General Knowledge of PE Pipe Extrusion Die
In the current pipe market, PE pipes are encountering and exceeding the amount of PVC pipe at a higher speed. It is necessary to understand the important part of the production of PE pipes - extrusion die - in production and in current or future projects. The two extrusion dies currently used are spiral mandrel dies and screen basket dies. These two types of dies are now discussed separately for your reference.
In the pipe extrusion line, whether it is PVC pipe or PE pipe, the die plays a very important role in product quality, and its function is to uniformly pass the melt extruded from the extruder through the entire die section. The pipe die is adapted to the requirements of the extruded product and plays a decisive role in the efficiency and economy of the entire production line. The two basic requirements for the die are:
1. The die must be adapted to the material being processed;
2. The die must be designed in strict accordance with the amount of extrusion and the section size of the pipe.
First, The Development Process of Polyolefin Die
The extrusion of polyolefin has evolved from stent-type dies to later spiral dies and basket-type dies. The purpose is to eliminate the mold clamping line caused by the die holder as much as possible, so that the material entering the die is well mixed and the melt is more even.
1, Bracket Type Die
The fluid-distributing part of the die is a bracket type, the die for producing a small-diameter pipe is a cross-bracket type, and the die for producing a large-diameter pipe is mostly six ribs or eight ribs. The biggest disadvantage of this type of die is that if the pressure of the front compression section is too small, the merge line is sometimes not very good. It is therefore necessary to have a sufficiently high pressure to recombine these separate streams. This requires a structure having a relatively high strength, so the overall weight of the die is high. This type of die is generally used for the extrusion of PVC pipes.
2, Bracket Die with Broken Board
The easiest way is to place a particle board behind the rack and the several streams caused by the rack are divided into a number of smaller streams. Therefore, the mechanical stress is pushed forward from the bracket portion, and the mold line is substantially eliminated by a large amount of small flow. This type of PE die is mostly used in the production of low density polyethylene and small diameter pipes. The die used in the low-density PE drainage watering pipe produced by our company in the West Factory was the same. Nowadays, such dies have been eliminated and are still in use in some private small factories.
3, Spiral Mandrel Die and Sieve Basket Die
Bracketed die with a broken board has rarely been used in the production of existing medium and high density polyethylene pipes. The utility model also uses a spiral mandrel die and a sieve basket die. These two types of heads have better solved the problem of eliminating the mold line. The core of the spiral mandrel die is a spiral mandrel, and the core of the basket die is a tubular body with a large number of perforations, ie a mesh stack. The structure and working principle of these two common dies are now explained separately.
Second, The Structure And Working Principle of The Spiral Mandrel Die
The spiral mandrel die is also called a spiral fluid split die. For spiral split flow dies, the melt stream entering the die is first diverted to the periphery of the head through a split system - either a star hole or other split system. The melt then enters a flow path that spirals around the outside of the mandrel. The depth of the spiral flow path gradually decreases as it approaches the outflow end of the die. At the same time, the gap between the core mold and the cavity of the die body is gradually increased. In this section, there is an overlap of axial and radial activity inside the stream. Along the direction of the die outflow, the proportion of radial activity decreases, while the proportion of axial activity continues to progress, with the ultimate total melt moving axially. In this way, the active imprint of the fluid separation channel is evenly distributed along the circumference of the pipe section, which means that no local loss of strength occurs.
The quality of the melt split around the section of the pipe is affected by a number of factors. These factors include the geometry of the headpiece, such as the number of runners, the twist angle of the runner winding, the pitch of the spiral, the depth and width of the groove, the width of the gap, the material being processed, especially its viscosity, and the amount of extrusion and material temperature. Thanks to the computer program and the experience of the operator, the spiral splitting fluid can be optimally designed to suit a specific application range. In this case, the wall thickness distribution is very uniform. In addition to the good elimination of the compounding line, this type of die has the upper hand in the following aspects:
1. The mechanical stress and thermal stress are low, and the product has good mechanical strength.
2, the structure is strong, it is suitable for the extrusion of high-viscosity materials, at the same time the assembly and disassembly of the die and easy to operate.
3, the core rod is easy to drill, so it is easy to use the internal cooling system.
These advantages make the spiral mandrel die more and more widely used.
The disadvantage of this type of die is that this type of die is sensitive to changes in the material due to its relatively complex structure. Even small changes in viscosity and rheological properties can cause changes in the activity in the spiral, and the change in pipe diameter has a similar effect. Therefore, the geometry of the spiral should be designed so that the die has the widest possible processing range to accommodate different materials and extrusions.
Third, The Structure And Working Principle of The Screen Basket Die
On a screen basket die, the stream is first passed through a support or a stream split system with multiple star holes. The melt then flows through the porous section. This element, the screen basket, where the stream does not flow in the axial direction, but flows radically inward from the inside. Since the screen basket has a certain length, a large overflow area is provided in the case where the diameter of the head is small. Thus, at relatively small head sizes, the pressure in the porous section is reduced. Within the screen basket section, the melt stream changes direction of flow twice and then becomes axial. The single stream can be well mixed and the melt is more homogenized.
The fusion between the streams is achieved by means of a blocking element and an adjacent slack buffer. Due to the large cross-sectional area of the screen basket, the head pressure of the basket type head is significantly lower than other structural forms. The material is subjected to a relatively small load and the material temperature is relatively low, so that the extrusion amount can be easily improved. The biggest advantage of this type of machine head is that it has better adaptability even in high extrusion volume, and can achieve good homogenization; pipe diameter and wall thickness change, PE material type, material viscosity, back pressure Under the change, we can produce high quality pipes. The biggest disadvantage of this type of die is that the fusion of all the materials in the die, the cleaning in the production, and the disassembly and assembly of the die are not as good and convenient as the spiral mandrel head. The PE pipe production line currently used by our company is produced by Magstar Machinery, and the head used is a sieve basket type head. It can be clearly seen in the production practice that the adaptability of the machine head is strong, regardless of PE80, PE100 or PE63. We also bought a special machine head to extrude PP-R pipe, which solved the inconvenience of cleaning the machine head frequently.
In short, these two types of die have their own advantages and disadvantages, it is recommended to choose the PE pipe die, take advantage of the length and complement, according to their own production conditions. The grade of the raw material is not fixed, the production volume is large, and when the color of the pipe is not changed frequently and the disassembly and assembly die is cleaned, it is better to use a sieve basket die. It is required that the wall thickness of the pipe is relatively uniform, and the grades of the raw materials are relatively fixed. When the color of the pipe is often changed and the die is cleaned, it is preferable to use a spiral mandrel die.
