Injection Mold Hot Runner Technology Knowledge Sharing

The hot runner can be independently heated and thermally insulated in the injection mold, which alone compensates for heat loss due to contact with the "cold" mold. Hot runner molds have been successfully used to process various plastic materials, and almost all plastic materials that can be processed with cold runner molds can be processed with hot runner molds.

The smallest parts are under 0.1 grams and the largest are over 30 kilograms. Hot runner molds are widely used in electronics, automotive, medical, daily necessities, toys, packaging, construction, office equipment and other fields.

A successful hot runner mold application project requires multiple links to ensure it.

The most important of these are two technical factors: the control of the temperature of the plastic and the control of the flow of the plastic.

A typical hot runner system consists of the following parts:

1) Hot runner plate (MANIFOLD)

2) Nozzle (NOZZLE)

3) Temperature controller

4) Auxiliary parts

The advantages of hot runner mold:

1) Shorten the molding cycle of parts;

2) Save plastic raw materials;

3) Reduce waste and improve product quality;

4) Eliminate subsequent processes, which is conducive to production automation;

5) Expand the application of injection molding process.

At the same time, there are also disadvantages such as rising mold costs, high requirements for manufacturing process equipment, and complicated operation and maintenance.

Types and applications of hot runner systems

When applying hot runner technology, the correct choice of gate type is crucial. The gate type directly determines the selection of hot runner system components and the manufacture and use of molds. Therefore, according to the different gate types, the hot runner system can be divided into three types: hot tip type hot runner system, sprue type hot runner system, valve type hot runner system, etc. Each type of hot runner system has its important application. Features and scope of application.

There are many factors that need to be considered when choosing the type of gate and hot runner system, the most important of which are the type of plastic matrix and additives, the weight and size of the part, the wall thickness of the part, the quality requirements of the part, the life of the tool and the production of the part.

1 Hot tip hot runner system (HOT TIP)

Its working principle is to precisely adjust and control the plastic molding processing temperature at the gate through the combination of the insert HOT TIP at the front end of the nozzle and the cooling system. Therefore, the manufacturing material and shape design of the nozzle insert HOT TIP are very important.

Hot tip (HOT TIP) hot runner system can be used to process most crystalline and amorphous plastics such as PP, PE, PS, LCP, PA, PET, PBT, PEEK, POM, PEI, PMMA, ABSPVC, PC , PSU, TPU, etc. Generally speaking, hot tip gates are mostly used for the processing of small and medium-sized parts, especially for the processing of small parts.

The gate section diameter is mostly between 0.5mm-2.0mm. The determination of the gate section diameter is mainly determined by the weight and wall thickness of the part, and of course the material and part quality requirements are also considered. If a gate with a smaller cross-sectional diameter is used, the gate will be closed quickly after the injection and filling stage, the gate trace on the part will be small, and the surface of the part will be beautiful and quality.

If the gate diameter is too small, the shear rate will be too high when the plastic flows through the gate, which will seriously damage the molecular chain structure of the plastic melt or the added materials in the plastic, resulting in unqualified product quality and unable to meet the use requirements. The usual practice in the selection of gate size is to preliminarily determine the gate size according to the wall thickness at the gate of the part: gate diameter = (0.75-1.0) wall thickness at the gate of the part. Take the smaller value for processing easy-flowing plastics, and take the larger value for processing difficult-to-flow plastics or shear-sensitive plastics.

Usually the hot tip gate is opened directly on the part, or it can be opened on the cold runner and then the cold gate is opened on the part. This is a mold system that combines hot runner and cold runner. When applying hot tip gates to make plastic parts, there will always be more or less gate marks on the part. In many cases, the gate marks will be higher than the surface of the part, affecting the beauty of the part or affecting the assembly with other parts. Therefore, when choosing a gate location, try to place the gate in a recessed recess on the part.

2 Sprue Hot Runner System (SPRUE GATING)

In the sprue type hot runner system, the plastic enters the mold cavity through an open runner (OPEN PIPE). The plastic flow pressure loss at the gate is small. The sprue type hot runner system is more suitable for the injection molding of parts with medium size and weight.

The advantage of using the sprue type hot runner system is that the shear rate experienced by the plastic when it flows through the gate is low, the residual stress after the part is formed is small, the degree of deformation is small, and the mechanical strength of the part is relatively good. Sprue gates are larger in size than hot tip gates, so gate traces may also be larger.

Therefore, in general, plastic injection molding is often used for molds that require stricter gate aesthetics; internal structural parts that do not require high gate aesthetics can be manufactured by sprue-type hot runner systems.

People also often use the sprue gate in combination with the cold runner, that is, the sprue nozzle is used as the main runner, and the sprue gate is opened on the cold runner. In this application, the sprue gate can be opened wider to facilitate plastic flow, since no one cares about the size of the gate trace on the cold runner.

Similar to the application of hot tip hot runner systems, the control of plastic temperature at the gate and mold temperature is extremely important. A separate cooling circuit is required around the gate. Because the size of the sprue-type gate is relatively large, if the temperature at the gate is not well controlled, the problem that the gate cannot seal the plastic flow after the mold is opened is more likely to occur.

The nozzle inserts of the sprue type hot runner system also have various sizes, shapes and manufacturing materials, which should be comprehensively considered in combination with the type of plastic to be processed (such as crystalline plastic or amorphous plastic) and the cooling of the gate.