Many PVC profiles have a variety of different types of internal ribs to form a variety of mold core structures. If the inner ribs of the profile are relatively small, the inner rib flow path design is relatively easy, however, the width to thickness ratio of some inner ribs tends to be greater than the width to thickness ratio of the outer wall. Therefore, under the principle that the pressure drop of each branch channel is the same, the confluence condition of the inner rib flow channel is relatively bad. The suitability of a set of mold core assembly is largely determined by the core structure processability. Since the inner rib generally cannot be directly cooled when entering the sizing die, the cooling speed is slow; and the outer wall of the profiled product is directly in contact with the sizing die, and the cooling speed is fast, so that the inner and outer cooling is uneven, which tends to cause deformation of the product, so the inner rib Design is one of the difficulties in mold design. To increase the cooling rate of the inner ribs, from the point of view of the sizing die design, the vents of the vacuum chamber can be opened at the position of the inner ribs. However, it is difficult to solve this problem fundamentally only by starting from the cooling system of the fixed mold. The quality of the inner ribs is essentially the rationality of the flow path structure of the die. To solve the deformation of the plastic profiled product caused by poor forming of the inner rib, the key is to improve the structure of the die flow passage in the mold. In addition, the flow rate of the runner material at the inner rib of the die has a appearance on the profile.
There is no relationship between shrinkage marks and profile deformation.

1 Internal rib die flow path analysis
The inner rib die flow passage of the plastic extrusion die can be roughly divided into a split section, a compression section, a confluence section, and a shaping section. Due to the structural characteristics of the internal ribs, it is difficult to ensure uniform splitting and smooth convergence. The internal ribs of different structural forms have different diversion sections, compression sections, confluence sections, and shaping sections. In this paper, the different types of internal ribs and the flow path structure and feeding mode of the internal ribs at different extrusion speeds are discussed.

2 Common forms and designs of internal reinforcement
2. 1 support type internal reinforcement
The supporting inner ribs mainly play the role of strengthening, supporting and connecting the plastic profile (Fig. 1 (a)). Under the normal extrusion speed, the support type internal rib die flow passage of the general mold can be designed in the form of two sides feeding.

[Figure 1 (b)]. That is, at the position of each inner rib, starting from the tail cone, two wide materials are processed on the upper and lower surfaces to be compressed into a trough of 4 to 7. 5 mm, and the trough is gradually narrowed from the tail cone to the outlet surface of the die to Meet the requirements of the compression section of the material to ensure that the pressure of other materials in the die is consistent. The two materials meet at a distance of 20 to 40 mm from the exit surface of the die to meet the requirements of the material setting section. The mold using this feeding form has a simple structure and is easy to process. In the ordinary PVC modified pellet extrusion speed, the traction speed is generally not more than 1.2 to 1.5 m / min [1], the die flow channel structure can meet the feeding requirements of the internal reinforcement, and Because the inner ribs are discharged too fast, the inner ribs are too thick or curved, which affects the molding of the product. At the high-speed extrusion speed, if the supporting inner rib of the mold is also in the form of PVC pellets on both sides, the extrusion speed will not go up. If the extrusion speed is forcibly increased, it will inevitably lead to the shortage of the inner ribs. Defects in shape molding. Therefore, the support type internal ribs for high-speed extrusion dies should be designed in the form of central feeding (Fig. 1 (c)).

To ensure the supply of the stream. The so-called central feeding mode is to divide the flow path of the inner rib into the feeding section, the compression section and the shaping section to respectively process, from the tail cone part of the inner rib position, make a square feeding hole in the central part, and then gradually pass the mouth. The compression section and the shaping section are extruded from the exit surface of the die. This design method ensures that a separate material is supplied to the inner rib at the joint head, thereby avoiding the implicature between the molten material supplied to the outer wall and the molten material of the inner rib in the die, and the flow paths of the respective sections are relatively independent. The entire flow path transition is relatively flat, thereby increasing the design extrusion speed of the mold. However, this design method has high processing requirements, smooth transition of each section of the core, and the smoothness of each joint surface cannot be lower than 0.8. When the die is assembled, each section is fastened with screws and positioned with a round pin to ensure the core. Each segment is tightly assembled, otherwise it is prone to flashing. In addition, in the design of a product containing a supporting inner rib, the thickness of the rib should be at least 20% thinner than the outer wall, which is a prerequisite for good formation of the inner rib.

2. 2 sheet type internal reinforcement
Due to the thin wall, the wide width and the inner ribs, the PVC sheet type inner rib products have a large number of stagnation points in the flow path, and are easy to stick and paste [3]. Its design difficulty has two points: First, there are many internal ribs, and all internal ribs must have uniform wall thickness; second, improper design of the internal ribs will cause shrinkage marks on the shape. At ordinary speeds, the flow path design of the plate-type inner ribs can be in the form of a partial core partial feed (as shown in Figure 2 (b)).

