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The Influence of the Type of Polypropylene and the Length of the Flow Path on the Structure and Properties of Injection Molded Parts With the Weld Lines.

INTRODUCTION

One of the disadvantages of injection molded parts are the weld lines, created in the areas of collision of two flow fronts of plastic in the mold cavity and are often impossible to eliminate [1-4]. In these areas, moldings have lower strength properties and a worse surface condition. Deterioration of moldings' properties is the result of a weak connection of flow fronts, mainly caused by the orientation of polymer macromolecules and filler on the stream fronts during the fountain flow and air bubbles entrapped between them [4-7]. The mechanism of the formation of weld lines depends on the conditions of polymer flow in channels of the injection mold. The liquid material flowing through the cold channels of the injection mold quickly solidifies at the wall of the cavity because of the heat conduction, and furthermore on the stream front it has a lower temperature than the material in the core. When the fronts of the cooled material collide with each other, a V-shaped notch forms on the surface of the part [8-11]. The formation of this notch is promoted by the incomplete removal of air or other gases from the space between the fronts of the material streams during the phase of cavity filling. The occurrence of the V-notch results in increased surface roughness of the part in this area, changed color and gloss, and is a place of stress concentration, which results in a reduction of the strength properties of the part.

The reasons for the weld lines occurrence in molded parts can be divided into the ones related to the molded part construction, the properties of the injected material, the construction of the injection mold, and the parameters of injection molding process [12]. Weld lines can be created in moldings of the shape and size that require multipoint feeding of the material to the cavity, moldings with holes, with different wall thicknesses and with a complicated design causing a different polymer flow rate in the individual areas of the cavity [6, 13, 14]. In these cases, it is important to correctly design the injection mold, especially its cooling system that ensures an even temperature distribution in the cavity, the way of the mold venting, the gate location and the shape and size of the gate [15, 16].

Strength properties of the weld line area depend on the type of material being processed. It was shown that amorphous polymers are characterized by higher strength than semicrystalline ones [13, 17]. The viscosity of the polymer at the processing temperature plays a significant role. In polyethylene examinations [15], it was found that the tensile strength of moldings with weld lines made of polyethylene with the melt flow rate (MFR) = 0.35 g/10 min is about 16% lower with respect to the strength of moldings without this area. While in the case of polyethylene characterized by a high MFR = 21.30 g/10 min, these differences were small and only ranged from 3% to 5%. Fillers and large particles of the coloring agent may also increase the viscosity of the material, cause worse cavity filling and, as a result, weaken the area of joining streams [7].

By changing the injection molding parameters, it is possible to influence the properties of the material in the weld line area [2, 18-20]. The low injection speed, too low mold temperature, and the injection temperature of the material as well as the low holding pressure favor poor connection of the plastic streams [8, 21].

The areas of weld lines in injection molded parts are usually not the result of imperfections of the processed material, the injection molding machine, the mold or the technological process, and result mainly from the part design. Manufacturers of polymeric components often do not have the influence on the part design or the choice of material used for injection. They are therefore forced to look for new technological solutions. A certain solution to the problem of weld lines in parts is the use of unconventional injection molding methods, such as push-pull injection molding [22, 23], cascade [24, 25], vibration [6, 26, 27], and injection molding with variable temperature [26, 28-32]. However, each of these methods involves higher costs of moldings production, associated with a more complicated mold design and the need to ensure proper control of the injection molding machine. Therefore, it is advisable to study the mechanism of the formation of weld line areas and the possibilities of reducing the negative effects of their occurrence, especially in the conventional injection molding process.

In recent years, there has been a growing interest in the production of porous injection moldings with many advantages, such as lower weight, better dampening properties, higher stiffness, lower plastic consumption compared to solid moldings. Some research is carried out to determine the conditions for their production, enabling the obtainment of moldings with a porous structure, but also characterized by favorable mechanical properties and good surface condition. Porous moldings are produced by the injection method using physical blowing agents (C[O.sub.2], nitrogen), water, or chemical blowing agents [33-36], Injection molding using chemical blowing agents is becoming more and more popular, due to the possibility of conducting the process using conventional injection molding machines, only with changed injection parameters [37]. For this reason, the study of the influence of the foaming process on the structure and properties of injection moldings with the weld line areas was taken into consideration. It was expected that the melt streams, colliding in the cavity, would create a stronger connection due to the higher flow velocity resulting from the foaming process.

