Blown film production

The performance curve of advanced blown film is more and more complicated. Extruded lines must be able to process more different types of materials and therefore require a large number of mechanical components. This article analyzes the impact of the design of each part in the film blowing machine on the quality of the film.

The performance curve of advanced blown film is more and more complicated. Extruded lines must be able to process more different types of materials and therefore require a large number of mechanical components. This article analyzes the impact of the design of each part in the film blowing machine on the quality of the film.

Nowadays, the ability to quickly provide a variety of films is more important than ever. The dramatic reduction in average production time in recent years is a good example. Moreover, the variety of films continues to increase, and hundreds of films are currently on the market. Therefore, when investing in a new extrusion line, it is impossible to predict what kind of film will be produced on the extrusion line. In other words, the blown film extrusion line components and the automatic system must adopt a modular structure so that the facility is more flexible and easy to retrofit and expand in the future. When considering the design of blown film lines, modular components must be considered, and the ability of the extruder to process various polymers should also be considered. Furthermore, blown film dies equipped with subsequent steps of cooling, film clamping, winding, etc. must be able to produce an unstructured film having a wide range of thickness and width and excellent optical performance.

Extruder - From pellets to polymer melts

The main task of a single extruder is to provide a uniform melt of temperature and material. Moreover, it is also necessary to handle a large number of suitable polymers without replacing the screw and avoiding prolonged cleaning time. In single-layer and triple-layer yarns where polyolefin extrusion is carried out, a grooved feed section is generally used. This method can increase the output at a lower temperature (no change in screw diameter), operate linearly with respect to the number of revolutions, and does not rely on back pressure. Smooth extruders for the feed section are used in a wide range of applications and are therefore often used in processing process polymers. This type of machine can also handle critical materials of various thicknesses, including EVOH, Polyamine 6, PET, and COC. Moreover, the extruder with a smooth feeding section can also increase the degree of homogenization of the melt. As with all extruders, the mixing section should use low shear forces to fully homogenize the melt in order to ensure a lower melt temperature and improve bubble stability. For example, this requirement can be achieved using a grooved transfer mixer (CT) or even a low shear mixer (VLS).

Die Head - Flow from Spiral Flow Groove to Annulus

The blown film die molds the melt output from each extruder around its perimeter to form a multilayer annular flow. Pre-dispensers should guarantee a smaller low temperature range and shorter pause times. At the same time, lower pressure drop and excellent self-cleaning performance should also be ensured. The ideal solution is a binary stacker, where the highest viscosity melt passes through each part and becomes the lowest melt. As a result, the temperature is homogenized. Moreover, with the use of stacked pre-separation, the blown film die design can be relatively compact.

However, the overall thickness of the film may be influenced by the weight in the longitudinal direction, controlled by a curve perpendicular to the longitudinal direction, the layer structure depending on the design of the spiral mandrel and the processing precision of the blown film die. Of particular importance is the design and accuracy of the internal gaps in the merged layers of each layer. Inside the axial spiral mandrel, the mold opening force that may affect the size of the internal gap is much lower than the mold opening force in the radial stacking die or the conical die head. In addition, the combined flow path of the multilayered fluid is shortened in the axially designed spiral mandrel. Due to the above two reasons, the axially designed spiral mandrel bar can improve the accuracy of the layer structure and make the layer thickness distribution more uniform. As the temperature of the PA layer increases, the thermal load of sensitive polymers such as EVOH can be reduced.

Cooling ring - from melt to film bubble

Cooling affects the optical properties and bubble stability of the film, which has a major effect on the overall characteristics of the product. Due to the use of materials such as polyamines, EVOH, and polypropylene, bubble stability is extremely important in blown film production. For example, the dual-die lip cooling ring has a specific structure and is particularly suitable for the above uses. The air flow at the lower slot suctions the bubble into the first lip by a non-contact method, so it has a stabilizing effect. The air flow from the upper slit cools the bubble and stabilizes it again (non-contact) on the lips of the second cooling ring. Due to the reduced bubble movement, blown film processing can be carried out without failure, as well as with low film tolerances and high cooling performance requirements (ie, increased throughput).

