How to improve the working life of the plastic machine screw?

Update:06 Feb 2021

The screw barrel for Injection molding machine is working for a long time under high temperature, high pressure, high mechanical torque and high friction environment. The first few factors are required by the process conditions, and the loss caused by friction is inevitable. Generally, screws have undergone surface nitriding treatment to increase surface hardness, that is, to improve wear resistance. However, if the cause of wear is ignored and the wear is not minimized, the working life of the screw will be greatly reduced.

The following explains the causes of screw wear and methods to reduce wear

1. Each plastic has an ideal plasticizing temperature range, and the barrel processing temperature should be controlled to make it close to this temperature range. Granular plastic enters the barrel from the hopper and first reaches the feeding section. Dry friction will inevitably occur in the feeding section. When these plastics are not heated enough and melted unevenly, it is easy to increase the abrasion of the inner wall of the barrel and the surface of the screw. Similarly, in the compression section and homogenization section, if the molten state of the plastic is disordered and uneven, it will also cause increased wear.

2. The speed should be adjusted properly. Because some plastics are added with strengthening agents, such as glass fibers, minerals or other fillers. The friction of these substances on metal materials is often much greater than that of molten plastics. When injection molding these plastics, if they are made with a high rotation speed, it will increase the shearing force to the plastic, and at the same time, the reinforcement will produce more shredded fibers. The shredded fibers contain sharp ends, causing wear Power is greatly increased. When inorganic minerals slide at high speed on the metal surface, their scraping effect is not small. Therefore, the speed should not be adjusted too high.

3. Check out the debris in the plastic. Generally speaking, there is no debris in the fresh plastic purchased originally, but after transportation, weighing, drying, color mixing, and especially adding recycled materials, it may be mixed with debris. It is as small as metal scraps, as large as heating coil nuts, paper clips, and even a bunch of warehouse keys, which have been mixed into the barrel. The damage to the screw is self-evident (the barrel will also be damaged at the same time) Therefore, it is necessary to install the magnet rack, and strictly control and monitor the feeding.

4. The water content in the plastic has a certain effect on the wear of the screw surface. If the plastic is not completely drained of water before injection, when the remaining water enters the screw compression section, it will form "steam particles" with high temperature and pressure that are previously melted and mixed in the molten plastic. As the screw advances during the injection process, the From the chemical section to the screw head, these "steam particles" relieve pressure and expand during the injection process, just like tiny impurity hard particles, which cause friction and damage to the wall.

In addition, for certain types of plastics, under high temperature and pressure, moisture may become a catalyst that promotes the cracking of the plastic, producing harmful impurities that can attack the metal surface. Therefore, the drying work before plastic injection molding not only directly affects the quality of the parts, but also affects the working life of the screw.

Screw type of injection molding machine

The screw can be selected according to the functions and requirements of the injection molding machine:

The commonly used ones are:

①Reciprocating screw (three-section screw) is divided into gradual change type, experimental type and general type.

② Screws for the production of imitation marble products.

③Exhaust type injection molding machine screw.

④Screw of pre-plasticized injection molding machine.

⑤ Mixing screw.

1. Conventional injection molding screw: The general injection molding machine uses a standard medium diameter screw, and two high and low pressure conventional screws, which can be applied to the processing of a wide range of general plastics and engineering plastics. This type of screw is also three-stage Types: Feeding section, compression section, and homogenization section are all determined by considering the working characteristics of the pre-molding and injection molding materials of the injection molding machine.

2.Longer injection screw: It is a general-purpose injection butterfly rod with higher plasticizing ability and better mixing effect. Compared with conventional screws, its characteristics are: large aspect ratio (20-25), measuring section length (30%) , The metering section groove is shallow (0.04~0.05D), and the compression is relatively large (0.11~012D), so it is beneficial to the plasticization of various temperature processes.

3. Type hard PVC injection screw: The characteristics of this screw are: small length-to-diameter ratio, short metering section and conveying section, longer compression section, deep groove in the metering section, smaller compression, and suitable For injection molding of plastics with poor thermal stability such as hard polyvinyl chloride, in order to reduce the strong shearing effect and material storage, and prevent overheating decomposition, this kind of seat screw is suitable for the "Z" type screw head with melt flow void .

4. Injection screw without metering section: The light metering injection molding screw is also called "ZM" screw. It is characterized by no metering section, only feeding section and transition section, so this is a two-stage screw, because there is no metering section It can reduce melt overheating, increase plasticizing capacity and mixing effect, so it is suitable for injection molding ABS and other non-crystalline materials that prevent shear overheating.

For example, it is used to process flame-retardant and impact-resistant polyethylene, polycarbonate, filled polyethylene, polyvinyl, and filled nylon, etc. The length to diameter ratio of the screw without metering section is generally 20, and the length of the feeding section and the transition section Each accounted for 50%, and the depth of each segment is recommended.

5. Separate injection screw: (BM screw) The separate injection screw is called "BM" screw, which has been widely used in injection molding. The main type of screw is characterized by adding a water pair in the melting section (transition section, contraction section) Thread, divides the main thread groove into two parts. When the screw rotates, the melt film scraped by the main thread flows into the groove of the secondary thread from the gap between the screw edge and the barrel, which separates the solid bed from the molten pool and keeps it fixed. The body bed has good contact with the inner wall of the barrel, which is convenient for transportation and heat transfer.

6. Corrugated screw (HPM screw): Corrugated screw is divided into "single wave" and "double wave". The characteristic of single wave screw is that the bottom diameters of adjacent grooves are equal, but they are eccentric and eccentric The pitch is equal, and the screw axis is symmetrically opened. Because the depth of the screw groove changes periodically along the axial direction, the material is subjected to strong extrusion and strong shear during pre-plasticization, which absorbs a large amount of mechanical energy and performs energy conversion; injection molding should be made of heat-sensitive materials such as vinyl chloride, which can reduce degradation.

7. Mixing type injection screw: The mixing type injection screw is characterized by setting mixing elements in the metering section. Based on the analysis of ordinary screws, it is shown that in the transition section, the polymer is initially melted, and a melt film is also formed at the trailing edge of the thread. Therefore, the entire conglomerate bed is actually surrounded by the melt film, which reduces the melting efficiency. It is better to emphasize the stable melting and conveying effect. After the material passes through the transition section and melts, the barrier type mixing element (the straight groove mixing element is also called the Maddook element); other barrier type mixing elements, (divided manifold mixing elements) (The mixing element is also called DIS-shaped mixing element)

8. Combined screw: The combined screw is mostly used in the core pumping of the screw metering section. According to the different properties of the material, the mixing element group with different functions is installed. The mixing element is made into the form of a key and connected by a key and mind. The end face of the sleeve It must be precisely matched to prevent the melt from entering.

 

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