Extrusion molding of plastic pipe​

日期:2022/8/12 9:27:09 / 阅读: / 来源:本站

Extrusion molding of plastic pipe
5.1 equipment
     At present, single screw extruder is widely used in plastic pipe extrusion. The production line of hard plastic pipe extruder composed of single screw extruder is shown in Fig. 5-1. The production line of soft plastic pipe extruder is shown in Fig. 5-2. The auxiliary machines on the plastic pipe extruder production line mainly include: molding die, cooling water tank, traction device, cutting machine and finished product storage or coiling device.
5.1.1 pipe forming mold
    There are many structural forms of plastic pipe extrusion molding dies. As shown in Fig. 5-3, it is a common die structure for PVC rigid pipes. During production, the internal pressure method is used to determine the diameter of the pipe blank. Figure 5-4 shows the structure of the extrusion polyolefin plastic pipe molding die. Usually, this mold structure is used in combination with the vacuum cooling sizing sleeve. The extrusion molding of soft poly (vinyl chloride) 2 pipe is usually of the die structure shown in Fig. 5-5, which is a straight through type drooping structure. It can be seen from the three typical mold structure diagrams for forming pipes that the plastic pipe forming mold structure is mainly composed of mold body, shunt cone, shunt cone support, die, mandrel, gland, sizing sleeve, adjusting screw, intake pipe joint and other main parts. 51.11 shunt cone the structure of shunt cone is shown in Fig. 5-6. In the figure, cone length L = (1 ~ 15) d, where D is the diameter of pipe blank; The taper angle a < 60 ° is widely used. If the melt viscosity is low, the taper angle a can be greater than 60 ° in order to reduce the mold structure size. The shunt cone is the part that first contacts the molten material in the molding die. The molten material enters the mold cavity after being extruded from the perforated plate of the inner diameter of the barrel. The split cone divides the molten material of the split cone structure shown in Fig. 5-6 into annular bobbins and flows to the mold outlet after passing through the split cone. At the same time, the thickness of the simple tube gradually becomes thinner.
5.1.1.2 split cone support
     The structure of shunt cone support is shown in Fig. 5-7. The separation of flow cone and shunt cone support is mainly considered from the perspective of convenient machining. In practical application, there are many structural forms in which the shunt cone and the bracket are designed as a whole. Figure 5-7 structure of shunt cone supportThe shunt cone support is a positioning part that connects and supports the shunt cone and the core rod. The working position of the shunt cone and the core rod is fixed by the shunt cone support to ensure that the combined center line of the shunt cone and the core rod coincides with the center line of the screw, so as to facilitate the smooth molding of the product in the mold.
5.1.1.3 mandrel
      The structure of the core rod is shown in Fig. 5-8. This kind of core rod part structure is used in extrusion molding of small-sized pipes (generally, the pipe diameter is less than 50mm). The core shaft of the core rod for forming large-sized pipes shall be provided with a resistance heating rod placement hole to heat the core rod body to reach the process temperature required for tube blank forming in the early stage of production. In Figure 5-8, the core rod shrinkage angle value: when PVC material is used for forming pipe, a = 15 ° ~ 30 °; When PP material is used for forming pipes, a = 25 ° ~ 40 °. The length L of the straight section is equal to the length of the straight section of the die, generally L - (20 ~ 40) 8 (8 is the wall thickness of the pipe). (a) Mandrel with external thread (b) mandrel with internal thread Fig. 5-8 mandrel structure L: - length of straight section of mandrel; 50: A core rod cone portion is long; L - groove depth; A-shrink angle mandrel and die are combined to form the melt channel cavity for extruded pipe, and the melt forms the parison of the pipe at the straight part of this section. The mandrel is a part for the inner diameter and surface dimension of the shaped tube products.
5.1.1.4 die
The structure of the die is shown in Fig. 5-9. The inner diameter D of the die is determined by the outer diameter of the forming pipe, and d-kd (k = 0.95 ~ 1.05) is taken; The length of the straight section of the die is equal to the length of the straight section of the mandrel. It can also be calculated as l = (0.5 ~ 3) d (D is the outer diameter of the pipe).
After the die is assembled with the mandrel, the molten material flow channel cavity is formed. When the molten material passes through the cavity, it will be subjected to pressure due to the gradual reduction of the cavity volume, and the tube blank will be formed when it is extruded from the cavity. The inner diameter of the die is the surface dimension part of the outer diameter of the forming pipe.
(a) Non threaded die (b) threaded die Fig. 5-9 die structure
5.1.1.5 calibrating sleeve
      Several common structural forms of sizing sleeves are shown in Fig. 5-10. Fig. 5-10 (a) is a submerged sizing sleeve, which is used for sizing and cooling of small-diameter pipes (below 50mm). Generally, the extruded pipe blank can be directly led into the cooling water tank; Fig. 5-10 (b) shows the application of water jacket sizing meal when internal pressure sizing is generally adopted; 5-10 (c) type vacuum cooling sizing sleeve structure is usually used for extrusion sizing of polyolefin plastic pipes. The inner diameter and length of the sizing sleeve can be selected according to Table 5-1. Table 5-1 inner diameter and length of sizing sleeveOuter diameter of pipe D / mm inner diameter of sizing sleeve D / mm length of sizing sleeve L / mm polyolefin (1.00 ~ 1.04) DD < 40 polyvinyl chloride (1.00 ~ 1.02) d l ≈ 10d d > 40 (1.008 ~ 1.012) d l < 10d d d > 100L = (3 ~ 5) D note: 1. For vacuum sizing sleeve, d = KDL value can be slightly larger, k = 1.007 ~ 1.01 for PVC material, k = 1.02 ~ 1.04 for PE material and K = 1.02 ~ 1052 for PP material. The vacuum degree is 0.02 ~ 0.08mpa.Within the range, 002-0.04mpa for small straight pipes and 0.06-0.08MPa for large diameter pipes. 3. The diameter of the vacuum hole is 0.6 ~ 1.1mm (the wall thickness is the largest). The sizing sleeve is mainly used to cool the tube blank of the extrusion molding die, fix the outer diameter of the damaged tube and trim the surface finish of the tube.
5.1..6 mold details
There are various structural forms of the mold, as shown in Fig. 5-11, which are the two commonly used mold. Fig. 5-11 (a) shows that mold parts can only be installd from one side of the mold, and Fig. 5-11 (b) shows that parts in the mold can be installed into the mold from both sides of the mold.
The specific function of the die is to assemble and fix various parts in the pipe forming die. The machine barrel and the sizing sleeve are connected into a whole by bolts through the die.
5.1.1.7 precautions for use of molding die
      ① The steel used for parts manufacturing shall be wear-resistant, corrosion-resistant, with small deformation in high temperature environment and certain toughness ② the working surface shall be smooth, flat and free of stagnation. If necessary, the surface shall be plated with hard chromium layer. ③ When disassembling and cleaning the sticking materials of all parts in the mold, it is not allowed to knock the working surface with a hammer, and it is not allowed to clean the scraps stuck on the working surface with a scraper. There is no scratch and burr on the working surface.
④ Note that the process temperature control of each part in the mold shall meet the requirements of the tube blank forming process, and the core rod in the large-size tube forming mold shall be equipped with a heating temperature control device. ⑤ The center line of each part in the mold shall coincide with the center line of the mold and the screw after assembly. ⑥ When the molding die is not used for a long time, all parts shall be cleaned. Apply a layer of antirust oil and seal the inlet and outlet for storage after assembly. No heavy objects are allowed on the stored mold body to prevent mold deformation.

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