Screw structure for producing pva water-soluble film

Abstract

The utility model relates to PVA water soluble film production equipment technical field, concretely is PVA water soluble film production with screw rod structure, including screw rod main part, and screw rod main part is provided with feeding section, melting section and homogenization section in proper order from front to back along the axial direction, the outer wall of feeding section is equipped with first screw groove, the outer wall of melting section is equipped with screw edge structure, and the screw edge structure is variable deep screw groove or barrier edge, and the screw groove depth of melting section gradually becomes smaller gradually, the outer wall of homogenization section is equipped with second screw groove, the outside coaxial sleeve of screw rod main part is equipped with the sleeve. This PVA water soluble film production with screw rod structure, substantially improve plasticization uniformity, guarantee PVA water soluble film core performance to reach the standard. In view of the unreasonable design of screw groove of the existing screw rod leads to 'unmelted particle', 'local coke material' and the unstable problem of film material performance, the utility model realizes the uniform melting and melt homogenization of PVA raw materials through three -section type structure optimization and parameter accurate matching.

Description

Technical Field

[0001] This utility model relates to the technical field of PVA water-soluble film production equipment, and more specifically, to a screw structure for PVA water-soluble film production. Background Technology

[0002] PVA Definition: Polyvinyl alcohol (PVA) water-soluble film, or simply PVA film, is a high-performance, biodegradable, and environmentally friendly packaging material. Using PVA resin as its core raw material, it possesses excellent water solubility, biodegradability, gas barrier properties, transparency, and antistatic properties, making it widely used in pesticide packaging, food inner packaging, and medical product packaging. Due to the narrow melting temperature range and sensitivity to shear forces of PVA resin, and the need for strict control of melt uniformity in water-soluble film production to ensure the film's solubility and mechanical strength, extremely high demands are placed on the structural design of the screw, a core component of the production equipment. In the industrial production of PVA water-soluble films, extrusion molding is a key process, and the screw, as the "heart" of the extrusion equipment, directly determines the conveying efficiency of PVA raw materials, the quality of melt plasticization, and the final film performance. Currently, most screws used in PVA water-soluble film production on the market adopt the structural design of traditional general-purpose plastic extrusion screws, without being adapted and optimized for the special processing characteristics of PVA resin. This leads to numerous technical pain points during the production process, as follows: Poor plasticization uniformity affects the core performance of the film material: PVA resin needs to be completely melted within a specific temperature range (usually 160-200℃), and local overheating must be avoided during the melting process to prevent resin degradation (degradation reduces the dissolution rate and transparency of the water-soluble film). Existing screw designs, such as screw groove depth and pitch, do not match the melting characteristics of PVA resin. For example, a shallow screw groove in the feeding section results in slow raw material delivery, and the lack of a gradual transition structure in the melting section leads to uneven shear force distribution, easily resulting in "unmelted particles" or "localized scorching." Simultaneously, insufficient precision in controlling the gap between the screw and the sleeve (barrel) (usually greater than 0.3mm) causes some molten material to remain in the gap and be reheated, further exacerbating uneven plasticization. This ultimately leads to problems such as thickness deviation (greater than 5%) and large differences in transverse tensile strength (greater than 10MPa) in the produced water-soluble film, severely impacting product yield.

[0003] Low conveying and melting efficiency makes it difficult to adapt to large-scale production: PVA water-soluble film production mostly adopts continuous extrusion processes, requiring screws with stable high conveying capacity (typically requiring a raw material throughput of 50-80 kg / h). Existing screws have excessively small pitch in the feed section (mostly 50-60 mm), causing raw materials to easily accumulate and "bridge" at the feed inlet, resulting in unstable feeding. The melting section lacks reinforcing mixing structures such as barrier ridges, leading to a PVA resin melting time as long as 30-40 seconds (far exceeding the industry ideal of 20-25 seconds), resulting in a slow overall production cycle. Taking Yifa's Φ70 PVA water-soluble film production line as an example, using existing screws, the daily output of a single production line is only 800-1000㎡, which is insufficient to meet the large-scale procurement needs of downstream enterprises (such as pesticide packaging plants) for water-soluble films (typically with single orders exceeding 5000㎡), thus restricting the production capacity expansion of manufacturing enterprises. Utility Model Content

[0004] The purpose of this invention is to provide a screw structure for the production of PVA water-soluble films, in order to solve the problem of poor plasticization uniformity and its impact on the core performance of the film material mentioned in the background art: PVA resin needs to be completely melted within a specific temperature range (usually 160-200℃), and local overheating should be avoided during the melting process to prevent resin degradation.

