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

1. Each plastic has an ideal plasticizing temperature range, and processing temperature of barrel should be controlled to make it close to this temperature range. Granular plastics enter barrel from hopper and first reach feeding section. Dry friction will inevitably occur in feeding section. When these plastics are not heated enough and melted unevenly, it is easy to cause increased wear on inner wall of barrel and surface of screw. Similarly, in compression section and homogenization section, if molten state of plastic is disordered and uneven, it will also cause increased wear.
2. Speed should be adjusted properly. Because some plastics are added with strengthening agents, such as glass fibers, minerals or other fillers. 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, while increasing shearing force to plastic, it will also make reinforcement correspondingly produce more shredded fibers. Shredded fibers contain sharp ends, which greatly increase abrasion force. When inorganic minerals slide at high speed on metal surface, their scraping effect is not small. Therefore, speed should not be adjusted too high.
3. Check out debris in plastic. Generally speaking, there is no debris in fresh plastic purchased originally, but after transportation, weighing, drying, color mixing, especially addition of 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 barrel. Damage to screw is self-evident (barrel will also be damaged at the same time). Therefore, it is necessary to install a magnet frame, strictly manage and monitor feeding.
4. Water content in plastic has a certain effect on wear of screw surface. If plastic is not completely drained of water before injection, when remaining water enters screw compression section, it will form "steam particles" with high temperature and pressure that are previously melted and mixed in molten plastic. With advancement of screw during injection process, from homogenization section to screw head, these "steam particles" will relieve pressure and expand during injection process, like tiny hard particles of impurity, which will cause friction and damage to wall.
In addition, for certain types of plastics, under high temperature and pressure, moisture may become a catalyst that promotes cracking of plastic, producing harmful impurities that can attack metal surface. Therefore, drying work before plastic injection molding not only directly affects quality of parts, but also affects working life of screw.

 

Screw can be selected according to functions and requirements of injection molding machine:
Commonly used ones are:
Reciprocating screw (three-section screw) is divided into a gradual change type, a modified type, and a general type.
Screws for production of imitation marble products.
Exhaust type injection molding machine screw.
Pre-plasticized screw for injection molding machine.
Mixing screw.
1. Conventional injection screw: General injection molding machine uses a standard medium diameter screw, two high and low pressure conventional screws, which can be applied to processing of a wide range of general plastics and engineering plastics. This type of screw is also a three-stage type: feeding section, compression section, and homogenization section, all of which are determined by considering working characteristics of pre-molding and injection molding materials of 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), measurement section length (30%) , measurement section groove is shallower (0.04~0.05D), compression rate is relatively large (0.11~012D), so it is beneficial to plasticization of various temperature processes.
3. Type hard polyvinyl chloride injection screw: Characteristics of this screw are: small length-to-diameter ratio, short metering section and conveying section, longer compression section, deep groove in metering section, smaller compression, which is suitable for plastics with poor thermal stability such as hard polyvinyl chloride. In order to reduce strong shearing effect and material storage, prevent overheating decomposition, this kind of seat screw is suitable to be equipped with a "Z" type screw head with melt flow. .
4. Injection screw without metering section: 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., length-to-diameter ratio of screw without metering section is generally 20, length of feeding section and transition section each accounted for 50%, and depth of screw groove is recommended for each section.
5. Separate injection screw: (BM screw) Separate injection screw called "BM" screw has been widely used in injection molding. Main type of screw is characterized by addition of a water secondary thread in melting section (transition section, constriction section), which divides main thread groove into two parts. When screw rotates, melt film scraped by main thread flows into groove of secondary thread from gap between screw edge and barrel, so that solid bed is separated from molten pool to maintain good contact between fixed body bed and inner wall of barrel, which is convenient for transportation and heat transfer.
6. Corrugated screw (HPM screw): Corrugated screw is divided into "single wave" and "double wave". Characteristic of single wave type screw is that bottom diameters of adjacent grooves are equal, but they are eccentric, eccentricity is equal, and screw axis is symmetrical. Since depth of screw groove changes periodically along axial direction, material is subjected to strong extrusion and strong shearing during pre-molding, absorbing a large amount of mechanical energy and performing energy conversion; injection molding should be made of heat-sensitive materials such as vinyl chloride, which can reduce degradation.
7. Mixing type injection screw: Mixing type injection screw is characterized by setting mixing elements at position of metering section. Based on analysis of ordinary screws, it is shown that in transition section polymer is initially melted, a melt film is also formed on trailing edge of thread. Therefore, entire conglomerate bed is actually surrounded by melt film, which reduces melting efficiency. It is better to emphasize stable melting and conveying effect. After material passes through transition section and melts, barrier type mixing element (straight groove mixing element is also called Maddook element); other barrier type mixing elements, (Split manifold mixing element is also called DIS-shaped mixing element)
8.Combined screw: Combined screw is mostly used in core pumping of screw metering section. According to different properties of material, install mixing element groups with different functions. Mixing elements are made into form of keys, connected with keys and minds. End faces of sleeves should be precisely matched to prevent melt from entering.