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1. Shrinkage phenomenon
The viscoelastic behaviour of the polymer melt when the film is calendered with a T-die head causes
(i) problems such as the shrinkage phenomenon, where the width of the film narrows;
¢Ú the problem of stretchability of the film, which is the phenomenon of material surging or breaking during stretching in air (i.e. in the air gap). The difference between the width of the die head seam and the width of the film is usually defined as shrinkage. The greater the shrinkage, the thicker the edge of the film (the rolled edge) and therefore the yield of the product decreases accordingly with the increase in the edges.
As you know, the shrinkage is related to the surface tension and elastic modulus of the molten film and is therefore caused by the shrinkage of the film. So the greater the tensile force required to shrink the cast film when the film is led out of the roll, the smaller the shrinkage will be on the surface of the quench roller. The degree of film cross-sectional shrinkage is related to the properties of polypropylene and its calendering conditions, such as the temperature of the molten film, the length of the air gap and the width of the die slit. The higher the density or the higher the melt flow index, the greater the shrinkage, provided the calendering conditions are constant. In addition, the shrinkage of molten films with a large off-die expansion ratio is reduced due to the Barus effect. With regard to the flow conditions, the larger the air gap, the wider the die opening, the faster the exit speed and the higher the temperature of the molten film, the greater the shrinkage.
The relationship between air gap and shrinkage. In general, shrinkage and stretchability show roughly opposite tendencies. Melt film cast at high exit speeds will shrink more and become thinner.
2. Curling phenomenon
The mechanism of curl formation: Polymer cast films are produced by melt extruding the polymer through a uniformly thin die slit onto a quench roller. A thick edge, called a rolled edge, is formed between the die and the quench roller. This edge must be cut from the film and either disposed of as scrap or recycled. Curling is often caused by three factors: surface tension, off-die swelling and edge stress effects. Surface tension and release swell can be important factors when the material is at low viscosity and elasticity respectively. The main cause of rolled edges is also the edge stress effect, which occurs when the film is stretched between the die head and the rollers. The edges of the film elongate under uniaxial stress while the material in the centre elongates in plane tension.
The following assumptions were made when modelling the formation of rolled edges:
(i) Transients will be neglected as the curling edge occurs during steady flow, so the analysis is limited to steady flow;
(ii) The edges and flow lines must not move during steady flow.
(iii) Again the flow rate of the fluid coming out of the die head must be equal to the flow rate of the fluid passing through the roll, because the flow lines are steady and the volume flow rate between the two flow lines at the die head must be equal to the flow rate between the two flow lines at the roll as described above.
The relationship between the shrinkage and the edge of the roll, if the film is very wide, then the flow line at the centre of the film will be straight. At the edge, the flow lines are closer together at the rollers than at the die head.
The formation of a rolled edge is due to three factors: release swell, surface tension and edge stress effects.
(1) Swelling of the release film
The viscoelastic fluid increases to a thickness greater than the die lip gap. Elastic stresses accumulate at all die walls in the die head and the edges also accumulate swell from more die walls. Swelling occurs everywhere along the width of the die head. If the swell is 2.1mm in the centre of the film, the edge may be 2.5mm and the centre-edge ratio of the film will be 1.19mm. Since the size of the curl is similar to the effect of high die swell from the centre to the edge, the curl may be less than 2 even in the worst case scenario. This is independent of the width of the die slit and the distance to the roll.
This does not depend on the width of the die slit and the distance to the rollers.
For example, the film thickness is usually less than 1 mm, so it is rare for the thickness of the rolled edge to be more than 5 mm due to off-die expansion. Since the width of the rolled edge is usually a few centimetres, it does not appear to be the result of off-die expansion.
(2) Surface tension
The surface tension on the film must be balanced by the stress on the edge. This stress pushes the material at the edge towards the film, causing the edge to thicken. For most polymer cast films, the surface tension number is at most less than 1, i.e. the surface tension is less than the adhesion force. This means that surface tension will play an important role in films made with very low viscosity fluids at low velocities, but not in most cast films. Therefore, surface tension is almost never applicable in cast films, but is more important in extrusion coating.
(3) Edge stress effects
The surface tension on a film must be balanced by stress at the edges. This stress pushes the material at the edge towards the film, causing the edge to thicken. The stress and strain conditions near the edge are different from those in the middle of the film. Comparing the stress conditions in the centre and at the edges shows that the rolled edge is related to the amount of stress at the edge. Near the centre, the flow lines are straight and parallel. At the edges, there is no stress in the transverse or thickness direction and the extension ratio is the same in those directions. The free edge state results in a variation in thickness between the centre and the edge.
The main reason for curled edges is the edge stress effect. All polymer films and coatings whose thickness is controlled by drafting will have curled edges as a result of this effect. Other factors, such as surface tension and release swell, affect the curl, but the size of the curl is much smaller and the width of the curl is at the same level as the thickness of the film. The width of the curl due to the edge stress effect is about the same as the distance from the die head to the rollers. The edge stress effect occurs between the die head and the rollers. Curling due to surface tension and high die expansion only occurs under special conditions such as low viscosity with elastic fluids.
Nanjing Sumino Precision Machinery Co., Ltd. has professional cast film production lines, the company's main cast film manufacturing lines are: TPU hot melt adhesive film production line, CPP/CPE cast film production line, EVA solar encapsulation adhesive film production line, PE hygiene breathable film production line, PVDF cast film production line, PP three-dimensional film production line, medical blood purification film production line, multi-layer co-extrusion barrier film production line, LLDPE winding film production line, etc.
