Blown Film Production Equipment Introduction

 

The molding equipment for extrusion blow molding film is mainly composed of extruder, machine head, cooling device, traction device, and coiling device, etc.

 

 

Blow-molded film generally uses a single-screw extruder, the screw diameter is mostly 45 ~ 150mm, and the length-to-diameter ratio is usually

20 ~30, but the length-to-diameter ratio of the extruder for extruding PVC film should not be too large, usually 20. In order to improve the compounding efficiency

rate, sometimes a mixing device is added to the head of the screw, and the length-to-diameter ratio of the screw should be larger, taking more than 25.

To produce blow molded film, an extruder suitable for the specifications should generally be selected according to the folding diameter and thickness of the film

Good economic benefits will be obtained. For example, the production of thin and narrow plastic films with large extruders is not easy to achieve

cooling under fast traction; Conversely, using a small extruder to produce thick and wide films will cause plastic melts

If the time is too long at high temperature, it will have a great impact on the quality of the film, and the productivity will not meet the requirements

One extruder is only suitable for extruding a few sizes of products. Table 1 lists the extruder specifications and film rulers

The relationship between inches. The relationship between the screw diameter of the extruder and the diameter of the head die of the film blowing machine is shown in Table 2.

 

Table 1 Relationship between extruder specification and film size

Screw diameter/mm	Film folding diameter/mm	Screw diameter/mm	Film folding diameter/mm
30 45 65 90	50~300 100~500 450~900 700~1200	120 150 200	<2000 <3000 <4000

 

Table 2 Relationship between screw diameter and die diameter of film blowing machine

Screw diameter/mm	45	50	65	90	120	150
Mouth die diameter/mm	<100	75~120	100~150	150~200	200~300	300~500

In addition, the selection of the extruder should also consider the physical properties of the processed material, mainly the selection of the extruder screw configuration. For example, when processing heat-sensitive PVC plastics, avoid the material staying in the barrel for too long.

 

In order to avoid the accumulation of material between the screw head and the perforated plate, the screw head should be designed with a pointed point, and the screw should not choose a barrier type, so as not to cause the material to decompose due to excessive shear force. For polyolefin materials, high-efficiency spirals can be used The rod improves quality and yield without decomposition problems.

 

 

1. Machine head structure

The machine head for blowing film (referred to as the blown film head) has a variety of structural forms, and the commonly used side feed mandrel is the one that is commonly used Type machine head, cross type machine head with central feeding, spiral machine head, rotary machine head, co-extrusion compound machine head, etc.

(1) Mandrel type machine head (side feed) The structure of the mandrel type film blowing machine head is shown in Figure 1, plastic melt
After being compressed by the neck, the flow to the mandrel is divided into two streams, and after flowing 180° along the mandrel to both sides, it is at A
Reconvergence. The combined material flow wraps the mandrel and flows along the circular channel of the machine head to the mold mouth and is squeezed in the form of a thin tube blank Exit, which is blown into a film by compressed air. This kind of machine head has a simple structure, less material in the runner, and only one material flow is assembled Thread, plastic is not prone to overheat decomposition, suitable for blow molding of heat-sensitive plastic films such as PVC. The disadvantages are:

1) The uneven thickness of the film is easily caused by the unequal flow velocity in the machine head and the joint part of the material flow.

2) The mandrel is prone to the phenomenon of bias (the mandrel is not concentric from the mouth mold).

3) The mouth die gap is not easy to control. If the gap is too large, it is necessary to achieve the set film thickness and folding diameter

It is necessary to increase the drafting ratio and blowing ratio, which causes operational difficulties; If the gap is too small, the internal reflection pressure in the machine head is large, resulting in output

Lower. The general gap is 0. 4 ～1. 2mm.

