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 enough sampling points or lines.
On most injection molding machines, temperature is sensed by an electric thermocouple. A thermocouple is basically formed by connecting ends of two different wires. If one end is hotter than the other, a tiny electrical signal will be generated: the more heated, the stronger signal.

Thermocouples are also widely used as sensors in temperature control systems. On control instrument, set desired temperature, display of sensor will be compared with temperature generated at set point. In this simplest system, when temperature reaches set point, it turns off, power turns back on when temperature drops. This kind of system is called an on-off control system because it is either on or off.

Melting temperature is very important, temperature of injection cylinder used is only a guideline. Melt temperature can be measured at nozzle or using air jet method to measure. Temperature of injection cylinder is set to depend on melt temperature, screw speed, back pressure, injection volume and injection cycle.
If you do not have experience in processing a specific grade of plastic, please start with the lowest setting. In order to facilitate control, shooting cylinder is divided into zones, but not all of them are set to same temperature. If operation time is long or operation is at high temperature, please set temperature of first zone to a lower value, which will prevent plastic from melting and shunting prematurely. Before injection, make sure that hydraulic oil, hopper closure, mold and injection cylinder are at correct temperature.

This is pressure that causes plastic to flow, which can be measured with a sensor on nozzle or hydraulic line. It has no fixed value, and the more difficult it is to fill mold, injection pressure increases. Injection line pressure is directly related to injection pressure.

In filling phase of injection cycle, high injection pressure may be required to maintain injection speed at required level. After mold is filled, high pressure is no longer required. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), structure will deteriorate due to sudden pressure change, so there is sometimes no need to use secondary pressure

In order to resist injection pressure, clamping pressure must be used. Do not automatically select maximum value available, but conside projected area and calculate a suitable value. Projected area of injection molded part is the largest area seen from application direction of 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 rough empirical value, because once injection molded part has any depth, sidewall must be considered.

This is pressure that screw needs to generate and surpass before retreating. Although use of high back is conducive to uniform color distribution and plastic melting, it also prolongs return time of middle screw, reduces length of fiber contained in filled plastic, and increases stress of injection molding machine; therefore, the lower back pressure, the better. Under no circumstances should it exceed 20% of injection pressure (maximum rating) of injection molding machine.

Nozzle pressure is pressure inside nozzle. It is approximately pressure that causes plastic to flow. It does not have a fixed value, but increases as difficulty of mold filling increases. There is a direct relationship between nozzle pressure, line pressure and injection pressure. On a screw injection molding machine, nozzle pressure is about ten percent less than injection pressure. In piston injection molding machine, pressure loss reaches 50%.

This refers to filling speed of mold when screw is used as a punch. When injecting thin-walled products, a high rate of fire must be used so that mold can be completely filled when melt is not solidified to produce a smoother surface. Filling period uses a series of programming and firing rate to avoid defects such as jetting or trapped air. Injection can be carried out under open-loop or circulation control system.
No matter what kind of injection speed is used, speed value and injection time must be recorded on record table. Note that time refers to time required for mold to reach predetermined first-stage injection pressure, which is part of screw advance time.

Due to rapid filling of mold, mold must allow gas to escape. In most cases, gas is just air in cavity. If air cannot be discharged, it will be melted and compressed, temperature rise will cause plastic to burn. Exhaust position must be set up near water trap and final injection part. Generally, exhaust position is a groove with a width of 6 mm to 13 mm and a depth of 0.01 to 0.03 mm, which is usually located at parting surface of one of mold halves.

During filling phase of injection cycle, high injection pressure may be required to maintain injection speed at required level. After mold is filled, it enters holding phase, at which time screw (acting as a punch) pushes in additional plastic to compensate for plastic shrinkage. This can be done at lower or equally high pressures. Generally, if high pressure is used in the first stage, lower pressure is used in second stage. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), due to pressure fusion, crystal structure will deteriorate, so there is sometimes no need to use secondary pressure.

Many injection molding machines use a mixture of new plastics and recycled recycled plastics (known as nozzle materials). Surprisingly, use of recycled plastic can improve performance of injection molding machine, that is, its use produces more consistent injection molded parts. However, it is worth noting that it is better to remove dust from recycled plastics before use, so as not to cause differences in the amount of plastic feed and lead to deviations in color distribution of injection molded parts. Exact use ratio of recycled plastics should be determined according to number of experiments. This information must be obtained without affecting physical properties of injection molded parts. General empirical value is between 15% and 25%.

Final starting point (weight and size) of injection molded part is closely related to production conditions: such as size of gasket, injection pressure and flow rate. This means that in many cases, it is possible to check whether injection molded part is satisfactory without actually taking any measurements on injection molded part. In each injection, selected parameters are measured and set or stored values are compared. As long as measured value is within pre-selected range, control system determines that injection molded part is acceptable.
If measurement exceeds filling limit, injection molded part will be discarded, or if it is only a little exceeded, it will be stopped and waited for a second inspection by a qualified person. Current injection molding machine is equipped with a video recorder and a computer system, so that each injection molded part is compared with stored required image during injection molding. Each injection molded part has to be compared with standard injection molded part's dimensions and visual defects.

Never forget that purpose of an injection molding machine is to produce injection molded parts that meet quality requirements at a specified cost within a specific time. To do this, it is basically to make accurate records. This can be done with a button on many injection molding machines. If there is no button, an appropriate record sheet should be completed and a sample of injection molded part should be kept for future reference.

The most important thing is to take a reasonable shutdown process, which can save a lot of time and money. If you need to shut down, such as burning plastic, then there is no need to pour out plastic, you may save cost of completely shutting down and cleaning injection molding machine.
Temporary pause
If injection molding machine is suspended, it is necessary to spray remaining plastic several times or allow other plastics to pass through injection molding machine to clean remaining plastic in shooting cylinder. In case of plastic fading, number of spray cleaning will increase. When performing minor repairs, heater of injection cylinder must be adjusted to the lowest value to minimize possibility of thermal decomposition. On more modern injection molding machines, process may start automatically.

Before injection of thermoplastics (such as PS), if machine has been shut down for one night in advance, just turn off slide plate and heater of injection cylinder at the bottom to spray injection cylinder clean. After nozzle is completely cleaned, try to cool injection cylinder as high as possible. After injection molding machine has cooled down, turn off all equipment, injection molding machine will be fully prepared for heating again.

If plastic decomposes or burns in injection molding machine, it will eventually change color, making injection molded parts waste. In this case, injection molding machine must be completely shut down and spray clean. Prevention method is to use a plastic with high thermal stability to spray clean heat-sensitive plastic, so that it can resist permanent heating afterwards. In order to cope with problem of plastic oxidation, operator can fill injection cylinder with plastic, such as PE.

Injection cycle refers to time required for injection molding machine to complete a specific set of actions. Therefore, action time of each part may affect entire cycle time. To shorten cycle and improve production efficiency, consider each part of action separately in order to identify parts that may shorten time. This often saves a little time for each part. Although this saving may be small, when these times are added together, overall reduction in percentage will be very significant.