Membrane extraction - from bubble to double flat membrane

The purpose of the clip holder and the pull-out device is to form the bubble into a good-quality double-sided flat film. Obviously, different materials require different clamping frames. Therefore, the modular structure is particularly important. Only the use of modularity allows the extraction device to handle a wide variety of materials, from HDPE for shirt bags to complex multi-layer barrier films used in food packaging and the pharmaceutical industry. In order to meet the stringent requirements of film roll quality, it is recommended to use a 360 with a horizontal steering lever. Steering lead-out device. The overall thickness of a thermoformable sheet barrier film may exceed 200μ and the polyamine content may be 30-40. Therefore, the rigidity is large. Especially in the above applications, the longer the clip frame, the better the quality of the film in terms of no wrinkles and flatness. This principle also applies to surface protection, label films and laminates. In addition, the temperature of the film should be evenly controlled during the film clamping process. The aluminum roller has high thermal conductivity and is therefore not suitable for this application. Carbon fiber plastic press rolls have low thermal conductivity, but they also have the problem of insufficient heat dissipation. Therefore, when the process continues, the temperature will become higher and higher, and after a long time, the film will sag easily.

The configuration of the press roller of the brush has many advantages. The surface area of ​​the brush contacting the film is small. In other words, similar to the air cushion, the brush can only absorb a small amount of heat through the bristle tips. The absorbed heat is discharged through a larger surface that is proportional to the brush stem. The larger outer diameter of the brush roller also reduces drag. Another advantage is the softness of the bristles. The lateral movement of the film is small, so that the print can be further reduced or eliminated.

Winder - from film to film roll

In order to ensure that the film leaves the extruding line and maintains a good quality, a tight high-speed winder must be used. The winding is mainly determined by the film produced. The difference is:

- Pressure-sensitive film is contact wound;
- Convenient film roll hardness control using 100-2000N constant coating pressure (CLP) contact roll-up;
- Anti-slip film with central winding and 100-2000N CLP contact winding;
- Anti-slip film, surface-sensitive film and EVA, PP copolymer or film with high content of PP, easy to shrink after shrinkage using gap;
- Sensitive thin films and multi-base products are wound in a permanent center (combined slitting at the beginning of the winding).

Different thicknesses are required for different applications. Therefore, the winder must be modularly designed to be upgraded as needed. In all rewinding systems, high-speed production of multiple films at the same time should be ensured under the premise of automatic roll change.

Automation - Full Process Equipment and Control

For low-cost film production, film thickness curve control is also important in addition to general control functions such as weight, temperature, run length, overlap base width, and web orientation control. In general, the difference between the three major systems is:

- partially affects the melt temperature around the die;
- partially affects the cooling air temperature around the cooling ring;
- Partially affects the air flow around the cooling ring.

Mechanical and automation systems must be modularly designed to be combined with one of the three major systems. Only in this way can the production of various types of films be completed.

In addition to the modular structure of the automation system, the ease of operation of the production line is also important for the low cost production of extruded films. In the case of frequent replacement of products, the operational errors and the complexity of the processes all lead to high costs. If the central control station is configured in the entire system, the above drawbacks can be avoided. The central control station exchanges data with all components on the line through a stable bus system. In this way, the operator can see all related functions at a glance. By using a touch-screen panel and a traditional window interface, operator errors can be avoided, making new employees easy to learn and use.

Product replacement is a very critical operation. Therefore, from a cost perspective, the automation system should support the operator throughout the process. For example, the position of the product can be quickly determined in the integrated database through the adjustable search and classification function. In addition, the existing, existing, and current mechanical settings must also be displayed at the same time.

For continuous quality control, all relevant production data must be stored and stored for several months. With the help of information systems that are integrated and networked with automation systems, data access can be achieved for different users. In this way, you can quickly identify and avoid errors. Moreover, it must also be connected via modem for remote diagnostics in order to quickly diagnose and eliminate potential problems.