Temperature measurement and control are very important in injection molding. Although these measurements are relatively simple, most injection molding machines do not 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 signal, the stronger it will be.

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

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

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

During filling phase of injection molding cycle, high injection pressure may be required to maintain injection molding speed at required level. Once 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 injection pressure, clamping pressure must be used. Do not automatically select maximum value available, but consider projected area and calculate an appropriate value. Projected area of an injection molded part is the largest area seen from direction of application 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 number and should be used as a very rough rule of thumb because once molded part has any depth, then side walls must be considered.

 

This is pressure that needs to be generated and exceeded before screw retreats. Although using high back pressure is beneficial to uniform distribution of color material and melting of plastic, it also prolongs return time of screw, reduces length of fibers contained in filled plastic, and increases stress on 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.

This refers to filling speed of mold when screw is used as a punch. When injecting thin-walled products, a high injection rate must be used to completely fill mold before melt solidifies and produce a smoother surface. A series of programmed firing rates are used during 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, speed value must be recorded on record sheet together with injection time. Injection time refers to time required for mold to reach predetermined first-stage injection pressure, which is part of screw advancement time.

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

During filling phase of injection molding cycle, high injection pressure may be required to maintain injection speed at required level. Once mold is filled, it enters holding phase, where 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 high pressure is used in the first stage, lower pressure is used in second stage. However, when injection molding some semi-crystalline thermoplastics (such as PA and POM), sudden changes in pressure will deteriorate crystal structure, so sometimes there is no need to use sub-stage pressure.

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

There is a close relationship between final characteristics of 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 molded part is satisfactory without actually taking any measurements of injection molding chamber. In each injection molding, selected parameters are measured and compared with set or stored values. As long as measured value is within a pre-selected range, control system determines that injection molded part is acceptable. If measurement exceeds the set limits, 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 recorders and computer systems, so that during injection molding, each plastic part is compared with stored requirement image. Each injection molded part is compared to standard injection molded parts for size and visual imperfections.

Never forget that 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 molded sample retained for future reference.

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

If injection molding machine is suspended, it is necessary to spray out remaining plastic several times or let other plastics pass through injection molding machine to clean remaining plastic in injection tank. If plastic fades, number of times of spray cleaning will increase. When performing minor repairs, heater of injection cylinder must be adjusted to the lowest setting to minimize possibility of thermal decomposition. On more modern injection molding machines this process may start automatically.

 

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

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

(1) Thickness of finished product is different, and difference is too large, resulting from different shrinkage rates.
(2) Uneven injection pressure transmission occurs due to high and low density (gate location and type).
(3) Mold temperature distribution is uneven, and cooling system near gate should be cooler, and vice versa.
(4) Molecular alignment gap is too large.
(5) Post-crystallization (crystalline plastic).
(6) Internal stress is too high.

Clamping pressure must be greater than the total pressure of plastic injected into mold. If it is too low, plastic may overflow from parting surface. If pressure is too high, it will damage machine, mold and waste electricity. Therefore, appropriate clamping force is based on principle that finished product is injected into mold without burrs on parting surface.

Screw has functions of transporting, kneading, exhausting, dehumidifying, melting and measuring raw materials. Seventy percent of heat required for melting plastic raw materials comes from friction heat generated when screw rotates, and 30% comes from heat added by electric heating plate. Low viscosity, small screw, and melt speed should be accelerated. Speed of melting glue with high viscosity and large screw should be slowed down. Composite materials need to be slowed down.

Speed of injection mainly determines flow of raw materials in the runner of mold and mold cavity. If speed is too fast, it will cause hair to be oversaturated, burned and stuck to mold. If speed is too slow, it will easily cause short shot shrinkage and obvious joint lines. It must be adjusted in sections according to actual needs.

Injection pressure and injection speed have some common effects, both of which determine how raw materials can flow evenly and completely to all corners in mold. If pressure is too low, short shot shrinkage will occur. If pressure is too high, it will produce burrs, mold sticking, residual internal stress, deformation, rupture, and easy damage to molds and machines.

Heat and temperature required to properly melt raw materials during molding vary depending on melting temperature and specific heat of each raw material. If temperature is too low, raw materials will melt unevenly, resulting in short shots, uneven color, and high internal stress in finished product. If heating is too high or for too long, finished product will easily develop lint due to excessive fluidity, and finished product will shrink due to difference in cooling temperature. In severe cases, raw materials will decompose, deteriorate or even burn.

Raw materials put a large amount of heat into mold, and finished product dissipates part of heat into air. Therefore, if you want to keep mold at a constant temperature, pass chilled water, cold water, hot water, hot oil, or electric heating rods through mold to balance heat entering and exiting mold, maintain a constant temperature. If mold temperature is too low, finished product is prone to short shots, rough surface, high internal stress, and mold sticking. If mold temperature is too high, finished product will easily shrink and sink, and cycle will be prolonged. Therefore, cooling time and mold temperature can be set based on experience.