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 fill velocity segment, and achieving a smooth transition from one set point to another, a stable melt surface velocity can be guaranteed to produce desired molecular weight 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 fast filling in critical area (such as runner) 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 estimated indirectly 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 stresses, 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 temperature reduces viscosity of material and reduces 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.
Geometry of mold is also a determining factor: thin walls require maximum injection speed; thick-walled parts require a slow-fast-slow speed curve to avoid defects; in order to ensure part quality meets standards, injection speed should be set to 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, it should ensure that melt throughput increases 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 sufficient 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 the 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 (waves) are caused by flow instability due to viscosity changes. Distorted flow can result in wavy or uneven haze, depending on degree of flow instability.
High-speed injection of the 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, and 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, frictional heat due to high shear heat increases melt temperature and slows rate of thickening of outer layer of 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. Control of multi-stage injection molding programs:
Modern injection molding products have been widely used in various fields. Shapes of products are very complex, and properties of polymers used are also very different. Even if it is a product of same material, due to different geometry of runner system and each part, different parts have requirements for flow (speed, pressure) of filling melt, otherwise it will affect rheological properties of melt in this part or crystallographic orientation of polymer, as well as apparent quality of product.
In an injection process, when screw pushes melt to mold, it is required to realize control of process parameters such as different injection speeds and different injection pressures at different positions. This injection process is called multi-stage injection.

 

Digital dial injection molding machine is relatively backward, with only one or two stages of injection, one stage of pressure holding, and one stage of control procedures for melting glue. For some products with complex structures and high requirements on appearance quality, it is difficult to set and control injection speed and other process conditions, resulting in some appearance defects of injection parts that cannot be improved by adjusting injection parameters.
In order to meet needs of improving appearance quality of injection molded parts and overcome above problems, injection molding machine manufacturers have developed and produced injection molding machines with multi-stage injection, multi-stage pressure holding, and multi-stage glue melting functions, which is a breakthrough technological progress in injection molding industry.
At present, most of injection molding machines have multi-stage control of injection speed. Usually, full injection stroke can be divided into 3 or 4 regions, and each region can be set to its own appropriate injection speed.
Use low speed in initial stage of injection, use high speed in cavity filling, and use low speed injection when filling is nearing end. Through control and adjustment of injection speed, various undesirable phenomena such as burrs, jet marks, silver streaks or scorch marks can be prevented and improved in the appearance of product.
Multi-stage injection control program can reasonably set multi-stage injection pressure, injection speed, pressure holding pressure and melting method according to structure of runner, form of gate and structure of injection molded parts, which is conducive to improving plasticizing effect and improving product quality, reduce defect rate and extend mold/machine life.
By controlling oil pressure, screw position and screw speed of injection molding machine through multi-stage programs, it can seek to improve appearance of molded parts, improve corresponding measures for shrinkage, warpage and burrs, and reduce size non-uniformity of each injection molded part of each mold. .
Basic knowledge of injection molding
Injection molding is an engineering technique that transforms plastics into useful products that retain their original properties. Important process conditions of injection molding are temperature, pressure and corresponding respective action time that affect plasticizing flow and cooling.

1. Barrel temperature: Temperature that needs to be controlled in injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect plasticization and flow of plastics, while the latter temperature mainly affects flow and cooling of plastics.
Each plastic has a different flow temperature. For same plastic, due to different sources or grades, its flow temperature and decomposition temperature are different. This is due to difference in average molecular weight and molecular weight distribution. Plasticizing process in machine is also different, so selection of barrel temperature is also different.
2. Nozzle temperature: Nozzle temperature is usually slightly lower than maximum temperature of barrel, which is to prevent "drooling phenomenon" that may occur in straight-through nozzle. Temperature of nozzle should not be too low, otherwise it will cause premature solidification of melt and block nozzle, or performance of product will be affected due to injection of early solidified material into mold cavity.
3. Mold temperature: Mold temperature has a great influence on intrinsic performance and apparent quality of product. Temperature of mold depends on presence or absence of plastic crystallinity, size and structure of product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

Pressure in injection molding process includes plasticizing pressure and injection pressure, and directly affects plasticization of plastics and quality of products.
1. Plasticizing pressure: (back pressure) When a screw injection machine is used, pressure on the top of screw when screw rotates and retreats is called plasticizing pressure, also known as back pressure. Magnitude of this pressure can be adjusted by relief valve in hydraulic system. In injection, size of plasticizing pressure is unchanged with speed of screw, so when plasticizing pressure is increased, temperature of melt will be increased, but speed of plasticizing will be reduced.
In addition, increasing plasticizing pressure can often make temperature of melt uniform, mixing of colorant is uniform, and gas in the melt can be discharged. In general operation, decision of plasticizing pressure should be as low as possible under premise of ensuring quality of product. Specific value varies with variety of plastic used, but usually rarely exceeds 20 kg/cm 2 .

