[Abstract]
  The hot runner system is an important component of plastic mold production equipment, used in conjunction with the molding of various types of plastic parts. Its working principle is to install a manifold and hot nozzles in the injection mold, and use the principles of heating and temperature control to keep the runner part of the mold in a molten state. The gate of the product is just like directly contacting the nozzle of the injection molding machine, so the finished product will not have one or more sprue tails when demolded. A complete hot runner system consists of a balanced manifold, hot nozzles, and a high-precision temperature controller. With the development of the mold industry, the hot runner system has also developed rapidly. This article details the application and selection considerations of mold hot runner systems.
Keywords: Mold; Hot Runner Technology; Manifold; Temperature Controller; Temperature Control
Mold; hot runner technology; shunting plate; temperature control
1. Introduction
Modern molds are different from traditional molds. They not only have very complex shapes and structures but also require higher technical standards. It is obviously difficult to manufacture them using traditional mold making methods. It is necessary to rely on modern science and technology and adopt advanced manufacturing technologies to meet the technical requirements. The current development characteristics of the entire industrial production are a large variety of products, rapid updates, and fierce market competition. To meet the market's urgent requirements for short delivery times, high precision, and low costs in mold manufacturing, cavity molds will have the following development trends:
(1) Increasingly high mold precision.
Ten years ago, the precision of precision molds was generally 5 μm, now it has reached 2~3 μm, and soon 1 μm molds will be on the market. With the miniaturization of parts and the improvement of precision requirements, some molds require machining precision within 1 μm, which requires the development of ultra-precision machining.
(2) Increasingly large molds.
On the one hand, this is due to the increasing size of parts molded by molds, and on the other hand, it is due to the multi-cavity mold (some now have hundreds of cavities per mold) required for high production efficiency.
(3) Increasing number of high-grade molds.
First, nano-level precision machining for automobiles, aircraft, and precision machinery; second, submicron-level (0.01 μm) precision machining for disk and drum manufacturing; third, millimicron-level (0.001 μm) precision machining for ultra-precision electronic devices.
Further increase in gas-assisted molds and high-pressure injection molding molds.
With the continuous improvement and development of plastic molding processes, gas-assisted molds and high-pressure injection molding molds have also developed to improve injection quality.
(4) Increasing proportion of plastic molds.
With the continuous improvement of the performance of plastic raw materials, the process of replacing steel with plastic and wood with plastic for parts in various industries will further accelerate, and the proportion of using plastic molds will increase day by day.
(5) Increasing number of extrusion molds and powder forging molds.
Due to the lightweight development of products such as automobiles, vehicles, and motors, such as replacing steel with aluminum, non-full density molding, polymer materials, composite materials, engineering ceramics, and ultra-hard material molding and processing. The adoption of new materials not only changes the product structure and performance but also brings about fundamental changes in the production process. Correspondingly, liquid (semi-solid) extrusion molds and powder forging molds have emerged. These molds require high manufacturing precision.
(6) Increasingly widespread application of standard mold parts.
The standardization of molds and the application of standard mold parts will greatly affect the mold manufacturing cycle, and also improve the quality of molds and reduce mold manufacturing costs.
(7) Vigorously develop rapid mold manufacturing.
Currently, it is an era of multi-variety and small-batch production. On the one hand, the product service cycle is shortened, and on the other hand, the variety update is fast, which requires the mold production cycle to be as short as possible. Therefore, rapid prototyping molds will attract more and more attention.
(8) Expand the application of hot runner technology.
Molds adopting hot runner technology can improve the production efficiency and quality of parts, and greatly save raw materials for parts. Therefore, the application of hot runner technology has developed rapidly abroad. More than 50% of the molds produced by many plastic mold factories adopt hot runner technology, even more than 80%, with very obvious effects. Hot runners have also been used in production in China, and the utilization rate of some enterprises has reached 20%~30%.
2. Working Principle of Hot Runner System.
The hot runner system is mainly composed of a manifold, hot nozzles, and a temperature control box. By means of heating the system through which the injection material passes and precise temperature control means, the molten plastic material is directly delivered to the mold cavity through a precisely designed runner and hot nozzles, effectively reducing the pressure in the runner (thus a smaller injection molding machine can be used, saving costs), improving melt fluidity, uniform material density, reducing internal stress of the product, resulting in smaller product deformation, better product surface quality and mechanical properties. It is more advantageous for transparent parts, thin parts, large parts or plastic parts with special requirements (such as engineering plastic parts). Another feature of the hot runner system is that the hot nozzles directly inject molten plastic material into the mold cavity, forming an automatic sprue break during demolding, eliminating all or most of the runner waste caused by traditional injection processes, no post-processing is required, and a large amount of materials are saved. In addition, due to the elimination of additional heat from waste, the cooling time of the mold is only the cooling time of the plastic part, shortening the molding cycle, improving the degree of production automation and production efficiency. More importantly, the saving of materials and energy is of greater significance for environmental protection.
