Extrusion Blow Molding Machine Die Head Structure, Parison Control Principle & Maintenance Method

Figure 1
Figure 1
Figure 2
Figure 2

The storage type head is divided into a center feed type and a side feed type. Lateral feed is a very widely used method. In the storage type side feed hollow blow molding machine, the function of the machine head is to fuse the melt and form a parison. The key to the fusion of the melt is the flow path in the machine head. At present, some enterprises in China have developed a double-layered heart-shaped envelope flow channel, but the double-layered heart-shaped envelope flow path has a short length and a large pressure drop, and the circumferential uniformity is poor after the fusion of the melt. On this basis, a double-layer double spiral flow path has been developed in China.

In Figure 1,  the outer core is provided with two outer spiral flow paths which are symmetrically wound at 180 degrees, and the inner core is provided with two inner spiral flow paths which are symmetrically wound at 180 degrees, and the inner spiral flow path and the outer spiral flow path are 180 degrees. Symmetrically arranged, the spiral angle of each spiral flow path (inner spiral flow path and outer spiral flow path) on the outer core and the inner core is 360 degrees, so that the outer core and the inner core are melted on the 360 ​​degree outer wall surface. The material is fully and uniformly blended. During operation, the molten material enters the outer cylinder of the machine head from the inlet, passes through the diverting hole of the diverting core sleeve, and the diversion cone on the outer core smoothly completes the diversion, and the molten material flows to both sides of the diverting cone, and is turned to 90 through the guiding flow passage. After the degree, the flow is diverted to the inner and outer cores, and a part of the melt flows downward from the outer spiral flow path on the outer core, and the other part flows into the inner spiral flow path of the inner core through the flow-through hole, and the melt is contained therein. When flowing in the outer spiral flow channel, a part of the molten material flows downward with the inner and outer spiral flow channels, and another part of the molten material flows down the inner and outer core walls to the storage chamber, so that the molten material can be ensured at 360 degrees. so that the melt is evenly fused.

The automatic control of the wall thickness of the parison has two kinds of axial control and radial control. For the radial control technology, China is still in the research stage. Relatively speaking, the research on axial control is mature. In this paper, the axial control of parison wall thickness is studied for large-capacity hollow blow molding machines above 200L. The axial control of the parison wall thickness uses a closed loop control technique. The user sets the axial curve of the parison wall thickness on the touch screen panel of the wall thickness controller, and the PLC controller transmits the corresponding voltage or current signal to the electro-hydraulic servo valve according to the curve, and the electro-hydraulic servo drive servo cylinder control center rod Move up and down to change the gap of the die core.

At the same time, a sensor (magnetic levitation electronic ruler) is mounted on the piston rod of the servo cylinder connected to the center rod, and the electronic ruler can sense the size of the head die gap and feed back to the PLC controller, compare it with the standard signal in the PLC controller, and then The servo power amplifier is sent to the electro-hydraulic servo valve, and then the servo cylinder is driven by the servo valve. The cylinder drives the center rod to move, and finally controls the opening of the die to complete the control of the parison wall thickness. The structural drawing of the parison wall thickness control is shown in Figure 2. The parison wall thickness control system studied in this paper is a position control system consisting of an electro-hydraulic servo system. The core part of the control is the position of the center rod, and the center rod position control accuracy is the key to determine the effect of the parison wall thickness control. Therefore, the research focus of this system is the central rod position control accuracy, that is, the control precision of the parison wall thickness and the response speed of the system.

The control method of the parison wall thickness is to divide each parison forming process into several points and control the wall thickness of these points respectively. The fewer the number of control points, the faster the response speed, but the number of points is too small, the desired wall thickness control accuracy is not achieved, and a welded seam (ring pattern) is formed around the parison; too many points will cause the system response time to be too long. The servo cylinder is too late to reflect the received signal, and the parison has already come out. The traditional 200L plastic bucket wall thickness controllers are 100 points or 300 points. In this paper, the 200 L double L ring barrel is studied. It is verified by experiments that the wall thickness control accuracy is ±1mm, and the response time of each point is 0.3~0.4 seconds. .

