Program Control of Technological Conditions of Injection Molding Machine

2021-05-28 11:06

As we all know, injection molding machines are programmed in a variety of ways, including injection speed control, injection pressure control, control of the amount of plastic injected into the cavity, control of back pressure and screw speed, etc. Purpose The purpose of process control is to improve the quality of plastic parts and maximize the performance of the injection molding machine.

Injection speed program control

1. The program control of the injection speed is to divide the injection stroke of the screw into 3 to 4 stages, and in each stage, use its own appropriate injection speed. For example, the injection speed slows down when the molten plastic first passes through the gate, uses high speed during mold filling and slows down at the end of mold filling. By using this method, you can prevent overflow, eliminate flow marks and reduce residual stress in molded parts.

2. When filling the mold at low speed, the flow rate is stable, the size of the plastic part is more stable, the fluctuation is smaller, the internal stress of the plastic part is low, and the internal and external stress of the plastic part tends to be the same (for example, the PC plastic part is immersed in carbon tetrachloride, and the high-speed injection plastic part has Cracking tendency, no cracking at low speed). Under slower filling conditions, the temperature difference in material flow, especially before and after the gate, is large, helping to avoid shrinkage and sinking. However, the longer filling time makes the molded parts more prone to delamination and poor adhesion, resulting in melt marks, which not only affect the appearance, but also greatly reduce the mechanical strength.

3. During high-speed injection, the material flow rate is fast. When the high-speed mold filling is smooth, the melt fills the cavity quickly, the material temperature drops less, and the viscosity drops slowly. Lower injection pressure can be used, which is a kind of hot material. filling situation. The high-speed filling improves the gloss and smoothness of the molded part, eliminates the phenomenon of melting marks and delamination, and has low shrinkage pressure and uniform color, which ensures the fullness of the larger part of the molded part. However, it is easy to cause fat foaming or yellowing, or even scorching of plastic parts, or cause difficulty in demolding, or uneven filling of the mold. With high viscosity plastics, there is a risk of melt fracture, resulting in cloud points on the surface of the plastic part.

High-speed and high-pressure injection can be considered in the following cases

1. For long-flow plastic parts with high melt viscosity and fast cooling speed, low pressure and low speed cannot completely fill all corners of the cavity.

2. For plastic parts with too thin wall thickness, the melt is easy to condense and stagnate when it reaches the thin wall. High-speed injection must be used to make a large amount of melt consume energy before entering the cavity immediately.

3. For plastics reinforced with glass fiber or containing a large amount of fillers, due to poor fluidity, high-speed and high-pressure injection must be used in order to obtain a smooth and uniform surface of the plastic parts.

4. For high-grade precision plastic parts, thick-walled plastic parts, plastic parts with large wall thickness changes, and plastic parts with thick protrusions and reinforcements, it is best to use multi-stage injection, such as two, three, four or even five. level.

Program-controlled injection pressure

1. Usually the injection pressure control is divided into primary injection pressure, secondary injection pressure or injection pressure control of more than three levels.

2. Proper pressure switching timing is very important to prevent excessive pressure in the mold and prevent overflow or lack of material. The specific volume of the molded part depends on the melt pressure and temperature when the gate is closed during the packing phase. If the pressure and temperature are the same each time the molded part is switched from the holding pressure to the cooling phase, the specific volume of the molded part will not change. At a constant molding temperature, the most important parameter that determines the size of the molded part is the holding pressure, and the most important variables that affect the dimensional tolerance of the formed part are the holding pressure and temperature. For example, at the end of filling, the holding pressure is immediately reduced, and when the surface layer forms a certain thickness, the holding pressure rises again, so that large and thick-walled moldings can be formed at low clamping force, eliminating shrinkage and fly edge.

3. The holding pressure and speed are usually 50%~65% of the maximum pressure and speed when the plastic is filled with the mold cavity, that is, the holding pressure is about 0.6~0.8MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low during the holding time, which prolongs the service life of the oil pump and reduces the power consumption of the oil pump motor.

4. The three-stage pressure injection can not only make the plastic parts fill the mold smoothly, but also will not appear melt marks, shrinkage depressions, flash and warpage deformation. It is beneficial for thin-walled plastic parts, multi-cavity small parts, large plastic parts with a long injection process, and even plastic parts with less balanced cavity configuration and injection not too close to the mold.

Program Control of Injection Plastic Cavity Filling Volume

Preadjusted metering is adopted, so that there is still a small amount of melt (buffer amount) at the end of the screw near the end of the injection stroke, and further injection pressure (second injection pressure or third injection pressure) according to the filling situation in the mold is used to supplement a small amount of melt. This can prevent the plastic part from sinking or adjust the shrinkage of the plastic part.

Screw back pressure and speed program control

High back pressure allows the melt to obtain strong shearing force, and low speed also allows the plastic in the barrel to obtain a longer plasticizing time. As a result, backpressure and speed are now more often programmed to be controlled simultaneously. For example, start with high speed and low back pressure throughout the screw metering stroke, then switch to low speed and high back pressure, then switch to high back pressure and low speed, and finally plasticize at low back pressure and low speed, releasing most of the pressure of the melt at the front of the screw , reduce the rotational inertia of the screw, thereby improving the screw metering accuracy. Excessive back pressure typically results in increased colorant discoloration; increased mechanical wear on the preforming mechanism and barrel screw; longer preforming cycles and reduced productivity; nozzles tend to bleed and increase regeneration; even with self-locking nozzles, if backing Pressures higher than the designed spring-loaded locking pressure can also cause fatigue damage. Therefore, the back pressure must be properly adjusted.

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