Close relationship between injection speed and product quality makes it a key parameter of injection molding.
By determining beginning, middle, end of filling speed segment, realizing a smooth transition from one set point to another set point, a stable melt surface speed can be ensured to produce desired molecule and the smallest internal stress.
Following principle of speed segmentation is recommended:

 

Velocity of fluid surface should be constant.
Quick glue injection should be used to prevent melt from freezing during glue injection process.
Injection speed setting should take into account rapid filling in critical area (such as runner) while slowing down speed at water inlet.
Injection speed should be guaranteed to stop immediately after cavity is filled to prevent over-filling, flashing and residual stress.
Basis for setting speed segment must take into account geometry of mold, other flow restrictions and unstable factors. Speed setting must have a clear understanding of injection molding process and material knowledge, otherwise, product quality will be difficult to control. Because melt flow rate is difficult to directly measure, it can be calculated indirectly by measuring screw forward 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 glue injection speed is very helpful for forming heat-sensitive materials such as PC, POM, UPVC, and their blending ingredients.
Geometry of mold is also a decisive factor: thin-walled parts need maximum injection speed; thick-walled parts need a slow-fast-slow speed curve to avoid defects; in order to ensure that quality of parts meets standard, injection speed setting should ensure melt front flow rate constant.
Melt flow speed is very important, because it will affect molecular arrangement direction and surface state of part; when front of melt reaches intersecting area structure, it should be slowed down; for complex molds with radial diffusion, melt throughput should be increased in a balanced manner; long runners must be filled quickly to reduce cooling of melt front, but injection of high-viscosity materials, such as PC, is an exception, because too fast speed will bring cold material into cavity through water inlet.
Adjusting injection speed can help eliminate defects caused by slowing of flow at water inlet. When melt passes through nozzle and runner to water inlet, surface of melt front may have cooled and solidified, or melt will stagnate due to sudden narrowing of runner, until enough pressure has been established to push melt through water inlet, which will cause pressure through water inlet to peak.
High pressure will damage material and cause surface defects such as flow marks and scorching of water inlet. This situation can be overcome by decelerating just before water inlet. This deceleration can prevent excessive shear at water inlet, then increase rate of fire to original value. Because it is very difficult to precisely control rate of fire to slow down at water inlet, it is a better plan to decelerate at the end of runner.
We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling injection speed at the end. Deceleration at the end of filling can prevent overfilling of cavity, avoid flash and reduce residual stress. Air traps caused by poor exhaust or filling problems at the end of mold flow path can also be solved by reducing exhaust speed, especially exhaust speed at the end of injection.
Short shot is caused by slow speed at water inlet or blockage of local flow caused by solidification of melt. This problem can be solved by increasing injection speed just after passing water inlet or local flow obstruction.
Flow marks, scorching of water inlet, molecular rupture, delamination, peeling and other defects that occur on heat-sensitive material are caused by excessive shear when passing through water inlet.
Smooth parts depend on injection speed, glass fiber filling materials are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by unstable flow caused by viscosity changes. Distorted flow can cause wavy lines or uneven fog. What kind of defects are produced depends on degree of instability of flow.
When melt passes through water inlet, high-speed injection will cause high shear, and heat-sensitive plastic will be scorched. This scorched material will pass through cavity and reach flow front, appearing on the surface of part.
In order to prevent injection patterns, injection speed must be set to fill runner area quickly and then pass through water inlet slowly. Finding this speed conversion point is essence of problem. If it is too early, filling time will increase excessively, and if it is too late, excessive flow inertia will cause appearance of jetting. The lower melt viscosity and the higher barrel temperature, the more obvious tendency of this kind of pattern to appear. Since small water inlet requires high-speed and high-pressure injection, it is also an important factor leading to flow defects.
Shrinkage can be improved through more effective pressure transmission and smaller pressure drop. Low mold temperature and too slow screw advancement speed greatly shorten flow length, which must be compensated by high shooting speed. High-speed flow will reduce heat loss, and due to frictional heat generated by high shear heat, it will cause melt temperature to rise and slow down thickening speed of outer layer of part. Cavity intersection must have enough thickness to avoid too much pressure drop, otherwise shrinkage will occur.
In short, most injection molding defects can be solved by adjusting injection speed, so skill to adjust injection process is to set injection speed and its segmentation reasonably.