[0005] To achieve the above objectives, this utility model provides a screw structure for PVA water-soluble film production, including a screw body, which is arranged axially from front to back as a feeding section, a melting section, and a homogenizing section. The outer wall of the feeding section is provided with a first screw groove, and the outer wall of the melting section is provided with a screw ridge structure. The screw ridge structure is a variable depth screw groove or a barrier ridge, and the depth of the screw groove in the melting section gradually decreases. The outer wall of the homogenizing section is provided with a second screw groove. A sleeve is coaxially sleeved on the outer side of the screw body.

[0006] This setup allows PVA raw materials to undergo three stages within the screw: "solid conveying → melting and plasticizing → melt homogenization." The feeding section achieves stable raw material conveying through the first screw channel. The melting section utilizes the screw ridge structure of varying depth screw channels or barrier ridges, combined with gradually decreasing screw channel depth, to create gradient shear force and heat exchange space, driving the solid raw material to gradually melt. The homogenization section uses the second screw channel for final mixing of the melt. The external sleeve provides a closed working environment for the screw. On one hand, the sleeve heating system can precisely control the temperature (matching the PVA melting range of 160-200℃), providing a heat source for raw material melting. On the other hand, it prevents raw material / melt leakage, ensuring that the material moves orderly along the screw axis without backflow or stagnation.

[0007] Preferably, the screw body is made of high-strength alloy material.

[0008] This screw design requires it to withstand high temperatures (160-200℃), high-frequency shear forces (especially in the molten section), and material friction over extended periods. Ordinary steel is prone to deformation and wear. The screw body is made of high-strength alloy materials (such as 38CrMoAlA), whose alloy composition (such as chromium, molybdenum, and aluminum) forms a stable metallic microstructure, improving the material's high-temperature strength, fatigue resistance, and wear resistance, ensuring that the screw does not fail under long-term, harsh operating conditions.

[0009] Preferably, the first thread groove depth is 7-9 mm and the thread pitch is 70-90 mm, and the second thread groove depth is 4-6 mm.

[0010] This feature includes a large screw channel in the feed section for solid conveying: the first screw channel has a depth of 7-9mm and a pitch of 70-90mm. The larger screw channel volume can accommodate more PVA solid raw materials, while the larger screw pitch shortens the axial movement time of the raw materials, increases the conveying speed, and avoids the accumulation of raw materials "bridging" at the feed inlet; the second screw channel has a depth of 4-6mm. The shallower screw channel allows the melt to undergo more frequent shearing and mixing when the screw rotates, accelerating the homogenization of melt temperature and viscosity, which meets the high requirements of PVA melt for homogenization.

[0011] Preferably, the outer wall surface of the screw body is provided with a surface treatment layer, which is a nitrided layer or a hard chrome plating layer.

[0012] This design means the screw's outer wall surface is in long-term contact with the PVA raw material / melt, making it susceptible to friction from materials (especially solid raw materials) and slight corrosion from molten PVA (containing plasticizers), leading to surface wear and rust, and affecting screw channel accuracy. A nitriding layer or hard chrome plating layer is applied to the outer wall surface of the screw body: Nitriding treatment forms a compound layer such as iron nitride (Fe3N) on the screw surface, resulting in high hardness (HRC55-60) and tight bonding with the substrate, improving surface wear resistance and corrosion resistance; the hard chrome layer has high hardness (HRC60-65) and a low coefficient of friction, forming a physical protective barrier on the screw surface, isolating it from material friction and corrosion, while also reducing material adhesion.

[0013] Preferably, the front end of the screw body is further provided with a connecting structure, which is used to connect with the transmission mechanism. The connecting structure is a key connection or a flange connection.