In general, using a hot runner system during injection can save 10%~20% of raw materials, reduce costs by about 30%, while the investment only increases by 10%~30%. Therefore, hot runner systems have been widely used abroad, and have also been regarded as one of the promotion technologies for the future development of injection molds in China.
3. Composition, Advantages and Disadvantages of Hot Runner System
Although there are many hot runner manufacturers and various hot runner product series in the world, a typical hot runner system consists of the following major parts: 1. Manifold; 2. Nozzle; 3. Mold temperature controller, auxiliary parts (others).
(1) Advantages of Hot Runner Molds
Hot runner molds are widely used in industrial developed countries and regions around the world. This is mainly because hot runner molds have the following significant characteristics: 1. Shorten the part molding cycle. Because there is no limitation of cooling time of the runner system, the part can be taken out immediately after molding and solidification. The molding cycle of many thin-walled parts produced by hot runner molds can be within 5 seconds. 2. Save plastic raw materials. In pure hot runner molds, there is no cold runner, so there is no production waste, which is particularly significant for application projects with expensive plastic prices. In fact, major international hot runner manufacturers have developed rapidly in the era of expensive oil and plastic raw materials in the world, because hot runner technology is an effective way to reduce waste and material costs; 3. Reduce waste products and improve product quality. In the molding process of hot runner molds, the temperature of plastic melt is accurately controlled in the runner system, and the plastic can flow into each cavity in a more uniform state, resulting in parts with consistent quality. Parts formed by hot runners have good gate quality, low residual stress after demolding, and small part deformation. Therefore, many high-quality products on the market are produced by hot runner molds, such as many plastic parts in Motorola mobile phones, HP printers, and DELL laptops that people are familiar with are made by hot runner molds; 4. Eliminate subsequent processes and facilitate production automation. After molding by hot runner molds, the parts are finished products, without processes such as trimming gates and recycling and processing cold runners, which is conducive to production automation. Many foreign product manufacturers combine hot runners with automation to greatly improve production efficiency; 5. Expand the application scope of injection molding processes. Many advanced plastic molding processes are developed on the basis of hot runner technology, such as PET preforming, multi-color co-injection in molds, multi-material co-injection processes, stack mold, etc.
(2) Disadvantages of Hot Runner Molds
Although hot runner molds have many significant advantages compared with cold runner molds, mold users also need to understand the disadvantages of hot runner molds. Summarized as follows: 1. Increased mold cost. Hot runner components are relatively expensive, and the cost of hot runner molds may increase significantly. If the part output is small and the proportion of mold tooling cost is high, it is not cost-effective economically. For mold users in many developing countries, the high price of hot runner systems is one of the main problems affecting the widespread use of hot runner molds; 2. High requirements for hot runner mold manufacturing process equipment. Hot runner molds require precision processing machinery as a guarantee, and the integration and coordination between the hot runner system and the mold are extremely strict. Otherwise, many serious problems will occur during mold production, such as plastic overflow due to poor plastic sealing, damage to hot runner components and interruption of production, poor relative position between nozzle inserts and gates leading to serious decline in product quality, etc. 3. Complex operation and maintenance. Compared with cold runner molds, hot runner molds are complex to operate and maintain. If used and operated improperly, it is easy to damage hot runner parts, making production impossible and causing huge economic losses. For new users of hot runner molds, it takes a long time to accumulate experience in use.
4 Key Technologies for Hot Runner Application
A successful hot runner mold application project requires multiple links to be guaranteed. Among them, the two most important technical factors are: one is the control of plastic temperature, and the other is the control of plastic flow.
(1) Control of plastic temperature. The control of plastic temperature is extremely important in the application of hot runner molds. Many processing and product quality problems in the production process are directly caused by poor temperature control of the hot runner system. For example, poor product gate quality when using hot pin gate injection molding, inconsistent part filling time and quality in multi-cavity molds, etc. If possible, try to choose a hot runner system with multi-zone separate temperature control to increase the flexibility and adaptability of use.
Moreover, with the increasing requirements for molds, higher requirements are put forward for temperature control. At present, there are not many units that can invest in R&D and production in this field. China is still using imitation products imported from abroad in the early 1980s, while Suzhou Bozhen Technology Co., Ltd.'s independently developed eMTC series adopts single-chip microcomputer principle to realize multi-channel parallel control, with higher control precision, greater integration, sensitive response, volume only one-fourth of ordinary temperature control equipment, and weight only one-fourth of other equipment, becoming a professional enterprise in this field. It is currently the only product that can be replaced with international equipment.
