Usage of injection speed and notes on machine adjustment

Proportional control of injection speed has been widely adopted by injection molding machine manufacturers. Although computer-controlled injection speed segmentation control systems have existed for a long time, the advantages of such machine settings have rarely been exploited due to the limited information available on the subject. This article will systematically explain the advantages of applying multi-stage speed injection molding and outline 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 in injection molding. By determining the beginning, middle, and end of the filling speed segment and achieving a smooth transition from one set point to another, a stable melt surface velocity can be ensured to produce the desired molecular pickup and minimum internal stress.

We suggest the following principles for such velocity segmentation.

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

2) Fast 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 (e.g. flow channel) while slowing down the speed at the inlet level.

(4) The injection speed should ensure that the cavity is filled and stopped immediately to prevent overfilling flying edges, and residual stress.  

The basis for setting speed segments must take into account the mold geometry, other flow constraints, and instability factors. The speed must be set with a clear understanding of the injection molding process and material knowledge, otherwise, the quality of the product will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be indirectly deduced by measuring the screw forward speed, or cavity pressure (to make sure there is no leakage of the check valve). 

Material properties are very important because polymers may degrade due to different stresses. Increasing the molding temperature may lead to intense oxidation and degradation of the chemical structure, but at the same time, the degradation caused by shear becomes less because the high temperature reduces the viscosity of the material and decreases the shear stress. Undoubtedly, multi-stage injection speeds are useful for molding heat-sensitive materials such as PC, POM, UPVC, and for their blending. 

The geometry of the mold is also a determining factor: the maximum injection speed is needed at thin walls; thick-walled parts need a slow-fast-slow type speed profile to avoid defects; and to ensure that the part quality is up to standard, the injection speed setting should ensure a constant melt front flow rate.

The melt flow rate is very important because it affects the molecular alignment direction and surface condition in the part; the melt front should be slowed down when it reaches the cross area structure; for complex molds with radial diffusion, a balanced increase in melt throughput should be ensured; long runners must be filled quickly to reduce cooling of the melt front, with the exception of injecting high-viscosity materials such as PC, where too fast a speed will bring cold material into the cavity through the inlet.  

Adjusting the injection speed can help eliminate defects caused by slowing flow at the inlet level. As the melt passes through the nozzle and runner to the inlet, the surface of the melt front may have cooled and solidified, or the melt may stall due to a sudden narrowing of the runner until enough pressure is built up to push the melt through the inlet, which can cause the pressure through the inlet to the peak.

The 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 entry point and then increases the injection rate to the original value.

Since it is very difficult to precisely control the injection speed to slow down at the inlet level, deceleration at the end of the runner is a better solution.  

We can avoid or reduce defects such as fluttering, scorching, and air trapping by controlling the injection speed at the end of the runner. Slowing down at the end of filling can prevent overfilling of the cavity, avoid flutter and reduce residual stress. Air trapping caused by poor venting or filling problems at the end of the mold flow path can also be solved by reducing the venting speed, especially at the end of the injection stage.  

The short shot is caused by slow speed at the inlet or local blockage of flow caused by melt solidification. This problem can be solved by speeding up the injection speed just after the water inlet or when the local flow is obstructed.

Defects that occur in heat-sensitive materials such as flow marks, inlet scorch, molecular breakage, delamination, and flaking are caused by excessive shear when passing through the inlet. 

Smooth parts depend on injection speed, and glass-filled materials are particularly sensitive especially nylon. Dark spots (wavy patterns) are caused by flow instability due to changes in viscosity. Distorted flow can lead to wavy patterns or uneven haze, and the exact defect produced depends on the degree of flow instability.  

High-speed injection as the melt passes through the inlet can result in high shear, and thermosensitive plastics will experience scorching, and this scorched material will travel through the cavity to the flow front and present itself on the part surface.  

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

The lower the melt viscosity and the higher the barrel temperature the more pronounced the tendency for this shot pattern to appear. Small inlets are also an important factor in flow defects because they require high-speed and high-pressure injection.  

