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die lines in plastics extrusion: film blowing experiments and numerical simulation.

by:Top-In     2020-07-28
Extrusion is the most important unit operation in the plastic processing industry.
This is a convenient way to continuously manufacture complex cross sections based on strict dimensional tolerances, mechanical properties and appearance features.
Extrusion is complex and sensitive, even with minor changes to the operating parameters.
The problems encountered during the extrusion process range from aesthetic defects to major faults that can close the production line.
Many surface defects occur during extrusion.
These will destroy the aesthetic and optical, electrical and mechanical properties of the extrusion.
In film, paper, pipes and wire and cable coatings with important surface properties, cosmetic issues will challenge profitability.
Even if the surface defect is purely a surface defect, it may be an expensive obstacle to sales.
The mold line is a longitudinal dent or projection formed on the extrusion surface (1, 2).
The stripes may last for several hundred feet.
When the mold line seriously interferes with extrusion, the processing line must be closed to clean the mold.
This is expensive if needed on a regular basis.
Despite the commercial significance of these defects, there is little literature on the death line, either in experimental or theoretical studies.
We have previously reviewed the mechanism and influencing factors of the formation of the death line, as well as the methods to suppress these stripes (1,2).
Here we first give a brief introduction to how to measure and characterize a straight line (
Mold line measurement)(2).
Because the mold line is usually observed on the blowing film, we then conduct the blowing film experiment to study how the diameter depends on the size, shape and position of the mold defect and various operating parameters.
The empirical relationship between the mold line and the defects, material properties and the operating conditions affected was explored using a non-dimensional group.
The results quantitatively determine the importance of these factors in the formation of mold lines.
In order to better understand the fluid dynamics of the molding line formation, we used fimap to simulate the free surface flow of the polymer melt leaving the mold lip (version 8. 52)(3).
In particular, we study how mold defects, material properties and operating conditions affect the mold line.
The results were compared with the experimental results.
The die line metering die line is a long surface stripe.
Like other surface defects, they are visible because their scattered light is different from the rest of the surface.
Therefore, optical surface metering helps to develop mold line metering (2).
In our mold line metering, we classify the mold line as an independent stripe on a smooth surface, or a distributed stripe on the entire surface (2).
We measure the mold line using a mechanical profiler.
The isolated and distributed mold lines can be described in this way.
Measuring the shape of the mold line also helps to deepen our understanding of the fluid dynamics of the formation of the mold line.
There are three ways to describe the mold line (2): 1.
Use the complete mold line profile. 2.
Visually compare the mold line with the scratch standard.
This is water sports or semi-water sports.
Quantitative methods focusing on mold line cosmetics. 3.
Mold Line statistics.
The first two methods are suitable for isolated die lines on smooth surfaces, and the third method is suitable for distributed die lines.
Here, we focus on the isolated die lines on the blown film, which are characterized by the use of their profiles as well as the basic width and height.
Blown film equipment blown film is carried out on the \"Yellow Jacket\" blonfilm Tower 6536.
Using a Killion extruder, a 19mm-inch ironing board, a constant pitch of 19mm and 17. 7[degrees]flight angle.
The ratio of barrel length to diameter is 24:1.
The depth of the screw channel varies from 3.
5mm to 1 in the feeding area.
Measurement Area 3mm.
In order to study how the operating conditions affect the mold line, the screw speed (throughput Q), take-off speed ([V. sub. f]), Amplification ratio (BUR)
And melttemperature ([T. sub. e])
Change independently.
The temperature of the three-cylinder heater and the die head heater can be set separately.
These temperature changes are less than 5 in each extrusion experiment [degrees]C. [
Figure 1 slightly]
There are external blow film molds (barrel)and inner (mandrel)pieces(see Fig. 1).
The polymer is extruded through the loop.
Provide fixed volume air from below, through special channels of the core rod.
This was done at the beginning.
Fix the bubble diameter and then occasionally fix the bubble diameter.
Monitor the internal air pressure of the bubble using a pressure gauge.
The defects of the core shaft and the lip are the main reasons for the mold line (1,2).
To investigate how they trigger the mold line, we have processed two mandraven defects that control the shape, size, and position.
Both mandala are made of 1018 low carbon steel.
All the core rod defects are very large, resulting in a mold line with a profile large enough to characterize.
