teflon fluoropolymers, all parts of the extrusion equipment that are in contact with the melt must be made of special corrosion-resistant materials, which can be nickel-based alloys and are not affected by acids.

In addition, DuPont pointed out that hardened nickel plates may be used when processing fluorinated polymers, but chrome cylinders and screws are not recommended, and pointed out that holes, notches or cracks on the plates may affect the performance of the product.

NatureWorks, which produces Ingeo bioplastics, recommends that all processing equipment use stainless steel to minimize corrosion. At the same time, the company pointed out that PLA (PLA) should not be placed in extruders, polymer filters, transmission lines, or other components. Extrude the system for a long time at melting temperature.

The coefficient of thermal expansion (CTE) of corrosion-resistant materials is lower than that of standard steel, which can cause problems when processing high-temperature resins such as fluorinated polymers. When the thermal expansion coefficient of the barrel is different from the thermal expansion coefficient of the screw, the gap between the screw and the barrel will change, causing problems of screw engagement and damage to the barrel. Therefore, it is important to keep the screw and barrel matching. The manufacturer of the screw and barrel may recommend these parts.

Through the use of hard wear-resistant alloys and coatings, the wear on the surface of the barrel and screw caused by tough reinforcing materials and other hard particles can be reduced. For example, tungsten alloy used for barrel lining and hardened surface of screw can provide excellent protection.

The carbon content affects the hardness of the alloy. For screws, medium carbon heat-treated steel is usually used as the bottom layer, and the hardened surface is welded to the top of the thread.

Screws are usually surface-treated with hardened cobalt or nickel-based welds, or they can be made of surface hardened or fully hardened tool steel. Colmonoy 56 is a nickel/chromium/boron alloy, usually used to protect screws from slight corrosion and abrasion resin. The surface hardening of Colmonoy 83 provides more protection, but at the same time it is more costly.

The wear-resistant alloy bimetal barrel lining can provide protection against various wear conditions and provide a series of cost/performance options. The best corrosion resistance is the hot isostatic press barrel (hereinafter referred to as "HIP") , For injection, even fluoropolymers will not pose a threat to the barrel. The barrel is provided with a nickel-rich boron alloy protective layer, which contains a matrix of molybdenum and borides and carbides.

It is said that when using corrosion-resistant screws, the HIP barrel significantly reduces the amount of iron particles absorbed from the barrel chamber. Of course, this level of protection requires a higher price. Nickel/chromium and tungsten cylinders can be used as long as the discharge end of the cylinder is cast with chromium/nickel/iron (hereinafter referred to as "Inconel"), and the surface of the transducer hole and flange are also made of Inconel. At the same time, the price of these cylinders is lower than that of HIP cylinders.

Preventive maintenance

Excessive wear and tear can place a heavy burden on the processor. Excessive clearance between the thread and the surface of the barrel will affect the performance, efficiency and energy consumption of the machine. In extrusion, the occurrence of abrasion may lead to a reduction in the amount of extrusion, so a higher screw speed and energy consumption are required. At the same time, this is usually accompanied by a higher melting temperature and more quality problems.

Using a good PM plan carefully monitors critical screw/barrel gaps to ensure maximum extrusion and unplanned downtime. When producing high-quality components, such as medical catheters and medical device components, PM planning becomes even more important.

The inspection process is to pull the screw out and clean it thoroughly, and check the outer diameter of the thread and the inner diameter of the barrel while it is hot. The plastic material on the surface of the screw and barrel can be removed from the screen with a steel wire brush and copper wire. The precision wear measuring instrument can realize accurate measurement, is convenient to use, and has a variety of specifications and functions to choose from.

First remove the plastic from the cylinder, and then insert the electronic aperture meter into the cylinder to read the ID data of each measurement position. The thread micrometer can measure the thread diameter and thread depth, and the digital display model tester provides liquid crystal readings for recording data to the computer.

In order to be able to predict when the screw or barrel needs to be replaced or repaired, and to schedule a shutdown at an appropriate time, regular measurements are required. The recommended gap range is 0.01016~0.01524cm, and the size for a new pair of screw and cylinder is 5.08cm.

According to experience, the reasonable gap between the screw and the barrel is generally 1/1000 of the size. The screw should be reassembled when the maximum allowable gap is exceeded. Repair service can put the worn parts back into use at a lower cost. In addition, the worn part of the injection molding machine barrel can be replaced with a new bushing, while the extruder barrel usually replaces the entire bushing, so the maintenance cost is high, so it is not Cost-effective.

Once the screw wears out at a certain point, the melt returns to the thread, causing the screw and barrel to wear faster. Severe wear leads to a lot of waste of resin and reduces production efficiency. However, severe wear and tear can be prevented by regular PM testing. The best maintenance strategy is to keep the screw/ Conical Screw Barrel within a reasonable range.
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