Injection molding process parameters and their adjustment

I. The injection molding process can be simply expressed as follows

The previous cycle is finished - mold closure - filling - holding pressure - glue back - cooling - mold opening - mold release - start the next cycle

 

After filling the cavity, the pressure of the cavity rises with time, and after filling the cavity, the pressure will remain in a relatively static state to replenish the insufficient amount of rubber due to shrinkage, in addition, this pressure can prevent the backflow of rubber due to the lowering of injection, which is the pressure-holding stage, the pressure of the cavity gradually decreases after the pressure-holding, and can theoretically drop to zero with time, but the actual is not zero, so after demolding, the product internal memory stress, so some products need to be post-treated to remove the residual stress.

 

The so-called stress is the force of the free movement of the chain or chain segment, i.e. bending deformation, stress cracking, shrinkage, etc.

 

II. the main parameters of the injection molding process

1, injection molding temperature, melt temperature plays a major role in the fluidity of the melt, because the plastic does not have a specific melting point, the so-called melting point is a molten state of temperature, the structure and composition of the plastic molecular chain is different, and thus the impact on its fluidity is different, rigid molecular chain by the temperature is more obvious, such as PC, PPS, etc., and flexible molecular chain such as: PA, PP, PE and other fluidity by changing the temperature is not obvious, so it should be adjusted according to the different materials to a reasonable injection molding temperature.
 

2, injection speed is the melt in the barrel (also for the screw advancement speed) speed (MM / S) injection speed to determine the product appearance, size, shrinkage, flow conditions distribution, etc., generally slow - fast - slow, that is, first with a higher speed is the melt more through the main flow channel, manifold, into the gate, in order to achieve a balanced injection of rubber, and then a rapid filling mode to fill the entire cavity, and then a slower speed to supplement the shrinkage and counterflow caused by the lack of rubber phenomenon, until the gate freeze, so as to overcome scorching, air lines, shrinkage and other poor quality generated.

 

3, the injection pressure is the melt to overcome the resistance required to move forward, directly affect the size, weight and deformation of the product, different plastic products require different injection pressure, for materials such as PA, PP, etc., increased pressure will make its liquidity significantly improved, injection pressure size determines the density of the product, that is, the appearance of glossy.

 

4, mold temperature, some plastic materials due to high crystallization temperature, slow crystallization, the need for higher mold temperature, some due to control the size and deformation, or the need for mold release, to a higher temperature or lower temperature, such as PC generally requires more than 60 degrees, and PPS in order to achieve a better appearance and improve fluidity, mold temperature sometimes requires more than 160 degrees, thus the mold temperature to improve the appearance of the product, deformation, size, rubber mold has an inestimable role.

 

III. the meaning of injection professional parameters

1. Injection volume

 

Injection volume refers to the amount of melt injected into the mold by the injection machine screw during the injection.

 

Injection volume = screw advance volume *ρ*C

 

ρ is the density of the injected material

 

C is 0.85 for crystalline polymers and 0.93 for non-crystalline polymers

 

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

 

2. Measuring stroke (pre-molding stroke)

 

After each injection process is terminated, the screw is at the front end of the barrel. When the pre-molding process is reached, the screw starts to rotate and the material is sent to the head of the screw, and the screw backs up under the counteraction of the material and touches the limit switch.

 

The size of the injection volume is related to the accuracy of the metering stroke, too small, the injection volume is not enough, too large, so that the front of the barrel is too large after each injection so that the melt temperature is not uniform or overheating decomposition.

 

After pre-molding metering real melt in its longitudinal temperature and radial temperature are temperature differences, screw rotation, pre-molding backpressure and barrel temperature will have a greater impact on the melt temperature and temperature differences.

 

3. the amount of delay prevention

 

Anti-delay amount refers to the screw metering in place, and then straight backward a distance, so that the specific volume of the metering chamber becomes larger, the internal pressure drops to prevent fluid from the metering chamber out.

