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159Cellular Polymers, Vol. 35, No. 4, 2016

The Influence of Blowing Agent Addition, Glass Fiber Filler Content and Mold Temperature on Selected Properties, Surface State and Structure of Injection Molded Parts From Polyamide 6


Pawel Palutkiewicz1* and Tomasz Garbacz2

1Department of Polymer Processing, Czestochowa University of Technology, 19c Armii Krajowej Ave., 42-201 Czestochowa, Poland

2Department of Mechanical Engineering, Lublin University of Technology, 36 Nadbystrzycka Str., 20-816 Lublin, Poland

Received: 22 June 2015, Accepted: 8 October 2015

SummARy

The effects of blowing agent and glass fiber addition and also mold temperature on selected properties of PA6 molded parts was presented in this work. Blowing agent was dosed to plastic in amounts 0.5 - 1%, and glass fiber was added in amounts 15 - 30%. Furthermore, molded parts from unfilled and unfoamed PA6 was also investigated. The experimental plan was prepared using Design of Experiments (DoE) method. The results of selected part properties: molded parts weight, thickness, hardness, impact strength, tensile strength, elongation at maximum force, and also surface state of molded parts (gloss and color) was presented. The article also presents microscopic investigations (using SEM method) of molded parts with blowing agent and glass fiber. It was found, that the glass fiber content has a larger impact on mechanical properties of parts than addition of blowing agent. The use of the blowing agent in an amount of 1% wt. will allow the reduce injection cycle time by reducing the hold pressure and hold time, without significant worsening their properties. The mold temperature has an impact especially on the gloss of molded parts and the pore size.

*Corresponding author: [email protected]

160 Cellular Polymers, Vol. 35, No. 4, 2016

Pawel Palutkiewicz and Tomasz Garbacz

IntROductIOn

Foaming injection molding is one of the unconventional methods of injection molding process. In this method, molded parts with a porous inner core and a solid external skin are obtained, having a closed-cell structure [1-7].

The structure of porous molded parts can be obtained through chemical reactions, with the use of chemical blowing agents (known abbreviated CBA - chemical blowing agent, or CFA - chemical foaming agents) [1-7]. Chemical blowing agents are usually categorized in terms of the nature of the thermal decomposition, for exothermic and endothermic. In this method, the blowing agent is added to the processed plastic, in form of masterbatch or powder, in small amounts of 0.1 - 2% wt. The second method for obtaining porous molded parts is physical foaming [1-11]. In this method, the inert gas (PBA - physical blowing agent, or also known as PFA – physical foaming agent) is added directly to liquid plastic in the mold cavity or to the barrel of injection molding machine (e.g. MuCell method [9]) or extruder. It is also known physical foaming process with the use of microspheres (e.g. Expancel microspheres [12]). In this method, the masterbatch in granular form, from EVA with the gas inside (hydrocarbon) is added to processed polymer. At a high processing temperature gas expands, creating pores in the molded parts. This method can also be used in the extrusion process.

The foaming injection molding process with use of chemical blowing agents has many advantages: eliminate sink marks in molded parts, caused by the shrinkage, reducing cycle time, lowering molded parts weight and material saving. This is important from economic and ecological point of view (less amount of used plastic, lower energy consumption and less waste) [13]. The process of formation and the growth of pores is very complex, depending mostly on the type of blowing agent and used polymer, injection conditions, design of cavities of mold, shape and length of the runners, dimensions of gates, etc. The pore size and their distribution in polymer affect on many molded parts properties, especially mechanical and physical (e.g. thermal) and surface state [14-18].

The application of multi-types of polymer modification allows the improvement of various molded parts properties such as mechanical and other [19-25]. Modifications may be made in a chemical, physical or physico-chemical way. The main goal of the modification is to improve the useful properties, processability, dimensional stability, stiffness, and many others. Often it is not possible to achieve all improvements at once in of the all aspects. For an example, glass or carbon fibers are used to reinforce plastics products, while powder fillers, like talc or carbon black additives contributes to improving processability and reducing the shrinkage anisotropy.



