A Novel Hybrid Foaming Method for Low-Pressure Microcellular Foam Production of Unfilled and Talc-Filled Copolymer Polypropylenes

Unfilled and talc-filled Copolymer Polypropylene (PP) samples were produced through low-pressure foam-injection molding (FIM). The foaming stage of the process has been facilitated through a chemical blowing agent (C6H7NaO7 and CaCO3 mixture), a physical blowing agent (supercritical N2) and a novel hybrid foaming (combination of said chemical and physical foaming agents). Three weight-saving levels were produced with the varying foaming methods and compared to conventional injection molding. The unfilled PP foams produced through chemical blowing agent exhibited the strongest mechanical characteristics due to larger skin wall thicknesses, while the weakest were that of the talc-filled PP through the hybrid foaming technique. However, the hybrid foaming produced superior microcellular foams for both PPs due to calcium carbonate (CaCO3) enhancing the nucleation phase.


Introduction
The use of plastics has enabled "quick wins" for automotive manufacturers, along with other similar industry sectors, in terms of metal replacement. Ensuring that the use of polymers meets the growing legislative requirements and reduction in environmental damage requires the use of novel processing innovation. The manufacture of polymer components with a microcellular cross-section provides a novel solution that reduces component weight, material usage, and processing energy requirements [1][2][3][4][5]. Foam-injection molding (FIM) shows high potential in creating parts with higher dimensional accuracy with reduced final weight [6].
In comparison to conventional injection molding (IM), the benefits of using FIM components and the subsequent processing are witnessed in the weight reduction of the resulting parts which ranges from 0 to 15% [7]. Also, FIM produces parts with higher impact strength, higher energy absorption and better thermal and acoustic insulation [8].
The process of FIM is very similar to IM except that the packing phase can be either completely removed, or lowered considerably, depending on whether low-pressure or high-pressure FIM is being conducted, respectively [9,10]. The four additional steps for low-pressure FIM from IM are: gas mixing and dissolution within the polymer melt (occurs during the plasticizing stage), cell nucleation (during injection stage), growing of the cells and shaping within the mold (during polymer cooling) [11]. The benefits to using low-pressure FIM is that large component weight savings can be achieved. For low-pressure FIM the resulting component weight-saving is lower however the cell structure of high-pressure foaming shows better microcellular structure [12]. Towards nano-cellular microstructure The CBA used was TecoCell ® H1 Chemical Blowing Agent, supplied by Trexel Inc. It creates an endothermic, chemical reaction at 200 °C between monosodium citrate (C6H7NaO7) and calcium carbonate (CaCO3) to release carbon dioxide (CO2) into the IM barrel [38].

Processing
Production of the injection molded parts were performed using an IM machine (e-Victory 120, ENGEL, Warwick, UK) with a clamping force of 1200 kN and a screw diameter of 40 mm. Alongside this IM machine, there is a gas dosing unit (T100, Trexel GmbH, Gummersbach, Germany) which enables MuCell ® IM to be performed. The SCF used for this research was nitrogen (N2) with 99.998% purity. Prior to any molding, the polymer was dried at 80 °C for a minimum of 4 hours in a commercial drier.
For both unfilled PP and talc-filled PP, the first processing performed was a conventional IM part. This was to get the benchmark figures to base the rest of the research from. After the conventional IM parts were produced, CBA was added to the process in increments of 1%, 2% and 5%. Prior to the experiment, preliminary experiments were conducted to ensure the appropriate stroke length was used based on the underpinning CBA content. For all the experiments performed on each polymer, everything was kept constant, and repeatability was performed by producing 50 parts for each experimental setting. The only changes made to the processing for each stage was the shot volume; this was lowered to accommodate for the extra foaming which was occurring.
The weight of the parts was taken and then the resulting weight was matched with the PBA. Here, the processing changes for each stage included the shot volume along with the SCF dosing amount. For higher weight reduction percentages; a higher SCF dosing was applied along with a compensation in the shot volume. For all the FIM processed in this paper, low pressure was used: using no holding pressure. This was only performed on the conventionally molded parts. The key processing data for each stage is listed in Table 1 for unfilled PP and Table 2 for talc-filled PP. The resulting average weight for each of these settings is also shown. All the other main processing parameters (Table 3) were used based on information from the material supplier recommendations and preliminary studies prior to these experiments.
There are 2 tensile bars that are produced with each production, both complying to Type A1 within BS EN ISO 20753:2014 ( Figure 2). The mold is made from Grade 6061 Aluminum. The CBA used was TecoCell ® H1 Chemical Blowing Agent, supplied by Trexel Inc. It creates an endothermic, chemical reaction at 200 • C between monosodium citrate (C 6 H 7 NaO 7 ) and calcium carbonate (CaCO 3 ) to release carbon dioxide (CO 2 ) into the IM barrel [38].