 

(2) Cross type head (center feed type) The structure of the cross head is divided into horizontal type and right angle type

as shown in Figures 2 and 3. The two structural molding parts are basically the same, but the feeding method is different. level

The type is used for flat extrusion and flat blowing, and the right-angle type is used for flat extrusion up blowing method or down blowing method.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The characteristics of the cross type machine head are that the pipe billet is uniformly discharged when extruded from the mouth die, and the film thickness is easy to control, mold The core is not subject to side pressure, and there will be no "center" phenomenon. However, due to the presence of shunt brackets, the film is patched together There are many lines, the gap in the machine head is large, there is a lot of stock, and it is not suitable for processing plastics with poor thermal stability, which are often used in PP, PE,Blow molding of films such as PA.
 

 

(3) Spiral head As shown in Figure 4, the spiral head opens 3 ~ 8 threaded grooves on the mandrel, the melt enters from the bottom center, and rotates and rises along the spiral groove, enters the circular gap runner, and after the melt eliminates the weld marks in the annular buffer groove, it enters the forming area to squeeze into a film blank, and is immediately blown into a film by compressed air. The main advantages of this machine head are uniform discharge, no stitching on the film, and easy to control the thickness. However, due to the long stay time of the material in the machine head, heat-sensitive plastics cannot be processed, and are often used to process PP, PE and other plastics with low melt viscosity and not easy to decompose.

 

(4) Rotating machine head Any machine head that can be rotated by core die or mouth die is usually called rotary machine head. The rotating machine head can effectively overcome the influence of the welding line on the quality of the film, and can make the melt stay time in the runner nearly consistent, so as to ensure the temperature of the extruded material and the uniformity of the film blank. Therefore, rotary heads are mostly used in the production of high-performance blow molded films. For example, the thickness tolerance of PP film produced by rotating machine head can reach 0. 1μm. The rotation mode of the machine head is that the mouth die rotates, and the mandrel does not rotate; The mandrel rotates, and the mouth die does not rotate; The mouth die and mandrel rotate together in the same direction or in the opposite direction. Commonly used rotary heads include mandrel type, spiral type and cross-shaped rotary head.

 

1)Mandrel rotary head. Fig. 5 shows an internally rotating (mandrel rotating) mandrel rotating head with agitators 2 and 10 provided on the mandrel 11. The agitator can be a stirring wing or a stirring rod, which can be flat or propeller-type. The agitator is driven by a DC motor 14 by a coupling 13.

 

2)Spiral rotary head. The typical structure of a spiral rotary head is shown in Figure 6. The alignment of the head body 8 is ensured by a pressing sleeve 1 inserted into the wear-resistant pad sleeve 15. The large nut 3 applies pressure to the inner surface of the wear-resistant pad sleeve 15 through the bearing to prevent the melt from overflowing from the runner. The torque of the motor 2 is transmitted to the head housing through gear 5, and the head body 8 can rotate 270° ~ 360°. After the plastic melt from the extruder enters the center of the machine head, it flows into the distribution channel of the spiral body 6 through the radial runner, and after uniform mixing, it is distributed along the circumference of the molding gap. At present, this type of machine head is widely used for the production of tubular film with a width of 200 ~ 6000mm.

 

3)Cross-shaped rotary head. The typical structure of the cross-shaped rotary head is shown in Fig. 7, which is mainly composed of adjustment screw 3, mouth die 4, core die 5, head housing 7 and core die bracket 6. The head housing 7 is driven by the transmission device 10 through the gear 11. This type of machine head is mostly used for the production of narrow films with a folding diameter of less than 1000mm, and the thickness tolerance can reach ±5μm. If other factors are not considered, the rotation speed of the rotating part should make the volume of material flowing along the circumference of the rotating part in a unit time larger than the volume supplied from the screw extruder.

 

 

(5) Co-extrusion compound head Co-extrusion
The composite head can form multiple layers of film, each layer
It can be different colors or different resins, and more than two extruders are added to form multi-color or multi-layer composites
Combine the film. The co-extrusion blow molding compound method is widely used in agricultural film, industrial film and barrier packaging film, and it can be rooted.

