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The influence of injection molding parameters and blowing agent addition on selected properties, surface state, and structure of HDPE parts.

INTRODUCTION

The foam injection molding of polymers containing the chemical blowing agent (CBA) is one of the nonconventional methods of injection molding, in which porous parts are produced. This method allows us to obtain molded parts with solid external skin and porous inner core (1-6). This process can be carried out using conventional injection molding machines and conventional mold, which is important due to the cost of manufacturing. Porous plastics are widely used in many areas including the insulating elements, filters, packaging, etc. Blowing agents are also used as modifiers of injection molding process, for example, to eliminate sink marks in molded parts, caused by the shrinkage (1). Application of blowing agents allows therefore to reduce the amount of defective products, which is important both for economic and ecological aspects (less amount plastic needed to manufacture specific portion of parts, lower energy consumption, less waste, etc.). Blowing agents may also be used in the extrusion process (1), (2), (7), (8).

The formation and the growth of pores is a complex process, depending inter alia on the type of polymer, the type of blowing agent, injection molding conditions, runners, and cavities design. The examination of the impact of these factors on the state of injection molded parts is difficult, because of the variety of the physical and chemical phenomena occur in the foaming process of thermoplastics. The size and distribution of pores in polymer affect molded parts properties.

In the available literature the approximate values of the parameters of foam injection molding process are often found, however, there are few research works (9-20), which provide the results of the influence of these parameters on the properties and structure of injection molded parts. The research works (9), (10) present the results of the influence of blowing agent content in the polymer and the injection conditions (injection temperature, mold temperature, injection velocity) on the formation of the structure and properties of molded parts from polypropylene. It was proved, that the effectiveness of the blowing agent (estimated by weight., density, and sink marks reduction in parts), is dependent on processing conditions. It was shown that a lower mold temperature (20-32.5[degrees]C) has an advantageous effect on the structure of molded parts. The blowing agent (CF Hydrocerol in a form of granules) in amount of 0.5 to I% allows to produce high-quality parts, without sink marks, well imaging the shape and dimensions of the cavity, with no significant worsening of their properties. In an article (9) it has been shown that the low processing temperature has a beneficial effect on the structure of the porous molded parts. At the lower mold temperature (20[degrees]C) and lower injection temperature (200-210[degrees]C), molded parts having a solid skin layer and a fine cellular core were obtained.

In this work (1), mechanical indices of solid and porous molded parts from polypropylene with 0.5 to 2% addition or Hydrocerol CF40 blowing agent, obtained at different injection pressure and mold temperature were evaluated. It was found, that the addition of 0.5% of blowing agent causes considerable decrease in mechanical properties of molded parts compared with solid parts, but further increase in the amount of blowing agent in the polymer did not result in a significant decrease in strength of those parts. The addition of blowing agent in amount of 0.5% to a polymer also contributed to a decrease in impact resistance of molded parts up to 70% compared with solid ones, but with a further increase of the content of blowing agent in the polymer, the impact strength of molded parts was increasing.

The results of the influence of processing parameters on the properties and structure of the molded parts obtained from polypropylene filled with wood fiber, with the addition of a chemical blowing agent are presented in articles (11-15). It was shown, that 2% of the blowing agent content allows obtaining fine porous structure. The use of blowing agent caused a reduction in weight of parts, with no significant effect on their tensile strength. Together with an increase of the mold temperature, the surface roughness of parts decreased. In an article (16), the effect of mold cavity design and the way of cavity feeding on the mechanical properties and structure of the molded parts was investigated. It was found that the ratio of core to skin thickness was the same in the whole part, only in the corners a larger fraction of solid plastic occurred. A larger participation of foam structure was observed in the high thickness parts. Injection time had small effect on their mechanical properties.

In an article (17), a significant influence of 0.5 to 3.75% blowing agent addition on reducing mechanical properties: tensile strength, elongation at break, stress at break, and impact resistance of molded from HDPE was proved.

In another work (18), authors stated that the content of blowing agent and injection pressure have the significant influence on the quality of porous structure, whereas the mold temperature significantly influences the thickness of solid skin layer of molded parts from LDPE.

Studies conducted by authors (8) showed a beneficial effect of the endothermic blowing agents (Hydrocerol series) on weight reduction, surface state, shrinkage, sink marks, and warpage of molded parts from PS, ABS, and HDPE/PP, EVA/PE/TPE polymer blends.

