Basic knowledge and application technology of plastic molds

1 Introduction

The various tools and products used in our daily production and life, ranging from the base of the machine tool, the body shell, to a small screw, button and the shell of various household appliances, are all 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 determines the quality of these products. Because of the different materials, appearance, specifications and uses of various products, molds are divided into non-plastic molds such as casting molds, forging molds, die-casting molds, and stamping molds, as well as plastic molds.

In recent years, with the rapid development of the plastics industry and the continuous improvement in strength and precision of general and engineering plastics, the application range of plastic products is also expanding, such as: household appliances, instrumentation, construction equipment, automobile industry, Japan In many fields such as hardware, the proportion of plastic products is increasing rapidly. A reasonably designed plastic part can often replace multiple traditional metal parts. The trend of plasticization of industrial products and daily-use products is on the rise.

2. The general definition of the mold

In industrial production, various presses and special tools installed on the press are used to make metal or non-metallic materials into parts or products of the required shape through pressure. Such special tools are collectively called molds. .

3. Description of the injection molding process

A 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, the molten plastic is injected into the molding cavity, cooled and shaped in the cavity, and then the upper and lower molds are separated, and the product is ejected from the cavity and out of the mold through the ejection system, and finally the mold is closed For the next injection, the entire injection process is carried out in cycles.

4. General classification of molds: 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 mold-faucet, pig iron platform
• B. Forging die-car body
• C. Stamping die-computer panel
• D. Die casting mold-super alloy, cylinder block

(2) Plastic molds are divided into different production processes and products

• A. Injection molding mold-TV shell, keyboard buttons (most commonly used)
• B. Blow mold-beverage bottle
• C. Compression molding mold-bakelite switch, scientific porcelain dishes
• D. Transfer molding mold-integrated circuit products
• E. Extrusion mould-glue pipe, plastic bag
• F. Thermoforming mold-transparent molded packaging shell
• G. Rotary forming mold-soft doll toy

Injection molding is the most commonly used method in plastic processing. This method is suitable for all thermoplastics and some thermosetting plastics. The number of plastic products produced is unmatched by other molding methods. As one of the main tools of injection molding processing, the injection mold is in terms of quality accuracy, manufacturing cycle and injection molding process. The level of production efficiency and other aspects directly affects the quality, output, cost and product update of the product, and also determines the ability and speed of the enterprise’s response in market competition.
The injection mold is composed of several steel plates with various parts, and is basically divided into:

• A Forming device (concave mold, convex mold)
• B positioning device (guide post, guide sleeve)
• C Fixing device (I-shaped board, code mold pit)
• D Cooling system (water hole)
• E Constant temperature system (heating tube, heating wire)
• F Runner system (pump nozzle hole, runner groove, runner hole)
• G ejection system (thimble, ejector)

5. Die can be divided into three types according to different types of pouring system

• (1) Large nozzle mold: The runner and gate are on the parting line, and they are demolded together with the product when the mold is opened. The design is the simplest, easy to process, and low cost, so more people use the large nozzle system to work.
• (2) Fine nozzle mold: runners and gates are not on the parting line, generally directly on the product, so it is necessary to design an additional set of nozzle parting lines, the design is more complicated, and the processing is more difficult. Generally, the selection depends on the product requirements. Fine nozzle system.
• (3) Hot runner mold: The structure of this type of mold is roughly the same as that of the nozzle. The biggest difference is that the runner is in one or more hot runner plates and hot nozzles with constant temperature. There is no cold material demoulding, runner and pouring. The port is directly on the product, so the runner does not need to be demolded. This system is also called a nozzleless system, which can save raw materials, and is suitable for situations where raw materials are more expensive and products require higher requirements. Design and processing are difficult, and mold costs are higher.

The hot runner system, also known as the hot runner system, is mainly composed of a hot sprue sleeve, a hot runner plate, and a temperature control electric box. Our common hot runner system has two forms: single-point hot gate and multi-point hot gate. The single-point hot gate uses a single hot sprue sleeve to directly inject the molten plastic into the cavity. It is suitable for plastic molds with a single cavity and single gate; the multi-point hot gate branches the molten material to each The sub-heat sprue sleeve then enters the cavity. It is suitable for single-cavity multi-point feeding or multi-cavity molds.

Advantages of hot runner system

• (1) No nozzle material, no need for post-processing, so that the entire molding process is fully automated, saving working time and improving work efficiency.
• (2) The pressure loss is small. The temperature of the hot runner is equal to the nozzle temperature of the injection molding machine, which avoids the surface condensation of the raw materials in the runner, and the injection pressure loss is small.
• 3) The repeated use of the nozzle material will degrade the performance of the plastic, and the use of the hot runner system without the nozzle material can reduce the loss of raw materials, thereby reducing the cost of the product. The temperature and pressure in the cavity are uniform, the stress of the plastic part is small, and the density is uniform. Under a small injection pressure and a short molding time, a better product can be injected than the general injection molding system. For transparent parts, thin parts, large plastic parts or high-demand plastic parts, it can show its advantages, and can produce larger products with smaller models.
• (4) The hot nozzle adopts standardized and serialized design, equipped with various nozzle heads to choose from, and has good interchangeability. The uniquely designed and processed electric heating ring can achieve uniform heating temperature and long service life. The hot runner system is equipped with hot runner plates, thermostats, etc., with exquisite design, diverse types, convenient use, and stable and reliable quality.

Shortcomings of hot runner system application

• (1) The closing height of the overall mold is increased, and the overall height of the mold is increased due to the addition of hot runner plates, etc.
• (2) The heat radiation is difficult to control. The biggest problem of the hot runner is the heat loss of the runner, which is a major issue that needs to be solved.
• (3) There is thermal expansion, thermal expansion and contraction are issues that we must consider when designing.
• (4) The cost of mold manufacturing increases, and the price of standard parts of the hot runner system is higher, which affects the popularization of hot runner molds.

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