Knowledge about hot runner molds

classification:

The hot runner system is divided into an adiabatic flow channel and a micro semi-hot runner system. The design of the adiabatic runner is complex, but the effect is good and the maintenance cost is very low. The structure of the micro semi-hot runner is simple, stable and easy to use, and the failure rate is low. Because of the simple structure, the maintenance cost is low, and the stability of production is more guaranteed.

Hot runner classification:

Open (for micro semi-hot runners), needle valve (for adiabatic runners).

Hot runner systems generally consist of hot nozzles, manifolds, temperature control boxes and accessories. Hot nozzles generally include two types: open hot nozzles and needle valve hot nozzles. Since the form of the hot nozzle directly determines the selection of the hot runner system and the manufacture of the mold, the hot runner system is often divided into an open hot runner system and a needle valve hot runner system. The manifold is used in a multi-cavity or multi-point feed, single point feed but level offset. The material is usually P20 or H13. The manifold is generally divided into two categories, standard and non-standard. The structure is mainly determined by the distribution of the cavity on the mold, the nozzle arrangement and the gate position. The temperature control box includes the main unit, cables, connectors, and wiring male and female sockets. Mold Daren WeChat small hot runner accessories usually include: heaters and thermocouples, runner seals, connectors and junction boxes.

advantage:

Shorten cycle

The molding cycle of the part is shortened. Because there is no limitation of the cooling time of the sprue system, the parts can be ejected in time after solidification. Many thin-walled parts produced with hot runner molds have a molding cycle of less than 5 seconds.

Save plastic

In the pure hot runner mold, there is no cold runner, so there is no production cost. This is especially significant for applications where plastics are expensive. In fact, the major hot runner manufacturers in the world have experienced rapid development in the world when oil and plastic raw materials are expensive. Because hot runner technology is an effective way to reduce material costs and reduce material costs.

Reduce waste

Reduce waste and improve product quality. During the hot runner mold forming process, the plastic melt temperature is accurately controlled in the runner system. Plastics can flow into the cavities in a more uniform state, resulting in consistent quality parts. The hot runner forming parts 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. Many familiar plastic parts such as MOTOROLA phones, HP printers, and DELL laptops are made with hot runner molds.

Production automation

Eliminating subsequent processes is conducive to production automation. After the workpiece is formed by the hot runner mold, it is the finished product, and there is no need to trim the gate and recycle the processed cold runner. Conducive to production automation. Many foreign manufacturers have combined hot runners with automation to significantly increase production efficiency. Many advanced plastic molding processes have been developed based on hot runner technology. Such as PET preforming, multi-color co-injection in the mold, a variety of materials co-injection process, STACK MOLD.

Disadvantages:

Although hot runner molds have many significant advantages over cold runner molds, mold users also need to understand the shortcomings of hot runner molds. To sum up, there are the following points.

Rising costs

Hot runner components are more expensive, and hot runner mold costs can increase significantly. If the production of parts is small, the cost of mold tools is high and economically uneconomical. Mold Daren WeChat Editor For many mold users in developing countries, the high cost of hot runner system is one of the main problems affecting the widespread use of hot runner molds.

High equipment requirements

The production process equipment is required to be high, and the hot runner mold requires precision processing machinery as a guarantee. The integration and cooperation requirements of the hot runner system and the mold are extremely strict, otherwise the mold will have many serious problems in the production process. If the plastic seal is not good, the plastic overflows and damages the hot runner components, and the relative position of the nozzle insert and the gate is not good, resulting in serious deterioration of the product quality.

Operation and maintenance complexity

The hot runner mold operation and maintenance is complicated compared with the cold runner mold. If the operation is improper, it will easily damage the hot runner parts, making production impossible, resulting in huge economic losses. For new users of hot runner molds, it takes a long time to accumulate experience.

The defects of most molded products are caused during the plasticizing and injection molding stages, but sometimes they are also related to improper mold design. Possible factors include: cavity number, design of cold/hot runner system, type of injection port, position and Size, and the structure of the product itself. Therefore, in order to avoid product defects caused by mold design, we need to analyze the mold design and process parameters when making the mold.