The addition of blowing agent to the processed polymer can affect the appearance of the molded parts, their color, and gloss. Authors of dissertations (10), (15), (19), (20) have shown, that the blowing agent worsens gloss of parts and causes the change in their color. It is therefore necessary to take into account this phenomenon during the designing of manufacturing process of molded parts.

EXPERIMENTAL

The examination of the influence of injection molding conditions (injection velocity, mold temperature, and injection temperature) and blowing agent addition on parts weight, density, mechanical properties, surface state, and structure of molded parts was the aim of this work. Properties of solid injection molded parts were compared with properties of porous parts.

The Hostalen GC 7260 (Basell Polyolefins) HDPE was used in experiments. Its properties were the following: 72[degrees]C Vicat softening temperature (method B50: 50[degrees]C/h; 50 N), and 8 g/10 min melt flow rate (MFR), determined at the temperature 190[degrees]C and 2.16 kg load. The porous structure of molded parts was obtained by addition of Hostatron P 1941 (Claim[degrees] endothermic chemical blowing agent (in a form of granules containing 40% of blowing agent and 60% PE). Plastic granules were mixed with the blowing agent before the injection molding process. Specimens, in the shape of tensile bars of 4 mm thickness, were produced according to PN-EN ISO 527-2:1998 standard, using two-cavity mold mounted on KraussMaf-fei KM65-160 C4 injection molding machine.

Using StatSoft Inc. Statistica 8 software and Design of Experiments (DoE) module, the experimental design with 16 input variables arrangements (Table 1) was created. The three independent input variables were: injection velocity ([v.sub.w]), mold temperature ([T.sub.f]), and injection temperature ([T.sub.w]). Their values were established on the basis of preliminary investigations, in such a way that the parts were characterized by good quality, did not have burn marks, sink marks, etc. Obtaining the correct porous molded parts required the use of relatively small value of packing pressure (20 MPa). Constant parameters of injection were the following: injection time: 0.4-3.5 s (depending on injection velocity); hold time: 31 s for A series, 24 s for B series, 16 s for C series; cooling time: 15 s for A series, 10 s for B series, 5 s for C series. Hold time and cooling time was reduced with an increase of the blowing agent percentage in the polymer.

TABLE 1. Plan of experiment.

Arrangement   Injection      Mold       Injection temperature,
number       velocity    temperature    [T.sub.w] ([degree]C)
             [v.sub.w]   [T.sub.f]
             (mm/s)      ([degree]C)

1                    20           20                       200

2                    20           20                       260

3                    20           70                       200

4                    20           70                       260

5                   120           20                       200

6                   120           20                       260

7                   120           70                       200

8                   120           70                       260

9                    20           45                       230

10                  120           45                       230

11                   70           20                       230

12                   70           70                       230

13                   70           45                       200

14                   70           45                       260

15 (C)               70           45                       230

16 (C)               70           45                       230

C, Central point of plan of experiment.


The samples were injected in the three series: A, B, and C. The A series of samples were prepared from HDPE without blowing agent, in accordance with 1.6 points of the plan of experiment, in the B series with 2% of blowing agent (0.8% active ingredient), and in the C series with 4% of blowing agent (1.6% active ingredient).

RESULTS AND DISCUSSION

Molded Pans Weight

The influence of input variables (injection velocity, mold temperature, and injection temperature) on the molded parts weight obtained in series: A, B, and C was determined. The weight of molded parts from two mold cavities was determined and the average value was calculated. The average weight of parts in each series were determined, respectively [m.sub.A], [m.sub.B], [m.sub.C]. The Sartorius CP225 weighing machine with close measurement space was used and the weight of parts was determined with [+ or -] 0.1 mg accuracy.

The investigations showed that the mold temperature has the most significant influence on the weight of solid parts and parts from HDPE with blowing agent. Figure 1 shows an example of the Pareto chart for the parts from HDPE with the 4% addition of blowing agent. It can be seen, that with the mold temperature increase, the weight of parts also increases.

