Deep understanding of blown film technology: diagnosing and solving film quality problems for blown film.

Deep understanding of blown film technology: diagnosing and solving film quality problems for blown film.

This article focuses on film quality issues. Some of these problems can be observed intuitively, and some require instrumental detection. As for checking appearance of film, best method is to keep a sample of film and record it when a problem is found, to enable people to learn and practice later. For example, a company's quality control department must maintain a log book that contains various patterns of defective film such as crystallization points, runner lines, and melt breaks. For film performance test, standard record of product performance test can be done well, and subsequent random test results only need to be compared with standard value.

1. Melt failure

Melt failure means appearance of roughness on surface of film. Some image titles like orange skinned shark skin. Melt cracks can also be wavy lines on film. According to most studies, source of melt failure is that melt is subjected to excessive shear stress as it passes through die. Thus, reducing shear stress can reduce or eliminate melt failure defects.

Shear stress is product of melt viscosity and shear rate. Therefore, any method of reducing viscosity and shear rate during processing leads to destruction of melt. The simplest is to raise temperature of melt, thereby reducing viscosity of melt, and then shear stress on melt flowing through die will decrease. Generally speaking, raising die temperature is best solution. Another solution is to add lubricant to formula. The mechanism of action of lubricant mainly consists of two aspects, one of which is to reduce internal flow resistance of polymer, that is, to reduce viscosity; adhesion to road.

There are two ways to prevent melt breakdown by reducing shear rate, but neither is simple. The first method is to increase die gap. As a result, it is necessary to increase drawing speed in order to achieve same film thickness and bubble diameter as before. Another way is to reduce auger speed and therefore throughput. This is incompatible with goal of obtaining high returns.

2. Uneven film thickness

It can be said that one of most important requirements for operators is appointment of film products that meet requirements of thickness uniformity. Many designs and configurations of blown film equipment are designed to produce film that meets thickness uniformity requirements and remains consistent over time.and entire production process. Excessive thickness variation can result in unsatisfactory product quality, poor performance of downstream equipment, and large amounts of film waste. Of course, a large variation in thickness significantly reduces profitability of film, because a thicker film has to be made to ensure mechanical properties.

Thickness variation can be divided into two types: variation with time or variation with position. In case of change in time, thickness deviation is mainly in longitudinal direction, and the main reason is extrusion fluctuation. (This is easy to understand.) In addition, temperature fluctuations can also cause long-term changes in extrusion volume, resulting in deviations in longitudinal film thickness. This phenomenon can be monitored by recording temperature of melt every few hours.

Finally, bubble instability can also lead to longitudinal thickness deviations. The phenomenon of bubble instability refers to change in shape of bubbles over time. The phenomenon may be a periodic change in diameter of bubble or height of line of frost, or an erratic movement of bubble, which can be called "tortuous movement". These changes in shape directly affect expansion of melt below freezing line, that is, thickness. In some cases, when frost line is too high, increasing air volume or lowering temperature of cooling air can stabilize film bubble. In some cases, especially when air ring outlet is very close to bubble, reducing cooling efficiency or increasing extrusion rate can stabilize bubble.

Waller's article "What to Do When a Bubble Doesn't Work" provides an overview of bubble instability.

Deep understanding of blown film technology: diagnosing and solving film quality problems for blown film.

Figure 1. The author defines 7 different types of membrane bubble instability

"Stretch resonance" is manifested by a constant change in diameter of bubbles. Similar to other processing methods such as sheet extrusion, stretch resonance phenomenon is caused by melt stretching too fast, i.e., draw ratio is too high. The solution is to decrease draft coefficient, for example by increasing auger speed.

"Spiral Bubble" or Snake Bubble means that bubble has a bulge that comes out of wind ring and rotates in a circular motion with upward thrust. This condition is result of frost line being too low to allow cooling wind to escape. Increasing frost line height, such as increasing extrusion volume, may solve problem.

"Frost line oscillation" refers to phenomenon where position at which membrane bubble first reaches its maximum diameter moves up and down in height. There are several reasons for this, such as fluctuations in extrusion and changes in environment around bubble, changes in air flow. Extrusion fluctuations, as described above, can be stabilized by improving feed system and melting process.

"Bubble Descent" is rapid expansion of bubble after exiting mold and reaching its maximum diameter over a small height range. This is due to insufficient cooling or other reasons that cause frost line to be too low.

"Bubble burst" refers to a situation where stretching speed is too high and bubble bursts at die site. Tearing can occur if film is pulled too fast or cooled too quickly. The best processing methods include raising die temperature and lowering draw ratio.

"Bubble Vibration" is caused by severe wind turbulence from wind ring due to low frost line. The solution is to increase height of frost line, for example by lowering fan speed.

"Bubble breathing" is caused by periodic increase and decrease in volume of gas inside membrane bubble. The main reason is related to internal cooling system. Because internal cooling system of film bubble continuously exchanges air inside film bubble with outside air. The solution is to check various valves, fans, and sensors in internal cooling system.

