There are three types of hot runners: manifold, nozzle, and temperature control box. Main gate feeds material and flows through nozzle through manifold. Temperature control box is temperature controlled.
Hot runner molds are molds that use heating devices to keep melt in runner from solidifying. Because it has a shorter forming cycle than traditional molds and saves more raw materials, hot runner molds are widely used in industrially developed countries and regions in the world today.
Concept classification
Hot runner systems are divided into fully hot runners and micro semi hot runner systems (semi hot runners). Design of adiabatic runner is complex, but effect is good and maintenance cost is very low. Micro semi hot runner structure is simple, stable and easy to use, with a low failure rate. Simple structure has low maintenance costs, which provides greater guarantees for stable production.
Hot runner classification
Open (for micro semi hot runners), needle valve (for adiabatic runners).
Hot runner system is generally composed of several parts such as hot nozzles, manifolds, temperature control boxes and accessories. Hot nozzles generally include two types: open hot nozzles and needle valve hot nozzles. Since form of hot nozzle directly determines selection of hot runner system and manufacture of mold, hot runner system is often divided into open hot runner systems and needle valve hot runner systems accordingly. Manifold is used when there are multiple cavities in one mold or multiple feeding points, or when material level is offset at a single feeding point. Material is usually P20 or H13. Manifolds are generally divided into two categories: standard and non-standard. Their structural form is mainly determined by distribution of cavity on mold, arrangement of nozzle and position of gate. Temperature control box includes main machine, cables, connectors, wiring male and female sockets. Hot runner accessories usually include: heaters and thermocouples, runner sealing rings, connectors and junction boxes, etc.
Single-hole installation

 

Multi-hole installation

 

Difference between three-plate mold and hot runner

 

Needle valve installation

 

Three-plate mold heated runner

 