The viscoelastic behaviour of the polymer melt when the film is calendered with a T-die head causes
(i) problems such as the shrinkage phenomenon, where the width of the film narrows;
¢Ú the problem of stretchability of the film, which is the phenomenon of material surging or breaking during stretching in air (i.e. in the air gap). The difference between the width of the die head seam and the width of the film is usually defined as shrinkage. The greater the shrinkage, the thicker the edge of the film (the rolled edge) and therefore the yield of the product decreases accordingly with the increase in the edges.
As you know, the shrinkage is related to the surface tension and elastic modulus of the molten film and is therefore caused by the shrinkage of the film. So the greater the tensile force required to shrink the cast film when the film is led out of the roll, the smaller the shrinkage will be on the surface of the quench roller. The degree of film cross-sectional shrinkage is related to the properties of polypropylene and its calendering conditions, such as the temperature of the molten film, the length of the air gap and the width of the die slit. The higher the density or the higher the melt flow index, the greater the shrinkage, provided the calendering conditions are constant. In addition, the shrinkage of molten films with a large off-die expansion ratio is reduced due to the Barus effect. With regard to the flow conditions, the larger the air gap, the wider the die opening, the faster the exit speed and the higher the temperature of the molten film, the greater the shrinkage.
The relationship between air gap and shrinkage. In general, shrinkage and stretchability show roughly opposite tendencies. Melt film cast at high exit speeds will shrink more and become thinner.
2. Curling phenomenon
The mechanism of curl formation: Polymer cast films are produced by melt extruding the polymer through a uniformly thin die slit onto a quench roller. A thick edge, called a rolled edge, is formed between the die and the quench roller. This edge must be cut from the film and either disposed of as scrap or recycled. Curling is often caused by three factors: surface tension, off-die swelling and edge stress effects. Surface tension and release swell can be important factors when the material is at low viscosity and elasticity respectively. The main cause of rolled edges is also the edge stress effect, which occurs when the film is stretched between the die head and the rollers. The edges of the film elongate under uniaxial stress while the material in the centre elongates in plane tension.
The following assumptions were made when modelling the formation of rolled edges:
(i) Transients will be neglected as the curling edge occurs during steady flow, so the analysis is limited to steady flow;
(ii) The edges and flow lines must not move during steady flow.
(iii) Again the flow rate of the fluid coming out of the die head must be equal to the flow rate of the fluid passing through the roll, because the flow lines are steady and the volume flow rate between the two flow lines at the die head must be equal to the flow rate between the two flow lines at the roll as described above.
The relationship between the shrinkage and the edge of the roll, if the film is very wide, then the flow line at the centre of the film will be straight. At the edge, the flow lines are closer together at the rollers than at the die head.
The formation of a rolled edge is due to three factors: release swell, surface tension and edge stress effects.
(1) Swelling of the release film
The viscoelastic fluid increases to a thickness greater than the die lip gap. Elastic stresses accumulate at all die walls in the die head and the edges also accumulate swell from more die walls. Swelling occurs everywhere along the width of the die head. If the swell is 2.1mm in the centre of the film, the edge may be 2.5mm and the centre-edge ratio of the film will be 1.19mm. Since the size of the curl is similar to the effect of high die swell from the centre to the edge, the curl may be less than 2 even in the worst case scenario. This is independent of the width of the die slit and the distance to the roll.
This does not depend on the width of the die slit and the distance to the rollers.
For example, the film thickness is usually less than 1 mm, so it is rare for the thickness of the rolled edge to be more than 5 mm due to off-die expansion. Since the width of the rolled edge is usually a few centimetres, it does not appear to be the result of off-die expansion.
(2) Surface tension
The surface tension on the film must be balanced by the stress on the edge. This stress pushes the material at the edge towards the film, causing the edge to thicken. For most polymer cast films, the surface tension number is at most less than 1, i.e. the surface tension is less than the adhesion force. This means that surface tension will play an important role in films made with very low viscosity fluids at low velocities, but not in most cast films. Therefore, surface tension is almost never applicable in cast films, but is more important in extrusion coating.
(3) Edge stress effects
The surface tension on a film must be balanced by stress at the edges. This stress pushes the material at the edge towards the film, causing the edge to thicken. The stress and strain conditions near the edge are different from those in the middle of the film. Comparing the stress conditions in the centre and at the edges shows that the rolled edge is related to the amount of stress at the edge. Near the centre, the flow lines are straight and parallel. At the edges, there is no stress in the transverse or thickness direction and the extension ratio is the same in those directions. The free edge state results in a variation in thickness between the centre and the edge.
The main reason for curled edges is the edge stress effect. All polymer films and coatings whose thickness is controlled by drafting will have curled edges as a result of this effect. Other factors, such as surface tension and release swell, affect the curl, but the size of the curl is much smaller and the width of the curl is at the same level as the thickness of the film. The width of the curl due to the edge stress effect is about the same as the distance from the die head to the rollers. The edge stress effect occurs between the die head and the rollers. Curling due to surface tension and high die expansion only occurs under special conditions such as low viscosity with elastic fluids.
Nanjing Sumino Precision Machinery Co., Ltd. has professional cast film production lines, the company's main cast film manufacturing lines are: TPU hot melt adhesive film production line, CPP/CPE cast film production line, EVA solar encapsulation adhesive film production line, PE hygiene breathable film production line, PVDF cast film production line, PP three-dimensional film production line, medical blood purification film production line, multi-layer co-extrusion barrier film production line, LLDPE winding film production line, etc.