Injection parameters and control

Temperature

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

On most injection molding machines, the temperature is sensed by an electric thermocouple. An electric thermocouple is two different wires connected at the end. If one end is hotter than the other, a small electrical signal is generated: the hotter it is, the stronger the signal.

Temperature control

Thermocouples are also widely used as sensors for temperature control systems. On the control instrument, the desired temperature is set and the display of the sensor is compared to the temperature generated at the set point. In this simplest system, the power is turned off when the temperature reaches the setpoint and turned back on when the temperature drops. This system is called an on/off control system because it is either on or off.

Melt temperature

The melt temperature is very important and the shot cylinder temperature used is only a guide. The melt temperature can be measured at the injection nozzle or by the air injection 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. The injection cylinders are divided into zones for ease of control, but not all are set to the same temperature. If operating for long periods 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 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 nozzle or hydraulic line. It has no fixed value, but the harder the mold is filled, the higher the injection pressure. There is a direct relationship between the injection line pressure and the injection pressure.

First Stage Pressure and Second Stage Pressure

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

Clamping pressure

To counteract the injection pressure, it is necessary to use clamping pressure. Instead of automatically selecting the maximum value available, calculate a suitable value considering the projected area. The projected area of the injection port is the maximum area visible from the direction of application of the clamping force. For most injection molding situations, it is approximately 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, since the sidewalls must be considered once the injection molded part has any depth.

Back Pressure

This is the pressure that has to be generated and exceeded before the screw recedes. Although a high back pressure is good for uniform color dispersion and plastic melting, it also prolongs the return time of the screw, reduces the length of the fibers in the filled plastic, and increases the stress of the injection molding machine; therefore, the lower the backpressure, the better, and under no circumstances should it exceed 20% of the injection molding pressure (maximum quota) of the machine.

Nozzle pressure

Nozzle pressure is the pressure inside the 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 10% less than the injection pressure. On piston injection molding machines, the pressure loss reaches 50%. 

Injection speed

This refers to the filling speed of the mold when the screw is used as the punch. When injecting thin-walled products, a high injection speed is necessary to allow the melt to fill the mold before it solidifies, producing a smoother surface. A range of programming and injection speeds are used to avoid defects such as jetting or air trapping during filling. The injection can be performed under an open-loop or *cyclic control system.

Regardless of the injection speed used, the speed value must be recorded on a log sheet along with the injection time, noting that time refers to the time required for the mold to reach the intended first stage of injection pressure and is part of the screw advance time.

Mold Exhaust

Due to the rapid filling of the mold, the mold must allow air to escape, which in most cases is just air in the mold cavity. If the air is not exhausted, it will be compressed by the melt, causing the temperature to rise and causing the plastic to burn. The venting level must be set up near the water trap and the final injection part. Generally, the air vent is a slot 6 mm to 13 mm wide and 0.01 to 0.03 mm deep, usually located at the parting surface of one of the mold halves.

Holding pressure

During the filling phase of the injection cycle, it may be necessary to apply a high injection pressure to maintain the injection speed at the required level. Once the mold is filled, it enters the holding phase, where the screw (acting as a press) pushes in additional plastic to compensate for plastic shrinkage. This can be done at lower or equally high pressures. Usually, if high pressure is used in the first stage, lower pressure is used in the second stage. However, in the injection molding of some semi-crystalline thermoplastics (e.g., PA and POM), secondary pressures are sometimes not necessary because of pressure fusion, which can deteriorate the crystalline structure.

Use of recycled plastics

Many injection molding machines use a mixture of new plastic and reused recycled plastic (commonly referred to as sparge material). Surprisingly, the use of recycled plastic can improve the performance of the injection molding machine, i.e., its use produces a more consistent injection molded part. However, it is important to note that it is best to remove dust from recycled plastics before use to avoid differences in the amount of plastic fed into the machine, which can lead to deviations in the color distribution of the injection molded parts. The exact percentage of recycled plastic to be used is determined by experimental data, which must be obtained without affecting the physical properties of the injection molded part. 

Quality Control

The final starting point (weight and size) of the injection molded part is closely related to the production conditions: the size of the mat, the injection pressure, and the flow rate. This means that in many cases it is possible to check whether the injection molded part is satisfactory without actually taking any measurements on it. In each injection, the selected parameters are measured and the set of stored values are compared. As long as the measured value is within the preselected range, the control system determines that the part is acceptable.

If the measurement is outside of the limit, the part will be discarded or, if it is just a little outside, it will be stopped and tested a second time by a qualified person. Today's injection molding systems are equipped with video cameras and computer systems so that each injection molded part is compared to a stored image of the requirements during injection molding. Each injection molded part is compared with the dimensions and visual defects of a standard injection molded part.

The purpose of the injection molding machine is to produce a quality part at a specified cost and within a specified time. To do this, it is essential to keep accurate records. On many injection molding machines, this can be done with the push of a button. If no button is available, a proper log sheet should be completed and a sample of the injection molded part should be kept for future reference. 

Stopping the machine

The most important thing is to adopt a reasonable shutdown process, which can save a lot of time and money. If you are shutting down, for example, to burn plastic, then there is no need to dispense the plastic and you may save the cost of completely shutting down and cleaning the injection molding machine.

Temporary stoppage

If the injection molding machine is suspended, it will be necessary to spray out the remaining plastic several times or allow other plastic to pass through the machine to clean the remaining plastic from the injection cylinder. In case of plastic fading, the number of times of spraying should be increased. For minor repairs, the heater of the injection cylinder must be adjusted to the lowest possible value to minimize the possibility of thermal decomposition. On more modern injection molding machines, this process may be activated automatically.

Overnight stoppages

If the machine has been shut down overnight before injection of thermoplastics (e.g. PS), simply close the bottom slide and the injection cylinder heater and clean the injection cylinder. After the nozzle is completely clean, the injection cylinder is cooled down as much as possible and all equipment is turned off when the machine is cooled down.

Heat-sensitive materials

If the plastic decomposes or burns in the injection molding machine, it will eventually change color and turn the injection molded part into a waste part. In this case, the injection molding machine must be completely shut down and sprayed clean. The prevention method is to spray the heat-sensitive plastic with a more heat-stable plastic, which will resist the subsequent heating. To cope with the problem of plastic oxidation, the operator can fill the injection cylinder with plastic, such as PE.

The injection cycle is the time it takes for the injection molding machine to complete a specific set of actions. Therefore, each part of the cycle time can affect the overall cycle time. To achieve a shorter cycle time, it is important to consider each part of the operation separately to identify the parts that may reduce the cycle time, so that each part can often save a little time. Although such savings may be small, when these times are added together, the reduction in time can be significant in terms of the overall percentage of reduction.