Last cycle is over - mold closing - filling - pressure holding - glue back - cooling - mold opening - demoulding - start next cycle
In filling and holding pressure drop section, cavity pressure increases with time. After filling cavity, pressure will remain in a relatively static state to supplement insufficient amount of glue due to shrinkage. In addition, this pressure can prevent phenomenon of colloid backflow caused by reduction of injection. This is pressure holding stage. After pressure holding is completed, cavity pressure gradually decreases, and can theoretically drop to zero over time, but it is not actually zero. Therefore, after demolding, internal stress of product is stored, so some products need to be post-processed to remove residual stress.

 

So-called stress is force that comes from free movement of Fugo chain or chain segment, that is, bending deformation, stress cracking, shrinkage, etc.

1. Temperature of injection molding compound and melt temperature play a major role in flow properties of melt. Since plastic does not have a specific melting point, so-called melting point is a temperature range in a molten state. Structure and composition of plastic molecular chain are different, so influence of fluidity is also different. Rigid molecular chains are obviously affected by temperature, such as PC, PPS, etc., while flexible molecular chains such as PA, PP, PE, etc., have less fluidity by changing temperature, so reasonable injection temperature should be adjusted according to different materials.
2. Injection speed is speed of melt in barrel (also advancing speed of screw) (MM/S). Injection speed determines appearance, size, shrinkage, flow distribution, etc. of product. Generally, it is slow first—fast—then slow, that is, first use a relatively high speed to make melt go through main channel, runner, and gate to achieve purpose of balancing injection, then fill the entire cavity with a fast filling method, and supplement insufficient rubber material caused by shrinkage and reverse flow at a slower speed until gate freezes, which can overcome occurrence of poor quality such as scorching, gas streaks, and shrinkage.
3. Injection pressure is resistance required for melt to overcome advance, which directly affects size, weight and deformation of product. Different plastic products require different injection pressures. For materials such as PA and PP, increasing pressure will significantly improve fluidity. Injection pressure determines density of product, that is, appearance and gloss.
4. Mold temperature, some plastic materials require higher mold temperature due to high crystallization temperature and slow crystallization speed, some require higher or lower temperature due to control of size and deformation, or need for demolding. For example, PC generally requires more than 60 degrees, while PPS in order to achieve better appearance and improve fluidity, mold temperature sometimes needs to be more than 160 degrees, so mold temperature has an irreparable role in improving appearance, deformation, size and plastic mold.

Injection volume refers to amount of melt injected into mold by screw of injection molding machine during injection molding.
Injection volume=screw advancing volume*ρ*C
ρ is density of injection molding material
C is 0.85 for crystalline polymers and 0.93 for amorphous polymers
Injection molding machine cannot be used to process products with less than 1/10 of injection volume or more than 70% of injection volume

After each injection program is terminated, screw is at the front end of barrel. When pre-plastic program arrives, screw starts to rotate, material is sent to head of screw, and screw retreats under reaction of material until it hits limit switch. This process is a metering process.
Size of injection volume is related to accuracy of metering stroke. If it is too small, injection volume is not enough, and if it is too large, residual material at the front of barrel will be too large after each injection, causing melt temperature to be uneven or overheated to decompose.
Longitudinal temperature and radial temperature of melt measured after pre-plasticizing have temperature differences, screw revolutions, pre-plastic back pressure and barrel temperature will all have a great influence on melt temperature and temperature difference.

Anti-delay amount means that after screw is metered in place, it will go back a distance in a straight line, so that specific volume of metering chamber will become larger, internal pressure will drop, and fluid will be prevented from flowing out of metering chamber.
Another purpose of anti-casting is to reduce pressure of nozzle runner system and internal stress when injection nozzle does not retreat for pre-molding, and it is easy to pull out material handle when mold is opened. Anti-elongation amount will cause air bubbles in measuring chamber, and anti-elongation amount may not be set for materials with high viscosity.
Above parameters can be adjusted reasonably to obtain products that meet quality requirements. For example, size can be achieved by injection pressure, mold temperature, injection speed, and back pressure.

 

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

Measurement and control of temperature is very important in injection molding. While making these measurements is relatively simple, most injection molding machines do not have sufficient temperature pick-up points or lines.
On most injection molding machines, temperature is sensed by thermocouples. A thermocouple basically consists of two different wire tails joined together. If one end is hotter than the other, a tiny electrical signal will be generated; the more heated, the stronger signal.

Melt temperature is important, and injection cylinder temperature used is only a guideline. Melt temperature can be measured at nozzle or using air jet method. Temperature setting of injection cylinder depends on melt temperature, screw speed, back pressure, injection volume and injection cycle.
If you have no experience processing a particular grade of plastic, start with the lowest setting. For ease of control, shot tank is divided into zones, but not all are set to same temperature. If operating for long periods of time or at high temperatures, set temperature in the first zone to a lower value, this will prevent premature melting and shunting of plastic. Before injection starts, make sure hydraulic oil, hopper closure, mold and shot cylinder are all at correct temperature.

This is pressure that causes plastic to flow and can be measured with sensors on nozzle or hydraulic line. It does not have a fixed value, and the more difficult it is to fill mold, the higher injection pressure. Injection line pressure is directly related to injection pressure.

During filling phase of injection cycle, high injection pressure may be required to maintain injection 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), due to sudden pressure change, structure will deteriorate, so sometimes it is not necessary to use secondary pressure.

In order to counter injection pressure, clamping pressure must be used, instead of automatically selecting maximum value available, calculate a suitable value considering projected area. Projected area of injection molded part is the largest area seen from direction of application of clamping force. For most injection molding cases, it's about 2 tons per square inch, or 31 meganewtons per square meter. However, this is only a low value and should be taken as a very rough rule of thumb, since as soon as injection moulded part has any depth, side walls must be taken into account.

This is pressure that needs to be generated and exceeded before screw retreats. Although use of high back pressure is conducive to uniform distribution of color material and melting of plastic, it also prolongs return time of middle screw, reduces length of fibers contained in filled plastic, and increases stress of injection molding machine; therefore, the lower back pressure, the better, and 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 acts as a punch. When injection molding thin-walled products, a high injection rate must be used so that mold can be completely filled when melt is not solidified, and a relatively smooth surface can be produced. Fill with a range of programmed firing rates to avoid defects such as jetting or trapping air. Injections can be performed under open loop or closed loop control systems.

Nozzle pressure is pressure inside the nozzle. It's roughly pressure that causes plastic to flow. It does not have a fixed value, but increases with difficulty of filling mold. 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 can reach about ten percent. In the case of piston injection molding machines, pressure loss can reach 50%.