Usage Of Injection Speed And Precautions For Machine Adjustment

Proportional control of injection speed has been widely adopted by injection molding machine manufacturers. Although the computer-controlled injection speed segmented control system has long existed, the advantages of this machine setup are seldom exploited due to the limited data available. This article will systematically explain the advantages of applying multi-speed injection molding, and briefly introduce its use in eliminating product defects such as short shots, trapped air, and shrinkage.

The close relationship between injection speed and product quality makes it a key parameter for injection molding. By determining the start, middle, and end of the fill velocity segment, and achieving a smooth transition from one set point to another, a stable melt surface velocity can be guaranteed to produce the desired molecular weight with minimal internal stress.

We recommend the following velocity segmentation principle:

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

2) Rapid injection should be used to prevent the melt from freezing during the injection process.

3) The injection speed setting should take into account the fast filling in the critical area (such as the runner) while slowing down the speed at the water inlet.

4) The injection speed should be stopped immediately after the cavity is filled to prevent overfilling, flash and residual stress.

The basis for setting the speed segment must take into account the geometry of the die, other flow constraints and instabilities. 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 estimated indirectly by measuring the advancing speed of the screw, or the cavity pressure (make sure that the check valve does not leak).

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

The geometry of the 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, the injection speed should be set to ensure that the melt front flow rate constant.

The melt flow rate is very important because it affects the molecular arrangement direction and surface state in the part; when the front of the melt reaches the cross-region structure, it should slow down; for complex molds with radial diffusion, the melt throughput should be guaranteed Increase evenly; long runners must be filled quickly to reduce cooling of the melt front, but injection of high-viscosity materials such as PC is an exception, because too fast a speed will bring cold material into the cavity through the water inlet.

Adjusting the injection speed can help eliminate defects caused by slow flow at the water inlet. When the melt passes through the nozzle and runner to the water inlet, the surface of the melt front may have cooled and solidified, or the melt may stagnate due to the sudden narrowing of the runner until sufficient pressure is built up to push the melt through the inlet. nozzle, this will cause the pressure through the inlet to appear peaked.

High pressure will damage the material and cause surface defects such as flow marks and inlet scorch, which can be overcome by slowing down just before the inlet. This deceleration prevents excessive shearing at the inlet level before increasing the rate of fire to its original value.

Because it is very difficult to accurately control the rate of fire to slow down at the water inlet, slowing down at the end of the runner is a better solution.

We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling the final injection speed. Deceleration at the end of filling prevents overfilling of the cavity, avoids flash and reduces residual stress. Trapped air caused by poor exhaust or filling problems at the end of the mold flow path can also be solved by reducing the exhaust speed, especially the exhaust speed at the end of the injection.

The short shot is caused by the slow speed at the water inlet or the local flow obstruction caused by the solidification of the melt. Speeding up the shot just past the 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 the 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 the degree of flow instability.

High-speed injection of the melt as it passes through the water inlet will cause high shear, and heat-sensitive plastic will scorch. This charred material will pass through the cavity, reach the flow front, and appear on the surface of the part.

In order to prevent shooting lines, the injection speed setting must ensure that the runner area is quickly filled and then slowly passed through the water inlet. Finding this speed transition point is the essence of the problem. If it is too early, the filling time will increase excessively, and if it is too late, excessive flow inertia will lead to the appearance of streaks.

The lower the melt viscosity and the higher the barrel temperature, the more pronounced the tendency of this shot to appear. Since the 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 the 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 the rate of thickening of the 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 the injection defects can be solved by adjusting the injection speed, so the skill of adjusting the injection process is to set the injection speed and its segmentation reasonably. Control of multi-stage injection molding programs:

Modern injection molding products have been widely used in various fields. The shapes of the products are very complex, and the properties of the polymers used are also very different. Even if it is a product of the same material, due to the different geometry of the runner system and each part, different parts have requirements for the flow (speed, pressure) of the filling melt, otherwise the rheological properties of the melt in this part will be affected. Or the crystal orientation of polymers, and the apparent quality of the product.

In an injection process, when the screw pushes the melt to the mold, it is required to realize the control of process parameters such as different injection speeds and different injection pressures at different positions. This injection process is called multi-stage injection.

The 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 the injection speed and other Due to process conditions, some appearance defects of injection molded parts cannot be improved by adjusting injection parameters.

In order to meet the needs of improving the appearance quality of injection molded parts and overcome the 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 in the injection molding industry. skill improved.

At present, most injection molding machines with multi-stage control of injection speed can usually divide the injection full stroke into 3 or 4 areas, and set each area to its own appropriate injection speed.

Use low speed in the initial stage of injection, use high speed in cavity filling, and use low speed injection when filling is nearing the end. Through the control and adjustment of the injection speed, various undesirable phenomena such as burrs, jet marks, silver streaks or scorch marks can be prevented and improved in the appearance of the product.

The multi-stage injection control program can reasonably set the multi-stage injection pressure, injection speed, pressure holding pressure and melting method according to the structure of the runner, the form of the gate and the structure of the injection molded parts, which is conducive to improving the plasticizing effect and improving the Product quality, reduce defect rate and extend mold/machine life.

