People use high-speed injection molding machines to shorten injection cycle and improve production efficiency. However, in production process of injection molded products, slippage of screw of injection molding machine is a big problem. Especially for hygroscopic materials (such as nylon), high water content of resin will cause screw of injection molding machine to slip. Improper drying will significantly reduce viscosity of material and generate water vapor in barrel, thus reducing conveying power of screw.
In this regard, a humidity analyzer can be used before processing to determine moisture content of dried material, so that drying level of material can reach recommended value of supplier.
Slippage of screw of injection molding machine will cause degradation of material, thereby affecting quality of product. What are reasons for screw slippage? How to "catch" slippery screw? When screw of barrel is slipping, materials probably gather at feeding port and cannot be conveyed to end of injection machine normally. When screw rotates and retreats in barrel to transport material and prepare for next injection, screw slippage will occur in plasticizing section.

 

At this time, rotation of screw is still inherited, but axial movement of screw will stop, that is, slipping occurs. Slippage of screw will often cause degradation of material before injection, quality of product will be reduced (such as lack of material), and molding cycle will be prolonged.
There are many reasons for screw slippage, which may be related to factors such as too high back pressure, overheating or too cold at the end of barrel, wear of barrel or screw, too shallow threads in feeding section, unfair hopper planning, blocked hopper, resin wetting, excessive resin lubrication, too fine materials, or unfair cutting of resin and recycled materials.
Too cold at the end of barrel is one of main causes of screw slippage. Barrel of injection machine is divided into 3 sections. At the end, feeding section, pellets will form a layer of melt film and stick to screw during process of heating and compression. Without this film, pellets are not easily transported to front end.
Material in feeding section of injection molding machine must be heated to critical temperature to form that critical melt film. However, usually residence time of material in feeding section is very short and cannot reach required temperature. And this kind of environment is generally produced on a small injection machine. Too short residence time will cause incomplete polymer melting and inclusion process, which will cause screw to slip or stall.

Screw of injection molding machine works under high temperature, high pressure, high mechanical torque and high friction environment for a long time. The first few factors are required by process conditions, 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.
Following will focus on causes of screw wear and methods to reduce wear:
Every kind of plastic has an ideal processing temperature range for plasticization, 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 increase abrasion of 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.

 

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.
To 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.
Small as metal shavings, as large as heating ring nuts, paper clips, and even a bunch of warehouse keys, have been mixed into barrel. This damage to screw is self-evident (barrel will also be damaged at the same time. ), so it is necessary to install a magnet holder, strictly manage and monitor feeding.
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, mixed in molten plastic. With advancement of screw during injection process, from homogenization section to screw head, these "steam particles" relieve pressure and expand during injection process, like tiny hard particles of impurities, which have frictional 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.