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How to efficiently change materials during injection process

TIME: 2019-06-18

How to efficiently change materials during injection process?
In the replacement of different materials, before and after the replacement of materials, we use the difference of melting viscosity of various materials to carry out the operation. The temperature control of the barrel should be decided beforehand.
For general thermoplastic materials, high temperature is easy to adhere to the metal surface, while low temperature is not easy to adhere to the metal surface. Replacement operation is to use the nature of materials, so that the material that wants to replace (want to extrude) is adhered to the inner wall of the hot heating barrel, and then the cold screw is supplied with high viscosity material (replacement material) to rotate. In addition to the material to be extruded, these are our basic considerations. At this time, it is better to keep the temperature of the screw side as low as possible, so that the material to be replaced does not stick to the screw is the most important.
Therefore, the replacement materials are suitable for low melting viscosity and compact materials, generally using high density PS or some special replacement materials.
The general sequence and precautions for replacement operations of dissimilar materials are indicated in the following process:
1. The temperature of the heating barrel should be kept as low as possible as the actual forming temperature until the end of the replacement operation.
2. The number of rotations of screw is as low as possible, and the back pressure of screw is lowered to prevent material temperature rising caused by friction heat.
3, every time the screw is supplied with a small amount of replacement material, as far as possible, no melt resin can be wound around the screw.
4. It is very effective to make the screw move forward in a short journey and return several times to inject resin by impact.
5. If the inner wall of the heating barrel and the outer diameter of the head of the screw are damaged, there is melting resin in this part, which makes it difficult to retain and replace.

Note on long time downtime for injection molding machine
I. injection molding machine and mold protection
Before the shutdown of the injection moulding machine, the mould should be opened to a distance of 5-10 mm, so that the hinge (crank arm) mechanism of the clamping machine is in a semi-straight state, and the hinge mechanism (crank arm) of the injection moulding machine is in a force-free state, so as to better protect the mould and the clamping mechanism.
Two. Cylinder cleaning.
Before shutdown, the plastics in the barrel should be cleaned up, and the residual plastics inside the barrel should be cleaned out by using one of the plastics of PP, PS and PE. After cleaning, screw stroke is loosed to the maximum stroke. This prevents the screw head from burning up when the machine starts heating up.
1. If the plastics used are HB ordinary transparent plastics and natural plastics, then the plastics inside the barrel can be shot directly without cleaning.
2. If the plastics used are colored, VO fire-proof grade plastics, corrosive plastics, and plastics with glass fibers, they must be cleaned out with one of the plastics of PP, PS and PE. After the cleaning work is completed, the temperature of the material tube can be closed directly.
Three, for the mold, cylinder cooler, hydraulic oil cooler drainage.
Firstly, the power supply of cooling water tower, water tower outlet pump, freezer equipment and mould temperature machine equipment is closed, and then the following four drainage protection works are done.
1. Close the inlet and outlet valves of the hydraulic oil cooler of the injection moulding machine, remove the inlet and outlet pipe joints, and use the air gun of the air compressor to blow out all the residual water in the cooler (drain out), so as to avoid rusting or oxidation inside the water ring.
2. Close the inlet and outlet valves of the barrel (material pipe) cooling water ring, remove the joint of the inlet and outlet water pipes, and blow out all the residual water in the cooling water ring (discharge) with the air gun of the air compressor to avoid rusting or oxidation inside the water ring.
3. Close the inlet and outlet valve of the mould (male/female) and remove the joint of the inlet and outlet pipe. Use the air gun of the air compressor to blow out all the residual water in the water conveying system of the mould (drain out), so as to avoid the rust of the mould core.
4, finally close the power supply of the air compressor equipment and release the residual air pressure.
Four. Electrical protection for injection molding machines
Before the holidays, please make sure that the power switch of each injection molding equipment has been disconnected, the doors and windows are closed, the workshop should also do a good job of anti-rat bite line measures, waterproof and moistureproof measures in the electric box. The injection control box is closed to prevent rats from damaging the computer mainframe.

Eight ways to remedy bending injection parts
The reason for the bending of injection parts is due to the defect characteristics of injection parts. The shape of injection parts is similar to that of the cavity, but it is a distorted version of the cavity shape.
The causes of bending problems of injection molded parts are:
(1) bending of injection molded parts is due to excessive internal stress in injection molded parts.
(2) mold filling speed is slow, resulting in bending of injection molded parts.
(3) plastic in mold cavity is insufficient, resulting in bending of injection molded parts.
(4) plastic temperature is too low or inconsistent, and it may also be bent by injection parts.
(5) injection parts are too hot at the time of ejection.
(6) the temperature is not enough, or the temperature of the moving and fixed die is not consistent.
(7) the structure of injection parts is unreasonable (such as stiffeners are concentrated on one side, but far apart).
Remedial method for bending of injection molded parts
(1) reduce injection pressure.
(2) reduce the screw forward time.
(3) increase cycle time (especially cooling time). Immersed in warm water (38oc) immediately after ejection from the die (especially the thicker injection parts) to allow the injection parts to cool down slowly.
(4) increase injection speed.
(5) increase the temperature of plastics.
(6) use cooling equipment to cool bending plastic parts.
(7) Increase the cooling time or improve the cooling conditions of the bending injection parts to ensure that the temperature of the moving and fixing moulds is the same as possible.
(8) according to the actual situation, the structure of bending injection parts can be improved if permitted.