Shrinkage can be improved by more efficient pressure transfer and smaller pressure drops. Low mold temperature and slow screw advancement greatly shorten the flow length and must be compensated for by high injection velocities. High flow speeds reduce heat loss and can cause an increase in melt temperature due to high shear heat-generating frictional heat, slowing the thickening of the outer layer of the part. The cavity crossover must be thick enough to avoid too large a pressure drop, otherwise, shrinkage will occur.  

In short, most 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 procedure. 

Modern injection molded products, which have been widely used in various fields, have very complex shapes and the properties of the polymers used vary greatly. Even for products of the same material, due to the different geometry of the sprue system and various parts, different parts have requirements for the flow (speed and pressure) of the molded melt, otherwise, the rheological properties of the melt in this part of the crystalline orientation of the polymer, as well as the apparent quality of the product, will be affected.

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

The digital dial-type injection molding machine is relatively backward, with only one or two injection stages, one pressure-holding stage, and one melt control program, which makes it difficult to set and control the injection speed and other process conditions for some products with complex structure and high appearance quality requirements, resulting in some appearance defects of the injection parts that cannot be improved by adjusting the injection parameters.

In order to meet the need 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 melting functions, which is a breakthrough technological progress in the injection molding processing industry. 

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

Low speed is used at the beginning of injection, high speed is used when the mold cavity is filled, and low speed is used near the end of filling. By controlling and adjusting the injection speed, various undesirable phenomena such as burrs, spray marks, silver streaks, or scorch marks can be prevented and improved. 

The multi-stage injection control program can reasonably set multi-stage injection pressure, injection speed, holding pressure, and melt method according to the structure of the flow channel, the form of the gate, and the structure of injection parts, which is conducive to improving the plasticization effect, improving product quality, reducing defect rate and prolonging mold/machine life. 

By controlling the oil pressure, screw position, and screw speed of the injection molding machine through a multi-stage program, it can seek to improve the appearance of molded parts, improve the countermeasures of shrinkage, warpage, and burr, and reduce the uneven size of each injection molded part. 

Basic knowledge of injection molding 

Injection molding is an engineering process that involves the transformation of plastics into useful products that maintain their original properties. The important process conditions for injection molding are the temperature, pressure, and corresponding individual action time that affects the plasticizing flow and cooling.

I. Temperature control

1, barrel temperature: injection molding process needs to control the temperature of the barrel temperature, nozzle temperature, and mold temperature, etc. The first two temperatures mainly affect the plasticization and flow of plastic, while the latter temperature mainly affects the flow and cooling of plastic.

Each kind of plastic has a different flow temperature, and the same kind of plastic, due to different sources or grades, its flow temperature and decomposition temperature is different, which is due to the average molecular weight and molecular weight distribution, the plastic plasticization process in different types of injection machines is also different, so the choice of barrel temperature is also different. 

2, nozzle temperature: nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the melt in the straight-through nozzle may occurring in the "salivation phenomenon". Nozzle temperature can not be too low, otherwise, it will cause early condensation of the melt and the nozzle will be blocked *, or due to early condensation of material into the cavity and affect the performance of the product 

3, mold temperature: mold temperature on the intrinsic properties of the product and the apparent quality has a great impact. The temperature of the mold is determined by 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.). 

Second, pressure control

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

1, plasticizing pressure: (back pressure) when using a screw injection machine, the top of the screw melt in the screw rotation backpressure is called plasticizing pressure, also known as backpressure. The size of this pressure can be adjusted by the relief valve in the hydraulic system. In injection, the magnitude of the plasticizing pressure is constant with the rotational speed of the screw, so increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the rate of plasticization.

In addition, increasing the plasticizing pressure often leads to a uniform temperature of the melt, uniform mixing of the color, and discharge of gas from the melt. In general operation, the decision of plasticizing pressure should be the lower the better under the premise of ensuring the excellent quality of the product, the specific value is different with the variety of plastic used, but usually rarely exceeds 20 kg / cm2. 

