Selection skill and use method of hot runner for mold

Time:2019-09-30 09:12:05 / Popularity: / Source:

Hot runner selection and use

Hot runner system is a relatively complex technology in the field of injection moulding process. There are a number of issues that need to be considered when plastic injection mold designer / custom plastic mold manufacturers / mold users choose and purchase hot runner systems. This article lists 15 principles you need to know specifically for these who want to buy a hot runner system:
hot runner 

Hot runner supplier selection

When designing a hot runner mold, custom plastic mold manufacturers does not simply go to hot runner supplier to buy a hot runner system and put it on mold. From conceptual design stage of hot runner mold to actual production process, to final use of mold, there are many close technical and trade coordination and cooperation between custom plastic mold manufacturers and hot runner supplier. Therefore custom plastic mold manufacturers and mold users should choose a good hot runner supplier.
When choosing a hot runner supplier, focus on two basic aspects:

1. Number and quality of hot runner components

Hot runner system has three types: hot tip type, SPRUEGATING and VALVE GATING. Each type can be subdivided into many product lines. For example, diameter of runner cross section in hot tip nozzle can vary from 4 mm to 12 mm, and nozzle is also made into a product series of different sizes. There are also many variations of nozzle gate inserts to meet different application requirements, so hot runner supplier's product range is as rich as possible. In this way, user can have greater selectivity of hot runner components, and type, size and weight of plastic moulded parts produced by hot runners are wider, which is more conducive to optimizing injection moulding process and product quality.
hot runner system 
Ordinary valve gating system
In the injection moulding process of plastic moulded parts, hot runner system and hot runner mold are operated under high temperature and pressure load conditions. There are many factors that cause failure of components of hot runner system, and injection moulding process is mainly applied to production of large-scale plastic moulded parts; once there is any shutdown, economic loss is very serious.
Ordinary open system
Therefore, quality and reliability of hot runner system is very important. Customers should have an in-depth understanding of quality and application history of hot runner components produced by hot runner suppliers. Some of better hot runner manufacturers have been certified by international organizations as ISO quality standards.

2. Technical support and after-sales service

In the use of hot runner mold, in addition to choice of a reliable hot runner system, users should also consider in case there is a problem with hot runner, whether it can get timely and effective technical support and after-sales service.
Many hot runner suppliers have a strong technical support and service network in the countries and regions where their company headquarters are located, and are equipped with full-time service staff who can access customers at any time for troubleshooting. However, in other countries and regions where their companies are away from their headquarters, these companies often only have offices that focus on sales of hot runner products. Technical support is relatively inferior, and users should pay attention to this factor.
hot runner 

Hot runner technical details

Users will involve many specific technical aspects when selecting and purchasing hot runner systems. If user has good technical knowledge related to hot runners, he can easily select and purchase the most suitable hot runner system to ensure smooth and improved product quality in subsequent injection moulding process.

1. Correct choice of hot runner product series

Hot runner suppliers often make their hot runner components into a product line according to size and weight of processed plastic moulded parts. For example, large plastic moulded parts are processed using large-sized nozzles, and small plastic moulded parts are processed with small-sized nozzles. Therefore, after user selects hot runner type correctly (whether it is a hot tip or a valve system), it is a choice of hot runner product series; and thus determines structural size and design of mold.
If hot runner product line is not selected correctly, it will be discovered in later stages of mold processing or in production of plastic moulded parts. Error is very serious and difficult to save. To help you properly select hot runner product line, each hot runner supplier has some instructive technical documentation for reference. Users should work closely with hot runner suppliers to select a hot runner product line.
hot runner system 

2. Injection pressure loss in hot runner system

Injection pressure loss in hot runner systems cannot be ignored. Many hot runner mold users have a misconception that injection pressure loss of hot runner is much smaller than that of cold runner. Reason is that plastic solution in hot runner is always hot throughout injection moulding process.
Cylinder wire rack system
In fact, in many cases, situation is just opposite: in hot runner mold, due to design of hot runner structure, activity interval of solution in hot runner system is greatly increased. Therefore, injection pressure loss in hot runner system is also often large. In practical applications, there are many cases in which injection moulding process is difficult due to excessive injection pressure loss in hot runner system. Therefore, for plastics with poor processing activity (such as PC, POM, etc.), large active spacing of solution in hot runner system, or large weight of workpiece, CAE software should be used for flow channel analysis and calculation.

3. Selection of standard and non-standard hot runner systems

Every hot runner manufacturers offer both standard and non-standard hot runner systems. If possible, users should try to choose a standard hot runner system.
Advantage is that standard hot runner system is less expensive than non-standard hot runner system and delivery time is much shorter. And parts are interchangeable, which is conducive to future use and maintenance. Once a part is broken, buy another standard part and install it. Common standard hot runner plates are 2 cavity in one raw, 4 cavity in one raw, 8 cavity in one raw, 4 cavity with X shape, 8 cavity with XX shape and so on.

4. Number of cavities on the mold and choice of layout

When designing hot runner mold and selecting number of cavities, in addition to increasing cavity to improve production efficiency, design of hot runner should also be considered. Choice of number of cavities on the mold and layout should be beneficial to balance of activity of plastic solution in hot runner system.
For example, if several cavities with same shape are arranged in a row, it is preferable to select two or four cavities instead of three. For mold in the form of 2-cavity or 4-cavity in a raw, hot runner can be designed as a completely natural balanced system. Conversely, 3-cavity mold requires a balance of human activities on hot runner manifold.
Balance of activity depends on the quality of work of specific hot runner design staff, so user should try to choose number of cavities that are conducive to balance of activity (such as selecting 16 cavities instead of 15 cavities, etc.) to eliminate any mistakes caused by balance of human design activities.