The structure and properties of injection molded parts with the areas of weld lines were analyzed, in particular the size of the V-shaped notch formed in this area in parts made of unfilled polypropylene (PP) and polypropylene filled with talc, solid and porous.

EXPERIMENTAL

The aim of the work was to examine selected properties and structure of moldings with weld lines, obtained from unfilled polypropylene and polypropylene with the addition of filler and blowing agent. The influence of the length of the plastic flow path in the cavity on mechanical strength of such moldings and the surface state in the weld line zone were also evaluated.

Tested Materials

Polypropylene with the trade name Moplen HP456J (manufactured by Basell Orlen Polyolefins) and polypropylene Xenoprene PP-TD-20 with 20 wt% talc content (produced by Spoldzielnia Pracy Chemikow XENON) were used for the examinations. Injection molded parts were made of unfilled PP, PP with the addition of the foaming agent and PP + talc without/with the foaming agent. In order to obtain porous moldings, the 2 wt% chemical blowing agent, Hydrocerol ITP 848, was added to both materials, in the form of a granulate with an endothermic course of decomposition. The amount of active substance in the granulate of this foaming agent was 70 wt% and its activation temperature was 160[degrees]C.

Samples

Samples in the form of small tensile bars for the uniaxial tensile tests were cut out of injection molded parts, in accordance with the PN-EN ISO 294-3 standard. Three samples from each molding, with the weld lines in the middle of them, formed at different flow paths from the gate, were obtained. The method of cutting out samples and the exemplary sample is shown in Fig. 1.

The moldings were produced using a single-cavity mold, mounted on a KraussMaffei injection molding machine KM 65-160 C4, with the injection parameters shown in Table 1, selected for the polypropylene without and with the foaming agent, respectively. During the processing of PP with the foaming agent, the holding phase was used because in the injection molding of this material in the cycle without the holding phase, parts with small sink marks were obtained. Applying twice lower holding pressure and much shorter holding time than for PP without the foaming agent did not escape the foaming process.

Measurement Methods

In order to determine the intensity of a foaming phenomenon, mass of moldings from all materials was determined using a laboratory weight type CP225 from Sartorius with a closed measuring space, with a measurement accuracy of [+ or -] 0.1 mg.

In order to visualize the internal structure of the samples, a high-resolution X-ray scanner GE PHOENIX v 1 tome 1 s was used. Test samples were cut from the center area of each sample, including the area of the weld line zone. The tests were carried out at a voltage of 150 kV and a current of 100 pA between the cathode and the anode of the X-ray tube. Then, 2000 projections (images) were obtained and a single radiograph was averaged from three images. The exposure time was set to 200 ms. The obtained resolution was 15 [micro]m (voxel size). Data acquisition was performed using the datos 1 x 2 acquisition program and the reconstruction was realized in the phoenix datos 1 2.0 reconstruction program. The VGStudio MAX2.0 package was used to analyze the results.

Mechanical properties of moldings were examined using the Inspekt Desk 20 testing machine from Hegewald & Peschke. The uniaxial tensile tests were carried out in accordance with the PN-EN ISO 527: 1998 standard, at a tensile speed of 50 mm/min.

The surface appearance of the weld line area and the occurrence and distribution of pores in the samples made of PP with the foaming agent were assessed in microscopic observations that were carried out using the optical microscope VHX-900F and the Olympus BX60M one.

Measurements of the geometric structure of moldings in the area of weld lines and analysis of the effects of collision of plastic streams in relation to the quality of moldings were carried out using the Taylor Hobson Talysurf Series 2 profilographometer. The measurements were based on a systematic scanning method on the field x - 4 mm and y - 1 mm at the sampling step [increment of x] = 1 [micro]m and [increment of y] = 2 [micro]m.

