I. Injection Pressure

Injection pressure is used to overcome the resistance encountered by the melt during its flow. The resistance in the flow process requires the pressure from the injection molding machine to counteract, providing the melt with a certain filling speed and compacting and compensating for shrinkage of the melt to ensure the smooth progress of the filling process.

II. Holding Pressure

When the injection process is about to end and the injection pressure is switched to the holding pressure, the holding stage begins. During the holding process, the injection molding machine feeds material from the nozzle into the cavity to fill the volume left vacant due to the shrinkage of the molded part. If no holding pressure is applied after the cavity is filled, the molded part will shrink by about 25%, especially at the ribs, where excessive shrinkage can lead to shrinkage marks. The holding pressure is generally around 85% of the maximum filling pressure, but it should be determined according to actual conditions.

The holding pressure curve is divided into two parts. One part is the holding at a constant pressure, which takes 2 - 3 seconds and is called the constant holding pressure curve. The other part is the gradual release of the holding pressure, which takes about 1 second and is called the delayed holding pressure curve. The delayed holding pressure curve has a significant impact on the molded part. If the duration of the constant holding pressure curve becomes longer, the volume shrinkage of the molded part will decrease, and vice versa. If the slope of the delayed holding pressure curve becomes larger and the delayed holding time becomes shorter, the volume shrinkage of the molded part will increase, and vice versa. If the delayed holding pressure curve is segmented and extended, the volume shrinkage of the molded part will decrease, and vice versa.

III. Back Pressure of the Screw

During the plastic melting and plasticizing process, the melt continuously moves towards the front end of the barrel (in the measurement chamber) and accumulates, gradually forming a pressure that pushes the screw backward. To prevent the screw from retreating too quickly and ensure uniform compaction of the melt, a reverse pressure needs to be applied to the screw. This reverse pressure that prevents the screw from retreating is called back pressure.

Properly adjusting the back pressure offers significant benefits for injection molding quality. During the injection molding process, appropriate adjustments to the back pressure can yield the following advantages:

It can compact the melt in the barrel, increase its density, and improve the stability of the injection volume, part weight, and dimensions.

It can expel the gas within the melt, reducing gas marks on the part surface and internal bubbles, and improving the uniformity of gloss.

It can slow down the screw's backward speed, allowing the melt in the barrel to be fully plasticized and enhancing the uniform mixing of colorants and masterbatches with the melt, thus avoiding color mixing in the parts.

Appropriately increasing the back pressure can improve surface shrinkage and the flow of material around the edges of the parts.

It can raise the melt temperature, improving the plasticizing quality of the melt and enhancing its fluidity during mold filling, resulting in a part surface free of cold flow marks.

IV. Clamping Force

The clamping force is set to resist the swelling force exerted by the plastic melt on the mold, and its magnitude is determined according to specific conditions such as the injection pressure. However, in practice, after the plastic melt is ejected from the nozzle of the injection molding machine, it passes through the main runner, sub-runners, and gate of the mold before entering the cavity, and there is a significant pressure loss along the way.

V. Barrel Temperature

The melt temperature must be controlled within a certain range. If the temperature is too low, poor plasticization of the melt will affect the quality of the molded part and increase the process difficulty. If the temperature is too high, the raw material is prone to decomposition. In actual injection molding processes, the melt temperature is often higher than the barrel temperature, with the difference depending on the injection rate and material properties, and it can be as high as 30°C. This is because the melt generates a large amount of heat due to shear when passing through the gate.