The results of investigation of selected properties of molded parts of the composite ABS with quartz sand (30% wt.) and with the addition of a blowing agent in amount of 2% wt. was presented in work [25]. The measurements of weight and thickness of the samples (in form of tensile bars) and the changes in their mechanical properties was presented. It was found, that the occurrence of a blowing agent and quartz sand in a molded parts from ABS decrease their impact resistance and tensile strength. In the studies, the special design mold with variable height cavity was used. The structure investigation using optical microscopy shown, that in the cavity areas of a greater thickness, the plastic shear conditions allowed for emergence and growth of the pores. The use of filler in the form of a close-grained quartz sand and its orientation along the direction of the polymer flow, may affect the reduction of foaming effect. This effect indicates to the possibility of intermolecular interactions in the polymer matrix, leading to a reduction the expansion impact of the blowing agent, during the cooling phase of molded parts from ABS with quartz sand.

Authors of work [26] presented the results of investigation of fiber breakage reduction during polymer processing with foaming. It was found, that for parts from polypropylene filed with glass fiber, the presence of a chemical blowing agent addition reduced the occurrence of the fiber breakage during processing on injection molding machine. As the concentration of chemical blowing agent was increased from 0 to 5% wt., the fiber length distribution of the reinforcing agent more closely resembled the original distribution found in the virgin resin. The reduction in fiber rotation as a result of the decreased shear region present within the flow field, decreased the occurrence of fiber breakage due to buckling and tensile fracture. Wide rheological investigation of these composites were also presented.

Research work [27] presents the effect of mold cavity design and the way of cavity feeding on the selected mechanical properties and structure of the molded parts made from polypropylene filled with 30% wt. of glass fiber filler and 0.07 and 0.35% wt. of the Genitron EPB exothermic blowing agent. In presented investigations, an experimental mold with various cavity height (from 6 to 20 mm) was used, also molded parts with weld lines were investigated. The samples were injected at two values of injection time (1.2 and 3.1 s), which allow to obtain two different injection velocities. 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.

The works of AK Błędzki and O. Faruk [28-31] are presented the results of many investigations of the influence of injection molding processing conditions on the properties and structure of the molded parts obtained from polypropylene filled with the wood fiber and addition of chemical blowing agents to obtain a



microporous structure. In work [30] are presented the studies of the properties and structure of parts obtained from polypropylene with the addition of wood fiber (added in amounts of 30 and 50% wt.) with a compatibilizer (maleic anhydride), solid, and with the 2 to 5% wt. of the exothermic chemical blowing agent (Hydrocerol 530). The blowing agent caused a significant reduction in weight of molded parts (up to 30%), without causing decrease in tensile strength. Furthermore, work [28] shown, that for the same molded parts, with the increase value of mold temperature from 80 to 110°C, the surface roughness was reduced by approximately 70%. It also has been shown, that the 2% wt. of the blowing agent addition allows to obtain fine microcellular structure. The properties of molded parts made of polypropylene with a filler in the form of wood fibers added in an amount of 30%, and endothermic blowing agent ESC 5313 added in amount of 4% wt. were also investigated in work [29]. It has been shown, that the most favorable processing temperature of the filed polymer with the blowing agent for achieving a fine microporous structure is 160°C, Microscopic observations have shown, that the structure of foamed molded parts is dependent on the injection locations and thus depends on gate positions in the mold cavity. In areas closer to the place of plastic feeding into the cavity, numerous pores were uniformly distributed, whereas in areas remote from that point, there were fewer pores. This contributes to the occurrence of different mechanical properties in various areas of the molded parts. The samples obtained from areas in close proximity to the injection location (which occurred intense foaming), were characterized by lower tensile strength than the samples obtained from areas further away from this location. However, in studies of flexural strength, favorable results were obtained for samples collected from areas closer to gates.