2. Injection pressure: In current production, injection pressure of almost all injection machines is based on pressure exerted on plastic by top of plunger or screw (converted from pressure of oil circuit). Role of injection pressure in injection molding is to overcome flow resistance of plastic flowing from barrel to cavity, give melt filling rate and compact melt.

 

Time required to complete an injection molding process is called molding cycle, also known as molding cycle. It actually includes following parts:
Molding cycle: Molding cycle directly affects labor productivity and equipment utilization. Therefore, in production process, relevant time in molding cycle should be shortened as much as possible under premise of ensuring quality. In the whole molding cycle, injection time and cooling time are the most important, they have a decisive influence on quality of product. Filling time in injection time is directly inversely proportional to filling rate, and filling time in production is generally about 3-5 seconds.
Holding time in injection time is pressure time for plastic in cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (for extra-thick parts, it can be as high as 5-10 minutes). Before molten material at gate is frozen, holding pressure time has an impact on dimensional accuracy of product, and if it is later, it has no effect. Dwell time also has an optimal value, which is known to depend on material temperature, mold temperature, size of sprue and gate.
If dimensions of sprue, gate and process conditions are normal, usually pressure value with the smallest fluctuation range of product shrinkage shall prevail. Cooling time is mainly determined by thickness of product, thermal and crystalline properties of plastic, and mold temperature.
End of cooling time should be based on principle of ensuring that product does not change when product is demolded. Cooling time is generally between 30 and 120 seconds. If cooling time is too long, it is not necessary. Difficulty in demolding, and even demolding stress will be generated when demoulding is forced. The other time in molding cycle is related to whether production process is continuous and automated, degree of continuous and automated.
General injection molding machines can be adjusted according to following procedures:
Adjust barrel temperature to middle of range and adjust mold temperature according to temperature range provided by raw material supplier's information.
Estimate required shot size and adjust injection molding machine to two-thirds of estimated maximum shot size. Adjust reverse cable (gluing) stroke. Estimate and adjust secondary injection time, and adjust secondary injection pressure to zero.
Preliminarily adjust primary injection pressure to half (50%) of limit of injection molding machine; adjust injection speed to the highest. Estimate and adjust cooling time required. Adjust back pressure to 3.5 bar. Remove degraded resin from barrel. Adopt semi-automatic injection molding mode; start injection molding process and observe movement of screw.
It is necessary to adjust injection speed and pressure appropriately. To shorten filling time, injection pressure can be increased. As mentioned earlier, since there will be a process before full mold filling, final mold filling pressure can be adjusted to 100% of primary injection pressure. Pressure must eventually be adjusted high enough that maximum speed that can be achieved is not limited by set pressure. If there is overflow, speed can be reduced.
After each observation cycle, adjust injection volume and switching point. Program so that 95-98% fill by shot weight can already be achieved in the first shot.
When injection volume, transfer point, injection speed and pressure of the first-stage injection are properly adjusted, adjustment procedure of second-stage holding pressure can be carried out.
Adjust holding pressure as needed, but do not overfill cavity.
Adjust screw speed to ensure that melt is complete just before cycle is complete and injection cycle is not limited.

Developing good injection molding machine operating habits is of great benefit to improving machine life and production safety.

(1) Check whether there is water or oil in electrical control box. If electrical appliance is damp, do not turn it on. Electrical parts should be dried by maintenance personnel before starting machine.
(2) Check whether power supply voltage conforms, generally it should not exceed ±15%.
(3) Check whether emergency stop switch, front and rear safety door switches are normal. Verify that directions of rotation of motor and oil pump are same.
(4) Check whether cooling pipes are unblocked, pass cooling water to cooling water jacket at the end of oil cooler and barrel.
(5) Check whether there is lubricating oil (grease) in each movable part, and add enough lubricating oil.
(6) Turn on electric heater to heat each section of barrel. When temperature of each section reaches requirements, keep temperature for a period of time to make temperature of machine tend to be stable. Holding time varies according to requirements of different equipment and plastic raw materials.
(7) Add enough plastic to hopper. According to requirements of different plastics for injection molding, some raw materials are best dried first.
(8) Cover heat shield on barrel, which can save electricity, prolong life of electric heating ring and current contactor.

(1) Do not cancel function of safety door at will for sake of convenience.
(2) Pay attention to observe temperature of pressure oil, and oil temperature should not exceed specified range. Ideal working temperature of hydraulic oil should be kept between 45~50℃, generally in the range of 35~60℃.
(3) Pay attention to adjusting each travel limit switch to avoid impact of machine during action.

(1) Before shutting down, plastics in barrel should be cleaned up to prevent remaining materials from being oxidized or decomposed by heat for a long time.

(2) Mold should be opened to keep toggle mechanism in a locked state for a long time.
(3) Workshop must be equipped with lifting equipment. Great care should be taken when assembling and disassembling heavy components such as molds to ensure production safety.