As the great wheel of the times moves forward continuously, rapidly, and cruelly, what is worse is that the labor market is hard to find, and the mold industry is generally short of people... It is worrying! Therefore, with limited human resources, how to increase your mold profits to cope with the increasing costs is the main problem everyone faces. Improving precision and automating mold making... is certainly a good way, but it requires Invest a lot of money to purchase equipment, train personnel... In response to the above situation, the simplest and easiest way to achieve it is to have a thorough understanding of "the use of hot runner". The processing of injection molding is a process of (plasticization) → (flow) → (forming) → (solidification and crystallization).
So the properties of plastics are particularly important. For example: solution temperature, pressure, viscosity, specific heat... etc. must be paid attention to. Since the field of plastics is very vast, it is impossible to go into it here, but we will explain the common sense part. The reason why plastic can be formed and processed is that it deforms under the action of temperature and pressure. Depending on the temperature at which it is heated, it can be divided into four states, namely glass state, highly elastic state (rubber state), and viscous flow state (plastic state). (chemical state), decomposition state, as shown in the figure:
Glass state: 0~T1, the molecules are in the frozen state, hard and brittle, and easily broken when exposed to pressure.
High elasticity state (rubber state): T1 ~ T2, it can be deformed due to external force, and it is not easy to form until it reaches the melted state.
Viscous flow state (plasticized state): T2~T3, which can be processed and formed at will.
Decomposition state: T3, the plastic begins to crack, gas decomposition products appear, and even reaches a burnt state. (Note) The following are the forming conditions for general plastics
For each different plastic, the relative forming area may be different, but the process analysis is the same. Therefore, for excellent mold designers, they should clearly understand the forming area and processing characteristics of each plastic. The flow state of general fluids (such as water, oil...) is based on Newton's definition. Plastic melt looks like an ordinary fluid, but it is actually a non-Newtonian fluid. For example: In a Newtonian fluid, although the shear stress changes, the viscosity does not change. As for plastic melt, when the shear stress changes, the viscosity also changes significantly. For example: in Newtonian fluid, when the pressure increases from 1 to 10, the outflow increases by 10 times. Do the same experiment with plastic melt. When the pressure increases from 1 to 10, the outflow may increase by 100 times, or 500 times, or even 1000 times (depending on different plastics).
Therefore, in this non-Newtonian flow, as the pressure increases, the flow resistance decreases. Therefore, during injection molding, although the gate is quite narrow, it is easy to fill the mold cavity. As for Newtonian fluid, there are two further classifications, as shown in the figure:
Injection molding is to use high-speed plastic solution A processing method that causes deformation. Because the plastic solution is compressible, it is easy to cause elastic pressure changes under high-speed flow. This phenomenon can be seen when the flow resistance changes rapidly. After this elastic pressure change occurs, the diffusion direction of the fluid front end is extremely chaotic and unstable. However, when high-speed filling is used, the plastic solution appears to be non-compressible. What causes this elastic pressure fluctuation (unstable pulsation)? The following analysis is shown in the figure:
When the flow of the plastic solution is similar to the laminar flow state, that is, the mold cavity is filled in a normal and stable state
In the figure, it is rich in compressibility The plastic solution is represented by a spiral spring. When the spring exerts pressure to move toward the center of the tube, the spring moves from left to right at the same speed. This is an ideal laminar flow state. Due to the injection pressure When in equilibrium with resistance, the spring moves smoothly. Such as C
But in some cases, when rapid filling is necessary, the injection pressure and speed will increase abnormally. Therefore, the elastic plastic solution (spring) withstands the compression of the process at the first moment, and causes strong resistance at the second moment. The reason is the fluctuation of pressure and the turbulence at the front end of the fluid. This flow condition It is called elastic turbulence. The plastic used should be selected at the beginning of designing the product, but most of the time the mold is not taken into consideration. But if possible, the materials selected should make the mold easy to manufacture.
The dimensional accuracy of those with small molding shrinkage (PS, ABS, PC) is easier to achieve.
Those with large molding shrinkage rates (PP, PE, POM) are more difficult to achieve dimensional accuracy (the tolerance of the mold is 1/6 of the tolerance of the molded product).
For those with a relatively large viscosity during flow (ABS, etc.), the solution is less likely to flow into the gap, but for those with a low viscosity (such as PA, POM), the solution can easily enter even if the gap is small.
Those with lower temperatures during molding (PS, etc.) are easier to form and have a faster molding cycle, but those with higher molding temperatures (PC) are slower.
Those that are not prone to deterioration or decomposition during molding (PS, PE, PP, etc.) will not easily cause defective products with unstable quality during mass production, but those that are prone to deterioration or decomposition during molding will not be strictly required for molding. Conditions (the mold can precisely control the forming conditions) make mass production impossible. This problem is particularly serious in the case of hot runner. Observed from the molecular structure, crystalline plastic is a linear polymer. According to its chemical structure, some parts of the molecules are assembled in an orderly manner, which is called crystalline plastic. Not all molecules become in this state, and depending on the cooling conditions, 40 to 80% of the molecules become crystalline by weight. This degree is called "crystallinity." Within the crystal, molecular chains called Lamella are bent and folded, while molecular chains that do not enter the crystalline part that produces unit crystals exist between Lamella or spherulites, resulting in a non-crystalline part. Amorphous plastic... Unlike crystalline plastic, molecules cannot assemble in an orderly manner. This is because the atomic groups forming the polymer chain are too large and the bridges hinder crystallization.
