You can't miss these injection molding process and parameter adjustment experience

2019-10-06  1206

First, the injection molding process can be simply expressed as follows:

The last cycle is over - closed mold - filling - pressure holding - rubber back - cooling - mold opening - demoulding - start the next cycle

In the filling pressure drop section, the cavity pressure rises with time, and the pressure will remain in a relatively static state after filling the full cavity to supplement the insufficient amount of glue due to shrinkage, and this pressure can be prevented due to injection. Reduced colloidal backflow phenomenon, this is the pressure holding phase, the cavity pressure gradually decreases after the pressure is maintained, and can theoretically fall to zero over time, but the actual is not zero, so the internal memory of the product after demolding Stress, and therefore some products need to be post-treated to remove residual stress. The so-called stress is the force that freely moves the high-order sub-chain or the segment, that is, bending deformation, stress cracking, shrinkage, and the like.

Second, the main parameters of the injection molding process

1, injection molding temperature

The melt temperature plays a major role in the flow properties of the melt. Since the plastic has no specific melting point, the so-called melting point is a temperature range in a molten state, and the structure and composition of the plastic molecular chain are different, so the influence on the fluidity is also different. Rigid molecular chains are more affected by temperature, such as PC, PPS, etc., while flexible molecular chains such as PA, PP, PE and other fluidity are not obvious by changing the temperature, so the reasonable injection temperature should be adjusted according to different materials.

2, injection speed

The speed of the melt in the barrel (also the speed of the screw) (MM / S) injection speed determines the appearance, size, shrinkage, flow distribution, etc., generally slower - faster - slower, First, a higher speed is used to pass the melt through the main channel, the runner, and the gate to achieve the purpose of balancing the shot, then filling the entire cavity with rapid filling, and then supplementing the shrinkage and countercurrent at a slower speed. The caused rubber shortage is not until the gate is frozen, which can overcome the poor quality of burning, gas, shrinkage and the like.

3, injection pressure

The melt overcomes the resistance required for advancement, directly affecting the size, weight and deformation of the product. Different plastic products require different injection pressures. For materials such as PA and PP, increasing the pressure will significantly improve the fluidity, and the injection pressure. The size determines the density of the product, that is, the gloss of the appearance.

4, mold temperature

Some plastic materials require high mold temperature due to high crystallization temperature and slow crystallization rate. Some require higher temperature or lower temperature due to control size and deformation, or the need for demoulding. For example, PC generally requires more than 60 degrees. In order to achieve better appearance and improve fluidity, the mold temperature sometimes needs to be more than 160 degrees, so the mold temperature has an inestimable effect on improving the appearance, deformation, size and rubber mold of the product.

Third, the meaning of the injection molding professional parameters

1, the amount of injection

The injection volume refers to the amount of melt injected into the mold during injection molding of the screw of the injection molding machine.

Injection volume = screw propulsion volume *ρ*C

ρ is the density of the injection molding material

C is 0.85 for crystalline polymer and 0.93 for amorphous polymer.

The injection molding machine cannot be used to process products that are less than 1/10 of the injection volume or 70% of the injection volume.

2, measuring stroke (pre-plasticized stroke)

After each injection procedure is terminated, the screw is at the foremost end of the drum. When the pre-plasticing procedure arrives, the screw starts to rotate and the material is sent to the screw head. The screw retreats under the reaction of the material and hits the limit switch. This process is a metrology process.

The amount of injection is related to the accuracy of the metering stroke. It is too small, the injection volume is not enough, and the residual material is too large after each injection in the front of the barrel, so that the melt temperature is uneven or overheated.

After pre-plasticizing, the melt in the gauge has a temperature difference between the longitudinal temperature and the radial temperature. The number of screw revolutions, the pre-plastic back pressure and the barrel temperature will have a great influence on the melt temperature and temperature difference.

3, anti-delay

The anti-delay means that after the screw is measured in position, it is linearly reversed by a distance, so that the specific volume of the measuring chamber becomes larger, and the internal pressure decreases, preventing fluid from flowing out of the measuring chamber.

