Injection volume refers to amount of material melt (g) injected into mold by injection molding machine screw (or plunger) during injection. Therefore, injection volume is determined by physical properties of polymer and propulsion volume in barrel of screw.
It can be seen that when selecting injection volume, on the one hand, it must fully meet the total material consumption of product and its gating system, and on the other hand, it must be less than theoretical injection volume of injection molding machine. If injection volume is too small, product will have various defects due to insufficient injection volume, but too large will cause energy waste. Therefore, injection molding machine cannot be used to process products with less than 10% of injection volume or more than 70% of injection volume. According to statistics, about 1/3 of energy of product manufacturers in the world is wasted on unreasonable machine model selection.

After each injection process ends, screw is at the front position of barrel. When pre-plasticization process arrives, screw starts to rotate, and material is transported to screw head. Screw retreats under back pressure of material until it hits limit switch.
This process is called metering process or pre-plasticization process. Distance screw retreats is called metering volume, which is also injection volume, and its metering stroke is also injection stroke. Therefore, injection volume required for product is adjusted by metering stroke.
It can be seen that injection volume is related to accuracy of metering stroke. If metering stroke is adjusted too small, injection volume will be insufficient. If metering stroke is adjusted too large, there will be too much residual material after each injection in the front of barrel, resulting in uneven melt temperature or overheating and decomposition. Repeatability of metering stroke will affect fluctuation of injection volume. Distribution of material temperature along metering stroke is uneven, increasing metering stroke will aggravate unevenness of material temperature.
Screw speed, pre-plasticization back pressure and barrel temperature will have a significant impact on melt temperature and temperature difference.

 

Before injection, melt temperature outside screw head metering chamber is the highest. Although there is a temperature difference, it is small at this time. After injection, temperature of melt in screw groove is the lowest, and melt temperature rises after staying for a period of time. This temperature difference can be improved by adjusting screw speed, axial back pressure or using a new screw.

After screw is injected, it is not desirable to inject all melt in screw head, but to retain some to form a residual material. In this way, on the one hand, it can prevent mechanical damage accidents caused by contact between screw head and injection, and on the other hand, repetitive accuracy of injection volume can be controlled by this residual material pad to achieve purpose of stabilizing quality of injection molded product.
If residual material pad is too small, buffering purpose cannot be achieved. If it is too large, residual material will accumulate too much. Modern injection molding machines control impulse through limit position at the end of screw injection: if actual value detected by displacement sensor exceeds setting range of buffer pad (generally 2-10mm).

Ejection amount refers to amount of screw metering (pre-plasticization) after it is in place, and then it linearly retreats a distance, so that specific volume of melt in metering chamber increases, internal pressure decreases, and melt is prevented from flowing out of metering chamber (through nozzle or gap). This retreat action is called ejection action. Ejection can be set according to polymer viscosity, relative density and product conditions. Excessive ejection amount will cause bubbles to be mixed with melt in metering chamber, seriously affecting quality of product.

Screw speed affects conveying of injection molding materials in screw; it is an important parameter that affects factors such as plasticizing capacity, plasticizing quality and molding cycle. Plasticizing capacity will increase with increase of speed. Increasing screw speed increases flow rate, but uniformity of melt temperature is improved. Melt temperature and screw speed increase with increase of screw speed.

 

Screw speed uses rated screw speed of injection molding machine according to injection molding conditions, 50%-60% of rated amount is used to calibrate plasticizing capacity and screw speed of material to be processed. When adjusting, gradually adjust from lower to higher speed.

Before entering next injection, screw rotates to transport molten material to the front of barrel for storage. At this time, screw rotates while retreating due to back pressure generated by material transported to the front of barrel. In order to adjust and control way screw retreats, a certain pressure opposite to molten material can be added to screw, which is screw back pressure.
Screw back pressure can improve melting and mixing effects of material, and at the same time, it can also ensure that molten material is completely filled in the front of screw to improve accuracy of injection metering. However, if screw back pressure is too high, it will cause material handling capacity to decrease, and material will also cause temperature to rise due to increased friction heat. On the contrary, if screw back pressure is too low, it will cause inaccurate injection volume measurement.
Effect of back pressure on melt temperature is very obvious.
Increasing back pressure helps to compact material in screw groove and drive away gas in material.
Increase of back pressure increases system resistance, slows down screw retraction speed, prolongs thermal history of material in screw, and improves plasticization quality.
However, excessive back pressure will increase backflow and leakage of melt in screw groove of metering section, reduce melt conveying capacity, reduce plasticization amount, and increase power consumption. Excessive back pressure will cause shear heat, excessive shear stress, cause polymer material to degrade and seriously affect quality of product. Therefore, back pressure is one of important parameters for controlling quality of injection molding machine.
Therefore, back pressure should be reduced at the beginning of screw pre-plasticization, and when metering is finished (2-3mm), screw speed should be slowed down in particular to reduce inertial impact and improve metering accuracy.