All mold defect sizes are listed in tables 1 and 2.
In order to improve efficiency, there are many defects on each core rod.
Defects are evenly distributed along the perimeter of the core rod to minimize their interaction.
Both Hearts have a height of 19.
05mm Anda in diameter 21. 08 mm.
The inner diameter of the cylinder is 26.
16mm, thus forming a ring gap 2. 540 mm.
There are no defects in the mold bucket.
Low linear material
Density Polyethylene (LLDPE)Dowlex[TM]2045 (DowChemical)and a high-
Density Polyethylene (HDPE)Paxon[TM](
Exxon Chemical Company)were used.
Table 3 lists their physical properties.
The experimental process must provide air slowly to obtain a stable bubble with a specific BUR.
Thin film bubbles are easily affected by ambient temperature and airflow.
Therefore, all experiments were performed at the same room temperature and at the smallest laboratory airflow.
When switching the test material, the screw speed of the new resin and 25rpm is used, and the screw passage and mold are about 30 minutes.
Since the construction of the dead lip-
Up can trigger the mold line and clean the mold lip with a brass tool before each extrusion to avoid scratches.
During each blown film process, after the foam is stable, squeeze the film out for 1 minute, collect and weigh it.
Sample preparation using an aluminum sample holder, in a flat and fastened film for mold line scanning (see Fig. 2).
The rectangular glass sheet in the middle provides a flat film holding area.
Then put FilmOn Ann on the glass with two sticks.
Surface roughness of glass 【R. sub. q]is 0. 04 [micro]
M, low enough, negligible error in Die line measurement.
The blowing film must be trimmed to fit the glass sheet on the holder.
In order to improve the measurement accuracy
Tape the film onto the glass sheet.
In order to prevent air retention under the film, adhesion must be fully carried out, which will deform the mold line.
Excessive power can also distort the line of death.
The mold line is kept perpendicular to the longitudinal direction of the bracket.
When it goes through the film, the outline stylus outlines their shape.
Measuring the shape of the mold line using a mechanical stylus profile analyzer [R]System 4000.
It has a stylus with a radius of 2. 54 [micro]
M, 200 mg
Scan speed of 0.
25 mm/s, normal magnification. [
Figure 2:
The Profiler was calibrated using two different standards.
Calibration is also required when adjusting the profile light beam or diamond stylus.
However, this is not necessary when it is just switching movie samples.
Before the analysis, to ensure the correct reading range, each film sample was pre-packed two or three times.
The stylus must then be re-written
All the way to the entire mold production line can be completed.
Draw the mold line shape on a grid map with a preset scale and automatically calculate the roughness height.
However, since the straight lines on the blown film are isolated and not distributed, they are balanced using their base width and height rather than statistics.
Unfortunately, our stylus analyzer is simulated.
All mold line profiles must be manually digitized from the chart and then characterized and compared using the appropriate height and width scale.
Experimental results and Discussion Figure 3 show microscopic images of typical mold defects and their diameters.
Measuring the mold line using a stylus Profiler shows excellent productivity (see Fig. 4).
Some of these differences are caused by digitization.
The shape of the mold line on our blown film is usually asymmetrical.
This may be due to 1)
Slight deformation of mold defects, 2)
Defects in sample preparation, 3)
Eccentric core shaft in blow film mold, or 4)
Sample misplacement during stylus analysis.
For our results, the boundless group in our dimensional analysis (2)were employed.
We believe that our observations quantitatively describe how various factors affect the formation of the mold line.
While the specific values will vary with the equipment and materials, we believe that our results will lead to the correct trend. [
Figure 3 slightly][
Figure 4 slightly][
Figure 5 Slightly]
Mold defect size mold wire size strongly depends on the mold defect size.
Figure 5 depicts the rectangular lip depression 13 and its mold line on the HDPEfilm.
The visual inspection shows that if there is no downstream stretching andBUR = 1, the size of the lip defect and its mold line has the same order of magnitude.
Figure 6 compares the mold lines on the LLDPE film caused by three lipind12, 13 and 14 on the core stick m1.
For indented I1, its Die line is raised on both blown film surfaces (
Neither side of the film is flat)
It is impossible to render the stylus contour (see Fig. 7).
The results of HDPE film are similar.
From Table 1, dents of 12, 13, and 14 have the same height, but have different widths, 12
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