The hot runner gating system can be understood as an extension of injection molding machinery. The function of the hot runner system is to deliver the thermoplastic melt to the vicinity of the molding die or directly into the die adiabatically.
The hot runner can be heated independently but is thermally insulated in the injection mold so that the heat loss caused by contact with the cold mold can be separately compensated.
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 part is less than 0.1 grams, and the largest part is more than 30 kilograms. Hot runner molds are widely used in the fields of electronics, automobiles, medical care, daily necessities, toys, packaging, construction, and office equipment.
A successful hot runner mold application project requires multiple links to be guaranteed. There are two most important technical factors, one is the control of plastic temperature; the other is the control of plastic flow.
A typical hot runner system consists of the following parts
Advantages of hot runner injection 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 technology.
At the same time, there are also shortcomings such as rising injection mold costs, high manufacturing process equipment requirements, and complicated operation and maintenance.
Types and applications of hot runner systems
When applying hot runner technology, the correct choice of gate type is very important. The gate-type directly determines the selection of hot runner system components and the manufacture and use of injection molds.
Therefore, according to the different gate types, the hot runner system can be divided into three types: hot tip hot runner system, sprue hot runner system, and valve hot runner system. Each type of hot runner system has its important features and scope of application.
Many factors need to be considered when selecting the types of gates and hot runner systems, the most important of which are the types of plastic matrix and additives, the weight and size of the parts, the quality requirements of the parts, the life of the tools, and the production requirements of the parts.
1. Hot Tip Hot Runner System
Its working principle is to combine the HOT TIP at the front end of the nozzle with the cooling system to precisely adjust and control the plastic injection molding processing temperature at the gate. Therefore, the manufacturing material and shape design of the nozzle insert HOT TIP is very important.
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, ABS PVC, 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 diameter of the gate section is mostly between 0.5mm-2.0mm.
The determination of the diameter of the gate section is mainly determined by the weight and wall thickness of the part, and of course, the material and part quality requirements must also be considered.
If a gate with a smaller cross-sectional diameter is used, the gate will be closed quickly after the injection filling stage is completed, the gate marks on the part will be small, and the surface of the part will be of good appearance 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 additive materials in the plastic, resulting in unqualified products that cannot meet the requirements of use.
In the selection of the gate size, the usual practice is to initially determine the gate size according to the wall thickness at the part gate: gate diameter = (0.75-1.0) the wall thickness at the part gate. The smaller value is used for processing easy-flow plastics, and the larger value is used for processing hard-flowing plastics or plastics that are sensitive to shear.
Usually, the hot tip gate is opened directly on the part, but it can also be opened on the cold runner and then the cold gate is opened on the part. This is a mold system that combines hot runners and cold runners.
When using hot tip gates to make plastic parts, there will always be more or fewer gate marks on the parts. Many times 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 selecting the gate location, try to place the gate in the recessed concealed part on the part.
2. Sprue gates
In the sprue 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 hot runner system is more suitable for injection molding of parts of medium size and weight. The advantage of using a sprue hot runner system is that the plastic undergoes a low shear rate when it flows through the gate and fills the mold, the residual stress after the part is formed is small, the degree of deformation is small, and the mechanical strength of the part is better.
Compared with hot tip gates, sprue gates are larger in size, so gate marks may also be larger. Therefore, under normal circumstances, molds with strict requirements for the aesthetics of the gate often use plastic injection molding; and internal structural parts that do not require high aesthetics for the gate can be manufactured by a sprue-type hot runner system.
People often use the sprue in combination with the cold runner, that is, the sprue nozzle is used as the main runner, and the sprue is opened on the cold runner.
In this application, the sprue gate can be opened larger to facilitate the flow of the plastic, because no one cares about the size of the gate marks on the cold runner.
Similar to the application of hot-tip hot runner systems, the control of the plastic temperature and mold temperature at the gate is extremely important. A separate cooling circuit needs to be set up around the gate.
Because the size of the sprue gate is relatively large, if the temperature at the gate is not well controlled, it is more likely that the gate cannot seal the plastic flowing after the injection mold is opened.
The nozzle insert of the sprue hot runner system also has a variety of sizes and shapes and changes in manufacturing materials, which must be considered in combination with the type of plastic to be processed (such as crystalline plastic or amorphous plastic) and the cooling of the gate.
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