[T.sub.f]             32.79

[T.sub.w]             14.27

[v.sub.w] [T.sub.w]    3.70

[v.sub.w] [T.sub.f]   -2.49

[v.sub.2.sup.2]        1.48

[T.sub.f] [T.sub.w]   -1.26

[T.sub.w.sup.2]        1.08

[v.sub.w]              0.85

[T.sub.f.sup.2]       -0.08

Standardized Effect Estimate (Absolute Value)

FIG. 1. Results of the Pareto analysis with reference to
molded parts weight (HDPE + 4% CBA.

Note: Table made from bar graph.


Figure 2 shows the change in parts weight, depending on the injection velocity, mold temperature, and injection temperature, on the example of parts obtained in the series C. It can be seen, that the lowest weight parts are obtained at a lower mold temperature and lower injection temperature. This can be explained by the fact, that the lower mold temperature favors the formation of small, numerous pores in the parts, on the other hand, the plastic at a lower temperature has a higher viscosity and more difficult fills the mold cavity. The injection velocity does not significantly affect the weight of parts.

Figure 3 shows the average values of parts weight obtained in the test plan agreements: 15 and 16, for A, B, and C series. It can be noticed, that in the solid parts and foamed parts with 2% content of blowing agent there are no significant differences in weight. Weight reduction, to the amount of 4% was achieved at the maximum blowing agent content, amounting 4%.

Molded Parts Density

Test samples used for density measurements were cut from the middle area of the tensile bars specimens, used in the measurement of parts weight. Pareto charts (Fig. 4) show that the mold temperature has the most significant influence on the density of parts obtained in series A, B, and C. When the mold temperature increases, the density of the parts also increases.

[T.sub.f]              2.99

[v.sub.w]             -2.05

[T.sub.f.sup.2]        1.06

[T.sub.w]             -0.76

[v.sub.w] [T.sub.w]    0.54

[T.sub.f] [T.sub.w]   -0.44

[v.sub.w.sup.2]       -0.41

[T.sub.w.sup.2]        0.29

[v.sub.w] [T.sub.f]   -0.13

Standardized Effect Estimate (Absolute Value) (a)

[T.sub.f]              3.89

[v.sub.w]              2.43

[T.sub.w.sup.2]       -1.58

[v.sub.w] [T.sub.w]    0.86

[T.sub.f] [T.sub.w]   -0.66

[T.sub.f.sup.2]       -0.61

[v.sub.w.sup.2]        0.26

[v.sub.w] [T.sub.f]   -0.14

[T.sub.w]             -0.04

p=0.05

Standardized Effect Estimate (Absolute Value) (b)

[T.sub.f]             15.91

[T.sub.w]              5.61

[T.sub.f] [T.sub.w]   -5.09

[v.sub.w]              4.91

[v.sub.w] [T.sub.f]   -3.41

[T.sub.w.sup.w]        2.45

[T.sub.f.sup.2]       -2.26

[v.sub.w] [T.sub.w]    1.41

[v.sub.w.sup.2]        0.12

Standardized Effect Estimate (Absolute Value) (c)

FIG. 4. Results of Pareto analysis with reference to the molded parts
density: (a) A series: HOPE without blowing agent, (b) B series: HDPE
+ 2% of blowing agent, (c) C series: HDPE + 4% of blowing agent.

Note: Table made from bar graph.


Figure 5 shows the dependence of the density of parts obtained in all series, in functions of the most relevant variables of processing conditions for processed polymer. It can be seen, that the highest density of parts was obtained at a higher mold temperature. For the parts obtained at A series, the differences occurring due to distinct injection conditions (injection velocity and mold temperature) were very small, about 0.3%. In the case of foam parts from HDPE with 2% of the blowing agent content, density of all parts was reduced of about 1%, compared with the solid parts. Differences in density due to distinct injection conditions (injection velocity and mold temperature) were rather small, and amounted to 0.6%.

From Fig. 6, showing the changes in the density of parts according to the percentage of blowing agent, it can be noticed, that in parts containing 4% of blowing agent the significant reduction in density compared with solid parts and parts with lower content of blowing agent addition occurs.

The phenomenon of increase in the density of the porous parts with increasing mold temperature can be explained by a tendency to create a few large pores in the structure of parts at high temperature. Also, higher temperature facilitates the mold cavity filling and allows for longer solidification of liquid plastic during the packing phase, resulting in a more densely packed polymer in the cavity. Furthermore, higher temperature favors the increase in the proportion of crystalline phase in the polymer, as result, with the increasing degree of crystallinity the density of parts increases.

