How is the plastic mold structure formed?

All kinds of tools and products used in our daily production and life, from the base of the machine tool and the shell of the machine body to the shell of an embryonic head screw, buttons and various household appliances, are closely related to the mold. The shape of the mold determines the appearance of these products, and the processing quality and precision of the mold also determine the quality of these products. Due to the different materials, appearances, specifications and uses of various products, molds can be divided into non-plastic molds such as casting molds, forging molds, die-casting molds, stamping molds and plastic molds.

1. In recent years, with the rapid development of plastic industry, the strength and precision of general-purpose plastics and engineering plastics have been continuously improved, and the application scope of plastic products has been constantly expanded, such as household appliances, instruments, construction equipment, automobile industry, daily hardware and many other fields, and the proportion of plastic products has increased rapidly. A reasonably designed plastic part can often replace many traditional metal parts. The plasticizing trend of industrial products and daily products is on the rise.

2. General definition of mold: In industrial production, metal or nonmetal materials are made into parts or products with required shapes by various presses and special tools installed on the presses. This special tool is collectively called the mold.

3. Description of injection molding process: Mold is a tool for producing plastic products. It is composed of several groups of parts, and there is a molding cavity in this combination. During injection molding, the mold is clamped on the injection molding machine, molten plastic is injected into the cavity of the molding mold, cooled and molded in the cavity, then the upper and lower molds are separated, and the product is ejected from the mold cavity through the ejection system, and finally the mold is closed for the next injection molding. The whole injection process is cyclic.

4. General classification of molds: it can be divided into plastic molds and non-plastic molds;

(1) Non-plastic molds include: casting molds, forging molds, stamping molds, die casting molds, etc.

A. Casting molds, faucets and pig iron platforms

B. Forging die-automobile body

C. Stamping die-computer panel

D. Die casting mold-high temperature alloy, cylinder block

(2) Plastic molds are divided into:

A. Injection mold-TV shell, keyboard keys (most commonly used)

B. Blow mold-beverage bottle

Mould-Bakelite Switch and Scientific Porcelain Dish

D. transfer mold-integrated circuit product

E. Extrusion die-hose and plastic bag

F. thermoforming mold-transparent molding packaging shell

G. rotational molding model-soft rubber doll toy

◆ Injection molding is the most commonly used method in plastic processing. This method is suitable for all thermoplastics and some thermosetting plastics, and the number of plastic products produced is also unmatched by other molding methods. As one of the main tools of injection molding, the quality accuracy, manufacturing cycle and production efficiency in injection molding directly affect the quality, output, cost and product renewal of products, and also determine the reaction ability and speed of enterprises in market competition.

◆ The injection mold consists of several steel plates and various parts, which are basically divided into:

Forming equipment (female die, male die)

B positioning device (guide post, guide sleeve)

C fixing device (I-plate, code pit)

Cooling system (water hole)

Constant temperature system (heating pipe, heating wire)

Runner system (nozzle hole, runner slot, runner hole)

G. Ejection system (thimble, ejector rod)

5. According to the different types of gating system, molds can be divided into three categories:

(1) large nozzle mold: the runner and gate are on the parting line, and the mold is demoulded together with the product when the mold is opened. The design is the simplest, the processing is easy and the cost is low, so many people use the large nozzle system for operation.

(2) Fine nozzle mold: The runner and gate are not on the parting line, but generally directly on the product. Therefore, it is necessary to design an additional nozzle parting line, which is complicated in design and difficult to process. In general, the fine nozzle system should be selected according to the product requirements.

(3) Hot runner mold: The structure of this mold is basically the same as that of the nozzle. The biggest difference is that the runner is located in one or more constant-temperature hot runner plates and heat pumps, and there is no cold material demoulding, so the runner and gate are directly on the product, so the runner does not need demoulding. This kind of system, also called nozzle-less system, can save raw materials and is suitable for occasions with expensive raw materials and high requirements for products. It is difficult to design and process, and the die cost is high.

Hot runner system, also known as hot runner system, is mainly composed of hot runner sleeve, hot runner plate and temperature control electric box. Our common hot runner systems include single-point hot runner and multi-point hot runner. Single-point hot gate is a plastic mold with a single hot gate sleeve to directly inject molten plastic into the cavity, which is suitable for single cavity and single gate. Multi-point hot runner branches the molten material into each hot runner sleeve, and then enters the cavity through the hot runner plate, which is suitable for single-cavity multi-point feeding or multi-cavity mold.

◆ Advantages of hot runner system

(1) There is no nozzle material and no post-treatment, which makes the whole molding process fully automatic, saves working time and improves working efficiency.

(2) Low pressure loss. The temperature of hot runner is equal to the nozzle temperature of injection molding machine, which avoids the surface condensation of raw materials in the runner and has low injection pressure loss.

(3) Reuse of nozzle materials will reduce the performance of plastics, and the hot runner system without nozzle materials can reduce the loss of raw materials, thus reducing the product cost. The temperature and pressure in the cavity are uniform, the stress of plastic parts is small, and the density is uniform. Under the condition of smaller injection pressure and shorter molding time, better products are injected than the general injection system. For transparent parts, thin parts, large plastic parts or demanding plastic parts, it can show its advantages, and it can produce larger products with smaller models.

(4) The hot nozzle adopts standardized and serialized design, and is equipped with a variety of nozzle heads to choose from, with good interchangeability. The uniquely designed and processed electric heating coil has uniform heating temperature and long service life. Hot runner system is equipped with hot runner plate, temperature controller, etc. , exquisite design, various types, convenient use, stable and reliable quality.

◆ Shortcomings in application of hot runner system

(1) The closing height of the whole mold is increased, and the overall height of the mold is increased due to the installation of the hot runner plate.

(2) Thermal radiation is difficult to control, and the biggest problem of hot runner is the heat loss of runner, which is a major issue to be solved.

(3) There is thermal expansion, which is a problem we should consider when designing.

(4) The manufacturing cost of molds increases, and the standard parts of hot runner system are expensive, which affects the popularization of hot runner molds.

University of Science and Technology Computer Mold Design Department