(2) Control of plastic flow. The plastic must flow balanced in the hot runner system, and the gates must open at the same time to make the plastic fill each cavity synchronously. For cavities with vastly different part weights, the runner size design must be balanced. Otherwise, some parts will have insufficient filling and holding pressure, while others will have excessive filling and holding pressure, excessive flash, poor quality and other problems. The runner size design of the hot runner should be reasonable. If the size is too small, the filling pressure loss will be too large; if the size is too large, the hot runner volume will be too large, and the plastic will stay in the hot runner system for too long, damaging the material performance and resulting in parts that cannot meet the use requirements after molding. There are already CAE software in the world to help users carry out optimal runner design, such as MOLDCAE.
(3) Application scope of hot runner molds.
a. Types of plastic materials.
Hot runner molds have been successfully used to process various plastic materials. Such as PP, PE, PBT, PA, PSU, PC, POM, LCP, PVC, PET, PM, ABS/PC, etc. Any plastic material that can be processed with cold runner molds can be processed with hot runner molds.
b. Part size and weight.
The smallest parts manufactured by hot runner molds are below 0.1g, and the largest are above 30Kg, with extremely wide and flexible applications.
c. Industrial fields.
Hot runner molds are widely used in electronic, automotive, medical, daily necessities, toys, packaging, construction, office equipment and other industrial sectors.
5 International Production Status of Hot Runner Molds
Hot runner mold production is extremely active in industrially developed countries and regions in the world. The proportion of hot runner molds is constantly increasing. Many small mold factories with fewer than 10 people produce hot runner molds. Overall, North America and Europe have used hot runner technology for a long time, with more experience and higher levels. In Asia, besides Japan, Singapore, South Korea, Taiwan, and Hong Kong are in a leading position. Although North America and Europe have high mold manufacturing levels, their prices are high and delivery times are long. In contrast, Asian hot runner mold manufacturers are more competitive in terms of price and delivery time, while China's hot runner molds are still in the initial stage, but are growing rapidly with an increasing proportion.
6 Precautions for Selecting and Purchasing Hot Runner Systems
Hot runner system is a relatively complex technology in the field of injection molding. There are many issues to consider when mold designers, manufacturers and mold users select and purchase hot runner systems.
6.1 Selection of Hot Runner Suppliers
When a mold manufacturing company designs and manufactures a hot runner mold, it is not simply buying a hot runner system from a hot runner supplier and installing it on the mold. In fact, between the mold manufacturing company and the hot runner supplier, from the conceptual design stage of the hot runner mold to the actual mold manufacturing process, and finally to the use of the mold, there are many detailed and close technical issues. Therefore, mold manufacturing companies and mold users should select a good hot runner supplier. When selecting a hot runner supplier, focus on two basic aspects: one is the variety, quantity and quality of hot runner components produced by the supplier, and the other is the technical support and after-sales service provided by the supplier in the region where the mold user is located.
(1) Variety, quantity and quality of hot runner components.
There are three main types of hot runner systems: HOT TIP, SPRUE GATING and VALVE GATING. In fact, each type can be subdivided into many product series. For example, the runner cross-sectional diameter in HOT TIP nozzles can often range from 4mm to 12mm, and the nozzles are made into different application series accordingly. Nozzle gate inserts also have many variants to meet different application requirements. Therefore, the richer the product series of the selected hot runner supplier, the better, so that users can have greater selectivity of hot runner components, and the types, sizes, weights and application ranges of plastic parts produced by hot runners will be wider, which is more conducive to optimizing the injection production process and improving product quality.
When processing plastic products by injection molding, the hot runner system and hot runner mold work under high temperature and high pressure dynamic load conditions. There are many factors leading to the failure of hot runner system components, and injection molding is mainly used for mass production of plastic parts. Once there is any production shutdown, the economic loss is very serious. Therefore, the quality and reliability of the hot runner system are very important. Customers should thoroughly understand and investigate the quality and application history of hot runner components produced by the supplier. Some excellent hot runner manufacturers have obtained ISO quality standard certification from international organizations.
(2) Technical support and after-sales service.