The U.S. patent [32] presented the use of blowing agents as a method to enhance the strength of molded parts with weld lines. In presented exemplary investigations, the molded parts from polyethylene with mica filler in amount of 25% wt. and an endothermic blowing agent (a mixture of a polycarboxylic acid and an inorganic carbonate) in amount 0.6% wt. were examined. It was shown, that the obtained parts are not cracked in areas of weld lines. In addition, the foamed parts with weld lines had a higher tensile strength than the solid parts without and with these weld lines areas.

Works [33-37] presents the results of the investigations of influence of different processing conditions (injection temperature, mold temperature, injection velocity, and blowing agent percentage) on selected properties of molded parts. The investigations shown, that the main influence on the porous structure (quantity and size of pores, and therefore also on parts weight, density, thickness, mechanical properties and the surface state) has the mold temperature.



Blowing agents mixed with various fillers may also be widely used in the extrusion process [38, 39]. During manufacturing process of foamed extrudates, as a nucleating agent, both talc as well as blowing agent may be used [40-42]. This contributes to obtain a fine cellular structure in extrudates (up to 95% of foam).

Foamed molded parts are manufactured massively and also quality requirements for those parts are increasing. An example, can be molded parts widely used in the automotive industry. The addition of a small amount of blowing agent to the processed plastic (with properly set injection conditions), allows to obtain parts with the finely-cellular structure with small pores in core, without causing a significant reduction of mechanical properties. It is also possible to set a shorter cycle time of injection by replacing the hold pressure by blowing effects of bubbles of the gas evolved from blowing agent. In contrast, application of fillers allows to improve the mechanical properties, and to save an amount of the used plastic. The use of fillers can afford to restore some mechanical properties of foamed molded parts to solid parts properties. Additionally, by proper selection of the injection conditions (such as mold temperature), can be affect the formation of the expected structure of the molded parts, and thus to their properties.

ExpERImEntAl

The main aim of this work, was to evaluate the influence, of the blowing agent addition, glass fiber addition and mold temperature on molded parts properties, their: thickness, weight, hardness, impact resistance, tensile strength, elongation at maximum force, surface state (color and gloss) and structure. Two kind of plastic granules were used to made a samples: the unfilled polyamide PA6 (Bazamid PA6 Natural) and polyamide PA6 with 30% wt. of glass fiber (Bazamid PA6 GF30 Natural), filled with fibers during manufacturing process of granulate. Used plastics were in the natural, white color. Plastic was dried prior to injection molding for 24 hours in temperature 100°C. Samples from PA6 with 15% wt. of glass fiber were obtained by mixing two kind of granules, unfilled and filled with 30% wt. of glass fiber. The porous structure of parts was obtained by addition of Hydrocerol CF (Clariant) endothermic blowing agent (in a form of powder). Plastic granules were mixed with the blowing agent before the injection molding process in the plastic mixing machine (with lockable, sealed mixing zone) installed above the hopper of injection molding machine. Specimens, in the shape of tensile bars of 4.1 mm thickness, were produced according to PN-EN ISO 527-2:1998 standard, using two-cavity mold mounted on KraussMaffei KM65-160 C4 injection molding machine.



The experimental design with 16 input variables arrangements (Table 1) was created using StatSoft Inc. Statistica 10 software and Design of Experiments (DoE) module. The three independent input variables were: blowing agent content (ba, %), glass fiber content (gf, %) and mold temperature (Tf, °C). 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 hold pressure (20 MPa). Constant parameters of injection process were following: injection temperature: 265°C, injection velocity: 60 cm3/s, injection time: 0.5 s, cooling time: 20 s. The hold pressure (pd, MPa) and hold time (td, s) were reduced with an increase of the blowing agent percentage in the polymer, therefore these parameters also affect the properties of the molded parts.

