The blowing film machine generally adopts a single screw extruder, from the process, the main equipment of film blowing molding is an extruder, head, cooling wind ring, traction, and takeup.
1. Extruder process points
Generally use a single screw extruder, screw diameter with a diameter of 45-120mm, the size of the diameter is determined by the film thickness and folding size. Output is affected by cooling and traction at two speeds, thin film with a small extruder, and thin film thick and wide with a large extruder.
The basic structure of the extruder includes a transmission device, charging device, barrel, screw, head, and die parts. The key to the extruder is the screw structure and length-diameter ratio of the screw.
The screw structure has a gradual change screw, mutation screw, and screw with a mixing diagram. For PE these three kinds of screws are applicable, with a mixing diagram screw effect is better. Screw length-diameter ratio, due to the limitation of mechanical processing in the past, the screw length-diameter ratio is short, it is affected by plastic plastination, one is not high yield, two is not good quality, now the length-diameter ratio develops to more than 30:1, length-diameter ratio, for product production, high yield, good quality, the length-diameter ratio should be more than 25 is better.
Screw heat treatment is good for long service life, preferably 38CrMnAI, after nitriding treatment. The capacity of the extruder is proportional to the diameter of the screw the capacity of the extruder is proportional to the speed of the extruder the capacity of the extruder is inversely proportional to the clearance of the barrel and the screw, and the clearance should be below 0.25mm.
2. Blow film head type
The head and mouth die are mainly used to blow the film of the head type of the steering type and the horizontal direction of the straight-through type two categories. Rectangular type is divided into mandrel type, spiral mandrel type, lotus petal type, rotary type, and so on. Straight type is divided into horizontal type and right Angle type, which is especially suitable for large melt viscosity and heat sensitive plastics.
(1) Advantages and disadvantages of a mandrel type head
The advantages of mandrel heads are less memory material, not easy to decompose, suitable for processing PVC, simple structure, easy manufacturing, convenient operation, and only one seam line.
Disadvantages: the mandrel is easy to produce bias so that the material flow at the right corner is slow, easy to produce uneven film thickness.
(2) Advantages and disadvantages of spiral mandrel head
Advantages of spiral mandrel head: good mechanical strength, stability, not easy to tilt, thin film thickness uniform; Disadvantages: large volume, unreasonable design, resulting in film suture line, easy to reduce the mechanical strength of the film.
(3) Advantages and disadvantages of the lotus flap type head
Lotus petal head advantages: simple structure, convenient processing, low cost, easy to operate and clean; Disadvantages: suture line, easy to reduce the strength of products.
(4) Advantages and disadvantages of the central feeding head
Advantages of center feeding head: film thickness is uniform, not easy to produce the skew phenomenon, suitable for processing PE, PP, PA; Disadvantages: organ memory material, suture line, the operation is not convenient.
(5) Advantages and disadvantages of the rotating head
Advantages of rotating head: the film thickness is uniform, not easy to produce the biased phenomenon, can make local parts of the film not exceed the standard, dispersed on the shaft roll so that the curled film is smooth, easy to print, high quality; Disadvantages: The structure is more complex, the cost is higher.
3. Blow-expansion ratio of the film
The blowing ratio is generally controlled within a certain range, the blowing ratio is too small, the product’s vertical and horizontal strength is not uniform, the blowing ratio is too large, difficult to operate, product thickness uniformity is difficult to control for LDPE, generally controlled between 1.5-2.5 is appropriate for HDPE, generally controlled between 3-5 is appropriate for PP film, control between 1.5-2.5 is appropriate for PA film, Control between 1.5-2.5 is appropriate for PVC film, control between 1.5-3 is appropriate.
4. Cooling device of blowing film machine
The cooling device in the film production process, the cooling of the bubble tube is very important, from the mouth due to the traction roll of only ten seconds, in this period, the bubble tube should reach a degree of actual cooling, otherwise the hot material by the traction roll compression easy to stick, there are many kinds of wind ring, ordinary wind ring, negative pressure wind ring, etc.
(1) General wind ring
One outlet air ring is the most common air ring, easy to manufacture, has a simple structure, low cost.
(2) negative pressure wind ring
The structure of negative pressure air is more complex than that of an ordinary air ring, but the cooling effect is good and the bubble tube is stable (with double tuyere).
(3) Cooling wind ring
The distance between the cooling air ring and the die is 30-100mm, and it is also attached to the die head. The inner diameter of the wind ring is generally 1.5-3 times the diameter of the head, and the gap width of the outlet is 1-4mm.
(4) Other cooling methods
The cooling method varies with the process, including water cooling, internal cooling, and so on.
5. Requirements for traction device of blowing film machine
The traction device is to press the film woven on the hermetic plate and sends it to the coiler to prevent air leakage in the bubble tube and ensure the stable shape and size of the bubble tube.
(1) Traction installation
The center of the press roll should be aligned with the center of the miter plate and the center of the head, otherwise, it will cause the film to be the distance between the traction roller, and the wrinkling phenomenon.
(2) The distance between the die and the traction roll
The distance between the die and the traction roll should be at least 3-5 times the diameter of the bubble tube. Otherwise, the film cannot be cooled and will adhere. At the same time, because the membrane tube changes from the circle to the level, the distance between the bubble tube at different points around the circle and the traction roll are different, and the film is easy to wrinkle and deformation after flattening.
(3) The requirements of the sandwich stick
One side of the clamping roller should be a steel roller, the other side should be a rubber roller, rubber hardness of the rubber roller should be controlled at 50-60 degrees if appropriate. Too hard, such as clamping roller deformation, due to the pressure is not tight, easy to leak, so that the film will produce uneven width.
(4) clamping roller spring adjustment
The spring on both sides of the roller should be able to adjust freely, otherwise, the roller will be tight, and loose, and the film will be pulled out of shape, and easy to wrinkle.
6. The role of the herringbone plate of the film blower
The hermetic plate is used to stabilize the bubble tube and guide the bubble tube into a flat film leading into traction. The size of the Angle of the hearing word plate plays an important role in the smoothness of the winding film.
Flat blowing is generally controlled at 30°, up and down blowing is generally large to control within 50°, and the Angle is easy to operate, but it will cause the film to wrinkle and there is a ruffled edge.
7. The role of the winding device in blowing film machine
The function of the winding device is to wind the product flat, neatly on both sides, and with moderate tightness onto the scroll, so it is required that the winding device can provide reliable step-less speed regulation of winding speed and moderate tightness tension.
The winding has a center (active) and surface winding (passive) whether the center or surface winding is required to achieve static balance so that the balance of volume.