RESULTS OF EXPERIMENTS

Measurement of Molding Mass

The results of mass measurement of injection molded parts are shown in Fig. 2. The average mass of moldings from unfilled polypropylene was 10.67 g, whereas the mass of moldings from polypropylene containing foaming agent was 10.30 g. The addition of the foaming agent resulted in a reduction of the mass of moldings by about 3.5%. In the case of solid moldings from PP with talc, the average mass of moldings was 12.64 g, while porous ones were about 2.5% lower and amounted to 12.33 g. The mass of parts made of PP filled with talc was about 2 g higher than the mass determined for unfilled polymer. From the data obtained, it appears that the efficiency of a foaming process was sufficient, despite the use of the holding phase in the injection molding cycle, although the addition of the foaming agent reduced the mass of the moldings to a small extent.

Mechanical Properties

Figure 3 shows the results of tensile strength measurements. It can be seen that in the case of moldings made of unfilled PP, the values of tensile strength for porous samples with the weld line formed in the Area A is lower by about 17% than the value obtained for solid samples, from the same area. A similar dependence occurs in the tests of samples with the weld lines created in Areas B and C. Lower strength of porous samples is related to the structure of polymer streams flowing in the mold channels, that is the porous core and the solid, supercooled skin layer at their fronts. During streams collision, due to the polymer fountain flow, macromolecules and also pores from the core are stretched along the flow lines and in the weld line zone are oriented parallel to each other. Examination of samples carried out by the X-ray scanner (Fig. 4) revealed the presence of numerous pores in the area of weld line zone in the case of moldings made of PP + foaming agent, which is the reason for reducing their tensile strength. Images presented in Fig. 4 show the porous structure in the layer located in the middle of the sample thickness.

With the increase of the length of the flow path to the joining zone of polymer streams, the tensile strength decreases both for samples made of polypropylene with and without foaming agent. The value of tensile strength in the case of solid samples from Area C is less than that obtained for samples from Area A by about 6%, whereas for the porous moldings by 5%. Smaller values of the tensile strength with the increase in the length of the material flow path from the gate to the weld line area can be explained by worse condition of the streams collision. The pressure and temperature of the material in the areas located farther from the gate is lower that results in worse stream connection and decreasing of sample mechanical properties. Such conditions are conducive to more intense foaming process. In Fig. 4, it can also be noticed that in the sample taken from the Area C, located the farthest from the gate, the number of pores is the largest, which explains the lowest tensile strength of these samples.

For samples formed from PP filled with tale, the properties practically did not change after the addition of the blowing agent. This can be explained by a smaller number of pores in the areas of weld lines or even no pores exactly in the stream connection line (Fig. 4), due to the hindering of the foaming process by the filler particles gathered on the melt stream front. In earlier work, it was reported that the talc addition to the neat PP generally improves the foaming effect [35]. Lower effects of the foaming in this case, bigger size of pores and lower cell density, can indicate the necessity of modification of the injection molding parameters. Further examinations in this field are desired, mainly experiments run with different melt and mold temperatures considered as the most influencing the foaming process [37] and also different contents of the foaming agent [36].

The moldings made of PP + talc were much less durable than those made of unfilled polymer were. The values of tensile strength for samples from PP filled with talc were less than determined for samples from unfilled PP by about 50% for solid samples and about 40% for porous ones. Generally, the worse mechanical properties of samples in the weld line zone are the result of the melt fountain flow in the mold cavity and the orientation of macromolecules on the fronts of the colliding streams. In the case of PP filled with talc, the talc particles are also oriented, like macromolecules, parallel to the fronts of the material streams that results in a deterioration of their strength properties. The orientation of the talc particles (light particles) is visible both in the area of weld line in solid and porous samples (Fig. 4).

Microscopic Observations of the Weld Lines Areas

The results of microscopic observation of examined samples are shown in Figs. 5 and 6. Figure 5a shows the weld lines in solid polypropylene samples in areas created at different lengths of the material flow path, while Fig. 5b shows the samples made of PP with blowing agent. In Sample A, in both the solid and porous moldings, the weld lines are the least visible. This can be explained by a better combination of plastic streams at a higher flow velocity, higher pressure, and higher temperature of the colliding fronts of plastic streams. The weld lines on the surface of the Sample C are more visible, because with a further flow of the material there are greater resistances to flow; there is a drop in pressure, flow velocity, and a decrease in the temperature of the material. In the porous moldings, the pores in their central part are clearly visible, with the width of the porous layer and the pore sizes increasing with the increase in the length of the material flow path in the cavity. The more effective foaming in Zone C is the result of the decrease in the polymer pressure in the areas located farther away from the gate.