It is designed as a multi-layer compound of different resins according to functional needs
Structure. For example, the anti-fog drop agricultural shed film is one layer containing anti-fog drops
Polyethylene, one layer is polyethylene containing anti-aging agents;
The middle layer of food packaging film is PVDC, which has very good barrier properties.
The symmetrical outward order is the adhesive layer resin and the outermost layer of polyethylene
The resin, PVDC in the middle acts as a good barrier, and the outer layer
Thepolyethylene makes bag making and heat closing easy.
The composite head has two forms: in-mold compounding and out-of-mold lamination.
Figure 8 shows the two plastic co-extrusion composite heads in the mold. Two types of plastic
The melt enters from the two inlet ports A and B respectively, and passes through the machine head
The self-shaped annular runner converges and extrudes in the mouth mold forming section. Figure 9
It is shown as an out-of-mold co-extrusion composite head. The molten resin is completely independent of each other
The flow channel flows through the mouth mold, and only gathers into one after leaving the mouth mold
Rise. In order to increase the composite adhesion, it can be used on the two membranes after leaving the mouth mold
Surfactant gas is introduced between the blanks. The co-extruded membrane of this structure is only
The outer material flow is adjusted.
The key to blow molding multi-layer film lies in the machine head, and one of the main problems in its design is to control the proportion of flow resistance in the machine head, which generally requires the linear velocity of each layer of film to be equal.
Another important problem is the bonding between the layers, the key is also temperature control, often the thickness of each layer is the same
Temperature and extrusion speed are sensitive. When designing the temperature control system of the machine head, it should be designed according to the requirements of high-temperature plastics.
and easy to adjust.
 

 

2. Main parameters of the machine head

No matter which structural form of the film blowing machine head is designed, the blowing ratio, the tensile ratio, and the width of the mouth mold gap must be considered
degrees and other structural parameters.
(1) Blow-up ratio The blow-up ratio refers to the ratio of the diameter of the tube bubble after blowing to the diameter of the nose mold. This is blowing
An important process parameter of plastic film is generally 1. 5 ～3. 0, for ultra-thin films, up to 6. Blowing up
The ratio is large, the transverse strength of the film is high, but the blowing ratio is too large, which can easily cause uneven film thickness, unstable tubular vesicles, and thinness
The membrane is prone to wrinkles and other adverse phenomena. During the production process, the compressed air must remain stable and constant
The inflating ratio.
(2) Tensile ratio The tensile ratio, also known as the traction ratio, refers to the ratio of the traction speed to the extrusion speed. Towing speed
It refers to the linear speed of the surface of the traction roller, and the extrusion speed refers to the linear speed of the melt leaving the mouth die. The tensile ratio increases,
The longitudinal strength of the film is thus increased. However, the tensile ratio should not be too large, otherwise it is difficult to control the thickness uniformity, even
It is possible to tear the film. The general stretch ratio is 4~6.
(3) Compression ratio The compression ratio refers to the cross-sectional area of the runner inside the neck and the cross-sectional area of the annular runner in the mold shaping area
The ratio should generally be greater than or equal to 2.
(4) Mouth die gap width is the unilateral clearance between the mouth die and the mandrel δ (see Fig. 1), generally 0. 4 ～1. 2mm, can also be selected according to the film thickness of 18 ~ 30 times. The gap width of the mouth die is too small, the material flow resistance is large, and the shadow is shadowed
Extrusion output; If it is too large, if you want to get a thinner film, you must increase the blowing ratio and the tensile ratio, but
If the blowing ratio and tensile ratio are too large, the film is unstable during production, easy to wrinkle and break, and the thickness is difficult to control.
Therefore, the gap width of the mouth mold is generally controlled at 0. 8 ～1. 0mm, and under special circumstances, it is greater than 1. 0mm, if used
The gap width of the mouth mold when LLDPE blow molded film is greater than 1. 2mm.
(5) The length of the mold and the mold shape In order to eliminate the weld seam, stabilize the material pressure, and the material can be evenly extruded, the length of the mold and the mold forming part L1 (see Figure 1) is usually the width of the mouth mold gap δ
(see Table 3). However, the material flow channel should not be too short, and normally, the material from the diversion is joined
The vertical distance from the point to the mouth die should not be less than twice the diameter of the mandrel at the shunt.
Table 3 Relationship between the length of the shaped section L1 and the gap width of the mouth mold δ

