Proportional control of injection speed has been widely adopted by injection molding machine manufacturers.
Although computer-controlled injection speed segmented control system has long existed, advantages of this machine setup are seldom exploited due to limited data available. This article will systematically explain advantages of applying multi-speed injection molding, briefly introduce its use in eliminating product defects such as short shots, trapped air, and shrinkage.
Close relationship between injection speed and product quality makes it a key parameter for injection molding. By determining start, middle, and end of filling speed segment, achieving a smooth transition from one set point to another, a stable melt surface velocity can be guaranteed to produce desired molecular orientation with minimal internal stress.

 

We recommend following velocity segmentation principle:
1) Velocity of fluid surface should be constant. .
2) Rapid injection should be used to prevent melt from freezing during injection process.
3) Injection speed setting should take into account rapid filling in critical areas (such as runners) while slowing down speed at water inlet.
4) Injection speed should be stopped immediately after cavity is filled to prevent overfilling, flash and residual stress.
Basis for setting speed segment must take into account geometry of die, other flow constraints and instabilities. 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 measure directly, it can be indirectly calculated by measuring advancing speed of screw, or cavity pressure (make sure that check valve does not leak).
Material properties are very important because polymers may degrade due to different stress, increasing molding temperature may lead to violent oxidation and degradation of chemical structure, but at the same time shear-induced degradation is smaller because high temperatures reduce viscosity of material and reduce shear stress. Undoubtedly, multi-stage injection speed is very helpful for molding of heat-sensitive materials such as PC, POM, UPVC and their blending ingredients.
Die Geometry
Geometry of mold is also a determining factor: maximum injection speed is required at thin walls; thick-walled parts require a slow-fast-slow speed curve to avoid defects; in order to ensure part quality meets standards, injection speed setting should ensure that melt front flow rate constant.
Melt flow rate is very important because it affects molecular arrangement direction and surface state in part; when front of melt reaches cross-region structure, it should slow down; for complex molds with radial diffusion, melt throughput should increase evenly; 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 a speed will bring cold material into cavity through water inlet.
Adjusting injection speed can help eliminate defects caused by slow 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 may stagnate due to sudden narrowing of runner until enough pressure is built up to push melt through water inlet, which can cause a pressure peak through water inlet.
High pressure will damage material and cause surface defects such as flow marks and inlet scorch, which can be overcome by slowing down just before inlet. This deceleration prevents excessive shearing at inlet level before increasing rate of fire to its original value. Because it is very difficult to accurately control rate of fire to slow down at water inlet, slowing down at end of runner is a better solution.
We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling final injection speed. Deceleration at the end of filling prevents overfilling of cavity, avoids flash and reduces residual stress. Trapped air 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 local flow obstruction caused by solidification of melt. Speeding up shot just past water inlet or local flow obstruction can solve this problem.
Defects such as flow marks, water inlet scorch, molecular breakage, delamination, flaking, etc. that occur on heat-sensitive materials are caused by excessive shearing through water inlet.
Smooth parts depend on injection speed, and fiberglass-filled materials are especially sensitive, especially nylon. Dark spots (wavy lines) are caused by flow instabilities due to viscosity changes. Distorted flow can result in wavy or uneven haze, depending on degree of flow instability.
High-speed injection of melt as it passes through water inlet will cause high shear, and heat-sensitive plastic will scorch. This charred material will pass through cavity, reach flow front, and appear on the surface of part.
In order to prevent shooting lines, injection speed setting must ensure that runner area is quickly filled and then slowly passed through water inlet. Finding this speed transition point is essence of problem. If it is too early, filling time will increase excessively, if it is too late, excessive flow inertia will lead to appearance of streaks.
The lower melt viscosity and the higher barrel temperature, the more pronounced tendency of this shot 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 by more efficient pressure transfer with less pressure drop. Low mold temperature and too slow screw advance dramatically shorten flow length, which must be compensated for by high firing rates. High-speed flow reduces heat loss, and frictional heat due to high shear heat increases melt temperature and slows rate of thickening of outer layer of the part. Cavity intersections must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur.
In short, most of injection defects can be solved by adjusting injection speed, so skill of adjusting injection process is to set injection speed and its segmentation reasonably.