Hot runner technology for injection molds

Hot runner pouring systems can be understood as an extension of injection molding machinery. The function of the hot runner system is to adiabatically deliver the thermoplastic melt near the molding mold or directly into the mold.

The hot runner can be heated independently and thermally insulated in the injection mold, thus compensating separately for the heat loss due to contact with the "cold" mold. Hot runner molds have been successfully used to process a wide variety of plastic materials, and almost any plastic material that can be processed with a cold runner mold can be processed with a hot runner mold.

The smallest of its parts is under 0.1 gram and the largest is over 30 kg. Hot runner molds have a wide range of applications in electronics, automotive, medical, daily necessities, toys, packaging, construction, office equipment, and other fields.

A successful hot runner mold application project requires multiple components to be guaranteed.

Two of the most important technical factors are the control of plastic temperature and the control of plastic flow.

A typical hot runner system consists of the following components.

1) Hot runner plate (MANIFOLD)

2) Nozzle (NOZZLE)

3) Temperature controller

4) Auxiliary parts

Advantages of hot runner molds. 

1)Shorten the molding cycle of parts;    

2) Save plastic material;    

3) Reduce scrap and improve product quality;    

4) Eliminate subsequent processes and facilitate production automation;    

5) Expand the application of the injection molding process.   

At the same time, there are also disadvantages such as the increase of mold cost, high requirement of production process equipment, and complicated operation and maintenance.    

Types and applications of hot runner systems    

In the application of hot runner technology, the correct choice of gate type is crucial. Gate type directly determines the choice of hot runner system components and the manufacture and use of the mold. Therefore, according to the different gate types, the hot runner system can be divided into three main types: hot tip hot runner system, gating hot runner system, and valve hot runner system, each type of hot runner system has its important application characteristics and scope of application.

There are many factors 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 wall thickness of the part, the quality requirements of the part, the tool life and the yield requirements of the part.   

1 Hot Tip Hot Runner System (HOT TIP)    

The principle of operation is the combination of a HOT TIP insert located at the front of the nozzle and a cooling system for precise adjustment and control of the plastic molding process temperature at the gate. Therefore, the material and shape of the nozzle insert HOT TIP are very important.   

Hot tip (HOT TIP) hot runner systems can be used to process most crystalline and non-crystalline plastics such as PP, PE, PS, LCP, PA, PET, PBT, PEEK, POM, PEI, PMMA, ABS PVC, PC, PSU, TPU, etc. Generally speaking, hot tip type gates are mostly used for processing small and medium-size parts, especially for processing tiny parts.

Most of the gate cross-section diameters are between 0.5mm-2.0mm. The determination of the gate cross-section diameter is mainly determined by the weight of the part and the wall thickness, but also by the material and the quality of the part. If a smaller gate diameter is used, the gate will be closed quickly after the injection phase, the gate marks on the part will be small and the surface of the part will be beautiful and of good quality.

If the gate diameter is too small, it will lead to a high shear rate when the plastic flows through the gate, which will seriously damage the structure of the plastic melt molecular chain or the added material in the plastic, resulting in unqualified products that cannot meet the requirements of use. In the selection of gate size, the usual practice is to determine the initial size of the gate according to the wall thickness of the part at the gate: gate diameter = (0.75-1.0) the wall thickness of the part at the gate. The smaller value is used for easy flow plastics, while the larger value is used for difficult 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 sprue and then the cold gate is opened on the part. This is a combination of a hot runner and cold runner mold system. When a hot-tip gate is used to make a plastic part, it always leaves more or fewer gate marks on the part. In many cases, the gate marks are higher than the surface of the part, affecting the appearance of the part of the assembly with other parts. Therefore, when choosing the location of the gate, the gate should be placed in the recessed and hidden place on the part as much as possible.   

2 Gating system (SPRUE GATING)    

In a sleeve-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 low. This system is suitable for injection molding of parts of medium size and weight and above.

The advantages of using a sleeve-type hot runner system are low shear rate experienced by the plastic when flowing through the gate to fill the mold, low residual stress after molding, less deformation, and better mechanical strength of the part. Compared to hot tip gates, sleeve-type gates are larger, so gate marks may also be larger.

Therefore, in general, the plastic injection molding method is often used for molds with strict requirements on gate aesthetics, while internal structural parts with low requirements on gate aesthetics can be manufactured by the sleeve type hot runner system.

It is often used in combination with a cold sprue, i.e. the sprue nozzle is used as the main sprue and the sprue is opened on the cold sprue. In this application, the sleeve gate can be opened larger to facilitate the flow of plastic, since no one cares about the size of the gate marks on the cold sprue.   

Similar to the application of a hot tip hot runner system, it is extremely important to control the temperature of the plastic at the gate about the temperature of the mold. A separate cooling circuit around the gate is required. Because of the large size of the sprue, if the temperature at the gate is not well controlled, it is more likely that the gate will not seal the plastic flow after the mold is opened.

The nozzle inserts of the sleeve-type hot runner system are also available in a variety of sizes, shapes, and manufacturing materials, depending on the type of plastic to be processed (e.g. crystalline or non-crystalline plastic) and the cooling of the gate.