Processing
Production of the injection molded parts were performed using an IM machine (e-Victory 120, ENGEL, Warwick, UK) with a clamping force of 1200 kN and a screw diameter of 40 mm. Alongside this IM machine, there is a gas dosing unit (T100, Trexel GmbH, Gummersbach, Germany) which enables MuCell ® IM to be performed. The SCF used for this research was nitrogen (N 2 ) with 99.998% purity. Prior to any molding, the polymer was dried at 80 • C for a minimum of 4 hours in a commercial drier.
For both unfilled PP and talc-filled PP, the first processing performed was a conventional IM part. This was to get the benchmark figures to base the rest of the research from. After the conventional IM parts were produced, CBA was added to the process in increments of 1%, 2% and 5%. Prior to the experiment, preliminary experiments were conducted to ensure the appropriate stroke length was used based on the underpinning CBA content. For all the experiments performed on each polymer, everything was kept constant, and repeatability was performed by producing 50 parts for each experimental setting. The only changes made to the processing for each stage was the shot volume; this was lowered to accommodate for the extra foaming which was occurring.
The weight of the parts was taken and then the resulting weight was matched with the PBA. Here, the processing changes for each stage included the shot volume along with the SCF dosing amount. For higher weight reduction percentages; a higher SCF dosing was applied along with a compensation in the shot volume. For all the FIM processed in this paper, low pressure was used: using no holding pressure. This was only performed on the conventionally molded parts. The key processing data for each stage is listed in Table 1 for unfilled PP and Table 2 for talc-filled PP. The resulting average weight for each of these settings is also shown. All the other main processing parameters (Table 3) were used based on information from the material supplier recommendations and preliminary studies prior to these experiments.  23 23 There are 2 tensile bars that are produced with each production, both complying to Type A1 within BS EN ISO 20753:2014 ( Figure 2). The mold is made from Grade 6061 Aluminum.

Simple Modeling
Prior to any microscopy visualization or mechanical testing, simple modeling can be performed using the processing settings and simple model equations. These equations are listed below in further detail and have been verified in previously published research journals [39]. The model assumes that the cross-sectional structure of the foamed parts comprises of a foamed core with a skin surface, modeled by the integral skin foams (ISF) [39]. However, this is a simplified model as in reality, the cross-sectional structure varies throughout. There exist larger cells in the center, leading to smaller and better distributed cells nearer the skin and then a transition between the skin and this foamed center.

Simple Modeling
Prior to any microscopy visualization or mechanical testing, simple modeling can be performed using the processing settings and simple model equations. These equations are listed below in further detail and have been verified in previously published research journals [39]. The model assumes that the cross-sectional structure of the foamed parts comprises of a foamed core with a skin surface, modeled by the integral skin foams (ISF) [39]. However, this is a simplified model as in reality, the cross-sectional structure varies throughout. There exist larger cells in the center, leading to smaller and better distributed cells nearer the skin and then a transition between the skin and this foamed center.
Enhancement of this modeling will be completed in further research but for this research, the model was used as a basic prediction to prove that it can provide prior knowledge to the experiments. The skin thickness was kept constant and as the samples used for this research were all semicrystalline, a nominal value of 10% [39] of the average part geometry was used; in this case 0.7 mm.
Tensile strength model equations: Enhancement of this modeling will be completed in further research but for this research, the model was used as a basic prediction to prove that it can provide prior knowledge to the experiments. The skin thickness was kept constant and as the samples used for this research were all semi-crystalline, a nominal value of 10% [39] of the average part geometry was used; in this case 0.7 mm.
Tensile strength model equations: where R tu is the ratio of tensile strength of unfilled foamed polymer to unfilled solid polymer, b is the sample width, h is the sample thickness, t is the sample average skin thickness and R is the ratio of the weight reduction for the foamed core. All the parameters are known (and t is kept constant at 0.7 mm) and then R can be calculated using Equation (2) whereby R w is the weight reduction of the whole part. Flexural strength model equations: Like the equations shown in the previous section, the flexural modulus of unfilled polymer shows proportionally to the square of the density when microcellular foamed. The flexural strength ratio, R fu , for unfilled polymer whereby all the inputs are the same as mentioned previously while the real reduction in the foamed core, R, can be calculated using Equation (4).