Table 1. Plan of experiment

Arrangementnumber

Independent input variables Other injection conditions changed in parallel to input

variables

Blowing agent content

Glass fiber content

mold temperature

Hold pressure

Hold time

ba, % gf, % tf, ºc pd, mpa td, s

1 0 0 60 70 15

2 0 0 90 70 15

3 0 30 60 70 15

4 0 30 90 70 15

5 1 0 60 10 2

6 1 0 90 10 2

7 1 30 60 20 2

8 1 30 90 20 2

9 0 15 75 70 15

10 1 15 75 10 2

11 0.5 0 75 10 2

12 0.5 30 75 20 2

13 0.5 15 60 10 5

14 0.5 15 90 10 5

15 (C) 0.5 15 75 10 5

16 (C) 0.5 15 75 10 5

C - central point of plan of experiment



Regression analysis was performed, which allowed to present the relationship between the dependent variable - test size (the expected response) and independent variables (input quantities - the percentage of blowing agent, glass fiber and mold temperature). This relationship is represented by the model Equation (1):

(1)

where: βn - coefficients of the model equation.

Analysis of the results using the Statistica software has made it possible Pareto charts and assessing the impact of input parameters on properties of molded parts.

RESultS And dIScuSSIOn

Molded Parts Weight

The influence of input variables (blowing agent content, glass fiber content and mold temperature) on the molded parts weight was determined. The weight of molded parts from two mold cavities was determined and the average value was calculated. The Sartorius CP225 weighing machine with close measurement space was used and the weight of parts was determined with ±0.1 mg accuracy.

The analysis of the residuals plays an important role in the investigation of adequacy of the fitted model. The residuals are differences between the designated experimental values of the mass, and their corresponding values calculated from the model equation. In the first stage of analysis are sought the simplest form of the model Equation (1) (only with linear main effects, without quadratic and their interactions). But in some case it does not provide good results. Therefore assumed a complex model Equation (1), taking into account linear and quadratic main effects and their interactions. Figure 1 shows the dependence of the expected value of the normal to the model residuals for weight of molded parts. The vast majority of the points depending are placed along a straight line, which indicates a proper fit equation model to test results.

Figure 2 shows the effects of standardized Pareto analysis for parts from PA6. Sign “+” or “-” at the absolute value determines respectively evaluate the effect of increasing or decreasing the value of the variable analyzed.



Figure 1. Expected normal value as a function of model residuals for weight of molded parts

Figure 2. Results of the Pareto analysis with reference to molded parts weight



The investigations showed, that the glass fiber content has more significant influence on the weight of parts than the blowing agent. These dependencies are also clearly indicated on the Figure 3. It can be seen, that the 30% amount of glass fiber, led to 20% increase in weight of parts. The addition of the blowing agent has less impact on weight change of parts.

Figure 3. The change of weight m of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)

Figure 4 shows the optimization of the injection molding process in terms of weight increase of molded parts from PA6. Showed the weight of the moldings on the amount of filler and blowing agent, and mold temperature. For all Figures 4, 8, 12, 16, 20 and 24 presenting the graphic interpretation of optimization of the injection molding process, the error bars represented confidence intervals.

For example, in the upper left corner of Figure 4 shows the relationship of the weight of the parts, depending on the percentage of blowing agent at optimum amount of fiberglass and the optimum mold temperature.

(2)

In the upper right corner of Figure 4, can be read the value of desirability of the input quantity m for optimum other parameters (ba-optim, gf-optim, Tf-optim).



The lower part of the figure shows the three correlations of total desirability (Ud), for each independent input variables. A sample graph in the lower right corner shows the dependence of the total utility (Ud) in relation to the mold temperature, the optimal amount of blowing agent content and fiberglass content, that is:

(3)

Optimization of the selection of the three input variables on weight of molded parts were carried out. The result is a set of optimal input variables: ba = 0%, gf = 30%, Tf = 60°C. It can be also seen, that the weight of parts first reduces and then increases, as the temperature was increased from 60 to 90°C. It can be explained by two phenomenon’s occurring at the same time. The first phenomenon is fact that at a higher values of mold temperatures, the porous structure is not as favorable (large, very few pores) as structure obtained at low temperature (fine pores). This phenomenon is described later in this paper. The second phenomenon is that the weight of molded parts should decrease with amount of blowing agent, by with increasing of mold temperature, the flow resistance of the liquid plastic also decreases, so that larger amount of polymer solidifies in the mold cavity.

