How to adjust the injection molding process parameters?

Temperature measurement and control are very important in injection molding. Although these measurements are relatively simple, most Injection Molding machines don't have enough temperature sampling points or circuits.

On most injection molding machines, temperature is sensed by thermocouples. A thermocouple is basically two different wires connected at the ends. If one end is hotter than the other, a tiny electrical signal will be generated, and the more heated the signal, the stronger it will be.

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

This is the pressure that causes the plastic to flow and can be measured using a sensor on the nozzle or hydraulic line. It doesn't have a fixed value, and the more difficult it is to fill the mold, the greater the injection pressure. There is a direct relationship between the injection line pressure and the injection pressure.

During the filling phase of the injection molding cycle, high injection pressure may be required to maintain the injection speed at the required level. Once the mold is filled, high pressure is no longer required. However, when injection molding some semi-crystalline thermoplastics (such as PA and POM), sudden changes in pressure can cause structural deterioration, so sometimes there is no need to use sub-stage pressure.

In order to resist the injection pressure, the clamping pressure must be used. don't automatically select the maximum value available, but consider the projected area and calculate an appropriate value. The projected area of an Injection Molded part is the largest area seen from the direction of application of the clamping force. For mostinjection molding cases it is about 2 tons per square inch, or 31 meganewtons per square meter, however this is a low number and should be used as a very rough rule of thumb because, once the molded part has any depth, the sidewalls must be considered.

Back pressure

This is the pressure that needs to be generated and exceeded before the screw retreats. Although using high back pressure is beneficial to the uniform distribution of color materials and melting of plastics, it also prolongs the return time of the screw, reduces the length of fibers contained in the filled plastic, and increases the stress on the injection molding machine. Therefore, the lower the back pressure, the better. Under no circumstances should it exceed 20% of the injection pressure (maximum rating) of the injection molding machine.

Nozzle pressure

Nozzle pressure is the pressure inside the nozzle. It's approximately the pressure that causes plastic to flow. It doesn't have a fixed value, but increases with the difficulty of mold filling. There is a direct relationship between nozzle pressure, line pressure and injection pressure. On a screw injection molding machine, the nozzle pressure is about ten percent less than the injection pressure. In piston injection molding machines, the pressure loss can reach about 10%. In piston injection molding machines, the pressure loss can reach 50%.

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 rate must be used to completely fill the mold before the melt solidifies and produce a smoother surface. Use a series of programmed firing rates when filling to avoid defects such as jetting or trapped air. Injection molding can be performed under open-loop or closed-loop control systems. No matter which injection speed is used, the speed value must be recorded on the record sheet together with the injection time.

Mold exhaust

Due to the rapid filling of the mold, the mold must allow gas to escape, which in most cases is just air in the mold cavity. If the air cannot be discharged, it will be melted and compressed, causing the temperature to rise and cause the plastic to burn. The exhaust position must be located near the water line and the final injection molded part. The general exhaust position is a groove 6 to 13 mm wide and 0.01 to 0.03 mm deep, usually located at the parting surface of one of the mold halves.

Keep pressure

During the filling phase of the injection molding cycle, high injection pressure may be required to maintain the injection speed at the required level. Once the mold is filled, it enters the holding phase, where the screw (which acts as a punch) advances additional plastic to compensate for plastic shrinkage. This can be done at lower or equally high pressure. Usually if a high pressure is used in the first stage, a lower pressure is used in the second stage. However, when injection molding some semi-crystalline thermoplastics (such as PA and POM), sudden changes in pressure will deteriorate the structure of the product, so sometimes there is no need to use secondary pressure.

Use of recycled plastics

Many injection molding machines use a mixture of virgin and recycled plastics (commonly known as nozzle materials). Surprisingly, the use of recycled plastic can improve the performance of injection molding machines, i.e. its use produces more consistent injection molded parts. However, it is worth noting that dust should be removed from recycled plastic before use to avoid differences in the plastic feed resulting in deviations in the color distribution of injection molded parts. The exact proportion of recycled plastic used must be determined based on experimental data. This data must be obtained without affecting the physical properties of the injection molded parts. The general empirical value is between 15% and 25%.

Quality control

There is a close relationship between the final characteristics of the injection molded part (weight and size) and production conditions such as pad size, injection pressure and flow rate. This means that in many cases it is possible to check whether the molded part is satisfactory without actually taking any measurements of the molded part. In each injection molding, the selected parameters are measured and compared with the set or stored values. As long as the measured value is within a pre-selected range, the control system determines that the injection molded part is acceptable. If the measurement exceeds the set limits, the molded part will be discarded or, if it is just a little outside of it, it will be stopped and waited for a second inspection by a qualified person. Today's injection molding machines are equipped with video and computer systems, so that during injection molding, each plastic part is compared with the stored required image. Each injection molded part is compared to standard injection molded parts for size and visual imperfections.

Record injection molding conditions

Never forget that the purpose of injection molding is to produce injection molded parts that meet quality requirements within a specific time and at a specified cost. To do this, it is basically necessary to keep accurate records. On many injection molding machines a button will do this. If there are no buttons, an appropriate record sheet should be completed and the molded sample retained for future reference.

Downtime

The most important thing is to have a sensible downtime process, which can save a lot of time and money. If you are shutting down, for example burning plastic, then there is no need to dump the plastic. You may save the expense of completely shutting down and cleaning the injection molding machine.

Temporary pause

If the injection molding machine is suspended, the remaining plastic must be sprayed out multiple times or other plastics must be passed through the injection molding machine to clean the remaining plastic in the material tank. If the plastic fades, the number of times of spray cleaning will increase. When making minor repairs, the tank heater must be adjusted to the lowest setting to minimize the possibility of thermal decomposition. On more modern injection molding machines the process may start fully automatically.

A pause throughout the night

Before injecting thermoplastics (such as PS), if the machine has been shut down overnight, you only need to turn off the bottom slide and the injection cylinder heater, and spray the injection cylinder clean. The injection nozzle should be completely clean, and the injection cylinder should be cooled as much as possible. After the injection molding machine cools down and all equipment is turned off, the injection molding machine will be ready to be heated again.

Heat sensitive plastics

If the plastic decomposes in the injection molding machine, it can burn and eventually change color, turning the injection molded parts into waste. In this case, the injection molding machine must be completely shut down and sprayed clean. The prevention method is to use a plastic with higher thermal stability to spray the heat-sensitive plastic, so that it can withstand permanent dirt and then reheat at any time. In order to deal with the problem of plastic oxidation, the operator can fill the material tank with plastic, such as PE.