Mechanical Properties

In tensile tests the tensile strength ([[sigma].sub.m]) and the elongation at maximum force ([[epslion].sub.m]) was determined. Examinations were carried out using Hegewald & Peschke Frame Desk 20 machine. The tension velocity was 50 Film/min. Pareto graphs made possible to investigate the influence of particular input variables on the [[sigma].sub.m] value which are presented in Fig. 7. The mold temperature has the most significant impact on the tensile strength of parts obtained in the all series of plan of experiment. Together with the mold temperature increase, the tensile strength increases, and is confirmed by the dependences presented in Fig. 8. A possible reason for increasing the tensile strength of solid and porous parts with increasing mold temperature is larger amount of crystalline phase in the parts obtained at higher value of this temperature. The addition of blowing agent lowered only slightly the tensile strength.

[T.sub.f]             16.74

[T.sub.f] [T.sub.w]   -3.54

[v.sub.w.sup.2]       -2.92

[v.sub.w]              2.76

[T.sub.w.sup.2]        1.44

[T.sub.f.sup.2]        1.25

[T.sub.w]             -1.18

[v.sub.w] [T.sub.w]   -0.82

[v.sub.w] [T.sub.f]    0.16

Standardized Effect Estimate (Absolute Value) (a)

[T.sub.f]             18.86

[T.sub.w]             -4.05

[T.sub.f] [T.sub.w]   -3.63

[T.sub.f.sup.2]        1.55

[v.sub.w.sup.2]       -1.16

[v.sub.w] [T.sub.w]    0.95

[T.sub.w.sup.2]        0.73

[v.sub.w] [T.sub.f]   -0.23

[v.sub.w]              0.05

Standardized Effect Estimate (Absolute Value) (b)

[T.sub.f]            25.91

[T.sub.f] [T.sub.w]  -4.75

[T.sub.w]             4.71

[v.sub.w.sup.2]       3.52

[v.sub.w] [T.sub.f]  -3.43

[v.sub.w] [T.sub.w]   2.47

[v.sub.w]             1.58

[T.sub.f.sup.2]       -1.0

[T.sub.w.sup.2]       0.20

Standardized Effect Estimate (Absolute Value) (c)

FIG. 7. Results of Pareto analysis with reference to the tensile
strength of molded parts: (a) A series: FIDPE without blowing agent.
(h) 13 series: HDPE + 2% of blowing agent, (c) C series: HDPE + 4%
of blowing agent.

Note: Table made from bar graph.


Changes in tensile strength and average elongation at maximum force of parts from HDPE obtained at the same injection parameters, specified in arrangement numbers 15 and 16 of plan of experiment, for series A, B, and C are shown in Fig. 9. The addition of blowing agent influenced significantly the mechanical properties of parts; reduced the tensile strength and increased elongation at maximum force. In parts from HDPE with the 4% of blowing agent content, the tensile strength decreased by approximately 11% (Fig. 9a), and elongation at maximum force increased approximately 6% (Fig. 9b), compared with the results for solid parts.

Gloss and Color of Molded Parts Surface

Surface state of molded parts was evaluated by measuring its gloss and color.

Gloss of Molded Parts Surface. The gloss examination was carried out on the Elcometer 406L glossmeter, using 60[degree] angle of light incidence. The results of gloss measurement are presented in gloss units (GU). The following notation was applied: [[alpha].sub.60[degree]]--the value of the gloss at the 60[degree] angle of light incidence.

From Pareto graphs showing the results obtained for the 60[degree] light angle (Fig. 10) it can be seen, that for the solid parts the main influence on [[alpha].sub.60[degree]] value has a mold temperature. With the increase of the mold temperature, the gloss of parts increases. High gloss of solid parts obtained at higher mold temperature can be explained by easily filling the mold cavity by polymer of a lower viscosity, fostered a better imaging of cavity surface.