To deal with various unstable bubble situations, Jung and Hyun created a mathematical model to describe shape of bubbles blown out of film. Their most interesting conclusion is perhaps "multiplicity of steady state". Briefly, a change in a process parameter such as degree of expansion can cause bubble to go from steady state to nest.state of mind and vice versa. They experimented with increasing cooling efficiency to stabilize bubble when it is unstable. But when cooling efficiency exceeds a certain critical point, bubble goes into another unstable state.

Positional thickness deviation refers to uneven film thickness at different positions in transverse direction of bubble. This situation can be caused by unstable bubbles, but is more likely due to non-concentric die heads, uneven cooling, uneven die temperature, and uneven exit melt velocity. Die head non-concentricity refers to difference in concentricity between outer annular portions of die head forming outer surface of bubble and inner annular portions of die head forming inner surface of bubble. This means that outlet gap is not uniform and melt outlet is large or small. Over a certain period of time, due to large output resistance, more melt flows out. Thus, final roll of film is thicker at locations corresponding to large holes and thinner at locations corresponding to small holes.

All die heads are equipped with a centering device. Die concentricity must be adjusted before commissioning and commissioning. However, download process still requires minor adjustments. A perfectly concentric head can form a completely centrosymmetric bubble. Of course, in this situation, there are also requirements for other processes, for example, cooling air velocity must be same around film bubble.

It is interesting that there is still confusion in industrial practice as to whether a large exit gap in a non-concentric die corresponds to a thick or thin swath. Figure 2 is a non-concentric die, extruder is in 12 o'clock direction and large gap is in 3 o'clock direction. The gap between head and bore of this head is 0.04" on average, and installation is currently based on a large gap of 0.06" and a small gap of 0.02". The lower part of Figure 7.2 represents situation where matrix blows bubbles, and bubbles corresponding to large gap on right have a swelling phenomenon. Here melt has more heat and film will stretch more than opposite side before it solidifies. The final film thickness test shows that film position corresponding to large die gap is twice as thick as other side, 0.003 inch and 0.0015 inch.

Deep understanding of blown film technology: diagnosing and solving film quality problems for blown film.

Figure 2. The shape of bubbles when various rods are turned on. Image source: Rational School report.

Uneven cooling at bottom of bubble can also cause film thickness to change across film. The amount of cooling at a certain bubble angle exceeds average amount of cooling, where melt solidifies quickly so that it is not completely drawn out and film thickness is greater here. This phenomenon is due to several factors. Duct lines with variable air resistance or heating some lines near a heat source can cause uneven cooling. In addition, air duct inside air ring must also ensure a constant volume of air around entire circumference. Some self-controlled air rings can achieve film thickness uniformity by adjusting air volume or air temperature at various points along circumference.

The last factor affecting film thickness uniformity is non-uniform melt exit rate. If melt speed at exit of die is lower than average speed, film thickness at corresponding point is thinner. This is due to fact that pulling speed is same around circumference. Some designs of molds are impractical, especially old ones, in which it is impossible to achieve a uniform release of melt around circumference. In addition, some of head outlets become stagnant, which also results in uneven film thickness.

3. Guide/exhaust line

Line defects can be distributed along bubble during machine operation. Runner lines are a big problem, degrading appearance of film, deteriorating optical properties such as gloss and transparency, and degrading physical properties such as tear strength. The most common cause of leading lines is that thread head is dirty. Over time, decomposed material will adhere to head flow channel, forming decomposed charred particles. As melt flows through chute, streamlines must span these focal points. At this time, melt flow separates and repolymerizes to form a weld, and a trough line is formed after outlet head. Remove and clean up mold in time to solve this problem. Another reason for ruler formation is scratches or rough die head gate wall, especially near die. re-polishing will help at this stage.

4. Gel

Gel, also known as fisheye, is small solid particles wrapped in or embedded in a film. Rice. 3. These particles look like unmelted solid particles, but they can also be regenerated from a fully molten melt. (Sometimes output matrixunmelted particles.) A gel consists of some degraded ultra-high molecular weight polymer or even a cross-linked product. The appearance of gels is mainly associated with two problems. First, it is a defect in appearance. Secondly, as a stress concentration point, it leads to poor performance of product.

Deep understanding of blown film technology: diagnosing and solving film quality problems for blown film.

Picture 3. Gel

Gels are obtained from raw materials or during processing. One case is that raw material pellets already contain gel during production. But this situation is very rare and can be quickly corrected. The gel is produced during production of new materials, and manufacturer will find and solve problem as soon as possible.

Most often the product will decompose due to excessive heat during extrusion resulting in a gel. Too high a process temperature can lead to gelation and, most commonly, excessive melt residence time and excessive shear. The polymer accumulates in screw or mouthpiece flow channel, which can be subjected to high temperature and pressure for hours, days, or even longer. As a result, decomposition occurs with formation of a gel, which is squeezed out of melt. "Gel Fill" (?) refers to a situation where many gels are squeezed out at same time. During disassembly and cleaning, it is often possible to find a place where gel appears, for example, a breakaway coil on screw or a flow channel without a smooth transition.