Figure 4. Graphic interpretation of optimization of the injection molding process for increasing the weight of the molded parts



Molded Parts Thickness

The thickness of molded parts was measured in the middle of molded parts using a digital micrometer with an accuracy of 0.01 mm. Height of the mold cavity was 4.1 mm. Figure 5 presents the results of the comparison of model residuals vs. values of the probability distribution. All points are arranged along a straight line, which is a prerequisite for further analysis.

Figure 5. Expected normal value as a function of model residuals for thickness of molded parts

Based on Pareto analysis (Figure 6), it can be concluded that the largest effect on the w molded parts thickness has the percentage of blowing agent.

From Figure 7 it can be seen, that parts obtained without blowing agent has the greatest thickness, because these parts were injected at high values of hold pressure and hold time. For parts obtained at mild amount of blowing agent, foaming process was insufficient to reduce the shrinkage, especially when hold pressure and hold time values were low, and obtained parts were about 4% thinner than the solid parts. For parts containing 1% of blowing agent, foaming process successfully eliminate the shrinkage effects, especially in parts unfilled glass fiber, but thickness of these parts was still less than parts from solid plastic.



Figure 6. Results of the Pareto analysis with reference to molded parts thickness

Figure 7. The change of thickness g of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



Optimization of the injection molding process (Figure 8) for increasing the thickness of the molded parts (and thus reduce shrinkage), showed the greatest thickness of the parts is obtained for parts from solid plastic. It is also worth mentioning, that the solid parts were a little greater thickness, because the hold pressure was higher and hold time was longer for those parts, comparing to foamed parts. The beneficial effects of use of blowing agent can be observed during comparing parts obtained with each other at a 0.5% and 1% of blowing agent. Parts from PA6 with a 1% of blowing agent have greater thickness than parts from polyamide with a 0.5% of blowing agent. This is because with an increase of the blowing agent percentage in the polymer, hold pressure and hold time was reduced. It was observed a little decrease in thickness due to the addition of filler in form of glass fiber. The use of fillers in the form of a fiber can contributes to increase the flow resistance of liquid polymer in the barrel, mold sprue, runners and cavities. Due to the flow resistance of the plastics less plastics solidifies in the mold. Another phenomena is a small effect of increasing of thickness with increasing mold temperature. At a higher mold temperatures, the porous structure is not very favorable, which reduces the thickness of the molded parts. This phenomenon is described later in this paper.

Figure 8. Graphic interpretation of optimization of the injection molding process for increasing the thickness of the molded parts



Hardness of Molded Parts

The hardness HB measurements of molded parts were carried out using ball indentation method according to PN-EN ISO 2039-1:2004 standard. From Figure 9 can be seen, that also in this investigation, all points are arranged along a straight line of function of model residuals. Figure 10 shows, that the

Figure 9. Expected normal value as a function of model residuals for hardness of molded parts

Figure 10. Results of the Pareto analysis with reference to molded parts hardness



biggest influence on the hardness of the molded parts is the interaction of blowing agent content and glass fiber content. The second most important factor is the percentage of the contents of the blowing agent. When blowing agent content increases, the hardness decreases. Porous molded parts are softer, this is due to the porous structure of the core.

From Figure 11 it can be seen, that glass fiber addition, especially in amount of 30% wt., contributes to increasing the hardness of the porous the parts, keeping their hardness on a par with molded parts from solid plastic with 30% of glass fiber. Foaming agent in an amount of 1%, added to the unfilled material, contributes to reduce the hardness of the parts by approx. 35%.