[T.sub.f]             3.24

[T.sub.f.sup.2]       2.71

[T.sub.f] [T.sub.w]   2.66

[v.sub.w.sup.2]      -1.87

[v.sub.w]             0.81

[v.sub.w] [T.sub.f]   0.70

[T.sub.w.sup.2]       0.65

[T.sub.w]            -0.58

[v.sub.w] [T.sub.w]  -0.18

Standardized Effect Estimate (Absolute Value)

[v.sub.w]            15.90

[T.sub.w]            -7.44

[v.sub.w.sup.2]      -7.06

[v.sub.w] [T.sub.f]  -3.89

[v.sub.w] [T.sub.w]   3.28

[T.sub.f]             1.57

[T.sub.f.sup.2]      -1.06

[T.sub.f] [T.sub.w]  -0.96

[T.sub.w.sup.2]       0.30

Standardized Effect Estimate (Absolute Value)

[v.sub.w]            21.42

[T.sub.w]            -8.41

[T.sub.f]             6.68

[v.sub.w.sup.2]      -3.83

[T.sub.w]            -2.43

[v.sub.w] [T.sub.w]  -2.28

[T.sub.f] [T.sub.w]   2.13

[v.sub.w] [T.sub.f]   1.61

[T.sub.f.sup.2]       0.62

Standardized Effect Estimate (Absolute Value) (c)

FIG. 10. Results of Pareto analysis with reference to the molded
parts gloss for the 60[degree] angle of light incidence: (a) A
series: HDPE without blowing agent, (b) B series: HDPE + 2% of
blowing agent, (C) C series: HDPE + 4% of blowing agent.
HDPE + 4% of blowing agent.

Note: Table made from bar graph.


The gloss of molded parts with blowing agent depends mainly on the injection velocity, and in a minor way on the injection temperature. With the increasing injection velocity the gloss of parts increases. High injection velocity causes a significant orientation of the polymer during cavity filling stage and increase in melt temperature as a result of intense friction, which makes the cavity lilting easy and allows to obtain parts good imaging cavity surface. At the lower injection velocity, the cooling of slow-moving plastic in the cavity is more intensive and the solidification of the surface layer is faster, which can lead to the slowing down the flow and the formation of surface defects visible as flow marks which impair the gloss of molded parts.

Figure 11 shows the change in the value of [[alpha].sub.60[degree]] depending on the content of blowing agent for the HDPE parts, obtained in the test plan arrangements: 15 and 16, for A, B, and C series. It can be noticed, that the solid parts and parts containing 2% of the blowing agent are almost the same gloss, whereas, for parts from HDPE with 4% of blowing agent content, the gloss decreased by approximately 30%.

Examination of Molded Parts Color. The examination of molded parts color was carried out using the X-rite SP60 colorimeter. The CIELab method of color estimation was used. The results of measurements obtained by this method are described by three coordinates: a, b, and L. The value "a" determines the change in color from green to red, and the value "b" from blue to yellow. The value L means the luminance. For "a" and "b" zero-values, parameter L determines the change of color from black (for L = 0) to white (for L = 100).

Pareto graphs, in relation to the value L, in the parts obtained at different injection molding conditions, are shown in Fig. 12. Samples were obtained from color less HDPE.

[T.sub.f]            19.53

[v.sub.w]            -7.00

[v.sub.w] [T.sub.f]   3.41

[v.sub.w.sup.2]       2.47

[T.sub.w.sup.2]       1.61

[T.sub.f] [T.sub.w]  -0.62

[v.sub.w] [T.sub.w]  -0.61

[T.sub.f.sup.2]      -0.31

[T.sub.w]            -0.04

Standardized Effect Estimate (Absolute Value)

[T.sub.f]            11.97

[v.sub.w]            -4.97

[v.sub.w] [T.sub.f]   4.73

[v.sub.w.sup.2]       2.41

[T.sub.w.sup.2]       1.92

[T.sub.f] [T.sub.w]  -0.74

[v.sub.w] [T.sub.w]  -0.60

[T.sub.f.sup.2]      -0.51

[T.sub.w]            -0.29

Standardized Effect Estimate (Absolute Value)

[T.sub.f]            -9.23

[T.sub.f] [T.sub.w]  -3.88

[T.sub.w.sup.2]       2.18

[v.sub.w] [T.sub.w]   1.81

[T.sub.w]            -1.78

[v.sub.w.sup.2]       1.65

[v.sub.w]             1.58

[v.sub.w] [T.sub.f]  -0.35

[T.sub.f.sup.2]      -0.08

Standardized Effect Estimate (Absolute Value)

FIG. 12. Results ot Pareto analysis with reference to the value
L: (a) A series: HDPE without blowing agent, (b) B series: HDPE + 2%
of blowing agent, (c) C series: HDPE + 4% of blowing agent.