Anti-casting also has the purpose of reducing the pressure of the nozzle runner system when the injection nozzle is not retracted, reducing the internal stress, and easily extracting the material during the mold opening. The anti-delay assembly makes the metering chamber noisy. For materials with high viscosity, there is no anti-delay.

The above parameters can be adjusted to meet the quality requirements through reasonable adjustment. For example, the size can be achieved by injection pressure, mold temperature, injection speed and back pressure.

Fourth, how to adjust the injection molding process parameters

1 temperature control

Thermocouples are also widely used as sensors for temperature control systems. On the control instrument, set the desired temperature and the sensor display will be compared to the temperature generated at the set point. In this simplest system, when the temperature reaches the set point, it will turn off, and the power will turn back on after the temperature drops. This type of system is called open-close control because it is not open or closed.

2 temperature

Temperature measurement and control are very important in injection molding. Although these measurements are relatively simple, most injection molding machines do not have sufficient temperature points or lines.

On most injection molding machines, the temperature is sensed by a thermocouple. A thermocouple consists essentially of the junction of two different wires. If one end is hotter than the other, a tiny telecommunication will be produced; the more heated, the stronger the signal.

3 melt temperature

The temperature of the melt is very important and the temperature of the shot cylinder used is only indicative. The melt temperature can be measured at the nozzle or measured using air jets. The temperature setting of the shooting tank depends on the melt temperature, screw speed, back pressure, shot volume and injection cycle.

If you have no experience in processing a particular grade of plastic, start with the lowest setting. For ease of control, the shot cylinders are zoned, but not all are set to the same temperature. If operating for a long time or at high temperatures, set the temperature in the first zone to a lower value, which will prevent premature melting and shunting of the plastic. Make sure the hydraulic oil, hopper closure, mold and shot cylinder are at the correct temperature before injection molding begins.

4 Injection pressure

This is the pressure that causes the plastic to flow and can be measured with sensors on the nozzle or hydraulic line. It has no fixed value, and the more difficult the mold is filled, the injection pressure is also increased, and the injection line pressure is directly related to the injection pressure.

First stage pressure and second stage pressure

During the filling phase of the injection cycle, high injection pressures may be required to maintain the injection speed at the required level. The mold is filled and no longer requires high pressure. However, when molding some semi-crystalline thermoplastics (such as PA and POM), the structure deteriorates due to sudden pressure changes, so sometimes it is not necessary to use the secondary pressure.

5 clamping pressure

In order to counter the injection pressure, the clamping pressure must be used. Do not automatically select the maximum value that can be used. Consider the projected area and calculate a suitable value. The projected area of the injection molded part is the largest area seen from the application direction of the clamping force. For most injection molding situations, it is about 2 tons per square inch, or 31 meganewtons per square meter. However, this is only a low value and should be used as a very rough empirical value, because once the injection molded part has any depth, the side wall must be considered.

6 back pressure

This is the pressure that needs to be generated and exceeded before the screw is retracted. Although the high back pressure is beneficial to the uniform dispersion of the pigment and the melting of the plastic, it also prolongs the return time of the middle screw, reduces the length of the fiber contained in the filled plastic, and increases The stress of the injection molding machine; therefore, the lower the back pressure, the better, under no circumstances can exceed 20% of the injection molding machine injection pressure (maximum rating).

7 Injection speed

This refers to the filling speed of the mold when the screw is used as a punch. When injection molding thin-walled products, high rate of fire must be used in order to completely fill the mold when the melt is not solidified, producing a smoother surface. A series of programmed firing rates are used during filling to avoid defects such as jets or trapped air. Injection can be performed under an open loop or closed loop control system.

8 nozzle pressure

The nozzle pressure is the pressure inside the nozzle. It is about the pressure that causes the plastic to flow. It does not have a fixed value, but increases with the difficulty of filling the mold. There is a direct relationship between nozzle pressure, line pressure and injection pressure. In a spiral injection molding machine, the nozzle pressure is about 10% less than the injection pressure. In the piston injection molding machine, the pressure loss can reach about 10%. In the case of a piston injection molding machine, the pressure loss can reach 50%.