The charts of interpretation of optimization of the injection molding process for increasing the harness of the molded parts are shown on Figure 12. As can be seen, the optimal processing values for hard molded parts were as follows: no blowing agent and glass fiber filler. The values of mold temperature had no significant effect on hardness of parts.

Figure 11. The change of hardness H of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



Figure 12. Graphic interpretation of optimization of the injection molding process for increasing the harness of the molded parts

Impact Strength

The Charpy impact strength (acU) test was carried out using specimens without notch. Tests were conducted using a hammer Charpy-Izod 25 J Model IT 503 (Toropol). Measurements were performed in accordance with PN-EN ISO 179-2: 2001 II 2001. Test samples for the measurement of the impact of dimensions 80 × 10 × 4 mm were cut from the samples for the tensile test. The unnotched samples were used. Figure 13 shows, that many measuring points is situated on a straight line, so the analysis of the results is subject to a small uncertainty of measurement.

Glass fiber content has a greatest influence on impact strength of molded parts (Figure 14). When filler content increase, the impact strength increases.

Figure 15 shows this dependence. Parts from unfilled plastic without blowing agent has no impact strength (parts were elastic, there are not cracked). Addition of glass fiber in amount of 15% wt. increases impact strength of parts. The blowing agent also contributes to increasing impact strength, but to a lesser extent.



Figure 13. Expected normal value as a function of model residuals for impact strength of molded parts

Figure 14. Results of the Pareto analysis with reference to molded parts impact strength



Molded parts obtained with small amount of blowing agent (0 - 0,25% wt.), and with large quantity of filler (30% wt. of glass fiber) become more brittle than parts without filler. This is illustrated in the graphs (Figure 16).

Figure 15. The change of impact strength acU of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf, (Tf = 75°C)

Figure 16. Graphic interpretation of optimization of the injection molding process for increasing the impact strength of the molded parts



Mechanical Properties

Selected mechanical properties were investigated in tensile tests. The tensile strength (σm) and the elongation at maximum force (εm) were determined. The tensile tests were carried out using universal testing machine Inspect 20 Hegewald & Peschke (class I) with 20 kN capacity of load cell, and ± 0.5% accuracy. Tests were carried out according to PN-EN ISO 527-2:1998 standard. The tension velocity was 50 mm/min. The results of examinations are presented in Figures 17 to 24.

Tensile Strength

From Figure 17, which shows the dependence of the expected value of the normal to the model residuals for tensile strength of molded parts, can be seen, that most of points are situated on a straight line, which means a proper fit equation model to test results.

Figure 17. Expected normal value as a function of model residuals for tensile strength of molded parts

Glass fiber addition has the most influence on tensile strength (Figure 18). While glass fiber content increase, the tensile strength increases to. The 30% of glass fiber content contributed to increase the strength of the molded parts by 220% compared to solid molded parts. For 15% of the filler, the strength



has increased by 100% compared to solid parts. The blowing agent has a low impact on tensile strength, compared to impact of glass fiber. Blowing agent slightly weakens the strength of the parts. For parts from PA6 with 30% fiberglass, blowing agent in amount of 1% contribute to 6% decrease of tensile strength. This can be easily seen also in Figure 19.

Figure 18. Results of the Pareto analysis with reference to molded parts tensile strength

Figure 19. The change of tensile strength (σm) of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



Optimal processing condition for obtaining high strength molded parts were following: low amount of blowing agent (0 – 0.25% wt.) and high filler content (30% wt.). Mold temperature has no impact on mechanical strength of parts (Figure 20).

Figure 20. Graphic interpretation of optimization of the injection molding process for increasing the tensile strength of the molded parts

Elongation at Maximum Force

In case of investigations of elongation at maximum force (Figure 21), points are not located exactly on straight line, results have a certain inaccuracy, but their analysis is still possible.

The Pareto graph (Figure 22) shows, that interaction of blowing agent content and glass fiber filler has a most significant influence on elongation at maximum force of molded parts. With increasing of blowing agent and glass fiber content, the elongation decreases.