Note: Table made from bar graph.


Figure 12a and b shows that for the molded parts from solid HDPE and from HDPE with the 2% content of blowing agent, the value of the luminance L increases with the increase of the mold temperature and slightly decreases with the increase of the injection temperature. This dependence does not occur for the parts from HDPE with 4% of blowing agent, for which the higher mold temperature causes a decrease in the L value (Fig. 12c).

Figure 13 shows the change of luminance L of parts from HDPE obtained in the test plan arrangements: 15 and 16, for A, B, and C series, depending on the blowing agent content. The value of L. has changed only for molded parts with the 4% content of blowing agent. These parts became whiter.

From Fig. 14, illustrating the results of Pareto analysis in relation to value "a" it can be seen, that for solid or po-rous parts, value "a" decreases with the increase of the mold temperature. In the C series (Fig. 14c), the most significant impact has the injection temperature. At the higher mold temperature parts with a shade of green are obtained.

[T.sub.f]            -7.35

[T.sub.f.sup.2]      -1.68

[v.sub.w.sup.2]       1.38

[T.sub.f]            -1.35

[v.sub.w] [T.sub.t]  -0.70

[v.sub.w] [T.sub.w]  -0.65

[T.sub.f] [T.sub.w]   0.56

[v.sub.w]             0.14

[T.sub.w.sup.2]      -0.07

p=0.05

Standardized Effect Estimate (Absolute Value)

[T.sub.f]            -8.70

[v.sub.w]            -4.73

[T.sub.w.sup.2]       4.32

[v.sub.w] [T.sub.f]  -3.29

[T.sub.f.sup.2]       3.22

[T.sub.w]            -2.47

[T.sub.f] [T.sub.w]   2.33

[v.sub.w] [T.sub.w]  -1.80

[v.sub.w.sup.2]       1.18

Standardized Effect Estimate (Absolute Value)

[T.sub.w]            -14.2

[T.sub.f]            -9.39

[T.sub.f] [T.sub.w]   5.38

[v.sub.w]            -5.33

[T.sub.w.sup.2]       4.61

[v.sub.w] [T.sub.w]   2.24

[v.sub.w] [T.sub.f]   1.61

[v.sub.w.sup.2]       1.32

[T.sub.f.sup.2]       0.82

Standardized Effect Estimate (Absolute Value)

FIG. 14. Results of Pareto analysis with reference to the value
"a" of molded parts from HDPE: (a) without blowing agent (A
series), (h) with 2% of blowing agent (B series), (C) with 4% of
blowing agent (C series).

Note: Table made from bar graph.


The results of Pareto analysis in relation to the value "b" in the parts from each series are shown in Fig. 15. For parts from HDPE without blowing agent, injection temperature and mold temperature have the most significant impact on the value "b," with the increase of those injection conditions parameters, value "b" increases, and the parts become yellowish (Fig. 15a). For parts with the 2% content of blowing agent, the most significant impact on the value "b" has injection temperature, however, change of the value "b" is not linear. It is also an important interaction between injection velocity and mold temperature. This interaction has also a significant influence on the value "b" for parts with the 4% content of blowing agent.

[T.sub.w]             6.17

[T.sub.f]             3.97

[T.sub.f] [T.sub.w]   2.08

[T.sub.w.sup.2]      -1.85

[v.sub.w.sup.2]       1.27

[v.sub.w] [T.sub.w]  -0.83

[v.sub.w] [T.sub.f]  -0.67

[T.sub.f.sup.2]      -0.42

[v.sub.w]            -0.41

Standardized Effect Estimate (Absolute Value)

[T.sub.w.sup.2]      -8.87

[v.sub.w] [T.sub.f]   5.23

[T.sub.f]             4.20

[T.sub.w]             3.44

[T.sub.f.sup.2]      -2.32

[v.sub.w.sup.2]       2.16

[v.sub.w]            -1.80

[v.sub.w] [T.sub.w]  -1.70

[T.sub.f] [T.sub.w]   0.92

Standardized Effect Estimate (Absolute Value)

[v.sub.w] [T.sub.f]  -5.29

[T.sub.w]             4.55

[v.sub.w] [T.sub.w]  -4.04

[v.sub.w.sup.2]       3.41

[T.sub.f.sup.2]      -2.99

[T.sub.w.sup.2]      -2.59

[T.sub.f] [T.sub.w]  -1.89

[T.sub.f]            -1.35

[v.sub.w]            -0.77

Standardized Effect Estimate (Absolute Value)

FIG. 15. Results of Pareto analysis with rclerence to the value "b"
of molded parts from HDPE: (a without blowing agent (A series). (b)
with 2% of blowing agent (B series), (c) With 4% of blowing agent
(C series).

