Close relationship between injection speed and product quality makes it a key parameter for injection molding. By determining beginning, middle, and end of filling velocity segment, achieving a smooth transition from one setpoint to another, a stable melt surface velocity can be guaranteed to produce desired molecular orientation and minimum internal stress. We recommend following speed segmentation principle:
Velocity of fluid surface should be constant.
Fast injection should be used to prevent melt from freezing during injection process.
Setting of injection speed should take into account that speed is slowed down at the water inlet level while filling quickly in critical areas (such as runners).
Injection speed should be guaranteed to stop immediately after cavity is filled to prevent overfilling, flashing and residual stress.
Basis for setting speed segment must take into account mold geometry, other flow restrictions and instability.
Speed setting must have a clearer understanding of injection moulding process and material knowledge, otherwise product quality will be difficult to control. Because melt flow rate is difficult to measure directly, it can be indirectly calculated by measuring screw advance speed or cavity pressure (make sure that check valve is not leaking). Material characteristics 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 degradation caused by shear becomes smaller, because high temperature reduces viscosity of material and reduces shear stress. Undoubtedly, multi-stage injection speed is very helpful for forming heat-sensitive materials such as PC, POM, UPVC and their blending ingredients.

 

1. Need maximum injection speed at thin walls;
2. Thick-walled parts need slow-fast-slow speed curve to avoid defects;
3. In order to ensure that quality of part meets standard, injection speed should be set to ensure that melt forward velocity does not change. Melt flow speed is very important because it affects  molecular arrangement direction and surface state in part;
4. When front of melt reaches structure of crossing area, it should slow down;
For complex molds with radial diffusion, melt throughput should be increased uniformly;
5. Long runner must be filled quickly to reduce cooling of melt front, but injection of high viscosity materials such as PC is exception, because too fast speed will bring cold material through water inlet into cavity.
Adjusting injection speed can help eliminate defects caused by slower flow at water inlet. When melt reaches water inlet through nozzle and flow channel, surface of melt front may have cooled and solidified, or melt will stagnate due to sudden narrowing of runner until sufficient pressure is established to push melt through water inlet, which will cause a peak shape of pressure through water inlet. High pressure will damage material and cause surface defects such as flow marks and scorching at water inlet. This situation can be overcome by decelerating just before water inlet. This deceleration can prevent excessive shearing at water inlet level, then increase injection speed to original value. Because it is very difficult to precisely control injection speed to slow down at inlet, deceleration at the end of runner is a better solution. We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling speed of last shot. Deceleration at the end of filling can prevent over-filling of cavity, avoid flashing and reduce residual stress. Trapped air due to poor exhaust or filling problems at the end of mold runner can also be solved by reducing exhaust speed, especially exhaust speed at the end of shot.

 

Short shots are caused by slow speeds at water inlet or local flow obstruction caused by melt solidification. Accelerating injection speed just after passing through water inlet or local flow obstruction can solve this problem. Defects in heat-sensitive materials, such as flow marks, scorching at water inlet, molecular rupture, delamination, and peeling, are caused by excessive shear when passing through water inlet. Smooth parts are dependent on injection speed, and glass fiber fillers are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by unstable flow due to viscosity changes. Twisted flow can lead to wavy or uneven haze. What kind of defects are produced depends on degree of flow instability. When melt passes through water inlet, high-speed injection will cause high shear, and heat-sensitive plastic will scorch. This scorched material will pass through cavity to reach flow front and appear on the surface of part. In order to prevent jetting, setting speed of injection must ensure that runner area is filled quickly and then passed through water inlet slowly. Finding this speed transition point is essence of problem. If it is too early, filling time will be excessively increased. If it is too late, too much flow inertia will cause appearance of ejection. The lower melt viscosity and the higher barrel temperature, the more pronounced tendency for this pattern to appear. Because small inlets require high-speed and high-pressure injection, they are also an important factor that causes flow defects. Shrinkage can be transmitted through more effective pressure, and less pressure will be improved. Low mold temperature and slow screw advance speed greatly shorten flow length, must be compensated by high rate of fire. High-speed flow will reduce heat loss, frictional heat due to high shear heat will cause melt temperature to increase, slowing down thickness of outer layer of part. Cavity crossing must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur. In short, most injection defects can be solved by adjusting injection speed, so technique for adjusting injection process is to set injection speed and its segmentation reasonably.