How to control pressure during injection molding process
Time：2018-05-09 來源： 海太機械
Whether oil pressure or electric injection molding machine, all the movement in the process of injection will produce pressure. Proper control of the pressure required to produce a reasonably quality finished product. The pressure control and measurement system is on the oil-pressure injection molding machine. All the movements are carried out by the oil road responsible for the following operations:
1、Screw rotation in the plasticizing stage.
2、Slide seat material path (the nozzle is close to the nozzle liner).
3、Axial movement of propellant screw during injection and pressure preservation.
4、Close the base material on the ejector rod until all extension of the elbow rod or the piston mold closing stroke has been completed.
5、Start to install the top of the top bar to push out the parts.
On a full-voltage machine, all movement is performed by a brushless synchronous motor equipped with a permanent magnet. The rotary motion is transformed into linear motion through the ball bearing screw used in the machine tool industry. The efficiency of the whole process depends in part on the plasticizing process in which the screw plays a key role.
Screw must ensure material melting and homogenization. This process can be adjusted by reverse pressure to avoid overheating. The mixing element must not produce high flow rate, otherwise, it will lead to polymer degradation. Each polymer has a different maximum flow rate, and if it exceeds that limit, the molecule will stretch and the polymer host chain will break. However, the emphasis is still on controlling the forward axial movement of the screw during injection and pressure preservation.
Subsequent cooling processes, including internal stress, tolerances and warping, are important to ensure product quality. All of this is determined by the quality of the mould, especially when optimizing the cooling passage and ensuring effective closed-loop temperature regulation. The system is completely independent and will not interfere with mechanical regulation. Mould movement such as closing die and ejecting die must be accurate and efficient. The velocity distribution curve is usually adopted to ensure the accurate approach of the moving parts.
Contact maintenance is adjustable. Therefore, it can be concluded that the product quality is mainly determined by the system controlling the forward movement of screw without considering the same energy consumption, mechanical reliability and additional conditions (such as mould quality). This adjustment is achieved by detecting oil pressure on an oil injection molding machine. Specifically, the oil pressure activates a set of valves through the control panel, and the fluid ACTS through the controller and is regulated and released.
Injection speed control includes open - loop control, half - loop control and closed - loop control. The open loop system relies on a Shared proportional valve. Proportional tension is applied to the required proportion of fluid, which creates pressure in the injector cylinder and allows the injection screw to move forward at a certain speed. Semi-closed loop system adopts closed loop proportional valve. The loop is closed at the position of the closing port, which controls the flow rate of oil by moving in the valve.
Speed sensor (usually potentiometer type) is used in closed - loop system to detect the decrease of tension. The flow of oil from the proportional valve can be adjusted to compensate for the speed deviation. Closed-loop control relies on specialized electronic components integrated with the machine. Closed-loop pressure control ensures uniform pressure during injection and pressure-keeping phases, as well as uniform backpressure throughout each cycle.
The proportional valve is adjusted by the detected pressure value and the deviation compensation is made according to the set pressure value. In general, hydraulic pressure can be monitored, but detecting melt pressure in the nozzle or die cavity is another effective method. A more reliable solution is to manage the proportional valve by reading the nozzle or cavity pressure readings. Adding temperature detection on the basis of pressure detection is especially beneficial to process management.
It is also helpful to predict the actual weight and size of the molded parts according to the set pressure and temperature conditions. In fact, by changing the pressure-retaining pressure, more materials can be introduced into the die cavity to reduce component shrinkage in line with design tolerances (including preset injection shrinkage). Near melting conditions, the semi - crystalline polymer showed a significant change in specific capacity. For this reason, overcharge will not block the parts in ejecting.