Note: Table made from bar graph.


The change of values "a" and "b" for parts from HDPE, obtained in the test plan arrangements: 15 and 16, for A, B, and C series, is shown in Fig. 16, It can be seen, that with the increase of the blowing agent content in polymer, value "a" decreases, which means a change of part color toward green, while the value "b" increase significantly, indicating a shill toward yellow.

Structure Investigations The structure of foam molded parts was examined using the Nikon Eclipse E200 microscope. Observations were carried out in the polarized light. Preparations used for the observation were in the form of cuttings of 25 microns in thickness, and were cut from the middle area of the tensile bars, perpendicularly to the polymer Row direction. Because the examinations have shown that the most significant influence on the weight of parts have mold temperature, the structure of parts obtained at extreme values of this temperature was investigated. Figure 17 shows the results of microscopic observation of parts from HDPE with the 2 and 4% content of blowing agent, at extreme values of mold temperature (20 and 70[degree]C).

From Fig. 17a and b it can be seen, that in the parts made of HDPE with the 2% of blowing agent content, the structure was obtained with few, large pores, and size is similar, regardless of the mold temperature. However, in the molded parts with the 4% of blowing agent addition (Fig. 17c and d), porous structure is largely dependent on the mold temperature. Structure with a small, densely spaced pores was obtained at a lower mold temperature, while the structure with less numerous, larger pores was obtained at higher mold temperature. This can be explained by the fact that at the higher mold temperature plastic solidification time is longer and the gas bubbles are linked together in a liquid polymer, while at the lower mold temperature, plastic solidifies more quickly limiting the growth of gas bubbles.

CONCLUSIONS

The investigation showed that the mold temperature have the main influence on the properties and surface state of molded parts from solid and foamed HDPE. With the increase of the mold temperature, the weight density, mechanical properties, and gloss of molded parts increases. In the foamed parts, it is related to their structure quantity and size of pores.

The blowing agent added to HDPE in the amount of 2% causes a slight decrease in weight, density, a slight worsening of the tensile strength, and gloss of the molded parts, and adding this amount of blowing agent results in small color change of parts. Significant changes occur in the parts molded from HDPE with 4% of the blowing agent. These parts were characterized by a smaller weight and density, but their tensile strength got worse, and the elongation at maximum force was increasing. Adding this amount of blowing agent also causes significant decrease in gloss and color change of parts. Molded parts from HDPE containing 4% of the blowing agent become duller and more yellow than the parts from HDPE without blowing agent and with its 2% content. It is therefore necessary to take into account this phenomenon on the stage of mold design, dye selection, and settings of the injection parameters.

The microscopic investigations showed that the mold temperature has a significant impact on the quantity and size of pores in the parts. At the lower mold temperature the numerous and fine pores occur in the core of the part made of HDPE with 4% of blowing agent, which is beneficial due to the mechanical properties of parts; however, their properties are lower than for parts made of HDPE without or with a smaller amount of the blowing agent.

Overall, the addition of 2% of the blowing agent for HDPE is sufficient to obtain parts with the good surface quality, and with the mechanical properties similar to the properties of solid plastic parts. The advantage of using this quantity of blowing agent is the possibility of reducing packing time from 31 s for solid parts to 24 s for porous parts, with no significant impact on their properties.

Correspondence to: Elzbieta Bociaga; e-mail: bociaga@ipp.pcz.pl

DOI 10.1002/pen.23316

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

[c] 2012 Society of Plastics Engineers

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Elzbieta Bociaga, Pawel Palutkiewicz

Department of Polymer Processing, Czestochowa University of Technology, Poland
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Title Annotation:high density polyethylene
Author:Bociaga, Elzbieta; Palutkiewicz, Pawel
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXPO
Date:Apr 1, 2013
Words:5812
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