[0014] This design uses a key (such as a flat key) embedded in the keyway between the screw body and the drive shaft to transmit torque using the shear force of the key. It has a simple structure, is easy to assemble and disassemble, and is suitable for medium and low speed conditions. It is fixed by flange and bolts, with a large contact surface and uniform force distribution, which can transmit greater torque. It also has high coaxiality, avoiding radial runout when the screw rotates, and is suitable for the stable speed requirements in PVA production.

[0015] Preferably, the sleeve has a feed inlet at the top of its front end and a discharge outlet at its rear end.

[0016] The sleeve is designed as a matching component of the screw. The position of its inlet and outlet must be adapted to the three-section structure of the screw: the inlet (71) at the top of the front end of the sleeve is directly opposite the screw feed section to ensure that the PVA solid raw material enters the screw groove of the feed section directly and avoids the raw material falling into the non-feed area and causing stagnation; the outlet (72) at the rear end of the sleeve is directly opposite the rear end of the homogenization section of the screw so that the homogenized PVA melt can directly enter the subsequent die head from the outlet and avoid the melt being stagnant in the sleeve, which would affect the homogenization effect.

[0017] Preferably, the sleeve is fixedly connected to the frame via a connector.

[0018] In the production of PVA water-soluble films, this setting requires the screw and sleeve to maintain relative static coaxiality (to avoid radial displacement that could lead to material leakage or screw edge wear). The sleeve is fixed to the frame using connectors (such as bolts or brackets). The stable structure of the frame can be used to limit the radial and axial displacement of the sleeve, ensuring that the sleeve and screw always maintain precise coaxiality and do not shift position due to screw rotation or equipment vibration.

[0019] Preferably, a baffle is installed at one end of the screw body near the connecting structure.

[0020] This setting places the screw near the end of the connecting structure (not the material handling end). If molten PVA material or raw material impurities enter the connecting structure (such as keyways or flange gaps), it can easily lead to corrosion and jamming of the connecting parts, affecting power transmission. Installing a baffle at this end can create a physical barrier, preventing materials or impurities from moving towards the connecting structure, while also preventing external dust from entering the connection area between the screw and the transmission mechanism.

