If the injection molding process is good, the firing rate adjustment is indispensable

The proportional control of injection speed has been widely used by injection molding machine manufacturers. Although the computer-controlled injection molding speed segmented control system has long existed, the advantages of this machine setting have rarely been brought into play due to the limited relevant data. This paper systematically explains the advantages of multi-stage speed injection molding, and briefly introduces its application in eliminating product defects such as short shot, gas trap and shrinkage.

The close relationship between injection speed and product quality makes it a key parameter of injection molding. By determining the beginning, middle and end of the filling speed segment and realizing the smooth transition from one set point to another, the stable melt surface speed can be guaranteed to produce the desired minimum internal stress.

We recommend the following speed segmentation principle:

1) The velocity of the fluid surface should be constant.

2) Rapid glue injection shall be adopted to prevent melt freezing during glue injection.

3) The setting of glue injection speed shall take into account the rapid filling in critical areas (such as flow channels) and slowing down at the water inlet.

4) The glue injection speed should ensure that it stops immediately after the mold cavity is filled to prevent overfilling, flash and residual stress.

The basis for setting the speed segment must take into account the die geometry, other flow constraints and instability factors. The speed setting must have a clear understanding of the injection molding process and material knowledge, otherwise the product quality will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be calculated indirectly by measuring the screw forward speed or cavity pressure (it is determined that the check valve has no leakage).

Material properties are very important because polymers may degrade due to different stresses. Increasing molding temperature may lead to severe oxidation and degradation of chemical structure, but at the same time, the degradation caused by shear becomes smaller, because high temperature reduces the viscosity of materials and reduces shear stress. Undoubtedly, the multi-stage glue injection speed is very helpful for forming heat sensitive materials such as PC, POM, UPVC and their blending.

The geometry of the mold is also a decisive factor: the maximum injection speed is required at the thin wall; Thick wall parts need slow fast slow speed curve to avoid defects; In order to ensure that the quality of parts meets the standard, the injection speed shall be set to ensure that the melt front flow rate remains unchanged. Melt flow rate is very important because it will affect the molecular arrangement direction and surface state in parts; When the melt front reaches the cross region structure, it should slow down; For complex dies with radial diffusion, the melt throughput should be increased evenly; The long runner must be filled quickly to reduce the cooling of the melt front, but the injection of high viscosity materials such as PC is an exception, because too fast will bring the cold material into the cavity through the water inlet.

Adjusting the injection speed can help eliminate defects caused by the slow flow at the water inlet. When the melt reaches the water inlet through the nozzle and flow channel, the surface of the melt front may have cooled and solidified, or the melt may stagnate due to the sudden narrowing of the flow channel until sufficient pressure is established to push the melt through the water inlet, which will make the pressure through the water inlet peak.

High pressure will damage the material and cause surface defects such as flow marks and scorched water inlet, which can be overcome by decelerating just in front of the water inlet. This deceleration can prevent excessive shear at the inlet, and then increase the fire speed to the original value. Because it is very difficult to accurately control the speed of fire at the inlet, it is a better scheme to decelerate at the end of the channel.

We can control the final injection speed to avoid or reduce defects such as flash, scorch, gas trap and so on. Deceleration at the end of filling can prevent over filling of the cavity, avoid flash and reduce residual stress. The gas trap caused by poor exhaust or filling at the end of the die flow path can also be solved by reducing the exhaust speed, especially at the end of the glue injection section.

The short shot is caused by the slow speed at the water inlet or the local flow obstruction caused by melt solidification. This problem can be solved by accelerating the injection speed when it just passes through the inlet or local flow obstruction.

Defects such as flow marks, inlet charring, molecular cracking, delamination and spalling on heat sensitive materials are caused by excessive shear when passing through the inlet.

Smooth parts depend on the injection speed, and glass fiber filling materials are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by flow instability caused by viscosity changes. Distorted flow can lead to wavy lines or uneven fog. What kind of defect depends on the degree of flow instability.

When the melt passes through the water inlet, high-speed injection will lead to high shear, and the heat-sensitive plastic will be scorched. This scorched material will pass through the cavity, reach the flow front and appear on the part surface.

In order to prevent injection lines, the injection speed setting must ensure that the runner area is filled quickly and then passes through the inlet slowly. Finding this speed transition point is the essence of the problem. If it is too early, the filling time will increase excessively. If it is too late, too much flow inertia will lead to the emergence of projectiles. The lower the melt viscosity and the higher the barrel temperature, the more obvious the trend of this kind of projection. Because the small inlet needs high-speed and high-pressure injection, it is also an important factor leading to flow defects.

Shrinkage can be improved by more effective pressure transmission and smaller pressure drop. Low die temperature and too slow screw propulsion speed greatly shorten the flow length, which must be compensated by high injection speed. High speed flow will reduce heat loss, and friction heat will be generated due to high shear heat, which will increase the melt temperature and slow down the thickening speed of the outer layer of the part. The cavity intersection must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur.

In short, most injection defects can be solved by adjusting the injection speed, so the skill of adjusting the injection process is to reasonably set the injection speed and its segmentation.