The Structure of the Mold

Injection molding is a processing method used in quantity production of some components with complex shapes. The specific principle is: the heated melting plastic raw materials are pushed by the injection molding machine screw into the mold cavity of the high-pressure plastic mold, and the plastic forming products will be obtained after cooling and curing.

In precision plastic injection molding, the plastic mold is composed of two parts: the moveable mold and the stationary mold. The moveable mold is installed on the moveable platen of the injection molding machine, and the stationary mold is installed on the stationary platen of the same machine. During the process of injection molding, the moveable mold and the stationary mold are closed to form the gating system and the cavity, and the moveable mold and the stationary mold are separated from each other when the mold is opened, so as to take out the plastic products. Although the structure of plastic molds may vary due to the varieties and properties of plastic, the shape and structure of plastic products and the type of injection machine, their basic structures are the same.

1. Classification of components of the plastic mold according to their functions:

The structure of plastic mold is divided into the following parts: the gating system, the temperature-regulating system, the system of molding parts, the exhaust system, the guiding system and the ejection system, among which the gating system and the molding part system are the parts that directly contact the plastics and that change with the plastics and products, regarding as the most complicated and most changeable parts in the plastic mold, with the highest requirement of finish and accuracy.

(1) The gating system:

It refers to the flow channel where plastic is injected into the mold cavity from the injection nozzle, including the main channel, the cold-slug cavity, the runner and the sprue etc.

(2) The system of molding parts:

It refers to the combination of various parts that constitute the shape of the product, including the moveable mold, the stationary mold, the cavity (the female mold), the mold core (the male mold), the molding screw etc. The mold core forms the internal surface of the product, while the cavity (the female mold) forms the external surface of the product. After the mold is closed, the mold core and the cavity constitute the cavity of the mold. According to the manufacturers that make prototypes requirements of process and manufacturing, some of the mold core and the female mold are composed of several pieces, and others are made into a whole. And mold inserts are only used in the parts which are vulnerable or intractable.

(3) The temperature-regulating system:

In order to meet the requirements of injection process  for the temperature of the mold, it is necessary to install a temperature-regulating system to adjust the temperature of the mold. For the injection mold of thermoplastic, what ranks first is to design a cooling system to cool the mold (or heat the mold). The most commonly used way to cool the mold is to set up a channel with cooling water in it, in which the heat of the mold will be taken away with the help of the cooling water circulation; in addition to adding hot water or hot oil to the cooling water, the electric heating elements can also be installed inside and around the mold in order to heat the mold.

(4) The exhaust system:

It is set up to send out the air in the cavity and the gas produced by the melting of plastic from the mold during injection molding. Defects like air marks (gas lines)on the surface of the products, scorch etc. will emerge and when the exhaust system does not work well; the exhaust system of the plastic mold is usually a groove-shaped air outlet on the mold to discharge the air originally stored in the mold cavity and the gas brought in by the molten material. When the molten material is injected into the mold cavity, the air originally stored in the mold cavity and the gas brought in by the molten material must be discharged out of the mold through the exhaust port at the end of the material flow. Otherwise, the product will have pores, poor connection, insufficient filling, and the product will be even burned due to the high temperature caused by compression of accumulated air. In general, the vent hole can be located either at the end of the material flow in the mold cavity or on the parting surface of the mold.

The latter is to set a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on the side of the female mold. During injection, there will not be a lot of molten material seeping out of the vent hole, because the molten material will cool and solidify here, blocking the channel. The aperture of the exhaust port should not be right towards the operator, in order to prevent accidental ejection of molten material which will result in personal injury. In addition, the gap between the ejector rod and the ejector hole can be used to exhaust gas as well as the gap between the ejector block, the demolding platen and the mold core.

(5) The guiding system:

It is set up to ensure that the moveable mold and the stationary mold can be accurately aligned when closing the mold, so the guide part must be set in the mold. In the injection mold, the guide part is usually formed by four sets of guide pillars and guide sleeves. Sometimes, it is necessary to set the internal and external conical surfaces that coincide with each other on the moveable mold and the stationary mold to assist positioning.