 

Another purpose of anti-delay is to reduce the pressure of the nozzle flow system when the injection nozzle does not retreat for pre-molding, reduce the internal stress, and easy to draw out the material handle when the mold is opened, the amount of anti-delay will make the metering chamber hostage to bubbles, the viscosity of large materials can be set without anti-delay amount.

 

The above parameters can be adjusted reasonably to get the products that meet the quality requirements, such as the size can be achieved by injection pressure, mold temperature, injection speed and backpressure.

 

IV, how to adjust the injection molding process parameters

- Temperature control

The thermocouple is also widely used as the sensor of the temperature control system. In the control instrument, the desired temperature is set and the display of the sensor will be compared with the temperature generated at the set point. In this simplest system, the power is turned off when the temperature reaches the set point, and turned back on when the temperature drops. This system is called on/off control because it is either on or off.

 

- Temperature

Temperature measurement and control is very important in injection molding. Although it is relatively simple to make these measurements, most injection molding machines do not have adequate temperature pick-up points or lines.

 

On most injection molding machines, the temperature is sensed by thermocouples. A thermocouple basically consists of two different wires connected at the end. If one end is hotter than the other, a tiny electrical signal is generated; the hotter it is, the stronger the signal.

 

- Melt Temperature

The melt temperature is very important and the injection cylinder temperature used is only a guide. The melt temperature can be measured at the injection nozzle or by using the air jet method. The temperature setting of the injection cylinder depends on the melt temperature, screw speed, back pressure, injection volume and injection cycle.

 

If you are not experienced in processing a particular grade of plastic, start with the lowest setting. Injection cylinders are divided into zones for ease of control, but not all are set to the same temperature. If operating for long periods of 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 fluid, hopper closers, molds and injection cylinders are all at the correct temperature before injection begins.

 

- Injection Pressure

This is the pressure that causes the plastic to flow and can be measured with a sensor on the injection nozzle or hydraulic line. It has no fixed value, and the more difficult the mold is to fill, the more the injection pressure increases. There is a direct relationship between injection line pressure and injection pressure.

 

- First stage pressure and second stage pressure

During the filling phase of the molding cycle, high injection pressure may be required to maintain the molding speed at the required level. After the mold is filled, high pressures are no longer required. However, in the case of semi-crystalline thermoplastics (such as PA and POM), a sudden change in pressure can deteriorate the structure, so sometimes it is not necessary to use the second stage pressure.

 

- Clamping pressure

In order to counteract the injection pressure, it is necessary to use the clamping pressure. Instead of automatically selecting the maximum value available, calculate a suitable value considering the projected area. The projected area of the injection part is the maximum area visible from the direction of application 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 a low value and should be taken as a very rough empirical value because, once the injection molded part has any depth, then the sidewalls must be taken into account.

 

- Back Pressure

This is the pressure that must be generated and exceeded before the screw retreats. Although a high backpressure is beneficial to the uniform distribution of color and melting of plastic, it also prolongs the return time of the middle screw, reduces the length of the fibers contained in the filled plastic, and increases the stress of the injection molding machine; therefore, the lower the backpressure, the better, and in no case should it exceed 20% of the injection pressure (maximum quota) of the injection molding machine.

 

- Injection speed

This refers to the filling speed of the mold when the screw is used as a punch. When injecting thin-walled products, a high injection speed is necessary to allow the melt to completely fill the mold before it solidifies and produce a smoother surface. A range of programmed injection speeds are used for filling to avoid defects such as jetting or trapped air. The injection can be performed under open-loop or closed-loop control systems.

 

- Nozzle Pressure

Nozzle pressure is the pressure inside the injection nozzle. It is approximately 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. On screw injection molding machines, the nozzle pressure is about ten percent less than the injection pressure. In the case of piston injection molding machines, the pressure loss can reach about ten percent. In the case of piston injection molding machines, the pressure loss can reach 50 percent.