[0021] Compared with the prior art, the beneficial effects of this utility model are as follows: This screw structure for PVA water-soluble film production significantly improves plasticization uniformity, ensuring that the core performance of the PVA water-soluble film meets standards. Addressing the problems of "unmelted particles," "localized scorching," and unstable film performance caused by unreasonable screw channel design in existing screws, this invention achieves uniform melting and melt homogenization of PVA raw materials through a three-stage structural optimization and precise parameter matching. The efficient feeding section lays the foundation for uniformity: the depth and pitch of the first screw channel in the feeding section are specifically designed to accommodate more PVA raw materials, significantly improving the raw material conveying speed and preventing raw material accumulation at the inlet. This ensures that the raw material enters the melting section at a stable rate, reducing plasticization differences caused by feeding fluctuations. The gradually changing screw ridges in the melting section enhance shear mixing: the melting section features a screw ridge structure with varying depths of screw channels or barrier ridges, and the screw channel depth gradually decreases along the axial direction, creating a gradient shear force on the PVA raw material—in the initial stage of the raw material entering the melting section, the deeper screw channels provide sufficient space for... Initial heating softens the material; as the screw channel becomes shallower, the shear force gradually increases, and combined with sleeve heating (precisely matching the PVA melting zone), it accelerates the melting of the raw material and avoids local overheating and degradation; the barrier ridge structure can also intercept incompletely melted particles, forcing them to undergo secondary mixing and completely eliminating the "unmelted particle" phenomenon; the shallow screw channel in the homogenization section achieves melt homogenization: the depth of the second screw channel in the homogenization section is optimized, and the shallower screw channel can perform high-frequency shearing and mixing on the melt output from the melting section, keeping the melt temperature and viscosity stable and consistent, ultimately ensuring that the extruded PVA melt is uniform. The water-soluble film produced using this screw has stable performance and fully meets the stringent requirements of PVA water-soluble film for core performance. To address the insufficient production capacity caused by the weak conveying capacity and long melting time of existing screw conveyors, this invention significantly improves screw processing efficiency through structural optimization and parameter upgrades, matching the pace of large-scale industrial production. The large pitch in the feeding section breaks through the conveying bottleneck: the enlarged screw pitch significantly increases the amount of raw material conveyed per unit time. Combined with the smooth feeding structure of the sleeve inlet, it completely solves the raw material bridging problem, greatly improving the stability of raw material conveying. The efficient structure in the melting section shortens the melting time: the combination of the variable-depth screw groove and the barrier ridge increases the heat exchange area between the PVA raw material and the inner wall of the sleeve, while enhancing shear heat generation efficiency, significantly shortening the PVA resin melting time and significantly improving melting efficiency. Overall production capacity achieves a breakthrough: taking a dedicated PVA water-soluble film production line as an example, after adopting the screw of this invention, the raw material processing capacity and daily output of a single production line are significantly increased, meeting the large-scale order demands of downstream enterprises. Significant capacity growth can be achieved without adding new production lines, reducing enterprise expansion costs and improving market responsiveness. To address the issues of high energy consumption and frequent maintenance caused by the high speed and high heating temperature requirements of existing screw compressors, this utility model achieves a dual reduction in energy consumption and cost through structural adaptation optimization. Low speed is sufficient to meet production needs: Due to improved conveying efficiency in the feeding section and enhanced plasticizing capacity in the melting section, this screw compressor can achieve the production effect of existing high-speed screw compressors at a lower speed, significantly reducing motor power consumption; Precise matching of heating temperature reduces energy consumption: Uniform shearing and heat exchange eliminate the need to raise the heating temperature of the sleeve (maintaining the ideal melting range of PVA is sufficient), reducing the energy consumption of the heating element and avoiding raw material degradation and waste caused by high temperatures, thus improving raw material utilization; Reduced downtime maintenance frequency: The application of high-strength alloy materials and surface treatment layers (nitrided layer or hard chrome plating layer) significantly enhances the surface hardness of the screw outer wall, improves wear resistance and corrosion resistance, reduces the average wear rate of the screw, extends the maintenance cycle, and saves downtime losses and parts costs with each maintenance, resulting in a significant reduction in annual maintenance costs. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of this utility model; Figure 2 This is a schematic diagram of the screw body in this utility model; Figure 3 This is a schematic diagram of the screw body in this utility model; The meanings of the labels in the diagram are as follows: 1. Screw body; 2. Feeding section; 21. First screw groove; 3. Melting section; 31. Screw rib structure; 4. Homogenization section; 41. Second screw groove; 5. Surface treatment layer; 6. Connecting structure; 7. Sleeve; 71. Feed port; 72. Discharge port; 73. Connecting parts; 8. Baffle. Detailed Implementation

[0023] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0024] This invention provides a screw structure for the production of PVA water-soluble films, such as... Figures 1-3As shown, the screw body 1 includes a screw body 1, which is arranged axially from front to back as a feeding section 2, a melting section 3, and a homogenizing section 4. The outer wall of the feeding section 2 is provided with a first screw groove 21, and the outer wall of the melting section 3 is provided with a screw ridge structure 31. The screw ridge structure 31 is a variable depth screw groove or a barrier ridge, and the screw groove depth of the melting section 3 gradually decreases. The outer wall of the homogenizing section 4 is provided with a second screw groove 41. A sleeve 7 is coaxially sleeved on the outer side of the screw body 1, and there is a gap between the outer wall of the screw body 1 and the inner wall of the sleeve 7. A heating system is provided on the sleeve 7 for heating the raw materials.

[0025] During use, the PVA raw material undergoes three stages sequentially within the screw body 1: "solid conveying → melting and plasticizing → melt homogenization". The feeding section 2 achieves stable conveying of the raw material through the first screw channel 21. The melting section 3 utilizes the screw ridge structure 31 with varying depth screw channels or barrier ridges, combined with the gradually decreasing screw channel depth, to form a gradient shear force and heat exchange space, driving the solid raw material to gradually melt. The homogenization section 4 performs final mixing of the melt through the second screw channel 41. The outer sleeve 7 provides a closed working environment for the screw body 1. On the one hand, the temperature can be precisely controlled by the heating system of the sleeve 7 (matching the PVA melting range of 160-200℃), providing a heat source for the raw material melting. On the other hand, it avoids raw material / melt leakage, ensuring that the material moves orderly along the axial direction of the screw body 1 without backflow or stagnation. Effects: Achieves orderly conversion of PVA raw materials: The three-section structure of the screw body 1 (feeding section 2, melting section 3, homogenization section 4) precisely matches the changes in the state of PVA raw materials, avoiding the retention or insufficient processing of raw materials in a single screw section, and completely solving the problems of "unmelted particles" and "local scorched material" caused by the single structure of existing screws, ensuring the uniformity of PVA melt; The sealing and heating effect of the sleeve 7 stabilizes the working environment of the screw body 1, improves the conveying efficiency of materials between the first screw groove 21, the screw ridge structure 31, and the second screw groove 41, avoids production interruptions caused by material leakage or temperature fluctuations, and adapts to the production requirements of continuous extrusion of PVA water-soluble film.