When using hot runner molds, in addition to first selecting a reliable hot runner system, users should also consider the important factor of whether timely and effective technical support and after-sales service can be obtained in case of any problems with the hot runner. Many hot runner suppliers have strong technical support and service networks in their home countries and regions, and are equipped with full-time service personnel who can visit customers at any time for troubleshooting. However, these manufacturers often only set up some offices focusing on hot runner product sales in other countries and regions far from their headquarters, and the technical support is relatively inferior. Users should pay attention to this factor.
6.2 Several Technical Issues in Selecting and Purchasing Hot Runner Systems
Users will involve many specific technical links when selecting and purchasing hot runner systems. If users have good technical knowledge related to hot runners, it will be easier to select and purchase the most suitable hot runner system, ensuring the smooth progress of the subsequent injection production process and improving product quality.
(1) Correct selection of hot runner product series.
Hot runner suppliers often make their hot runner components into product series according to the size and weight of processed plastic parts. For example, large plastic parts are processed with large-size nozzles, and small plastic parts are processed with small-size nozzles. Therefore, after users correctly select the type of hot runner (whether to use hot tip or valve pin system), the next step is the selection of hot runner product series, which in turn determines the structural size and design and manufacture of the mold. If the hot runner product series is selected incorrectly, and it is found later in mold processing or plastic part production, the error is very serious and difficult to remedy. To help correctly select the hot runner product series, each hot runner supplier has some guiding technical documents for reference. Users should closely cooperate with hot runner suppliers to select the best hot runner product series.
(2) Injection pressure loss in hot runner systems.
The injection pressure loss in the hot runner system cannot be ignored. Many users of hot runner molds have misunderstandings, believing that compared with cold runners, the injection pressure of hot runners is much smaller, because the plastic melt in the hot runner is always hot during the entire injection process, but in fact, the situation is often the opposite. In hot runner molds, due to the structural design requirements of the hot runner, the flow distance of the melt in the hot runner system will be greatly increased, so the injection pressure loss in the hot runner system is often relatively large. In practical applications, there are many cases where injection molding is difficult due to excessive injection pressure loss in the hot runner system. Therefore, for processing plastics with poor fluidity (such as PC, POM, etc.), large flow distance of melt in the hot runner system or large part weight, CAE software should be used for runner analysis and calculation. CAE software focusing on runner analysis and calculation includes MoldCAE, etc.
(3) Non-standard hot runner systems.
All hot runner manufacturers provide both standard and non-standard hot runner systems. If possible, users should try to choose standard hot runner systems, that is, try to choose nozzles, hot runner plates, gate inserts, etc. with standard lengths and sizes. The advantage is that standard hot runner systems are cheaper than non-standard ones, have much shorter delivery times, and parts are interchangeable, which is conducive to future use and maintenance. Once a part is damaged, a standard part can be bought and installed. Common shapes of standard hot runner plates include 2-cavity 1-row, 4-cavity 1-row, 8-cavity 1-row, 4-cavity X-shape, 8-cavity XX-shape, etc.
(4) Selection of the number and layout of cavities on the mold.
When designing a hot runner mold and selecting the number of cavities, in addition to placing as many cavities as possible to improve production efficiency, users should also consider the design of the hot runner. The selection of the number and layout of cavities on the mold should be conducive to the flow balance of plastic melt in the hot runner system. For example, if several cavities of the same shape are arranged in a row, it is best to choose 2 or 4 cavities instead of 3. Because a mold with 2 or 4 cavities in a row can have its hot runner designed as a completely naturally balanced system. On the contrary, a mold with 3 cavities requires artificial flow balance of the hot runner manifold, and different runner sizes are used for different flow paths on the hot runner manifold to try to achieve flow balance. The quality of flow balance depends on the work quality of specific hot runner designers. Therefore, users should try to choose the number of cavities conducive to flow balance (such as choosing 16 cavities instead of 15 cavities, etc.) to eliminate any mistakes caused by artificial design of flow balance.
(5) Limitations of minimum cavity distance.
When designing molds for producing micro parts, people generally hope to arrange the cavity distance as close as possible, so that the mold can be more compact and more cavities can be placed, but the minimum cavity distance will be limited by the minimum distance between hot runner components such as nozzles. Therefore, when designing molds with very close cavity distances, users should check the minimum allowable nozzle distance to avoid mold design errors.
(6) Type of processed plastic.
When selecting a hot runner system, the type of processed plastic is a very important consideration. For processing glass-reinforced plastics (such as glass-reinforced nylon materials, etc.), wear-resistant gate inserts should be selected; for processing plastics that are easy to thermally decompose (such as PVC), a hot runner system with unobstructed runners and no flow dead ends should be selected; for processing plastics with poor fluidity (such as PC), larger nozzle series and larger runner cross-sectional sizes in the hot runner plate should be considered.