After obtaining the results of the test, the operator usually needs to evaluate the specific conditions of the mold to avoid unnecessary cost and time in the process of modifying the mold. In most cases, this assessment also includes the setting of machine process parameters. That is to say, in order to make up for the deficiencies in the mold design, the operator may have made an incorrect setting without knowing it. In this case, the production process of the equipment is not normal, because the range of parameter settings required to produce a qualified product is very small. Once any slight deviation occurs in the parameter setting, the quality of the final product may exceed the allowable quality. The range of error, and the resulting actual production costs are often much higher than the costs incurred by prior mold optimization.

The purpose of the test is to find optimized process parameters and mold design. In this way, even changes in materials, machine settings, or the environment can ensure a stable and uninterrupted mass production environment, not just to get a good sample. this point is very important.

Tryout steps:

Step 1. Set the temperature of the drum

It should be noted here that the initial tank temperature setting must be based on the material supplier's recommendations. This is because the same materials from different manufacturers and different brands may have considerable differences, and material suppliers often have considerable research and understanding of their own materials. Users can make basic settings based on their recommendations and then make appropriate fine-tuning based on specific production conditions.

In addition to this, it is also necessary to use a detector to measure the actual temperature of the melt. Because the temperature of the barrel we set is often due to the environment, temperature sensor type and position depth, etc., it is not guaranteed to be consistent with the melt temperature of 100%. Sometimes the actual temperature of the melt and the set temperature of the drum vary greatly due to the presence of oil or other reasons (previously, we have had an example where the temperature difference between the two is as high as 30 °C).

Step 2. Set the temperature of the mold

Again, the initial mold temperature setting must also be based on the recommended values ​​provided by the material supplier.

It should be noted that the mold temperature we refer to refers to the temperature of the cavity surface, not the temperature displayed on the mold temperature controller. Many times, the temperature displayed on the mold temperature controller does not match the temperature of the cavity surface due to environmental and improper power selection of the mold temperature controller. Therefore, the temperature of the cavity surface must be measured and recorded before the formal test. At the same time, it is also necessary to measure the different positions in the mold cavity, check whether the temperature of each point is balanced, and record the corresponding results to provide reference data for subsequent mold optimization.

Step 3.

According to experience, parameters such as plasticizing amount, injection pressure limit value, injection speed, cooling time and screw speed are preliminarily set and optimized.

Step 4. Perform a fill test to find the transition point.

The switching point refers to the switching point from the injection phase to the pressure holding phase, which may be the screw position, the filling time, the filling pressure, and the like. This is one of the most important and basic parameters in the injection molding process. In the actual filling test, the following points need to be observed:

(1) The holding pressure and holding time during the test are usually set to zero;

(2) The product is generally filled to 90%~98%, depending on the wall thickness and the structural design of the mold;

(3) Since the injection speed affects the position of the turning point, the turning point must be reconfirmed each time the injection speed is changed.

Through the filling test, the user can see the flow path of the material in the cavity, thereby judging where the mold is easy to trap, or where to improve the exhaust.

Step 5. Find the limit value of the injection pressure.

In this process, attention should be paid to the relationship between injection pressure and injection speed. For hydraulic systems, pressure and speed are interrelated. Therefore, it is not possible to set both parameters at the same time so that they meet the required conditions at the same time.

The injection pressure set on the screen is a limit value of the actual injection pressure, and therefore, the limit value of the injection pressure should be set to always be greater than the actual injection pressure. If the injection pressure limit is too low, so that the actual injection pressure approaches or exceeds the limit of the injection pressure, then the actual injection speed will automatically drop due to the power limitation, thereby affecting the injection time and the molding cycle.

Step 6. Find the optimal injection speed.

The injection speed referred to here is that the injection speed is kept as short as possible while the filling pressure is as small as possible. In this process, you need to pay attention to the following points:

(1) Surface defects of most products, especially those near the gate, are caused by the injection speed.

(2) Multi-stage injection is only used if one injection does not meet the process requirements, especially during the trial phase.

(3) In the case that the mold is intact, the pressure point is set correctly, and the injection speed is sufficient, the speed of the injection speed is not directly related to the generation of the flash.

Step 7. Optimize the dwell time.

The holding time is also the condensation time of the gate. Generally, the condensing time of the gate can be determined by weighing to obtain different dwell time, and the optimal dwell time is the time when the product die weight is maximized.

Step 8. Optimize other parameters such as holding pressure and clamping force.

Finally, it is important to emphasize that the purpose and focus of the test is to optimize the mold and process to meet the requirements of mass production, not just to test a good product sample.