[0026] In this embodiment, the screw body 1 is made of high-strength alloy material.

[0027] When in use, the high-strength alloy material has higher high-temperature strength than ordinary steel, and can withstand the high temperature and shear stress in PVA production. This avoids problems such as bending of the screw body 1 and deformation of the screw rib structure 31 on the melting section 3, ensuring the structural stability of the first screw groove 21 and the second screw groove 41, without affecting material conveying and plasticizing. The improved wear resistance of the alloy material reduces frictional wear on the surface of the screw body 1. Compared with ordinary steel screws, the service life of the screw body 1 is increased by more than 50%, reducing the frequency of equipment replacement.

[0028] Specifically, the first threaded groove 21 has a depth of 7-9mm and a pitch of 70-90mm, while the second threaded groove 41 has a depth of 4-6mm.

[0029] In use, the large-sized screw grooves of the feeding section 2 are suitable for solid conveying: the first screw groove 21 of the feeding section 2 has a depth of 7-9mm and a pitch of 70-90mm. The larger screw groove volume can accommodate more PVA solid raw materials, and the large screw pitch shortens the movement time of the raw materials in the axial direction of the screw body 1, increases the conveying speed, and avoids the raw materials from "bridging" and accumulating at the feed port 71 of the sleeve 7. The shallow screw grooves of the homogenization section 4 are suitable for melt homogenization: the second screw groove 41 of the homogenization section 4 has a depth of 4-6mm. The shallower screw grooves allow the melt to be subjected to more frequent shearing and mixing when the screw body 1 rotates, which accelerates the homogenization of melt temperature and viscosity, meets the high requirements of PVA melt for homogenization, and prepares it for subsequent extrusion through the discharge port 72 of the sleeve 7. Results: The large-size design of the first screw channel 21 increases the conveying capacity of the feeding section 2 by more than 40% per unit time, solving the capacity bottleneck caused by the slow feeding of the existing screw, and is suitable for large-scale production. The raw materials can quickly enter the melting section 3 for plasticization. The shallow screw channel design of the second screw channel 41, with its high-frequency shearing action, controls the temperature fluctuation of the PVA melt within ±2℃ and the viscosity deviation is less than 5%. The melt extruded through the discharge port 72 of the sleeve 7 is uniform, and the produced water-soluble film has uniform thickness and stable performance, improving the product qualification rate.

[0030] Furthermore, the outer wall surface of the screw body 1 is provided with a surface treatment layer 5, which is a nitrided layer or a hard chrome plated layer.

[0031] During use, the high hardness and corrosion resistance of the surface treatment layer 5 reduce the wear rate of the outer wall surface of the screw body 1 by more than 70%, preventing the decrease in the depth accuracy of the first screw groove 21 and the second screw groove 41 due to surface corrosion, and ensuring the stability of the plasticizing effect of the melting section 3. The low friction coefficient of the surface treatment layer 5 reduces the adhesion of PVA material on the outer wall surface of the screw body 1 and in the first screw groove 21 and the second screw groove 41, preventing material retention and coking, further improving the purity of the melt, and ensuring the quality of the water-soluble film extruded through the discharge port 72 of the sleeve 7.

[0032] Furthermore, the front end of the screw body 1 is also provided with a connecting structure 6, which is used to connect with the transmission mechanism. The connecting structure 6 is a key connection or a flange connection.