By controlling the oil pressure, screw position and screw speed of the injection molding machine through multi-stage programs, it can seek to improve the appearance of the molded parts, improve the corresponding measures for shrinkage, warpage and burrs, and reduce the uneven size 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. The important process conditions of injection molding are the temperature, pressure and corresponding respective action time that affect the plasticizing flow and cooling.

1. Temperature control

1. Barrel temperature: The temperature that needs to be controlled in the injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics.

Each plastic has a different flow temperature. For the same plastic, due to different sources or grades, its flow temperature and decomposition temperature are different. This is due to the difference in average molecular weight and molecular weight distribution. The plasticizing process in the machine is also different, so the selection of the barrel temperature is also different.

2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the "drooling phenomenon" that may occur in the straight-through nozzle. The temperature of the nozzle should not be too low, otherwise it will cause the premature solidification of the melt and block the nozzle, or the performance of the product will be affected due to the injection of the early solidified material into the mold cavity.

3. Mold temperature: The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. The temperature of the mold depends on the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

 2. Pressure control

The pressure in the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastics and the quality of products.

1. Plasticizing pressure: (back pressure) When a screw injection machine is used, the pressure on the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The magnitude of this pressure can be adjusted by the relief valve in the hydraulic system. In injection, the size of the plasticizing pressure is unchanged with the speed of the screw, so when the plasticizing pressure is increased, the temperature of the melt will be increased, but the speed of plasticizing will be reduced.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the colorant is uniform, and the gas in the melt can be discharged. In general operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring the quality of the product. The specific value varies with the variety of plastic used, but usually rarely exceeds 20 kg/cm 2 .

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

3. Molding cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts:

Molding cycle: The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring the quality. In the whole molding cycle, the injection time and cooling time are the most important, they have a decisive influence on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.

The pressure holding time in the injection time is the pressure time for the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (the extra thick parts can be as high as 5-10 minutes). Before the molten material at the gate is frozen, the holding pressure time has an impact on the dimensional accuracy of the product, and if it is later, it has no effect. The dwell time also has an optimal value, which is known to depend on the material temperature, mold temperature, and the size of the sprue and gate.

If the dimensions of the sprue and gate and the process conditions are normal, usually the pressure value with the smallest fluctuation range of product shrinkage shall prevail. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature.

The end of the cooling time should be based on the principle of ensuring that the product does not change when the product is demolded. The cooling time is generally between 30 and 120 seconds. If the 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 the molding cycle is related to whether the production process is continuous and automated, and the degree of continuous and automated.

General injection molding machines can be adjusted according to the following procedures:

Adjust the barrel temperature to the middle of the range and adjust the mold temperature according to the temperature range provided by the raw material supplier's information.

Estimate the required shot size and adjust the injection molding machine to two-thirds of the estimated maximum shot size. Adjust the reverse cable (gluing) stroke. Estimate and adjust the secondary injection time, and adjust the secondary injection pressure to zero.

Preliminarily adjust the primary injection pressure to half (50%) of the limit of the injection molding machine; adjust the injection speed to the highest. Estimate and adjust the cooling time required. Adjust the back pressure to 3.5 bar. Remove degraded resin from the barrel. Adopt semi-automatic injection molding mode; start the injection molding process and observe the movement of the screw.

It is necessary to adjust the injection speed and pressure appropriately. To shorten the filling time, the injection pressure can be increased. As mentioned earlier, since there will be a process before full mold filling, the final mold filling pressure can be adjusted to 100% of the primary injection pressure. The pressure must eventually be adjusted high enough that the maximum speed that can be achieved is not limited by the set pressure. If there is overflow, the speed can be reduced.

After each observation cycle, adjust the injection volume and the switching point. Program so that 95-98% fill by shot weight can already be achieved in the first shot.

When the injection volume, transfer point, injection speed and pressure of the first-stage injection are properly adjusted, the adjustment procedure of the second-stage holding pressure can be carried out.

Adjust the holding pressure as needed, but do not overfill the cavity.

Adjust the screw speed to ensure that the melt is complete just before the cycle is complete and the injection cycle is not limited.

Develop good operating habits of injection molding machines

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

1 Before power on:

(1) Check whether there is water or oil in the electrical control box. If the electrical appliance is damp, do not turn it on. The electrical parts should be dried by maintenance personnel before starting the machine.

(2) Check whether the power supply voltage conforms, generally it should not exceed ±15%.

(3) Check whether the emergency stop switch and the front and rear safety door switches are normal. Verify that the directions of rotation of the motor and the oil pump are the same.

(4) Check whether the cooling pipes are unblocked, and pass cooling water to the cooling water jacket at the end of the oil cooler and the barrel.

(5) Check whether there is lubricating oil (grease) in each movable part, and add enough lubricating oil.

(6) Turn on the electric heater to heat each section of the barrel. When the temperature of each section reaches the requirements, keep the temperature for a period of time to make the temperature of the machine tend to be stable. The holding time varies according to the requirements of different equipment and plastic raw materials.

(7) Add enough plastic to the hopper. According to the requirements of different plastics for injection molding, some raw materials are best dried first.

(8) Cover the heat shield on the barrel