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

III. 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: molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, under the premise of quality assurance, we should try to shorten the molding cycle time. In the whole molding cycle, the injection time and cooling time are the most important, and they have a decisive influence on the quality of the products. The mold filling time in the injection time is directly inversely proportional to the mold filling rate, and the mold filling time in production is generally about 3-5 seconds. 

The holding time in the injection time is the pressure time of the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (up to 5-10 minutes for extra thick parts). Before the melt is frozen at the gate, the amount of holding time has an effect on the dimensional accuracy of the product, but if it is later, it has no effect. The holding time also has a minimum value, which is known to depend on the material temperature, the mold temperature, and the size of the main channel and gate.

If the size of the main channel and gate as well as the process conditions are normal, the pressure value that results in the smallest range of shrinkage fluctuation is usually the one. 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 to ensure that the product does not cause changes when the mold is released as a principle, cooling time is generally about 30 to 120 seconds between the cooling time is unnecessary, not only to reduce production efficiency, complex parts will also cause difficulties in demolding, forced demolding will even produce demolding stress. The other time in the molding cycle is related to whether the production process is continuous and automated, as well as the degree of continuity and automation.   

In general, injection molding machines can be adjusted according to the following procedures.      

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

Estimate the required injection volume and adjust the injection molding machine to two-thirds of the estimated maximum injection volume. Adjust the reverse cable (pumping) stroke. Estimate and adjust the secondary injection time and set the secondary injection pressure to zero. 

Initially adjust the primary injection pressure to half of the machine limit (50%); set the injection speed to the maximum. Estimate and adjust the required cooling time. Set the back pressure to 3.5 bar and remove the degraded resin from the barrel. Use semi-automatic injection mode; start the injection process and observe the screw action. 

Adjust the injection speed and pressure as needed, or increase the injection pressure if you want to shorten the filling time. As mentioned before, the final pressure can be adjusted to 100% of the primary injection pressure due to the process before the full mold filling. The pressure should eventually be adjusted high enough so 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 changeover point. The program is set so that 95-98% of mold filling by shot weight can be achieved at the first injection stage. 

Once the injection volume, changeover point, injection speed, and pressure of the first stage are properly adjusted, the second stage of the holding pressure adjustment procedure can be performed. 

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

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

Develop good injection molding machine operating habits 

Good injection molding machine operating habits are beneficial for improving machine life and production safety. 

1 Before starting the machine. 

(1) Check whether there is water or oil entering the electrical control box. If the electrical appliance is damp, do not turn on the machine. The maintenance personnel should blow-dry the electrical parts before starting the machine. 

(2) Check whether the supply voltage is in line with the general should not exceed ± 15%. 

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

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

(5) Check whether there is a lubricant (grease) in each moving part and add enough lubricant. 

(6) Turn on the electric heat and heat up each section of the barrel. When the temperature of each section reaches the requirement, then keep warm for a period of time to make the machine temperature stabilize. The holding time varies according to the requirements of different equipment and plastic materials. 

(7) Add enough plastic to the hopper. According to the requirements of injection molding different plastics, some raw materials are better to go through drying first. 

(8) To cover the heat shield on the barrel, which can save electricity and extend the life of the electric heating ring and current contactor. 

2 Operation process. 

(1) Do not cancel the safety door at will for the sake of convenience. 

(2) Pay attention to the temperature of the pressure oil, and the oil temperature should not exceed the specified range. The ideal working temperature of hydraulic oil should be maintained between 45~50℃, generally within the range of 35~60℃ is more appropriate. 

(3) Pay attention to the adjustment of each stroke limit switch to avoid the impact of the machine during the action. 

3 At the end of work. 

(1) Before stopping the machine, the plastic in the barrel should be cleaned up to prevent the leftover material from oxidation or long-term thermal decomposition. (2) The mold should be opened so that the elbow lever mechanism is in a closed state for a long time. 

(3) The workshop must be equipped with lifting equipment. The assembly and disassembly of the mold and other bulky parts should be very careful to ensure production safety.