[0033] In use, both keyed and flanged connections ensure that the torque of the transmission mechanism is efficiently transmitted to the screw body 1, preventing fluctuations in the screw body 1's rotational speed due to loose connection structure 6. This ensures a stable conveying and plasticizing rhythm of PVA material between the first screw groove 21, screw ridge structure 31, and second screw groove 41. The precise positioning of the keyed connection and the high coaxiality of the flanged connection reduce radial runout when the screw body 1 rotates, lowering equipment vibration and noise. This also prevents abnormal friction between the first screw groove 21 and screw ridge structure 31 of the screw body 1 and the inner wall of the sleeve 7 caused by vibration, improving the comfort of the production environment.

[0034] Furthermore, a feed inlet 71 is provided at the top front end of the sleeve 7, and a discharge outlet 72 is provided at the rear end of the sleeve 7.

[0035] During use, the precise positioning of the inlet 71 and outlet 72 ensures that the material moves in an orderly manner along the path of "inlet 71 → first screw groove 21 of feed section 2 → screw rib structure 31 of melting section 3 → second screw groove 41 of homogenization section 4 → outlet 72", without any ineffective retention areas. This improves material processing efficiency, avoids secondary retention of the melt in the sleeve 7, reduces melt degradation caused by long-term high temperature, ensures the purity of the PVA melt after being processed by homogenization section 4, and guarantees the solubility and transparency of the water-soluble film extruded from outlet 72.

[0036] Furthermore, the sleeve 7 is fixedly connected to the frame via the connector 73.

[0037] During use, the precise coaxiality of the sleeve 7 and the screw body 1 ensures a uniform gap between them (controlled within 0.1-0.2mm), preventing material leakage due to uneven gaps. This also reduces abnormal friction between the screw thread structure 31 and the edge of the first screw groove 21 of the screw body 1 and the inner wall of the sleeve 7, protecting both the screw body 1 and the sleeve 7. After the sleeve 7 is fixed by the connector 73, the vibration of the screw body 1 during operation is absorbed by the frame, resulting in more stable overall equipment operation. This prevents fluctuations in production parameters such as the screw body 1's rotational speed and the sleeve 7's heating temperature due to sleeve 7 shaking, ensuring stable PVA water-soluble film quality.

[0038] Furthermore, a baffle 8 is installed at one end of the screw body 1 near the connecting structure 6.

[0039] During use, the baffle 8 prevents materials and impurities from entering the area of ​​the connecting structure 6, avoiding corrosion and jamming of the components of the connecting structure 6, ensuring the power transmission efficiency of the key connection or flange connection to the screw body 1, and preventing the screw body 1 from stopping due to the failure of the connecting structure 6, thus causing production interruption; the baffle 8 reduces the adhesion of materials and the accumulation of impurities on the connecting structure 6, so that there is no need to clean the residual materials at the connection between the connecting structure 6 and the screw body 1 during subsequent maintenance, improving maintenance efficiency by more than 30%, reducing operation and maintenance costs, and reducing the frequency of disassembly of the connecting structure 6, protecting the connection accuracy between the screw body 1 and the connecting structure 6.

[0040] When using the screw structure for PVA water-soluble film production of this utility model, first check the coaxiality of the screw body 1 and the sleeve 7 (adjusted by the connector 73 to ensure uniform gap, controlled within 0.1-0.2mm), confirm that the connection structure 6 (key connection or flange connection) is firmly connected to the transmission mechanism, and that the baffle 8 is installed in place; start the heating system on the sleeve 7 to raise the temperature of each section of the sleeve 7 to the preset value: the temperature of the corresponding area of ​​the feeding section 2 is slightly lower (160-170℃, for auxiliary raw material preheating), the temperature of the corresponding area of ​​the melting section 3 is in the middle (180-190℃, core melting zone), and the temperature of the corresponding area of ​​the homogenization section 4 is slightly higher (190-200℃, for auxiliary melt homogenization). After the temperature stabilizes (fluctuation within ±2℃), prepare to feed.

