Knowledge sharing of injection mold hot runner technology

Time:2021-05-26 11:50:45 / Popularity: / Source:

Hot runner gating system can be understood as an extension of injection molding machinery. Function of hot runner system is to deliver thermoplastic melt to vicinity of molding die or directly into die adiabatically.
Hot runner can be heated independently, but thermally insulated in injection mold, so that it can individually compensate for heat loss caused by contact with "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 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.
Among the most important are two technical factors:
One is control of plastic temperature;
Second is control of plastic flow.
A typical hot runner system consists of following parts:
1) Hot runner plate (MANIFOLD);
2) Nozzle (NOZZLE);
3) Temperature controller;
4) Auxiliary parts.
injection mold hot runner technology 
Advantages of hot runner mold:
1) Shorten 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 application of injection molding technology.
At the same time, there are also shortcomings such as rising mold costs, high manufacturing process equipment requirements, complicated operation and maintenance.

Types and applications of hot runner systems

When applying hot runner technology, correct choice of gate type is very important. Gate type directly determines selection of hot runner system components, manufacture and use of molds. Therefore, according to different gate types, 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 application characteristics and scope of application.
Many factors need to be considered when selecting types of gates and hot runner systems, the most important of which are type of plastic matrix and additives, weight and size of parts, quality requirements of parts, life of tools and production requirements of parts.

1 Hot Tip Hot Runner System (HOT TIP)

Its working principle is to combine HOT TIP at the front end of nozzle with cooling system to precisely adjust and control plastic molding processing temperature at the gate. Therefore, manufacturing material and shape design of nozzle insert HOT TIP are 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, ABSPVC, PC , PSU, TPU, etc. Generally speaking, hot tip gates are mostly used for processing of small and medium-sized parts, especially for processing of small parts.
Diameter of gate section is mostly between 0.5mm-2.0mm. Determination of diameter of gate section is mainly determined by weight and wall thickness of part, of course material and part quality requirements must also be considered. If a gate with a smaller cross-sectional diameter is used, gate will be closed quickly after injection filling stage is completed, gate marks on part will be small, surface of part will be of good appearance and quality.
If gate diameter is too small, shear rate will be too high when plastic flows through gate, which will seriously damage molecular chain structure of plastic melt or added materials in plastic, resulting in unqualified products that cannot meet requirements of use. In the selection of gate size, usual practice is to initially determine gate size according to wall thickness at part gate: gate diameter = (0.75-1.0) wall thickness at part gate. Smaller value is used for processing easy-flow plastics, and larger value is used for processing hard-flowing plastics or plastics that are sensitive to shear.
Usually hot tip gate is opened directly on part, but it can also be opened on cold runner and then cold gate is opened on part. This is a mold system that combines hot runner and cold runner. When using hot tip gates to make plastic parts, there will always be more or less gate marks on parts. Many times gate marks will be higher than surface of part, affecting beauty of part or affecting assembly with other parts. Therefore, when choosing gate location, try to place gate in recessed concealed part on part.


In sprue hot runner system, plastic enters mold cavity through an open runner (OPEN PIPE). Plastic flow pressure loss at gate is small. Sprue hot runner system is more suitable for injection molding of parts with medium size and weight.
Advantage of using sprue hot runner system is that plastic undergoes a low shear rate when it flows through gate and fills mold, residual stress after part is formed is small, degree of deformation is small, mechanical strength of 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 aesthetics of gate often use plastic injection molding; and internal structural parts that do not require high aesthetics for gate can be manufactured by a sprue-type hot runner system.
People often use sprue gate in combination with cold runner, that is, sprue nozzle is used as main runner, and sprue gate is opened on cold runner. In this application, sprue gate can be opened larger to facilitate flow of plastic, because no one cares about size of gate marks on cold runner.
Similar to application of hot-tip hot runner systems, control of plastic temperature and mold temperature at gate is extremely important. A separate cooling circuit needs to be set up around gate. Because size of sprue gate is relatively large, if temperature at gate is not well controlled, it is more likely that gate cannot seal plastic flowing after mold is opened.
Nozzle inserts of sprue hot runner system also have a variety of sizes, shapes and changes in manufacturing materials, which must be considered in combination with types of plastics to be processed (such as crystalline plastics or amorphous plastics) and cooling conditions of gates.

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