Relationship between injection pressure and speed

Time:2020-04-09 09:19:25 / Popularity: / Source:

Movement of machine itself is actually a movement that overcomes resistance. That is to say, machine must first provide a power slightly larger than (slightly equal to) resistance. It is also called pressure on injection molding machine. Only when this force is provided can it be moved; Action is divided into speed, which involves flow. It is source of power. After resistance is overcome, flow rate increases as action increases, and the faster action, the greater resistance.
injection pressure
Therefore, it can be said that movement itself is looking for a balance between power and resistance.
injection pressure 
Injection itself is a process in which hot stream fills cavity. In theory, if you do not consider surface defects of product, mold factor should be as fast as possible. However, since hot stream must generate resistance during flow (pressure in cavity), machine must provide a power greater than or equal to this resistance, then it can fill hot stream into cavity, which means that action of injection is also an action to overcome resistance.
 injection speed 
Some people may ask, is this pressure constant from beginning to the end of injection? Certainly not, even at a constant rate (or a speed). Because contact surface is continuously expanding as flow fills cavity, that is, its force area is constantly expanding, injection pressure is changing at a constant speed. Injection pressure is certainly changing over single-stage speed injection, because rate of flow filling varies in addition to area of force being applied.
 injection speed 
I believe that some people will be confused, how can I set this injection pressure? In fact, setting injection pressure is to set pressure in cavity (theoretical), in the end how big, I don't know! Do you know how much pressure in cavity is, I believe it is not known, what should I do? First assume a condition, injection pressure can not be set in machine picture, only a pressure reducing valve can be used to adjust injection pressure, injection speed is adjustable. In this case, we will definitely say that safety first, adjust pressure reducing valve to not burst mold (this force has mold design limit value that can be checked), because it is too low to be filled. However, when cavity resistance (in theory, it is equal to hydraulic injection force) is less than limit of pressure reducing valve during actual injection, pressure reducing valve does not work. If pressure reducing valve limit is exceeded, pressure reducing valve forcibly lowers input value. When cavity resistance is lower than limit pressure of pressure reducing valve, flow completely fills cavity at a set speed. When resistance is greater than limit of set pressure reducing valve, action of pressure reducing valve will inevitably cause filling speed of flow to slow down naturally in order to find a new balance point between internal pressure resistance of cavity and pressure of injection liquid. In fact, injection molded product itself is such a process, as long as ultimate pressure can be set (ultimate pressure of pressure reducing valve), it does not necessarily need to be artificially given hydraulic pressure at the time of injection, and this is the case with many closed-loop controlled injection molding from Europe, because this control system can adjust input and output load of oil pump, and can push flow to fill cavity according to speed set by operator. Under condition of ensuring speed, a hydraulic pressure slightly equal to cavity resistance is provided. When cavity resistance tends to be greater than set limit force, machine will automatically reduce injection speed to achieve a new balance. This injection method can be said that injection speed dominates injection pressure.
However, in actual production, why do you usually have three or even four sets of injection pressures? Domestically produced machines are generally machines with quantitative or open-loop variable control systems, are controlled by two independent proportional adjustment methods of proportional flow valves and proportional pressure valves, and output load is always maximum design load. Therefore system can not confirm whether output state is consistent with setting requirements, and propulsion of stream itself requires hydraulic pressure as power source. Hydraulic pressure of control system cannot be automatically adjusted according to internal pressure of cavity, so it is necessary to artificially give a change in the pressure of liquid to promote change of flow rate, that is change of injection speed. At the same time artificial injection speed will affect hydraulic pressure during injection. Therefore, this method is contradictory between injection pressure and injection speed. In actual production process, it can only be adjusted by experience, so many people will encounter change of injection speed from 30% to 50%, product has not changed. In fact, it is not entirely a problem with machine itself. Only at this time, injection pressure you give is just about cavity resistance when injection speed is 30% (of course with other pressure losses, such as nozzle, mold inlet, etc.).
hydraulic pressure 
There will definitely be some friends who have contacted imported machine of variable closed-loop control method and will say that machine he is in contact with can be set to three pressures. Yes, and this is more and more brands tend to set pressure in this way. Reason is that there is always an abnormal situation in the filling of flow. For example, in multi-cavity mold, a cavity is blocked during production process. If this is also case where only limit pressure can be set as described above. At this time, it is inevitably cause other cavities parts to withstand local high pressure, which may cause damage or mold expansion of cavity. Therefore, if there are multiple sections to set pressure, maximum hydraulic pressure in a certain position, that is, injection pressure, can be determined according to structural characteristics of mold and position of flow filling to avoid some unexpected situations, so that it can be produced safely and smoothly.

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