That is, at a position of 60 to 100 mm from the exit surface of the die, the upper and lower surfaces of the inner rib are each milled with a circular transition trough from shallow to deep, and the diameter of the two circles can be determined according to the ratio of the enlargement coefficient of the outer wall. Since the wall thickness of the sheet-type inner ribs generally does not exceed 1.2 mm, the length of the transition section does not have to be too
Long, the actual design, can take 15 ~ 30mm, after the transition period, the two streams meet. Because the inner ribs of the plate are relatively large, in order to ensure that the speed of the exit dies of all the inner ribs is stable and consistent, the die shaping section of the plate type inner rib is generally longer than the support inner rib of the same thickness. This feeding form facilitates the processing accuracy and ensures the design requirements of the internal ribs at ordinary extrusion speeds. At the same time, we must also pay attention to the inner ribs in the central part of the product. Because the extrusion resistance is small, the flow is short, and the inner ribs are easy to form; while the inner ribs on both sides of the product have large resistance to extrusion and the flow is long, which easily causes the lack of internal ribs. . Therefore, in the design of the inner ribs, the length of the inner rib feeding portion should be gradually increased from the center to the sides. In the actual design, L = L 0 + X&tgα can be taken. Where: L is the length of the feeding part of any inner rib; L 0 is the length of the feeding part of the inner rib of the product; X is the distance from any inner rib to the center of the product; The angle between the length difference of L and L 0 and the horizontal direction is less than 6°.

At higher speeds, the design of the internal ribs can be in the form of separate cores (Fig. 2(c)).
The core is divided into several pieces for processing. Each piece is called a split core. Each split core is directly assembled on the die of the die with a dovetail insert, leaving a gap between the cores. The gap value is the wall thickness of the inner rib. The split core extends into the coupling head and is milled at the end to extend into the die transition section. The width TS2 of the square groove can be calculated according to the ratio of the wall thickness T2 of the die transition section to the wall thickness T2 of the die setting section. Actual design
Refer to the following formula: TS2 = K&( T2/ T1) & TS1 where: TS2 is the width of the square groove; TS1 is the wall thickness of the inner rib of the die shaping section; T2 is the wall thickness of the outer wall of the die transition section; T1 is The wall thickness of the outer wall of the die-shaped section; K is the empirical coefficient, generally taking K = 1 to 1.2. This design method can well solve the contradiction between the feeding of the inner ribs and the increase of the extrusion speed. However, this type of flow path design requires high processing precision and is difficult to assemble. Between the small cores of the split body, the core and the mold body are required to be matched with grinding. In addition, the product containing the plate type inner rib is the same as the support type inner rib. In the design of the product cross section, the thickness of the inner rib should be 50% to 75% of the thickness of the outer wall [2]. In order to improve the forming of the product and the bonding strength between the inner and outer walls, it is also possible to add rounded corners at the joint of the inner and outer walls. In addition to a good feed pattern and a good styling cooling system, this is where the outer surface of the sheet avoids shrink marks.

2. 3 protruding type reinforcement
The protruding inner ribs often play the role of decoration and reinforcement. In some non-closed cavity products, some protruding ribs can also reduce the shrinkage marks. Such a rib is not difficult to design as long as the section design of the product is reasonable. The flow path of the inner rib is usually taken in the form of a groove on the core to ensure the feeding. The length L of the shaping can be referred to the following formula: L = (50 ~ 100) & t where: t is the height of the rib. At the general extrusion speed, this empirical coefficient can be made smaller. In the high-speed extrusion die design, the empirical coefficient can be appropriately larger. However, the cross section of the product containing the protruding type reinforcing ribs should be reasonable, and the inner ribs of the protruding objects should not be larger than the wall thickness, otherwise it is easy to produce shrink marks on the outer surface of the product, which is disadvantageous for the molding of the product.

2. 4 cruciform inner ribs
In addition to the decorative and reinforcing effect, the inner ribs can be removed from the welding process during the assembly of the profile, and directly connected by screws. This kind of PVC inner rib has great difficulty in the design and processing of the core. Therefore, in the form of the flow channel, the flow path of the crucible inner rib is placed on the transition section of the template, and it is not extended into the joint. Inside, the corresponding core portion is concave to ensure the supply of the flow. A wire cutting process is used on the die plate to make a single small piece, and the protruding portion extends to the exit face of the die (Fig. 3(b)).

At ordinary extrusion speeds and higher extrusion speeds, the flow path of the cruciform inner ribs adopts the same structure. However, the length of the latter section is longer than the former, the former may take 20 to 30 mm, and the latter may take 25 to 40 mm. In addition to the extrusion speed, the length of the shaping section is also related to the wall thickness.

3 Conclusion
(1) Support type internal ribs Under the normal extrusion speed, the die flow path should adopt the structure of feeding on both sides; in the high-speed extrusion, in order to ensure the supply of the flow, the flow channel of the die should adopt the central key supply. The structural form.
(2) Plate-type internal ribs At the normal extrusion speed, the die flow path should adopt the structural form of integral core core feeding; at higher speed extrusion, the die-type flow path of the plate type internal rib should be adopted. The higher form of the split core is separately supplied.
(3) Due to the simple structure of the protruding type inner ribs, the die flow channel structure grooved on the core is used at the ordinary extrusion speed and the higher extrusion speed, but the protrusions in the high speed extrusion die The length of the die of the inner rib is much longer than that of the ordinary extrusion speed.
(4) Because of the difficulty in processing, the cruciform inner ribs are used as part of the stencil block and concave at the corresponding position of the core to ensure the supply of the flow.

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