How to select injection nozzle for injection molding machine
The nozzles can be divided into straight through nozzles, locking nozzles, hot runner nozzles and multi channel nozzles.
1. Accuracy requirements for injection parts
After assembly, the integral injection parts must be placed on the frame, and the nozzle and the main casting sleeve of the mould must be closely joined to prevent spillover, so that the center line of the injection parts should be concentric with the center line of the clamping parts; in order to ensure the matching accuracy of the injection screw and the inner hole of the barrel, the positioning of the two injection cylinder holes and the barrel must be ensured. The parallelism of the center hole and the symmetry of the center line must be guaranteed for the horizontal machine, and for the vertical machine, the parallelism and the symmetry of the center line must be guaranteed for the two guide holes of the base moving cylinder and the positioning center hole of the barrel.
The factors affecting the position accuracy are the dimension accuracy, geometric accuracy, manufacturing accuracy and assembly accuracy of the holes and shafts of the related components.
The clamping part is one of the important parts of injection moulding machine. Its function is to realize the opening and closing motion, make the elastic deformation of the system to achieve the clamping force, and lock the mould.
Two. Requirements for clamping parts.
(1) the movement of the opening and closing die of the kinetic model must be high speed, smooth and mute.
2. The clamping mechanism must meet the requirements of rated clamping force and reliably lock the mold.
3. The clamping parts have enough mold space and model stroke.
4. Motion model must be reliable and safe, protect the safety of people and mould, and set double insurance.
The clamping parts and their dies have sufficient strength and rigidity.

Relationship between injection quality and ejection velocity segmentation
The relationship between ejection speed and product quality makes it a key parameter in injection molding.
By determining the beginning, middle and end of filling velocity segment and realizing a smooth transition from one setting point to another, stable melt surface velocity can be guaranteed to produce desired molecule interrogation and minimum internal stress.
The following principle of speed segmentation is recommended:
1, the velocity of the fluid surface should be constant.
2, rapid ejection should be used to prevent melt freezing during gelatinization.
3. The setting of injection speed should take into account that the filling speed at the critical area (such as the runner) should be slowed down at the inlet position.
4. The injection speed should be guaranteed to stop immediately after filling the cavity to prevent overfilling, flying edge and residual stress.
The basis for setting the speed segment must take into account the geometric shape of the die, other flow restrictions and instability factors. The setting of speed must have a clear understanding of injection process and material knowledge, otherwise, the quality of products will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be calculated indirectly by measuring the screw forward speed or cavity pressure (to make sure that the check valve does not leak).
Material properties 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, the degradation caused by shear decreases, because high temperature reduces the viscosity of materials and shear stress. Undoubtedly, the multi-stage injection speed is very helpful for the formation of heat-sensitive materials such as PC, POM, UPVC and their blending. The geometrical shape of the die is also the decisive factor: the maximum injection speed is needed at the thin wall; the slow-fast-slow speed curve is needed for thick-walled parts to avoid defects; in order to ensure that the quality of the parts meets the standards, the injection speed setting should ensure that the melt front flow rate remains unchanged.
The velocity of melt flow is very important because it affects the orientation of molecule arrangement and surface state in parts; when the melt reaches the cross-region structure in front of it, it should be decelerated; for complex dies with radial diffusion, it should be guaranteed that the melt throughput increases evenly; and for long runners, it must be filled quickly to reduce the melt front. Cooling, but injection of high viscosity material, such as PC, is an exception, because too fast will bring the cold material through the water inlet into the cavity.
Adjusting the injection speed can help eliminate the defects caused by the slow flow at the inlet. When the melt passes through the nozzle and channel to the intake, the surface of the melt front may have cooled and solidified, or the melt stagnates because of the sudden narrowing of the channel until sufficient pressure is established to push the melt through the intake, which will cause the pressure through the intake to peak.
High pressure will damage the material and cause surface defects such as flow marks and entrance burning, which can be overcome by decelerating just before the entrance. This deceleration can prevent excessive shear at the inlet and then increase the firing rate to the original value. Because it is very difficult to control the jet speed accurately at the inlet, it is a better scheme to decelerate at the end of the channel.
Preventive measures for poor injection molding
The reasons for the smaller size of injection molded parts are:
Firstly, most materials have the characteristics of thermal expansion and cold contraction, and resin is the same. So the forming products begin to shrink after being formed, and then cool down to the same time as normal temperature. However, the problem is that if the final shrinkage is too large, the size of the forming products becomes too small, resulting in the size is too small and bad.
One of the reasons is that when determining the initial die size, the basic shrinkage rate of various resins is taken into account and its numerical value is calculated as a reference. However, even if the shrinkage rate is taken into account in the actual process, the size of the product shrinkage is smaller than the size of the specification. Therefore, it is necessary to meet the above situation. The mold is remade.
Secondly, it is related to the environment in which the material is formed, such as the dry state of the material, the mixing rate of the material, the forming conditions, etc. But the most important is the forming conditions, because the die temperature and injection pressure can increase the shrinkage, so it is necessary to pay attention to them.
Generally speaking, if the injection pressure is raised, the size of the product will become larger, and if the temperature of the mould is raised, the size of the product will tend to be smaller. These can be easily understood as follows: the size of the product is directly proportional to the injection pressure and injection time, and inversely proportional to the temperature of the mould.
3. In the phenomena often occurring in long-term batch production, the waste gas accumulated from the material during injection exists in the mould, so the filling property of the material is not good and the size of the material is too small. In the content related to the waste gas, the key problem is that the precision plastic of injection moulding can not be achieved without deficiencies. Because lack is a relatively important issue, we should consider it in detail in the future.
Countermeasures for forming:
1. Increase injection pressure.
2, reduce mold temperature
3. Extend injection and cooling time.
4. Confirm the type of the forming machine and whether the mold is suitable.
5, mold Countermeasures
6, the new mold in the case of re processing bad place.
In batch production, this kind of situation occurs because of the wear of the die or the blockage of exhaust gas. First, the die is cleaned completely and then the exhaust gas is cleaned out and then formed again. It must be well confirmed.