1. Briefly describe working state of check valve in process of plasticization, injection and pressure holding; Plasticization: On Injection molding: off Holding pressure: off
Injection pressure refers to pressure exerted on plastic melt by top of plunger or screw. Its function is to overcome flow resistance of plastic melt flowing from barrel to mold cavity, giving melt a certain filling rate, compacting and feeding melt. During injection process, pressure at nozzle of injection machine is the highest to overcome flow resistance in the whole process of melt. After that, pressure gradually decreases along flow length to front wave front of melt. If inside of cavity is well vented, final pressure at the front of melt is atmospheric pressure. On injection machine, gauge pressure is commonly used to indicate injection pressure, which is generally between 40 and 130MPa.
In the field of plastic injection molding, there is always a strange problem. Every time a mold is installed on machine, it takes tens of minutes or even half a day to adjust. It is difficult to produce according to mature parameters of other machines. In the era of standardized manufacturing, this phenomenon is unimaginable, but it comes in detail.
Radio bracket product is shown in Figure 1. Maximum external dimension of product is 297.13 mm * 114.59 mm * 66.69 mm, average thickness of plastic part is 2.50 mm, plastic part material is ABS+PC, shrinkage rate is 1.0065, and plastic part weight is 108.10 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.
F=Am*Pv/1000 F: Clamping force: TON Am: Cavity projected area: CM2 Pv: Filling pressure: KG/CM2 (General plastic material filling pressure is 150-350KG/CM2) (The lower value for good liquidity, the higher value for poor liquidity) Injection pressure=filling pressure/0.4-0.6 Example: Cavity projected area 270CM2 Filling pressure 220KG/CM2 Clamping force=270*220/1000=59.4TON
In injection molding plus process, it may be due to poor processing of raw materials, unreasonable design of plastic parts or molds, operators not mastering suitable process operating conditions, or due to mechanical reasons, often short shots and dents in plastic products. , flash, trapped air, cracking, warping and other forming defects.
Plastic melt rheology is science that studies flow and deformation of plastic melt. In classical mechanics, flow and deformation are physical concepts in two categories. Flow is a property of liquid materials, while deformation is a property of solid materials. General liquid flow follows Newton's law of fluid flow - shear stress on a material is proportional to shear rate, and general solid deformation follows Hooke's law - stress on a material is proportional to amount of deformation. Newtonian fluids and Hooke elastomers are two dominant classical models.
Positioning rack is a paper-passing part in desktop office printer, which plays role of limiting width of paper, positioning and guiding paper. This article takes positioning rack of a Japanese brand printer as an example to introduce design points of injection mold for positioning rack.
Reinforcing ribs are an indispensable functional part of plastic parts. Reinforcing ribs effectively increase rigidity and strength of product like an I-shaped iron without greatly increasing product section area, but it does not have shape problem of an undercut difficult to form like an I-shaped iron. Plastic products with torsion and bending are especially suitable. In addition, ribs can also act as internal runners to help fill mold cavity, and play a great role in helping plastics flow into branches of part.
Injection curve is an indicator light in injection molding process. Combining injection curve during process debugging can improve efficiency, reduce debugging waste and reduce debugging cost. From pressure-speed curve, we can see actual pressure and speed changes, pressure-speed state when pressure is switched, matching of pressure and speed, so as to assist debugging and scientifically set process. Injection pressure setting is maximum allowable injection pressure during injection.