Figure 6 illustrates the appearance of the weld lines in moldings from PP filled with talc, observed using an Olympus BX60M microscope. In solid samples, the weld lines are regular, while in porous ones, these lines have an irregular shape, indicating an uneven flow of material in the cavity. Such flow may be due to the presence of a filler and the simultaneous occurrence of the foaming process. The filler disturbs the polymer flow and it is possible to collect talc particles on the pores, which affects the uneven flow and irregular shape of the weld line.

Geometric Structure of the Weld Line Area

Measurements of the geometric structure of moldings in the area of weld lines, using the Taylor Hobson Talysurf Series 2 profilographometer, were carried out in points marked with A, B, and C, located in the areas of weld lines at different distances from the gate. Measuring points are shown in Fig. 7. The analysis of the measurement results was carried out in the TalyMap Universal software. The first stage of measurements included leveling samples and isolating surfaces of equal dimensions x = 2.2 mm and y = 0.8 mm, with the assumption that the V-notch formed in the area of the weld line will be in the central part of the sample (Fig. 8).

Figure 9 shows isometric images of the tested surfaces (three-dimensional [3D]) obtained using a contact profilographometer. It can be seen that in the areas of weld lines, in all tested samples, there are thickening of the wall being the effect of the plastic streams collision. The largest thickness in this area has samples covering the Area A, located closest to the gate, which can be explained by the highest flow velocity of colliding streams. In samples from unfilled polypropylene, from other areas, the thickening of the wall is small (B) or invisible (C). In the case of polypropylene filled with talc, both without and with the blowing agent, the flow of the material is hindered by the presence of filler particles, which results in larger wall thickness changes in the weld line area, regardless of the distance of this area from the gate. This effect may also be related to the smaller shrinkage of the polymer caused by the orientation of the filler particles perpendicular to the flow direction and their higher density in this area.

In the measurements carried out with the use of a contact profilographometer, the Pt parameter was determined, quantifying the total height of the raw profile. The calculation of the Pt parameter in the V-notch was carried out by extracting eight longitudinal profiles in accordance with the PN-EN ISO 4287: 1999 standard. The results of measurements of the total height of the raw profile (Pt) for the molded parts are given in Fig. 10.

It can be seen that the total height of the raw profile is much higher for samples made of polypropylene filled with talc, both solid and porous. For example, solid samples from Area C made of PP + 20% talc have a value of Pt more than six times higher than samples from unfilled PP, whereas in the case of porous samples it was about 3.5. It can further be observed that the Pt parameter increases with the increasing distance of the weld line from the gate. Samples covering the area farthest from the gate (C) have values of Pt parameter by 660%, 108%, 58%, and 17%, respectively, for PP, PP + blowing agent, PP + talc, and PP + talc + blowing agent, higher than values determined for samples with the weld line formed in the area closest to the gate (A). These results indicate a beneficial effect of the foaming agent in reference to the size of the V-notch formed in the area of weld line. The foaming process, also occurring in the area of weld line, increases the speed of the fronts of colliding polymer streams and is more intense in areas of lower pressure, that is, those more distant from the gate. In these areas, there are two opposite phenomena affecting the formation of the V-notch, namely, the increase in the polymer flow rate caused by the foaming process and the reduction of the flow rate due to the drop of the polymer temperature. Thus, the addition of a foaming agent to the material can effectively contribute to a better connection of the plastic streams in areas, especially located farther away from the gate.

CONCLUSIONS

The occurrence of weld lines in injection molded parts is a significant problem in the case of manufacturing elements with a complex shape, with high requirements regarding their mechanical properties and surface appearance. In weld line location, an area with structure and properties other than the rest of the molded part is formed and a surface defect is created in the form of a visible line of polymer streams joining.

