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Structural composition of plastic molding

Time：2019-03-23 09:34:34 / Popularity： / Source：

Mold injection molding is a processing method used in mass production of complex parts of certain shapes. Specific principle refers to: plastic raw material melted by heat is injected from screw of injection molding machine into cavity of plastic injection tooling, and after cooling, solidifying, plastic molded products is obtained.
Plastic molding consists of two parts: movable mold and fixed mold. Movable mold is mounted on moving template of injection molding machine, and fixed mold is mounted on fixed template of injection molding machine. In injection molding, movable mold and fixed mold close constitute a casting system and cavity, and when mold is opened, movable mold and fixed mold are separated to take out plastic molded products.
Although structure of plastic injection tooling may vary depending on type and performance of plastic, shape and structure of plastic molded products, and type of injection molding machine, basic structure is uniform.
  • First, plastic injection tooling structure is divided into functions, mainly consisting of: pouring system, temperature control system, forming part system, exhaust system, guiding system, ejection system and so on. Gating system and mold parts are in direct contact with plastic, vary with plastic and product. It is the most complicated and most varied part of mold, and requires highest degree of smoothness and precision.
1. Gating system: refers to part of flow channel before plastic enters cavity from nozzle, including  main channel, branch channel, cold material hole, gate, and so on.
2. Mold parts system: refers to various mold parts and components that make up shape of product, including dynamic mold, fixed mold and cavity (concave), core (protrusion), forming rod and so on. Core forms inner surface of product and cavity (female) forms outer surface shape of product. After clamping, core and cavity form cavity of mold. According to process and manufacturing requirements, sometimes core and die are made up of several pieces, sometimes made into a whole, and only inserts are used in parts that are easily damaged and difficult to process.
3. Temperature regulation system: In order to meet mold temperature requirements of injection process, a temperature adjustment system is required to adjust temperature of mold. For injection molding for thermoplastics, cooling system is primarily designed to cool mold (mold can also be heated). Common method of mold cooling is to open a cooling water channel in mold, and use circulating cooling water to remove heat of mold; In addition to use of cooling water to pass hot water or oil, heating of mold can also be installed with electric heating elements inside and around the mold.
4. Exhaust system: It is set up to remove gas generated by melting of air and plastic in cavity during injection molding process to outside of mold. When exhaust gas is not smooth, surface of product will form gas marks (gas lines), scorch, etc. Exhaust system of plastic molding is usually a slot-shaped air outlet opened in mold to discharge original cavity air and gas brought in by melt.
When molten material is injected into cavity, air originally stored in cavity and gas brought in by melt must be discharged to outside of die through exhaust port at the end of flow, otherwise product will have pores, poor welding, mold filling, and even accumulated air will burn product due to high temperature caused by compression. In general, vent hole can be disposed at the end of flow of melt in cavity or on parting surface of mold.
The latter is a shallow groove having a depth of 0.03-0.2 mm and a width of 1.5-6 mm on one side of die. During injection, vent hole does not have a lot of melt oozing out, because melt will cool and solidify there to block passage. Do not point opening of exhaust port to operator to prevent melt from accidentally squirting and injuring people. In addition, matching gap between ejector rod and ejector hole, top block/stripping plate and core may be utilized to exhaust.
5. Guide system: It is to ensure that movable mold and fixed mold can be accurately centered during mold clamping, and guide member must be provided in mold. In injection molding, four sets of guide columns and guide sleeves are usually used to form guide members, and sometimes  inner and outer tapered surfaces are respectively arranged on movable mold and fixed mold to assist positioning.
6. Ejection system: generally includes: thimble, front and rear ejector plate, thimble guide rod, thimble return spring, thimble plate locking screw and other parts. When product is molded and cooled in mold, front and rear molds are separated and opened, plastic molded products and its conglomerate in flow passage are pushed out or pulled out of mold cavity and flow path by push-out mechanism(ejector pin) under push rod of injection molding machine. In order to carry out next injection molding work cycle.
  • Second, plastic molding is generally composed of mold parts, mold cores, auxiliary parts, auxiliary systems, auxiliary settings, dead angle processing mechanisms, etc.
1. Mold base: Generally, we don't need to design. We can order directly from standard mold base manufacturer, which greatly saves time required to design mold, so it is called standard mold base of plastic injection tooling. It forms the most basic part of plastic injection tooling.
2, Mold core: mold core is core part of plastic mold, it is the most important part of mold. Molded part of plastic molded products is inside mold core, and most of time processing is also spent on mold core. However, compared to some relatively simple molds, it has no mold core and product is formed directly on template. Most of early plastic molding were like this, relatively backward.
3, Auxiliary parts: Common auxiliary parts for plastic molding include positioning ring, nozzle bushing, thimble, grab pin, support column, ejector guide pillar guide sleeve, garbage nails, etc., some of which are standard parts that can be ordered directly when ordering formwork, and some need to be designed by themselves.
4. Auxiliary system: There are four auxiliary systems for plastic molding: pouring system, ejection system, cooling system and exhaust system. Sometimes, because plastic material used needs to be heated at a high temperature, some heating systems exist in some molds.
5, Auxiliary settings: Auxiliary settings of plastic molding include ring hole, KO hole (top stick hole) and so on.
6, Dead angle treatment structure: When plastic molded products has a dead angle, mold will have one or more structures to deal with dead angles such as sliders, inclined tops, hydraulic cylinders, etc. In most books in China, this kind of mechanism for dealing with dead ends is called "core pulling mechanism".
In fact, plastic molding is not difficult, no matter how plastic molded products changes, for mold to form plastic molded products, its structure is nothing more than above aspects. Difference between molds is whether mold is large or small? Location or method of each auxiliary part, auxiliary setting, and auxiliary system is different. Method, structure, size of dealing with dead angles have changed. Of course, design experience is particularly important in order to make designed mold simple, easy to assemble, long in life, moderate in price, and good in forming products. Good experience can deal with problems in design and processing, and it is also quite sure to deal with changes.
  • Third, injection machine's structural composition: a general-purpose injection machine mainly includes injection device, clamping device, hydraulic transmission system and electrical control system.
Main function of injection device is to plasticize plastic evenly and inject a certain amount of melt into cavity of mold at a sufficient pressure and speed. Injection device is mainly composed of a plasticizing component (composed of a screw, a barrel and a nozzle), a hopper, a transmission device, a metering device, an injection and a moving oil cylinder, and so on.
Clamping device: its function is to realize opening and closing of mold, to ensure tight clamping of forming mold during injection, and to take off product. Clamping device mainly consists of front and rear fixed template, moving template, connecting rod for connecting front and rear templates, clamping cylinder, linkage mechanism, mold adjustment device and product ejection device.
Hydraulic system and electrical control system: Its function is to ensure that injection machine works accurately and effectively according to predetermined requirements (pressure, speed, temperature, time) and sequence of actions of process. Hydraulic system of injection machine is mainly composed of various hydraulic components, 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 power and control injection machine.
Last article：How to correctly distinguish and choose injection moulding materials PA6 and PA66? Next article：Case Study of Injection Moulding Defects - Automotive Molding

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