Mold injection molding is a processing method used in mass production of certain parts with complex shapes. Specific principle refers to: heated and melted plastic material is pushed by screw of injection molding machine and injected into cavity of plastic mold at high pressure. After cooling and solidification, a plastic molded product is obtained.
Plastic mold consists of two parts: a movable mold and a fixed mold. Movable mold is installed on movable mold plate of injection molding machine, fixed mold is installed on fixed mold plate of injection molding machine. During injection molding, movable mold and fixed mold are closed to form pouring system and cavity. When mold is opened, movable mold and fixed mold are separated to facilitate removal of plastic products.
Although structure of plastic molds may vary due to different types and properties of plastics, shapes and structures of plastic products, types of injection machines, basic structure is same.

 

1. Plastic mold structure is divided by function and mainly consists of: pouring system, temperature adjustment system, molded parts system, exhaust system, guide system, ejection system, etc. Among them, pouring system and molded parts are parts that are in direct contact with plastics and change with plastics and products. They are the most complex and changing parts of mold and require the highest processing smoothness and precision.
1. Gating system: refers to runner part before plastic enters cavity from nozzle, including main channel, cold material cavity, runner and gate, etc.
2. Molding parts system: refers to combination of various parts that constitute shape of product, including movable mold, fixed mold and cavity (concave mold), core (convex mold), forming rod, etc. Core forms inner surface of product, and cavity (die) forms outer surface shape of product. After mold is closed, core and cavity form cavity of mold. According to process and manufacturing requirements, sometimes core and die are composed of several pieces, sometimes they are made as a whole, inserts are only used in parts that are easily damaged and difficult to process.
3. Temperature adjustment system: In order to meet mold temperature requirements of injection process, a temperature adjustment system is required to adjust temperature of mold. For injection molds for thermoplastic plastics, cooling system is mainly designed to cool mold (mold can also be heated). Common way to cool mold is to open a cooling water channel in mold, and use circulating cooling water to take away heat of mold. In addition to using cooling water to pass hot water or hot oil to heat mold, electric heating elements can also be installed inside and around mold.
4. Exhaust system: It is established to discharge air in cavity and gas generated by melted plastic out of mold during injection molding process. When exhaust is not smooth, air marks, burnt and other defects will be formed on the surface of product; exhaust system of a plastic mold is usually a groove-shaped air outlet opened in mold to discharge air in original cavity and gas brought in by melt.
When molten material is injected into mold cavity, air originally existing in mold cavity and gas brought in by melt must be discharged out of mold through exhaust port at the end of material flow. Otherwise, product will have pores, poor connections, insufficient mold filling, even accumulated air will generate high temperatures due to compression and burn product. Under normal circumstances, exhaust hole can be located either at the end of melt flow in cavity or on parting surface of mold.
The latter is to open a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of die. During injection, a lot of molten material will not seep out of vent hole, because molten material will cool, solidify there and block channel. Exhaust port must not be opened facing operator to prevent molten material from accidentally spraying out and injuring people. In addition, matching gap between ejector rod and ejector hole, matching gap between ejector block and stripping plate and core can also be used to exhaust air.
5. Guide system: It is established to ensure that movable mold and fixed mold can be accurately aligned when closing mold. Guide components must be installed in mold. In injection molds, four sets of guide posts and guide sleeves are usually used to form guide components. Sometimes it is necessary to set mutually matching inner and outer cones on movable mold and fixed mold to assist positioning.
6. Ejection system: Generally includes: ejection pin, front and rear ejection pin plates, ejection pin guide rod, ejection pin return spring, ejection pin plate locking screw, etc. When product is formed and cooled in mold, front and rear molds of mold are separated and opened. Ejection mechanism, ejector pin, is pushed by ejector pin of injection molding machine to push plastic product and its condensate in runner out or pull it out of mold cavity and runner position for next injection molding cycle.
2. According to structure, plastic molds generally consist of several parts such as mold base, mold core, auxiliary parts, auxiliary systems, auxiliary settings, and dead-end processing mechanisms.
1. Mold base: Generally, we do not need to design it. You can order it directly from standard mold base manufacturer, which greatly saves time required to design mold, so it is called plastic mold standard mold base. It forms most basic frame part of plastic mold.
2. Mold core: Mold core part is core part of plastic mold. It is the most important component of mold. Forming part of plastic product is inside mold core, and most of processing time is spent on mold core. However, compared with some relatively simple molds, it does not have a mold core part, and product is formed directly on mold plate. Most of early plastic molds were like this and were relatively backward.
3. Auxiliary parts: Commonly used auxiliary parts for plastic molds include positioning rings, injection bushings, ejector pins, grab pins, support columns, ejection plate guide posts, guide bushes, garbage nails, etc. Some of them are standard parts and can can be ordered directly when ordering mold base, and some of them need to be designed by yourself.
4. Auxiliary systems: There are four auxiliary systems for plastic molds: pouring system, ejection system, cooling system and exhaust system. Sometimes, because plastic materials used need to be heated to a very high temperature, some molds will also have a heating system.
5. Auxiliary settings: Auxiliary settings of plastic mold include lifting eye holes, KO holes (top stick holes), etc.
6. Dead Angle Treatment Structure: When a plastic product has a dead Angle, mold will also have one or more dead Angle processing structures. Such as slider, inclined roof, hydraulic cylinder, etc. In most domestic books, this kind of mechanism for dealing with dead ends is called "plowing mechanism".
In fact, it is not difficult to make a plastic mold. No matter how plastic product changes, for mold used to form plastic product, its structure is nothing more than above aspects. Difference between molds is whether mold is large or small? Positions or methods of each auxiliary parts, auxiliary settings, and auxiliary systems are different. Method, structure, size, etc. of dealing with dead corners have only changed. Of course, design experience is particularly important in order to make designed mold simple to process, easy to assemble, long in service life, affordable, and have good formed products. With good experience, you can deal with problems that arise during design and processing, and you will be more confident about design changes.
3. Structural composition of injection machine: A general-purpose injection machine mainly includes an injection device, a mold clamping device, a hydraulic transmission system and an electrical control system. Main function of injection device is to plasticize plastic evenly and inject a certain amount of melt into cavity of mold with sufficient pressure and speed. Injection device is mainly composed of plasticizing parts (composed of screw, barrel and nozzle) as well as hopper, transmission device, metering device, injection and mobile cylinder, etc.
Mold clamping device: Its function is to realize opening and closing of mold, ensure that molding mold is reliably closed during injection, and eject product. Mold clamping device mainly consists of front and rear fixed mold plates, movable mold plate, tie rods used to connect front and rear mold plates, mold clamping cylinders, linkage mechanisms, mold adjustment devices and product ejection devices.
Hydraulic system and electrical control system: Their function is to ensure that injection machine works accurately and effectively according to predetermined requirements (pressure, speed, temperature, time) and action sequence of process. Hydraulic system of injection machine is mainly composed of various hydraulic components and circuits and other ancillary equipment. Electrical control system is mainly composed of various electrical appliances and instruments. Hydraulic system and electrical system are organically organized to provide power and control to injection machine.