Advantages: Shortened cycle
Part molding cycle is shortened, because there is no cooling time limit for runner system, part can be ejected in time after molding and curing. Many thin-walled parts produced by hot runner molds can be molded in less than 5 seconds.
Save plastic
In pure hot runner molds, there is no production cost because there is no cold runner. This is particularly important for applications where plastics are expensive. In fact, major international hot runner manufacturers have developed rapidly in the era when oil and plastic raw materials are expensive in the world. 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 hot runner mold molding process, plastic melt temperature is accurately controlled in runner system. Plastic can flow into each mold cavity in a more uniform state, resulting in parts of consistent quality. Gate quality of hot runner molded parts is good, residual stress is low after demolding, and part deformation is small. Therefore, many high-quality products on the market are produced by hot runner molds. For example, many plastic parts in familiar MOTOROLA mobile phones, HP printers, and DELL laptops are made of hot runner molds.
Production Automation
Eliminating subsequent processes is conducive to production automation. Parts are finished products after being molded by hot runner molds, there is no need to trim gates and recycle cold runners. It is conducive to production automation. Many foreign product manufacturers combine hot runners with automation to greatly improve production efficiency. Many advanced plastic molding processes are developed on the basis of hot runner technology. Such as PET pre-molding, multi-color co-injection in mold, multi-material co-injection process, STACK MOLD, etc.
Disadvantages
Although hot runner molds have many significant advantages compared to cold runner molds, mold users also need to understand disadvantages of hot runner molds. In summary, there are following points.
Cost increase
Hot runner components are relatively expensive, and cost of hot runner molds may increase significantly. If part output is small, mold tool cost ratio is high, which is not economically cost-effective. For many mold users in developing countries, high price of hot runner systems is one of main problems that affect widespread use of hot runner molds.
High equipment requirements
Production process equipment requirements are high, and hot runner molds need precision processing machinery to ensure. Integration and matching requirements of hot runner system and mold are extremely strict, otherwise mold will have many serious problems during production process. For example, poor plastic sealing leads to plastic overflow and damage to hot runner components, interrupting production, poor relative position between nozzle insert and gate leads to a serious decline in product quality.
Complex operation and maintenance
Compared with cold runner molds, hot runner molds are complex to operate and maintain. If operation is improper, it is very 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.
Disadvantages of mold testing
Most defects of molded products are caused in plasticization and injection stages, but sometimes they are also related to improper mold design. Possible influencing factors include: number of mold cavities, design of cold/hot runner system, type, position and size of injection port, and structure of product itself. Therefore, in order to avoid product defects caused by mold design, we need to analyze mold design and process parameters when making mold.
After obtaining mold trial results, operator usually needs to evaluate specific situation of mold to avoid adding unnecessary costs and time in process of modifying mold. In most cases, this evaluation also includes setting of machine process parameters. In other words, in order to make up for deficiencies in mold design, operator may make incorrect settings without knowing it. In this case, production operation process of equipment is abnormal, because parameter setting range required to produce qualified products is very small. Once any slight deviation in parameter setting occurs, quality of final product may far exceed allowable error range, and actual production cost resulting from this is often much higher than cost of prior mold optimization.
Purpose of mold trial is to find optimized process parameters and mold design. In this way, even if factors such as materials, machine settings or environment change, a stable and uninterrupted mass production environment can still be ensured, not just to obtain a good sample. This is very important.
Mold trial steps
Step 1. Set temperature of barrel
It should be noted here that initial barrel temperature setting must be based on recommendations of material supplier. This is because same material from different manufacturers and brands may have considerable differences, material suppliers often have a fairly in-depth study and understanding of their own materials. Users can make basic settings based on their recommendations, then make appropriate fine-tuning based on specific production situation.
In addition, a probe is needed to measure actual temperature of melt. Because barrel temperature we set is often due to environment, model and position depth of temperature sensor, etc., it cannot be guaranteed to be 100% consistent with melt temperature. Sometimes due to presence of oil or other reasons, actual temperature of melt and set temperature of barrel are very different (in the past, we have had examples where temperature difference between the two was as high as 30℃).
Step 2. Set temperature of mold
Similarly, initial mold temperature setting must also be based on recommended value provided by material supplier.
It should be noted that mold temperature we are talking about refers to temperature of mold cavity surface, not the temperature displayed on mold temperature controller. Many times, due to environment and improper power selection of mold temperature controller, temperature displayed on mold temperature controller is inconsistent with temperature of mold cavity surface. Therefore, before formal mold trial, temperature of mold cavity surface must be measured and recorded. At the same time, measurements should be taken at different positions in mold cavity to check whether temperature at each point is balanced, and corresponding results should be recorded to provide reference data for subsequent mold optimization.
Step 3. Based on experience, preliminarily set parameters such as plasticizing amount, injection pressure limit, injection speed, cooling time and screw speed, and optimize them appropriately.
Step 4. Perform a filling test to find the transition point.
Transition point refers to switching point from injection stage to holding stage. It can be screw position, filling time and filling pressure. This is one of the most important and basic parameters in injection molding process. In actual filling test, following points need to be followed:
(1) Holding pressure and holding time during test are usually set to zero;
(2) Product is generally filled to 90%~98%, depending on wall thickness and structural design of mold;
(3) Since injection speed will affect position of transition point, transition point must be reconfirmed every time injection speed is changed.
Through filling test, user can see flow path of material in the mold cavity, so as to determine where mold is prone to air entrapment, or where venting needs to be improved.
Step 5. Find limit value of injection pressure.
In this process, attention should be paid to relationship between injection pressure and injection speed. For hydraulic systems, pressure and speed are interrelated. Therefore, it is impossible to set these two parameters at the same time to meet required conditions at the same time.
Injection pressure set on the screen is limit value of actual injection pressure. Therefore, limit value of injection pressure should be set to always be greater than actual injection pressure. If injection pressure is limited too low, so that actual injection pressure approaches or exceeds limit value of injection pressure, then actual injection speed will automatically decrease due to power limitation, thereby affecting injection time and molding cycle.
Step 6. Find optimized injection speed.
Injection speed referred to here is injection speed that simultaneously satisfies requirements of making filling time as short as possible and filling pressure as small as possible. In this process, following points should be noted:
(1) Most surface defects of products, especially those near gate, are caused by injection speed.
(2) Multi-stage injection is only used when one injection cannot meet process requirements, especially in mold trial stage.
(3) When mold is intact, turning point is set correctly, and injection speed is sufficient, speed of injection has no direct relationship with generation of flash.
Step 7. Optimize holding time.
Holding time is also condensation time of gate. Generally, condensation time of gate can be determined by weighing, so as to obtain different holding times, and optimal holding time is the time when product mold weight reaches maximum.
Step 8. Optimize other parameters, such as holding pressure and clamping force.
Finally, it should be emphasized that purpose and focus of mold trial is to optimize mold and process to meet requirements of mass production, rather than just testing out good product samples.