From the observation of volume changes, thermoplastic plastics can also be divided into two categories, one is amorphous plastics and the other is crystalline plastics. Regarding the classification of crystalline and amorphous plastics, the properties of various plastics have been noted in the table. We can further understand the changes between volume and temperature from the following examples. For example: PS (representative of amorphous plastic) expands approximately 8.3% when heated from 20°C to 200°C. In terms of density, it decreases from 0.97 cm/g to 1.012 cm/g (representative of crystalline plastic) under the same conditions. There are the following changes:
Volume at 20℃: 1.03 cm/g
Volume at 200℃: 1.33 cm/g
Volume increase rate: 29%
The molten amorphous polymer can be greatly compressed using the injection molding machine used. Depending on the conditions, excess melt may be forced to fill the mold cavity. The molded product made under such conditions will solidify while retaining a large internal stress. It has a great impact on the performance of molded products. It will be destroyed at the moment of demoulding, and it is also easily damaged by slight external force or the action of chemicals.
Crystalline plastics completely melt the crystals due to heating, and the melt becomes amorphous, and its behavior is the same as that of amorphous polymers. It is worth noting that when the pressure becomes higher, the transition temperature from crystalline material to amorphous material will also increase. When molding crystalline plastics, one thing that is very important in terms of the quality of the molded product is that the polymer must complete the molding action when it is in an amorphous state. This matter, especially during the holding pressure period, the deformation during holding pressure is caused by flow.
After the melt of crystalline plastic is rapidly cooled, the recrystallization of some parts of the molded product is hindered. The recrystallization phenomenon cannot be completed instantly and continues at any time. Density and crystallization There is a direct relationship between the degree of crystallization. The higher the degree of crystallization, the higher the density. On the contrary, if the degree of crystallization is low, the density will be reduced. The part where recrystallization is hindered by rapid cooling will continue to undergo post-crystallization to a greater or lesser extent due to differences in temperature and time factors. Post-crystallization continues until the original density of this part is restored. Therefore, it can be understood that post-crystallization and post-shrinkage are related, and post-crystallization and post-shrinkage are also the causes of bending deformation and dimensional change of the molded product (the molded product becomes smaller).
If the surface temperature of the mold cavity is high, the molding shrinkage will be large at first, but there will be little change during heat treatment. Therefore, molded products made at a very high mold surface temperature have excellent dimensional stability even though they are used at high temperatures. Therefore, when determining the size of the mold cavity for crystalline plastics, it is necessary to consider the relationship between post-crystallization and post-shrinkage. What is important is that the surface temperature of the mold cavity must be correctly controlled from the beginning of molding. Of course, it is impossible to make the surface temperature of the mold cavity completely free of temperature differences, but an effective temperature control system can be used to minimize the temperature difference.
Usually, when the mold temperature is increased, the product size will shrink. But it is not absolute. Sometimes it is found that when the mold temperature is increased, the size will increase. In the end, it still depends on the actual molding effect. Hot runner mold in Today, it is widely used in all industrially developed countries and regions in the world. This is mainly because the hot runner mold has the following significant features:
1. Shorten the molding cycle of the part
Since there is no limitation on the cooling time of the runner system, the part can be molded and solidified in time. Push out. The molding cycle of many thin-walled parts produced with hot runner molds can be less than 5 seconds.
2. Save plastic raw materials
Since there is no cold runner in a pure hot runner mold, there is no production cost. This is especially significant for applications where plastics are expensive. In fact, all major international hot runner manufacturers have developed rapidly in an era when petroleum and plastic raw materials were expensive in the world. Because hot runner technology is an effective way to reduce material costs.
3. Reduce scrap and improve product quality
During the hot runner mold molding process, the plastic melt temperature is accurately controlled in the runner system. Plastic flows more uniformly into each mold cavity, resulting in consistent quality parts. Hot runner molded parts have good gate quality, low residual stress after demoulding, and small deformation of parts. 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 with hot runner molds.
4. Eliminate subsequent processes, which is conducive to production automation.
After the parts are formed by the hot runner mold, they are finished products. There is no need to trim the gates and recycle the cold runner and other processes. 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 process
Many advanced plastic molding processes are developed based on hot runner technology. Such as PET preform production, multi-color injection in the mold, multiple material injection processes, STACK MOLD, etc. Although hot runner molds have many significant advantages compared to cold runner molds, mold users also need to understand the shortcomings of hot runner molds. To sum up, there are the following points.
1. Rising mold costs
Hot runner components are relatively expensive, and the cost of hot runner molds may increase significantly. If the parts output is small, the mold tool cost ratio is high, which is not economically cost-effective. For many mold users in developing countries, the high price of hot runner systems is one of the main problems affecting the widespread use of hot runner molds.
2. Hot runner mold production technology and equipment require high requirements
Hot runner molds require precision processing machinery to ensure. The integration and cooperation requirements between the hot runner system and the mold are extremely strict, otherwise many serious problems will occur in the mold production process. For example, poor plastic sealing may cause plastic overflow and damage to hot runner components, interrupting production, and poor relative positioning of the nozzle insert and gate may result in a serious decline in product quality.
3. Complex operation and maintenance
Compared with cold runner molds, hot runner molds are complex to operate and maintain. Improper use and operation can easily 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.