(6) The ejection system:

It is usually composed of ejector pin, ejector platen, ejector platen guides, reset spring of the ejector, locking screw of the ejector platen etc. When the product is cooled and formed in the mold, the front mold and the rear one will be separated from each other, and the ejector pin--the ejecting part, driven by the ejector pin, will push the plastic product and its condensate out of the flow channel and the mold cavity, so as to carry out the next injection molding.

2.Classification of components of the plastic mold according to their structures:

When it comes to the structure, the plastic mold is generally composed of the mold base, the mold core, the auxiliary parts, the auxiliary system, the auxiliary setting and the section to handle blind angles.

(1) The mold base:

Generally, we don't need to design it by ourselves as they can be ordered directly from manufacturers who produce standard mold bases, which apparently saves the time needed for mold design, so it is called the standard mold base of plastic mold. It is the most basic framework of the plastic mold.

(2) The mold core:

The mold core of the plastic mold is the core and the most important part of the mold. It is where the plastic products form, and the product stays in the mold core for most of the time. However, for some relatively simple molds, they do not have the mold core, and the product directly forms on the mold. Most of the early plastic molds look like that, and they were backward.

(3) The auxiliary parts of the plastic mold

Commonly used auxiliary parts of the plastic mold include locating ring, sprue sleeve, pin, pin stop, support pin, guide pillar and guide sleeve of the ejector platen, garbage nail, etc. Some of them are standard parts, which can be ordered directly when ordering the mold base, and others need to be designed by ourselves.

(4) The auxiliary system:

There are four auxiliary systems in the plastic mold: the gating system, the ejection system, the cooling system and the exhaust system. Some molds have a heating system because the plastic material has to be heated to a very high temperature, sometimes.

(5) The auxiliary setting of the plastic mold:

The auxiliary setting of the plastic mold includes the swinging ring hole, the K.O. hole (the knock out), etc.

(6) The section to handle blind angles:

When plastic products have blind angles, the mold will be installed one or more structures to deal with blind angles, such as sliding blocks, inclined jacking, hydraulic cylinder and so on. This sector dealing with blind books is called "core-pulling mechanism" in most books in China.

In fact, the structure of the plastic mold is not so complex. No matter how the plastic product changes, the structure of the mold for forming it is nothing more than the above parts. Is the difference between molds is whether they are large or small? Their difference lies in the following aspects: the position or mode of each auxiliary part, auxiliary setting and auxiliary system, and the method, structure and size of the blind angle. Of course, the design experience is very important to make the designed mold simple to process and easy to assemble, with a long life and a moderate price. And good products can be produced in the rapid prototyping factory. The experienced can deal with the problems in design and processing, and are confident in handling design changes.

3.The structure of the injection machine:

The structure of the injection machine is composed of the injection device, the mold closing device, the hydraulic transmission system and the electric control system. The main function of the injection device is to plasticize the plastic evenly and injects a certain amount of molten material into the mold cavity with sufficient pressure and a suitable speed. The injection device is mainly composed of plasticizing parts (including screw, barrel and nozzle), the hopper, the transmission device, the metering device, the injection rod and the mobile oil cylinder, etc.

(1) The mold closing device

The mold closing device is mainly composed of the front and rear stationary platens, the moveable platen, the pull rod connecting the front and the rear platens, the mold closing cylinder, the connecting rod mechanism, the mold adjusting device and the product ejection device. Its function is to achieve the open and close of the mold, to make sure the mold close tightly when ejecting and to push out the product.

(2) The hydraulic system and the electrical control system:

Its function is to ensure that the injection machine can work accurately and effectively according to the predetermined requirements (pressure, speed, temperature and time) and the sequence of specific procedures of the process. The hydraulic system of the injection machine is mainly composed of various hydraulic components, circuits and other auxiliary equipment, while the electrical control system is mainly composed of various electrical appliances and instruments. The hydraulic system and electrical system are scientifically organized together to provide power for the injection machine and control it.