Plastic varieties	PVC	PE	PP	PA
L1	(16~30)δ	(25 ～40)δ	(25 ～40)δ	(15 ～20)δ

(6) Buffer groove size The buffer groove, also known as the storage tank, is usually opened at the entrance of the core mold shaping area, which eliminates a lot
The welding traces generated when the strands of melt converge are conducive to improving the uniformity of the flow of the film blank and improving the mechanics of the film
Performance. The cross-section of the groove is usually bowed, and the chord length (along the mandrel axis) is the width of the groove, which is (15 ~ 30) δ, chord
The height (along the radial direction of the core mold) is the groove depth, which is (4 ~ 8)δ.
(7) Runner expansion angle and diversion line bevel The plastic melt transitions from the runner to the forming section and shapes the core mold
The inverted cone angle α (see Fig. 1) is called the runner expansion angle, which is usually 80° ~100°, but the maximum is not more than 120°.
The value of the bevel β of the mandrel shaft splitter line (see Fig. 1) is related to the fluidity of the plastic, and should not be too small, no
It will make the discharging at the tip of the mandrel slow, forming superheated stagnant material decomposition, generally β = 40° ~60°.

 

 

The temperature of the membrane tube extruded from the machine head is high (above 160°C), it is in a semi-flowing state, and the diameter becomes larger after expansion.
Requires immediate cooling setting. Cooling efficiency directly affects the production capacity of extrusion molding and the optical properties of the film,
 

 

If the cooling is insufficient, the membrane tube is unstable, and the film is made
It is difficult to uniformly thicken and fold diameter, traction, and rolling
The film is easy to stick when taken.
The commonly used cooling devices for blown film are:
Wind ring, water ring, double air outlet pressure relief air ring,
internal cooling device, etc. 1. Cooling air ring
The cooling air ring is the main blow molding film
cooling system, up-blowing method, flat blowing method,
The downblowing method can be used to blow on various resins
Membranes can be used. The structure of ordinary wind rings is such as:
Figure 10.
The position of the wind ring is generally 30 ~ away from the nose
100mm, choose a large value when the film diameter increases.
The inner diameter of the wind ring is larger than that of the mouth die
150 ~300mm, small diameter choose small value, large
The caliber is large.
The function of the wind ring is to evenly, quantify, stabilize the pressure and speed of the compressed air from the fan along the circumference of the film
blow to the tube bubble in a certain direction. The air ring has at least three air inlets, and compressed air is blown in along the tangent direction of the air ring.
Several layers of baffles are set in the wind ring to buffer and stabilize the pressure, so that the incoming air flow blows at a uniform speed
Tubular vesicles. The gap of the air outlet is generally 1~4mm, which is adjusted by bolts to control the air output.
The angle between the air outlet and the pipe film extrusion plane (generally called the blowout angle) is 45° ~ 60°, so that the airflow can be
The bubble tube is held up, the film is easy to operate, and the bubble tube is stable. If the angle is too small, the bubble tube shakes seriously and the impact is thin
film thickness uniformity; If the angle is too large, it will affect the cooling effect of the film.
The air outlet should be determined according to the speed of the production line. For example, when the line speed of PVC film is 5m/min, it should
Equipped with a fan with an air volume of 5 ~ 10m3/min.
The cooling effect of ordinary air rings is relatively poor, and if the traction rate of the tube 

bubble is faster, two ordinary winds can be used

rings in series, while cooling the film.