Note from Brother Toughness: PE plastic should be more common in this case!

Excessive shear in extruder, such as serrated portion of barrel and shear portion of mixing element, can release a lot of heat into polymer and cause gelation. The temperature in these places rises sharply, and risk of degradation of polymer is high. Inclusion of antioxidants can effectively alleviate all conditions caused by gel. The mechanism of action of this additive is to prevent occurrence and transmission of degradation.

5. The mechanical properties of film are too low

Mechanical properties often tested for film products include tensile strength, tear strength, and toughness. The first two require measurements in both portrait and landscape orientations. The last one is simultaneous measurement in both directions.

The factors that affect above performance are as follows. The most important influencing factor is main raw material for film production. Polymers naturally have strength according to their molecular structure. For example, nylon has relatively high mechanical properties in all aspects, while low density polyethylene has relatively low mechanical properties. But in thin film applications, strength is not only consideration. In general, main raw material determines characteristics of film.

Second is recipe composition. The main characteristics of film are determined by main raw material, while various additives can significantly change characteristics of final product. Some additives, such as antistatic agents, have little to no effect on film properties., but most of them clearly improve mechanical properties of film. Reinforcing agents such as fiberglass can be very effective in improving various film properties, especially tensile strength. Elastomers increase toughness. There are also some additives that degrade performance of film. For example, for polymeric materials of secondary processing, as proportion of additives increases, previous thermal history will lead to a significant increase in risk of degradation and residual degradation from previous and multiple processes will affect newly formed film. Product performance has a greater negative impact.

Even if composition of film product does not change significantly during use, i.e. composition of film remains constant, performance will vary. This is due to various processing methods. The most important influence of processing technology on film properties is exerted by degree of molecular orientation during film stretching. As mentioned above, degree of stretch of film in longitudinal and transverse directions determines orientation of molecular chain, which, in turn, determines properties of film, such as tensile strength and toughness. Therefore, by adjusting degree of stretching of film in two directions, it is possible to control isotropic properties of film. The basic principle is that higher degree of stretch in a particular direction, higher molecular orientation, and more stretch in that direction, worse tensile strength. When molecular orientation of two directions is matched, impact resistance of film is at its best.

There are several factors that affect mechanical properties of film. Frost line height is an important symbol for measuring cooling rate of a polymer melt and has a great influence on crystalline phase of crystalline materials such as polyethylene. (The orientation of stretch during processing also affects crystallinity and alignment of crystalline phase.)

The crystalline region is denser and harder than amorphous region, so degree of crystallinity has a great influence on tensile strength and impact resistance. Interestingly, in industrial practice, operator can control crystallinity only by adjusting height of frost line. This is because products usually have width and thickness requirements. Therefore, in case of a constant head diameter and outlet gap, there is not much room for adjusting expansion ratio and draw ratio. However, there are still many ways to adjust frost line height, and different methods affect mechanical properties of film in different ways: cooling air temperature and air speed can be set by chiller and fan; draft speed and auger speed can be adjusted at same time to ensure a constantnew degree of pumping; setting process temperature can also control die temperature at melt outlet.

Technological factors that affect mechanical properties of film include residence time of melt in extruder and die maintenance practices. Residence time refers to length of time melt withstands high temperature and high pressure in extrusion system. This is mainly related to screw speed, but depends on screw design, die design and die pressure. For a given screw and die design, residence time decreases with increasing screw speed and increases with increasing die pressure. As average residence time increases, more of melt decomposes. This leads to a decrease in mechanical properties of film. Therefore, optimization of extrusion system must include minimizing residence time of melt.

Scientific and reasonable head maintenance can ensure mechanical properties of film. The usual situation in extrusion process is that, over time, melt adheres to wall of flow channel of die and decomposes to form solid particles. The streamlines are cut and reconnected as melt flows over pellets, creating guide lines. Even if there are no obvious line defects on film surface, these invisible lines of flow channels are still where film performance is weak. Since load is easily concentrated where runner is, breaking strength is low. To avoid this situation, it is best to regularly maintain extrusion head and clean up degraded material adhering to wall of flow channel in time.

6. Poor optical performance

The causes of poor optical performance can be divided into two categories: Materials and process. Materials is incompatibility of compositions and phase separation. Incompatibility occurs when foreign matter such as dust, decomposed particles, or moisture enters extrusion system, causing spots or streaks on film. Phase separation is due to wrong choice of ingredients or wrong choice of additives.

Process problems relate to gating lines, too low cooling rates, melt breakage, and interlaminar instability. Runner lines, as previously mentioned, have a bad effect as they degrade both optical and mechanical properties of film. The cooling rate has a great influence on crystallinity of polymers. Since higher degree of crystallinity, lower transparency of film, a higher cooling rate is usually required. Melt destruction effects are similar to gating lines. Defects on film surfaceand lead to a decrease in transparency of film. The same applies to defects at interface between layers.

Source: Rationalist report, author doesn't know real person very well, it's beautifully written, organized by Plastic Questions, thanks!

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