5. Minimum cavity spacing limit

When designing molds for production of small parts, it is generally desirable to arrange cavities as close as possible so that molds are compact and may have more cavities. However, minimum cavity spacing is limited by minimum spacing between hot runner components such as nozzles. Therefore, when designing molds with closely spaced cavities, users should pay attention to check minimum nozzle spacing to avoid rework of mold design.

6. Types of processed plastics

Type of plastic processed is a very important consideration when selecting a hot runner system. If processing glass-reinforced plastics (such as glass reinforced nylon materials, etc.), a wear-resistant gate insert should be used. If processing plastics that are easily thermally decomposed (such as PVC), a hot runner system with smooth runners and no dead ends should be used. If processing plastics with poor activity (such as PC), should consider use of larger nozzle series, and use of larger runner cross-section dimensions in hot runner plate.

7. Maturity of hot runner products

A new hot runner product takes a long time to gradually improve. Hot runner manufacturers will continue to introduce new products, and will also eliminate hot runner products that have been proven to be unsuitable. Therefore, users should try to choose hot runner products with good maturity, popular and long history. For such products, whether it is a hot runner manufacturer or another hot runner user, there are more experiences and successful cases for new users with less experience.

8. State of hot runner system before shipment

Some hot runner suppliers perform certain tests on system before shipping their hot runner system to user. Actual injection moulding process experiments are even performed on very important applications. However, each hot runner supplier has a different test range before hot runner system is shipped. Users should be aware of it.

9. Selection of multi-zone temperature control for hot runners

If users need to purchase large and complex hot runner systems, or process plastics that are sensitive to temperature and have narrow processing parameters, they should choose a hot runner design with multiple zones for temperature control. In this way, user can locally adjust and control temperature distribution as needed.
Temperature control box
An ideal hot runner system should have a uniform temperature distribution, but there are actually many reasons for difference in temperature throughout hot runner. For example, quality of hot runner heating element, excessive heat loss at the junction of hot runner system and mold, difference in the heat of plastic solution in hot runner. The larger the size of hot runner system, the more complex it is, and the more multi-zone temperature controlled hot runner system should be selected.
hot runner 

10. What should I do if I have a color change request?

Some users use same mold to produce plastic moulded parts of same variety with different colors, which is an application project with color change requirements.
When ordering hot runners, try to choose a hot runner system with a small sprue to speed up color change process and reduce waste. At the same time, any runner turns in hot runner system must be round and smooth with no dead ends. If valve type hot runner system is used to produce plastic moulded parts with color change requirements, there will always be a dead angle (DEAD SPOT) behind valve needle, which must be paid special attention!

11. One mold with multi-cavity

When designing one mold with multi-cavity, balance of activity must be considered.
If difference in the size and weight of parts is too large, and injection pressure difference of each cavity is above 200-300 BAR, it is difficult to achieve balance of activity by changing size of runner in hot runner system. If activity is unbalanced in a mold-shaped multi-cavity mold, there will be some problems in that filling of parts is insufficient, other parts are overfilled, flash is large, and residual stress is high. At this point, consider using a valved hot runner system or changing overall design of mold. Valve system can close the early filled cavity gates at the appropriate time to avoid overfilling of these cavities.

12. Proportion of cavity and hot runner volume

Runner volume of hot runner system must not be too large compared to volume of cavity. Otherwise, plastic solution will stay in hot runner system for too long and will thermally decompose. Unable to produce qualified plastic moulded parts.
If weight of plastic moulded parts is too small, hot and cold runners should be used. Because injection volume increases after using cold runner, it helps to improve proportional relationship between cavity and hot runner volume, and shortens residence time of plastic solution in hot runner system.

13. Using trial mold

Hot runner molds are more precious, especially hot runner molds with high cavity counts (such as 96, 128 cavities, etc.), and price is higher.
If you want to open up a new field of application, have insufficient experience, or try to use newer hot runner components (such as nozzles or new gates), you should consider making a simple, single-cavity trial mold. To verify feasibility study of program. After gaining sufficient experience, we will make a precious and formal work mold.
hot runner 

14. Drawing of hot runner components on mold drawing

Hot runner suppliers generally make their hot runner components into an electronic library for users to use. When designing mold drawing, injection mold designer can select required nozzle and other component drawings from hot runner component electronic library and put them on the appropriate mold position. Since hot runner supplier will frequently update its hot runner products, users should constantly request the latest library from hot runner manufacturers.
Although people have begun to design molds using 3D methods, most of current hot runner components' electronic libraries are still 2D. Some custom plastic mold manufacturers have begun to set up a 3D hot runner component library to meet needs of 3D mold design development.

15. Quality Warranty Period

Hot runner suppliers offer some warranty periods for their hot runner products. Warranty periods range from one to five years, depending on hot runner product items specifically purchased by hot runner manufacturer and user. During warranty period, if hot runner product has problems under normal operation of user, hot runner supplier will give a free replacement.
Obviously, the longer warranty period, the better for user. To ensure that quality warranty period is valid, users should respect hot runner.

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