 

2. Cooling water ring

In the flat extrusion downblow film production line, the melt is just separated
When opening the mold, it is first cooled with a wind ring to stabilize the tube bubbles, and then
Immediately cool with a water ring to obtain a thin layer with high transparency
Membrane. Figure 11 shows the production of PP equal knots by the downblowing method
Cooling water ring structure of crystalline plastic film. It is the inner diameter with
The jacket with the outer diameter of the membrane tube is matched, and the jacket is connected to the cooling water.
Cooling water overflows from the annular hole in the upper part of the jacket, along the water ring
It flows down between the wall and the outer surface of the film. film surface
The water droplets are removed by the adsorption of the wrapping guide roller.

 

3. Double air outlet pressure relief air ring 

The double air outlet pressure relief air ring is a kind of negative pressure air ring, and its working principle is the source
The principle is shown in Figure 12. It has two air outlets, each consisting of two
The blower is supplied with air separately, and the size of the air outlet can be adjusted. In the wind ring
partitions, which are divided into upper and lower air chambers; It is located between the upper and lower wind chambers
A decompression chamber is set up. The main structural parameters of the double air outlet pressure relief air ring
Including the inner diameter of the air ring and the blowing angle of the air outlet. In order to make the wind ring generate
Sufficient negative pressure is convenient for the operation of the film when driving, and the downdraft is recommended
The diameter D is 100mm larger than the diameter of the mouth die, and the diameter D of the upwind outlet is on the diameter D
According to the blowing ratio of the film, it is generally taken (1. 1 ～2. 0) Under D.
when the inflation is relatively high, take the upper limit; Conversely, take the lower limit. Upwind outlet
The blow angle is 60° ~ 70°, and the blow angle of the downdraft outlet
30° ~40°.
The double air outlet pressure relief air ring has the following advantages:
1) The "negative pressure effect" is used to increase the expansion degree of the tube bubble in the wind ring and increase the heat transfer area of the film expansion. composed
The early expansion of the tube vesicle reduces the thickness of the melt film, so that
The heat transfer effect is enhanced, thereby reducing the cooling of the tubular vesicles
Wire, which increases the rigidity and stability of the tubular vesicles.
2) Through the "negative pressure effect", the cooling air is accelerated
The flow of qi tends to be the most along the tube vesicles
improve the heat transfer effect.

 

4. Internal cooling device

Fig. 13 shows the heat exchanger type air in the tube bubble
Internal cooling device, a cylindrical type is installed on the nose mandrel
The heat exchanger has an air inlet door at the top and is equipped with electric air
Fan. The lower end is a ring air outlet, and the electric fan is turned on.
The air circulates in the membrane tube and flows through the heat exchanger
Cooling. The cooling medium for heat exchange is usually room temperature water or
The cooled cold water passes through the casing of the mandrel of the nose
Entry and discharge.

 

 

 

The function of the herringbone splint and guide roller is to stabilize the bubble tube and gradually flatten the cylindrical film into the traction load
Place. Herringbone cleats can be wooden boards, fiberboards, and metal plates. If it is a metal plate, the plate is cooled by water to the film
Better cooling. The angle of the herringbone splint can be adjusted, generally 15° ~ 45°, and the angle of the flat blowing method is better
small, usually 30°; The angle of the upper blowing method or the lower blowing method is larger, which can be about 50°. When the angle is large, the bubble exercise is induced
It is more convenient, but the angle is too large to cause film wrinkles. The angle of the herringbone plate, the length of the traction roller, and the diameter of the film are calculated
The relationship between the two is shown in Table 4.

Table 4 Relationship between the angle of the herringbone plate, the length of the traction roller and the folding diameter of the film

Traction roll length/mm	400	800	1100	1700	2200
Maximum film formation diameter/mm Herringbone board length/mm Calculate the diameter of the membrane tube/mm Calculate the angle of the herringbone board	300 500 190 18°	700 1000 446 25°	1000 1500 640 25°	1500 1700 958 30°	2000 2200 1280 35°

 

 When the diameter of the membrane tube is greater than 2m, the guide roller can be used instead of the herringbone plywood. The guide roller is about 50mm in diameter
The metal roller has a chrome plated surface of the guide roller, which reduces friction with the film. The guide rollers gradually decrease the spacing to make it thinner
Membrane flattening.