Parts become more brittle (which has been demonstrated in impact strength resistance investigations). Both types of molded parts: porous filled, and composite (blowing agent and filler) are characterized by brittle fracture. Plastic deformation occurred only in the case of solid, unfilled parts. It can be seen also in Figure 23.



Figure 21. Expected normal value as a function of model residuals for elongation at maximum force of molded parts

Figure 22. Results of the Pareto analysis with reference to molded parts elongation at maximum force



Optimal processing conditions for obtaining parts not characterized by brittle cracking were following (Figure 24): lack of blowing agent and filler, low mold temperature (60°C). Also, a small reduction of break with increase in

Figure 24. Graphic interpretation of optimization of the injection molding process for increasing the elongation at maximum force of the molded parts

Figure 23. The change of elongation at maximum force (εm) of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



temperature was observed. It can be explained by fact, that a higher mold temperature favors the growth of pores, and in this conditions the pores may also be combined with each other. As a result, plastic is solidified with a few large pores that give the “notch effect”, resulting in a reduction in the elongation at maximum force. This phenomenon may not be relevant from the point of view of applications (mostly molded parts are used in the elastic deformation area), but it allows to explore all of the features of the investigated material.

Surface State of Molded Parts

The surface condition of molded parts were evaluated in the investigations of color and gloss. Due to the limited volume of this publication in this part of the analysis, optimization was omitted.

Parts Color

The examination of molded parts color was carried out using the X-rite SP60 spherical 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). Due to the volume of this paper, the study of color and gloss shall not include charts showing expected normal values as a function of models residuals and also graphics interpretation of optimizations of the injection molding process.

The predominant effect on the luminance L of molded parts have the percentage of blowing agent (Figure 25). Luminance increases with increase of amount of blowing agent. Glass fiber addition is the second most important factor influences on this parameter. Also, with an increase of glass fiber addition luminance increases to. Filled, porous parts are brighter than the solid plastic molded parts. It can be easily seen on Figure 26.

The blowing agent and a filler also affects the color parameters of the parts. The biggest influence on the color change has a blowing agent content (Figures 27 to 30).



Figure 25. Results of the Pareto analysis with reference to L parameter

Figure 26. The change of luminance L of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



Figure 27. Results of the Pareto analysis with reference to a parameter

Figure 28. The change of a parameter of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



Figure 29. Results of the Pareto analysis with reference to b parameter

Figure 30. The change of b parameter of molded parts from PA6 in function of blowing agent addition ba and glass fiber addition gf , (Tf = 75°C)



It can be seen, that with increasing of blowing agent content, decreases the value of coordinate “a”, and increases the value of the coordinate “b”. Porous parts are greener and more yellow. With the increase of glass fiber content, but in a small extent, decreases the coordinate value of “a”, and growing coordinate value “b”. The obtained molded parts with glass fiber also change color to more green and yellow.

Gloss of Parts

The gloss examinations were carried out using the Elcometer 406L glossmeter. Examination consists of measuring the intensity of the reflected and diffuse light in a narrow range of angles of reflection. The intensity of the reflected light depends on the material and the angle of incidence of light. The results of gloss measurement are presented in gloss units (GU). High gloss is characterized by a large value of GU (up to 100), while a low GU value indicates matt surface (no gloss). Gloss measurement was carried out at an angle of incidence of 60° (α60°).

Gloss of molded parts mainly depends on the temperature of the mold. With the increase of the mold temperature, gloss increases (Figure 31). The addition of glass fiber has less impact on gloss, than the mold temperature. With increasing fiber content, the value gloss decreases, but only in a low mold temperature (60°C) (Figure 32).

Figure 31. Results of the Pareto analysis with reference to gloss parameter α60°



Figure 32. The change of α60° parameter of molded parts from PA6 in function of glass fiber addition gf and mold temperature Tf, (ba = 0.5% wt.)