The key technologies for the large-capacity hollow blow molding machine heads above 200L were studied. The double-layered double helix flow path is designed in comparison to other structural forms. It makes the fusion of the melt more uniform, the flow of the melt is smoother, the theoretical joint is eliminated, and the strength of the product is improved; according to the functional requirements, the process parameters and dimensions are determined by the optimization method, and the large-capacity hollow blow molding machine product is more than 200L. The optimization design lays the foundation; on the basis of the existing parison wall thickness control, the optimal control points and control points of the 200L double L ring barrel are studied, and the uniformity, precision and products of the wall thickness are improved. Strength of.

Maintenance Of  Extrusion Blow Molding Machine Die Head

Cleaning of the preform head. Maintaining the cleanliness of the parison head is an important prerequisite for extrusion blow molding of high performance products. Some polymers degrade during processing, especially if the melt is retained in the reservoir for a period of time during high processing temperatures or intermittent blow molding. In addition, the blow molding grade polymer contains a variety of additives which form by-products during the melting process. These degradants or by-products accumulate in the flow path of the handpiece, causing streaks on the surface of the parison, which affects the appearance of the article. The streamlined or chrome-plated nose flow reduces the appearance of parison streaks. The inflated parison has a higher temperature when it contacts the blow mold cavity, which can appropriately offset the influence of the parison strip on the appearance of the article. The use of copper sheets or the like in the production process to remove aggregates from the die head sometimes eliminates streaks on the parison. If not, stop it for a thorough cleaning. Here are a few ways to clean the machine head.

  • Manual Cleaning Method

Before disassembling the machine head, heat it to above the melting point of the plastic with an electric heater. It cannot be heated with an acetylene flame because it causes local overheating and warpage of the machine head, affecting the size and shape of the die and mandrel. After that, the heating is stopped, the heater is removed, and the head is removed. First remove the majority of the melt with a copper or copper squeegee, then use brass wool for the final cleaning. A high velocity gas stream can also be used to remove the melt from the handpiece, but the oxidized melt is still wiped off with brass wool, sometimes with a grinding wheel or by heating to remove the melt. The melt on the threads can be removed with an anti-sticking agent. Avoid scratching the flow path when cleaning the handpiece by hand, especially in the die area. This method has a large workload and will cause physical damage to the metal surface of the nose runner and is often a metallurgical damage. The following cleaning methods can be avoided.

  • Solvent Cleaning Method

It is washed by means of an acid or alkaline chemical, an organic or inorganic solvent. Among them, the equipment of the acid I or alkaline chemical cleaning method has low cost, but the cost of the chemical agent is high, and the cleaning efficiency is low, and the metal is generally corroded. The organic solvent has high cleaning efficiency. Solvent cleaning method should set up recycling equipment to avoid polluting the environment.

  • Ultrasonic Cleaning Method

This method has a good cleaning effect, but the cost of equipment and chemicals is high, and there are corrosion problems. It is best used for post-cleaning to remove inorganic residues. In addition, there are cleaning methods such as a salt bath, an oven or a fluidized bed.

Extrusion Blow Molding Machine Mold Maintenance

When blowing a transparent container, the cavity of the blow mold should be highly polished, and it should be polished once every other time during the production process. Polish the mold cavity with a polishing agent and a soft tissue paper, and wipe it with force to allow a small amount of polishing agent to penetrate into the surface of the cavity, and then polish it with a clean tissue until the mirror surface appears. Always change the tissue during polishing to avoid scratching the cavity. After the blow mold blank is worn, it must be repaired by skilled mold workers. The shear block on the neck of the blow mold and the shear sleeve on the intake rod are key components and should be kept in good condition and replaced if necessary. For example, after the cutting edge of the cutting block is unevenly worn, the formed container may leak in the neck during use. When the blow mold cooling tunnel affects the flow of the cooling medium due to blockage or corrosion, it should be cleaned immediately. The guide post and the guide bush of the blow mold should be regularly lubricated and replaced at least once a year to ensure the alignment of the two halves and improve the life of the mold. Whenever the production or blow molding mold is to be in stock, the cooling holes of the mold should be blown off with compressed air, and the mold cavity should be coated with a protective agent to avoid corrosion.

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