 

 

The traction roller is a pair of metal rollers covered with rubber, the diameter of the roller is usually 100 ~ 200mm, and the current traction roller length below 1700mm mostly uses rollers with a diameter of 150mm. The center of the contact line between the traction rollers
It should be aligned with the center of the herringbone plate and the center of the machine head to ensure that the membrane tube is stable and not skewed, otherwise it will cause the membrane tube to be around it
The difference between the point and the traction roller increases and causes wrinkles. It is best to pass cooling water in the traction roller to prevent the film from sticking.
The traction roller leads the film out of the machine head and flattens it, and changes the direction of the film into the winding device, and at the same time presses the film tightly.
Prevent compressed air from escaping in the bubble tube to ensure the stability of the shape and size of the bubble tube.
The traction roller should have a large speed regulation range, and the maximum speed should be slightly higher than that of the entire film blowing unit to reach the maximum production
The highest traction rate required when the production capacity is required, and the lowest rate should be convenient for the extraction operation. At present, the blown film produced in our country
The traction rate of auxiliary aircraft is mostly 2 ~ 20m/min. The maximum traction rate of some high-speed film blowing units abroad has been reached
60m/min, or even higher.
The center height of the traction roller (refers to the distance from the center of the traction roller to the extruder foundation plane) is the determination of the entire auxiliary machine
One of the main factors to ensure that the blow molded film is fully cooled, and the size is too small not only the film is not sufficiently cooled, but also will
This causes the adhesion of the film layer, and the distance difference between the points on the circumference of the membrane tube from the outlet of the machine head to the traction roller increases
large, the film is prone to wrinkles when flattened; The size is too large, the auxiliary machine is large and bulky, inconvenient to operate, and also increases
The height of the plant has been increased and the investment has been increased. At present, the current single-screw extruder in our country is using ordinary air rings
Under the condition of cooling, the center height of the traction roller is basically 5 ~ 7 times the diameter of the membrane tube, and the maximum is 8 ~ 9 times. Membrane tube
The multiple of the small diameter is high, and the multiple of the large diameter of the membrane tube is low.
 

 

After the film comes out of the traction roller, it passes
guide the roller into the winding device. Film rolls
Take the quality of the quality for future cutting and printing
brushing, etc. have a great impact. When winding, the film should be
Flat without wrinkles, the curled edge should be in a straight line
on, the tightness of the film on the winding shaft
should be consistent. Therefore, the winding device should be able to lift
Provide infinitely adjustable winding speed and tightness
Moderate tension. The winding device has a surface roll
Take and reel the center as shown in Figure 14.

 

1. Surface winding device

The surface winding device is shown in Fig. 14a. The motor transmits power and speed to the drive roller by the conveyor belt,
The winding roller is in contact with the driving roller, and the friction between the two drives the winding roller to roll the film on the winding roller.
This kind of winding device can keep synchronized with the traction rate, the structure is simple, the winding shaft is not easy to bend, but it is easy to damage the thin
Film, suitable for coiling thick films and wide films that are difficult to achieve center coiling.

 

2. Center winding device

The central winding device, also known as the active winding device, as shown in Figure 14b, the driving device directly drives the winding
Roll. This device is widely used at present, and the change of film thickness has little effect on coiling. During the winding process, due to the continuous change of winding diameter, in order to maintain a constant winding line speed and tension, the rotational speed of the winding shaft is used
It should become smaller with the increase of the diameter of the film coil, and the power of the torque motor as the winding shaft can meet this demand
needs. The simplest way is to use the friction clutch to adjust the speed of the coiling roller so that it follows the diameter of the film roll
Increase and decrease.
In modern industrial production, the maximum film roll diameter can reach 1500mm and the maximum width can reach 3200mm.