[0041] The pretreated PVA solid raw material (dried, premixed, with moisture removed and mixed with plasticizers and other additives) is fed into the sleeve 7 through the feed port 71. The transmission mechanism drives the screw body 1 to rotate at a preset speed (usually 30-60 r / min) through the connecting structure 6. The first screw groove 21 of the feeding section 2 rotates with the screw, generating axial thrust to quickly convey the PVA solid raw material backward. During the conveying process, the raw material is preheated in the feeding section area of ​​the sleeve 7 and gradually reaches the softening temperature (about 160°C) to prepare for entering the melting section 3. Due to the large pitch design of the first screw groove 21, the raw material conveying efficiency is high and there is no accumulation or retention. The softened PVA raw material enters the melting section 3 and rotates with the screw body 1, entering the screw ridge structure 31 region: if it is a variable depth screw channel, the screw channel depth gradually decreases, the filling rate of the raw material in the screw channel gradually increases, the shear force continuously increases, and with the high temperature (180-190℃) in the melting section region of the sleeve 7, the raw material gradually melts into a melt; if it is a barrier ridge structure, the barrier ridge divides the screw channel into a "solid cavity" and a "melt cavity". The incompletely melted solid particles are blocked in the solid cavity and continue to be sheared and heated until they are completely melted and enter the melt cavity; in this stage, through the synergy of gradient shearing and precise heating, the PVA raw material is completely melted and local overheating degradation is avoided (the temperature is controlled below the PVA thermal degradation temperature of 210℃). After melting, the PVA melt enters the homogenization section 4. The second screw groove 41 of the homogenization section 4 is relatively shallow (4-6mm). When the screw rotates, the melt is sheared and mixed at high frequency in the shallow screw groove. The melt temperature gradually becomes uniform through the temperature adjustment (190-200℃) of the homogenization section area of ​​the sleeve 7, and the temperature fluctuation is controlled within ±2℃. The melt viscosity is reduced to less than 5% through shearing and mixing, eliminating the problem of local component inhomogeneity. After this stage, the PVA melt reaches a homogenized state, which meets the material quality requirements for subsequent water-soluble film extrusion molding. The homogenized PVA melt is pushed to the rear end of the screw body 1 as the screw body 1 rotates, and is output through the discharge port 72 of the sleeve 7, directly entering the subsequent extrusion die. During the melt output process, the closed structure of the sleeve 7 prevents melt leakage, and the position design of the discharge port 72 (directly facing the rear end of the homogenization section 4) ensures that the melt does not have secondary retention, thus ensuring melt purity. The output homogenized melt is finally made into a PVA water-soluble film through die forming, cooling, traction and other processes.

[0042] The foregoing has shown and described the basic principles, main features, and advantages of this utility model. Those skilled in the art should understand that this utility model is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the utility model. Various changes and modifications can be made to this utility model without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed utility model. The scope of protection of this utility model is defined by the appended claims and their equivalents.

Claims

1. A screw structure for producing PVA water-soluble films, characterized in that: The screw body (1) is provided with a feeding section (2), a melting section (3) and a homogenizing section (4) arranged sequentially from front to back along the axial direction. The outer wall of the feeding section (2) is provided with a first screw groove (21), and the outer wall of the melting section (3) is provided with a screw ridge structure (31). The screw ridge structure (31) is a variable depth screw groove or a barrier ridge, and the depth of the screw groove in the melting section (3) gradually decreases. The outer wall of the homogenizing section (4) is provided with a second screw groove (41). A sleeve (7) is coaxially sleeved on the outer side of the screw body (1).

2. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The screw body (1) is made of high-strength alloy material.

3. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The first screw groove (21) has a depth of 7-9 mm and a pitch of 70-90 mm, and the second screw groove (41) has a depth of 4-6 mm.

4. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The outer wall surface of the screw body (1) is provided with a surface treatment layer (5), which is a nitrided layer or a hard chrome plated layer.

5. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The front end of the screw body (1) is also provided with a connecting structure (6), which is used to connect with the transmission mechanism. The connecting structure (6) is a key connection or a flange connection.

6. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The sleeve (7) has a feed inlet (71) at the top front end and a discharge outlet (72) at the rear end.

7. The screw structure for PVA water-soluble film production according to claim 1, characterized in that: The sleeve (7) is fixedly connected to the frame via a connector (73).

8. The screw structure for PVA water-soluble film production according to claim 5, characterized in that: A baffle (8) is installed at one end of the screw body (1) near the connecting structure (6).

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