If Putonghua is the language of communication between Chinese and Chinese, and English is the language of communication with crooked nuts, then physics is the language of communication between human beings and all things in the world! Pure physics has five branches: 1. Mechanics. 2. Electromagnetics. 3. Thermodynamics. 4. Relativity. 5. Quantum mechanics. Then multidisciplinary physics has seven branches: 1. Biophysics. 2. Chemical physics. 3. Medical physics. 4. Astrophysics. 5. Geophysics. 6. Economic physics. 7. Atmospheric physics. So physics is everywhere.
In this article, we will talk about what phenomena are involved in fluid mechanics in injection molding and how to solve them! First of all, let's introduce the concept of crystalline materials and amorphous materials.
We have said in the last article: in injection molding, thermodynamics and hydrodynamics are inseparable, and the fluidity of plastic fluid in thermoplastics (also known as thermosetting plastics) is not linearly proportional to temperature, but is similar to parabola proportional.
If the abscissa represents the temperature and the ordinate represents the fluidity, the relationship between them should be as shown in the figure.
Because the MFI index (melt flow index) of plastics is clear only to the manufacturers and sales personnel, and the ordinate (MFI index) in the figure is only convenient to see, and it is not necessarily correct, so we do not have to be true. It can be seen in the figure that amorphous materials (such as ABS) have no melting point. They will only soften slowly with the increase of temperature until they become fluids and then decompose into gases and carbides.
Crystal materials have a melting point, just like ice. Below 0 ℃ is solid, and higher than 0 ℃ is liquid (there are four forms of substances in nature: 1, solid 2 liquid 3 gas state 4 ionic state: flame, electric spark, etc.). However, PP and PET materials are between crystalline and amorphous, which we call semi crystalline materials.
Then some friends may ask, what is the use of understanding these things? Knowledge is useful! For example, if a pile of material is wet by rain and is in urgent need of production, how to use the shortest time to dry the material and ensure that the material will not agglomerate? What's the right temperature for the oven?
Special treatment for special cases, ABS need not think about, there is no special way, but PP can have, we all know that PP materials generally do not need to be dried, and few people know how much temperature this material can be used to dry without caking. After reading the diagram, I think you should have a number in mind. Anyway, I have used 150 ℃ to dry PP raw materials (granulation and returning materials must not be used).
The maximum baking temperature of POM is 160 ℃, and that of PA6 is 180 ℃. This is the raw material, granulation return can not be. Of course, if there is no need to play according to the routine! Because I've seen PA6 soften at 190 degrees, don't risk caking if you don't have to.
Now that we talk about fluid, we have to introduce the flow resistance: for example, the air in the mold cavity, the column (the hole on the product), the bulge, the corner and other structures. However, all the materials that can hinder the flow of the fluid are called the resistance fluid, and the viscosity coefficient of the fluid itself is also related to the resistance of the fluid flow, and the ratio of the fluid flow speed to the fluid viscosity coefficient It's called Reynolds coefficient.
When the Reynolds coefficient reaches a certain value, there will be a "Karman vortex street" phenomenon, that is, a row of vortices behind the blocking fluid, which will make the plastic fluid wrap air in the mold cavity. In the process of injection molding, there will be some bad problems, such as gas lines, material flowers, bubbles and so on.
In other words, as long as one of the flow velocity and viscosity coefficient of the fluid changes, it can be solved, and it is the easiest to slow down the flow speed by changing the process parameters.
Since it is a fluid, there must be laminar flow. Laminar flow: we can simply understand that the flow in each layer is different. Because this is the most popular understanding. However, we can not directly see the situation at different depths. If it is water, then because the water is transparent and there is no reference system, we can not judge the changes of the upper and lower layers.
If it's an opaque liquid, you can see the flow in the upper layer, but you can't see the lower layer. The most intuitive phenomenon is the flowing stream. The flow on both sides of the river moves slowly, and the flow in the middle is very fast. Therefore, the water flow varies with the depth. The same is true of plastic fluid, and the change of laminar flow is more obvious than that of water, because when plastic fluid flows, the outermost layer is also losing temperature, which means that the flow index of material is also reduced.
Simply speaking, the flow speed of the first layer attached to the mold is different from that of the middle layer. The flow rate of the layer attached to the mold is slow, and that of the middle layer is fast.
Once the flow is stopped, the crystallization speed of the material is very fast, of course, this is related to the thermal conductivity and thickness of the material. It is worth mentioning that uneven thickness of the product is the most likely to cause stress marks and deformation, and sometimes it may cause trapped gas. The phenomenon caused by trapped gas is shrinkage, which is not the time to add injection pressure and add point injection It can be solved in a short time. If you want to solve the problem, you should first find out the reason of the problem.
