Construction Of Injection Mold

What is the construction of an injection mold?

The plastic mold is composed of two parts, a movable mold and a fixed mold. The movable mold is installed on the moving template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine.

During injection molding, the movable mold and the fixed mold are closed to form a pouring system and a cavity. When the injection mold is opened, the movable mold and the fixed mold are separated to take out the plastic product.

Although the structure of the plastic mold may vary due to the variety and performance of the plastic, the shape and structure of the plastic product, and the type of the injection machine, the basic structure is the same.

1. Plastic mold structure is divided by function

It is mainly composed of: pouring system, temperature adjustment system, forming parts system, exhaust system, guide system, ejection system, etc.

Among them, the pouring system and the molded parts are the parts that are in direct contact with the plastic and change with the plastic and the product. They are the most complex and the most varied parts in the mold, which require the highest processing finish and precision.

1.1. Pouring system

This refers to the part of the flow path before the plastic enters the cavity from the nozzle, including the main flow path, the cold cavity, the flow path, and the gate.

1.2. Molded parts system

This refers to the combination of various parts that constitute the shape of the product, including moving mold, fixed mold and cavity (concave mold), core (convex mold), forming rod, and so on. The core forms the inner surface of the product, and the cavity (die) forms the outer surface shape of the product.

After the injection mold is closed, the core and the cavity form the cavity of the mold. According to the process and manufacturing requirements, sometimes the mold core and the die are composed of several pieces, sometimes made as a whole, and inserts are only used in parts that are easily damaged and difficult to process.

1.3. Temperature regulating system

In order to meet the mold temperature requirements of the injection molding process, a temperature regulation system is needed to regulate the mold temperature. For injection molds for thermoplastics, the cooling system is mainly designed to cool the mold (the mold can also be heated).

The common method of mold cooling is to set up a cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; in addition to using cooling water to pass hot water or hot oil, the heating of the mold can also be installed inside and around the mold Electric heating element.

1.4. Exhaust system

It is set up to exclude the air in the cavity and the gas generated by the melting of the plastic from the mold during the injection molding process. When the exhaust is not smooth, defects such as gas marks (gas lines) and scorch will be formed on the surface of the product; the discharge of the plastic mold The gas system is usually a slot-shaped air outlet set in the mold to discharge the air from the original cavity and the gas brought by the melt.

When the molten material is injected into the cavity, the air originally in the cavity and the gas brought by the melt must be discharged to the outside of the mold through the exhaust port at the end of the flow.

Otherwise, the product will have air holes, poor connection, Dissatisfied with mold filling, and even the accumulated air will burn the product due to high temperature caused by compression. Under normal circumstances, the vent hole can be located either at the end of the melt flow in the cavity or on the parting surface of the mold.

The latter is a shallow groove with a depth of 0.03-0. 2mm and a width of 1.5-6mm. During injection, there will not be a lot of molten material leaking from the vent hole, because the molten material will cool and solidify at this place to block the channel.

The opening of the exhaust port must not face the operator to prevent accidental injection of molten material and injury. In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the top block and the stripper plate, and the core can also be used to exhaust.

1.5. Guidance system

It is set up to ensure that the moving mold and the fixed mold can be accurately centered when the mold is closed, and the guide part must be provided in the mold. In the injection mold, four sets of guide columns and guide sleeves are usually used to form the guide part. Sometimes, the moving mold and the fixed mold need to be provided with internal and external conical surfaces to assist positioning.

1.6. Ejection system

Generally, it includes thimble, front and back thimble plate, thimble guide rod, thimble return spring, thimble plate locking screw, and so on. After the product is molded and cooled in the mold, the front and back molds of the mold are separated and opened.

The ejector mechanism-the thimble is pushed by the ejector of the injection molding machine to push or pull out the plastic products and the condensate in the flow channel. For the next injection molding cycle.

2. The plastic mold is generally composed of several parts such as mold base, mold core, auxiliary parts, auxiliary system, auxiliary setting, dead angle processing mechanism, and so on.

2.1. Formwork

Generally, we don’t need our design, we can directly order from the standard mold frame manufacturer, which greatly saves the time required to design the mold, so it is called the standard mold frame of plastic mold. It constitutes the most basic frame part of the plastic mold.

2.2. Mold Core

The mold part is the core part of the plastic mold. It is the most important part of the mold. The forming part of the plastic product is inside the mold core, and most of the processing time is also spent on the mold core.

However, there are relatively simple molds that do not have mold core parts, and the product is directly formed on the template. Most of the early plastic molds are like this, relatively backward.

2.3. Auxiliary parts

The commonly used auxiliary parts of plastic molds include positioning ring, sprue bushing, thimble, pick pin, support column, ejector plate guide column guide bush, garbage nail, etc., some of them are standard parts, you can directly order the mold frame When ordering together, some parts need to be designed by yourself.

2.4. Auxiliary system

There are four auxiliary systems for plastic molds: pouring system, ejection system, cooling system, and exhaust system. Sometimes, because the plastic material used needs to be heated at a high temperature, some molds will also have a heating system.

2.5. Auxiliary settings

Attached to the plastic mold are eyelets for holes, KO holes (top stick holes), etc.

2.6. Dead angle processing structure

When plastic products have dead corners, the mold will also have one or more structures to deal with dead corners. Such as slider, inclined top, hydraulic cylinder, and so on. In most domestic books, this kind of mechanism for dealing with dead ends is called “pulling mechanism”.

In fact, the plastic mold is not difficult, no matter how the plastic product changes, for the mold to form this plastic product, its structure is nothing more than the above aspects.

The difference between the molds is whether the mold is large or small?

The position or method of each auxiliary part, auxiliary setting, and auxiliary system are different. The method, structure, size, etc. of dealing with dead ends have changed.

Of course, the design experience is especially important to make the designed molds easy to process, easy to assemble, long in life, affordable, and well-formed products. A good experience can deal with problems in design and processing and is more confident about design changes.

3. The structure of the injection mold machine

A general-purpose injection machine mainly includes an injection device, a mold clamping device, a hydraulic transmission system, and an electrical control system.

The main function of the injection device is to evenly plasticize the plastic and inject a certain amount of melt into the cavity of the mold with sufficient pressure and speed. The injection device is mainly composed of plasticized parts (composed of a screw, barrel, and nozzle) as well as a hopper, transmission device, metering device, injection and moving oil cylinder, etc.

Clamping device

Its function is to realize the opening and closing of the mold, to ensure that the molding mold is closed tightly, and to release the product during the injection. , Mold adjustment device, and product ejection device.
Hydraulic system and electrical control system:
Its role is to ensure that the injection machine works accurately and effectively according to the predetermined requirements of the process (pressure, speed, temperature, time) and the sequence of actions. The hydraulic system of the injection machine is mainly composed of various hydraulic components and circuits and other auxiliary equipment.
The electrical control system is mainly composed of various electrical appliances and instruments. The hydraulic system and the electrical system are organically organized to provide power and realization control for the injection machine.