The structural foam injection molding, with using a chemical blowing agent, is not sufficiently advantageous due to the strength properties of the moldings with the areas of weld lines, in particular moldings made of unfilled polypropylene. The tensile strength of the samples from this material, both solid and porous, decreased with the increase in the length of the material flow path to the place where the stream welding region was formed. However, in the case of polypropylene filled with 20% talc, the tensile strength of solid and porous samples almost did not change, irrespective of the location of the weld line.

In the cross section of the molding wall, along the weld line, a V-notch is observed and the wall thickens at the point where the polymer streams collide. The size of this notch depends on the length of the polymer flow path from the gate to the area of weld line. With the increase of this length the notch depth, determined by the total height of the raw profile (parameter Pt), increases. However, the thickening of the wall in the weld line area decreases with the increase of the distance of this area from the gate.

The addition of a blowing agent to the injected polymer may be one of the methods facilitating the filling of the cavity and the joining of the material streams with a higher flow rate, and thus obtaining products with a better surface condition in the weld line areas, especially when they are located far from the gate.

When the shape of the molding prevents the problem of the weld lines from being avoided, only methods allowing the formation of these areas, for example, in unloaded or invisible places during the use of the part, can be used. It is important to consider the influence of the injection conditions, especially the course of cavity filling phase, on the quality of the connection of the colliding streams of material.

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Elzbieta Bociaga (iD), (1) Stawomir Kaptacz (iD), (1) Piotr Duda, (1) Anna Rudawska (2)

(1) Faculty of Computer Science and Materials Science, University of Silesia in Katowice, ul. Zytnia 12, Sosnowiec, 41-200, Poland

(2) Faculty of Mechanical Engineering, Lublin University of Technology, ul. Nadbystrzycka 36, Lublin, 20-618, Poland

Correspondence to: E. Bociaga; e-mail: elzbieta.bociaga@us.edu.pl

DOI 10.1002/pen.25170

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Preparation of test samples: (a) way of sample cutting and (b) sample for static tensile test; the Letters A, B, and C are marked with moldings with the weld line created at the different lengths of the polymer flow path from the gate.

Caption: FIG. 2. Comparing the mass of moldings from examined materials.

Caption: FIG. 3. Tensile strength of samples with weld lines formed at different lengths of the polymer flow path; designation of Samples A, B, and C as shown in Fig. 1.

Caption: FIG. 4. Structure of the weld line zone in the longitudinal section of samples from the examined materials obtained by using the X-ray scanner; the images from the layer located in the middle of the sample thickness.

Caption: FIG. 5. Weld lines in samples made of unfilled polypropylene (PP) (a) and polypropylene containing blowing agent (b), in the Areas A, B, and C, formed at different lengths of the polymer flow path (VHX-900F microscope).

Caption: FIG. 6. The weld lines in samples from polypropylene (PP) filled with talc; images obtained with the Olympus BX60M microscope.

Caption: FIG. 7. An injection molded part with marked locations for testing the geometrical structure of the weld lines areas.

Caption: FIG. 8. Two-dimensional images of the surface of the tested samples obtained using a Taylor Hobson Talysurf Series 2 profilographometer.

Caption: FIG. 9. Isometric images of the tested surfaces (3D) obtained using a Taylor Hobson Talysurf Series 2 profilographometer.

Caption: FIG. 10. The total height of the raw profile Pt for the V-notch formed in the area of weld lines in the molded parts.
TABLE 1. Injection molding parameters for Moplen HP456J and
Xenoprene PP-TD-20 polypropylene.

                                    Type of polypropylene

Process parameter                   Without          With
                                 foaming agent   foaming agent

Melt temperature ([degrees]C)         220             220
Mold temperature ([degrees]C)         20              20
Injection speed (mm/s)                50              50
Holding pressure (MPa)                30              15
Injection time (s)                   0.74            0.74
Holding time (s)                      10               2
Cooling time (s)                      20              20
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Author:Bociaga, Elzbieta; Kaptacz, Slawomir; Duda, Piotr; Rudawska, Anna
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXPO
Date:Aug 1, 2019
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