(7) Maturity of hot runner products.
The maturity and application history of each hot runner product are different. A newly launched hot runner product needs a long time to be gradually improved. While continuously introducing new products, hot runner manufacturers will also eliminate hot runner products that are proven to be inappropriate through practice. Therefore, users should try to choose hot runner products with good maturity, popularity and long application history. For such products, both hot runner manufacturers and other hot runner users have more experience and successful cases for new users with little experience to learn from.
(8) Status of hot runner system before delivery.
Some hot runner suppliers will conduct certain tests on the system before delivering it to users, and even conduct actual injection molding tests for very important application projects, but the test scope of each hot runner supplier before delivery is different, and users should understand it to be well-informed.
(9) Multi-zone temperature control of hot runners.
If users need to purchase large-size and complex hot runner systems, or process plastics that are sensitive to temperature and have narrow processing parameter ranges, they should choose a hot runner design scheme with multi-zone separate temperature control, so that users can locally adjust and control the temperature distribution as needed. An ideal hot runner system should have a uniform temperature distribution, but in fact, there are many reasons for temperature differences in various parts of the hot runner, such as the quality of hot runner heating elements, excessive heat loss at the junction between the hot runner system and the mold, different shear heat of plastic melt in various parts of the hot runner, etc. The larger and more complex the hot runner system, the more it should choose a hot runner system with regional temperature control.
(10) Application projects with color change requirements.
Some users produce the same type of plastic parts with different colors using one mold, which is an application project with color change requirements. When ordering hot runners, try to choose a hot runner system with a small runner volume to accelerate the color change process and reduce waste. At the same time, any runner turns in the hot runner system must be rounded and smooth without flow dead ends. If a valve hot runner system is used to produce plastic parts with color change requirements, there will often be flow dead ends behind the valve pin, which must be given special attention.
(11) Molds with multiple heterogeneous cavities in one mold.
When designing a mold with multiple heterogeneous cavities in one mold, the problem of flow balance must be considered. If the part sizes and weights differ greatly, and the injection pressure difference between each cavity is more than 200~300bar, it is difficult to achieve flow balance by changing the runner size in the hot runner system. If the flow is unbalanced in a mold with multiple heterogeneous cavities in one mold, some parts will have insufficient filling and holding pressure, while others will have excessive filling, large flash, high residual stress and other problems. At this time, consideration should be given to using a valve hot runner system or changing the overall mold design scheme. The valve system allows users to close the gates of cavities that are filled early at an appropriate time to avoid overfilling of these cavities.
(12) Proportional relationship between cavity and hot runner volume.
Compared with the cavity volume, the runner volume of the hot runner system should not be too large, otherwise the plastic melt will stay in the hot runner system for too long, causing thermal decomposition and unable to produce qualified plastic parts. If the weight of the plastic part is too small, a combined cold and hot runner scheme should be adopted. The use of cold runners increases the injection volume, which helps to improve the proportional relationship between cavity and hot runner volume and shorten the residence time of plastic melt in the hot runner system.
(13) Adoption of trial molds.
Hot runner molds are relatively expensive, especially high-cavity hot runner molds (such as 96, 128 cavities), which are even more expensive. If it is to open up a new application field with insufficient experience, or adopt relatively new hot runner components (such as nozzles or new gates), consideration should be given to first manufacturing a simple single-cavity trial mold to conduct feasibility research and verification of the scheme, and after gaining sufficient experience, then manufacturing the expensive formal working mold.
(14) Drawing of hot runner components on mold drawings.
Hot runner suppliers generally make their hot runner components into electronic libraries for users to use. When designing and drawing mold drawings, mold designers can select the required nozzle and other component drawings from the hot runner component electronic library and place them in the appropriate mold positions. Since hot runner suppliers often update and upgrade their hot runner products, users should pay attention to constantly obtaining the latest libraries from hot runner manufacturers. Although people have begun to design molds using 3D methods, at present, the vast majority of electronic libraries of hot runner components from various hot runner suppliers are still 2D. Some manufacturers have begun to establish 3D hot runner component libraries to meet the needs of 3D mold design development.
(15) Quality warranty period.
Hot runner suppliers will provide some warranty periods for their hot runner products, ranging from 1 year to 5 years, depending on the hot runner manufacturer and the specific hot runner product items purchased by the user. During the warranty period, if the hot runner product has problems under normal operation and use by the user, the hot runner supplier will replace it free of charge. The longer the warranty period, the more beneficial it is to the user. To ensure the validity of the quality warranty period, users should follow the hot runner use and maintenance regulations, and shall not modify or alter any hot runner components without the consent of the hot runner supplier.