Structure Investigations of Molded Parts

The structure of foamed parts was examined using the SEM microscope. Preparations used for the observation were cut from the middle area of the tensile bars, perpendicularly to the polymer flow direction. The results of microscopic observations are presented in Figure 33.

Figure 33 presents images of the structure of molded parts obtained at different amounts of blowing agent, various amounts of the filler (fiberglass), and different values of mold temperature, obtained also at different processing conditions (the hold pressure and hold time, but this should not affect significantly on the structure of investigated molded parts). For obvious reasons has been omitted the structure of solid, unfilled parts. Also, images of parts from agreements 3 and 16 are not shown, because parts from arrangements 3 and 4 differ only in the value of the mold temperature, which does not affect the orientation of fibers, and also, the parts from arrangements 15 and 16 are obtained with the same processing conditions (these were control samples). Photos 4 and 9 show the distribution of the fibers in the solid parts without a blowing agent content and filled with fiberglass, in quantities successively 30 and 15% wt. (arrangement number 4 and 9 of plan of experiment). Parts obtained from arrangement number 4



have a larger the amount of fibers in the structure, which is obvious. Photos number 5, 6 and 11 show the structure of the unfilled porous parts. Parts obtained from the arrangements numbers 5 and 6 differs only in value of mold temperature, successively 60 and 90°C. It can be seen, that for molded parts obtained at lower value of mold temperature, the structure of fine, closely spaced pores was obtained, while at a higher mold temperature, the structure of lower number of large pores was obtained, as well as for part from arrangement number 11. This can be explained by the fact, that at higher mold temperature, the cooling time and polymer solidification time is longer, and therefore there is also long process of foaming, which promotes growth of the gas bubbles, and their joining in a liquid polymer. However,

Figure 33. The SEM morphology of molded parts from PA6 with various amounts of blowing agent and glass fiber, obtained at different processing conditions, according to arrangement of plan of experiment



at the lower temperature plastic solidifies faster, limiting the growth of gas bubbles. Also, the investigations in works [33-37] showed that the mold temperature has a significant impact on the quantity and size of pores in the molded parts. Comparing SEM images can be seen, that the amount of blowing agent does not substantially affect the number of pores in the parts, however, the effect exists, as shown by the results of the investigations of parts weight. The relationship between the pore size and the temperature of the mold is also visible for parts filled with 30% wt. of filler (arrangement 7 and 8), and 15% wt. of filler (arrangements 13 and 14). Comparing parts with 15 and 30% wt. of fiberglass addition (arrangements numbers 12 and 15, with 0.5% wt. content of blowing agent and obtained at the same value of mold temperature), it can be seen, that the amount of filler does not greatly affect the size and number of pores in the parts.

cOncluSIOnS

The investigations presented show that the use of blowing agents for molded parts filled with fibrous filler (fiberglass) has a number of advantages. The use of the blowing agent in an amount of 1% wt. will allow the reduce injection cycle time (reducing the hold pressure and hold time), while maintaining good dimensional accuracy of parts. Moreover, the porous molded parts have lower weight than solid molded parts, with little reduction in strength properties, such as tensile strength and elongation at maximum force. Adding a blowing agent in the plastic also contributes to decrease the hardness of parts, which may be an advantage (more flexible products). However, the blowing agent addition significantly affected the color, so it is important to take account of this phenomenon during the design of the injection molding process. It has been shown that the presence of the fibrous filler (as fiberglass) in the processed plastic (PA6), does not significantly affect the occurrence of pores in the parts.

Glass fiber added to the plastic to the greatest extent affected the properties of the produced parts. It contributes to a significant increase in the weight of molded parts, a large increase in impact strength and tensile strength. The glass fiber reduces the values of elongation at maximum force, and also changes the color of molded parts.

Investigations have shown that the temperature of the mold has an impact primarily on the gloss of molded parts and the pore size. At a higher mold temperature, few large pores are obtained, as demonstrated by the SEM structure investigations.



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