Internal Cell Structure
The cell morphology of all the samples were examined at 85 mm along the part (midpoint). All parts were scorn in the required area and then submerged in liquid nitrogen for 4 hours before being snapped along the scorn line ( Figure 2). These surfaces were analyzed using a scanning electron microscope (SEM) (Evo LS 25, Zeiss, Cambridge, UK). Low Vacuum mode was used to gain a clear contrast while also obtaining a large depth of view.
Gwyddion data visualization and data analysis software was used in order to measure skin thickness, cell density and average cell diameter [40]. The skin thickness was measured until the first cells were seen to be visible [29].

Tensile Properties
The first of the mechanical properties analyzed on the samples was through tensile tests performed on a mechanical testing unit (H25 KS, Hounsfield, Surrey, UK). For each experimental setting performed, 10 parts were assessed. BS EN ISO 527-1:2012 was followed to obtain the maximum stress and an Axial Extensometer (3542, Epsilon, WY, USA), to calculate the Young's Modulus. The maximum force was divided by the cross-sectional area to obtain the maximum stress (Equation (5)), while the chord slope method was used to calculate the tensile modulus between strain values of 0.0005 and 0.0025 (Equation (6)). The experiment was performed at 1 mm/min until a strain of 0.0025 had been met, then increased to 10 mm/min until break.

Flexural Modulus Properties
The other mechanical property that was analyzed was the flexural modulus of the samples. Again, 10 samples of each experimental setting were tested using a mechanical testing unit (H25 KS, Hounsfield, Surrey, UK). Due to the high flexural nature of the polymers, BS EN ISO 14125:2011 was followed with a 3 point flexural structure to accommodate for the high deflection. The large deflection correction calculation was used (Equation (7)) as all the deflections performed in this research complied to the criteria of the deflection being greater a tenth of the span. 2 mm/min speed was performed until the maximum force had been met and started to decrease. Figure 3 shows the tensile strength results against the predicted result through the simple modeling method for unfilled PP and talc-filled PP. The error range for unfilled PP is between 22.2% to 9.5% while the talc-filled PP has greater inaccuracies between 26.0% to 12.4%. The main reasoning of this is that the equations have not been modified to accommodate for the incorporation of talc into the polymer, the equations that were previously derived were for the addition of talc to a polymer and not for a polymer with talc already incorporated into its polymer matrix. and (2), any minor changes to the t (thickness) value, results in a major difference in results. The skin thickness was averaged at 0.7 mm for all the samples in this research. However, Table 4 shows that the actual skin thickness varies from each experimental setting and thus, making the simple modeling a generalized prediction for this research. For both polymers, the models show greater accuracy in predictions for physical foaming and a combination of foaming methods and shows higher that 15% inaccuracies for all parts produced through chemical foaming.  Figure 4 is the flexural strength results of the experimental and predicted for unfilled PP and talc-filled PP. The largest error is 16.5% while the smallest error is 3.1% (seen as the dashed line, whereby the predicted data for the talc PP is an over prediction). Thus, showing that the flexural strength simple modeling gives improved results compared to that of the tensile strength for the polymers in this experiment. As with the tensile results, the least accurate comparisons come from the chemical foaming agent while the most accurate is that of the hybrid foaming method. All the modeled results are underestimations of the actual results. From analyzing Equations (1) and (2), any minor changes to the t (thickness) value, results in a major difference in results. The skin thickness was averaged at 0.7 mm for all the samples in this research. However, Table 4 shows that the actual skin thickness varies from each experimental setting and thus, making the simple modeling a generalized prediction for this research. For both polymers, the models show greater accuracy in predictions for physical foaming and a combination of foaming methods and shows higher that 15% inaccuracies for all parts produced through chemical foaming.  Figure 4 is the flexural strength results of the experimental and predicted for unfilled PP and talc-filled PP. The largest error is 16.5% while the smallest error is 3.1% (seen as the dashed line, whereby the predicted data for the talc PP is an over prediction). Thus, showing that the flexural strength simple modeling gives improved results compared to that of the tensile strength for the polymers in this experiment. As with the tensile results, the least accurate comparisons come from the chemical foaming agent while the most accurate is that of the hybrid foaming method.

Unfilled PP Analysis
As seen in Figure 5, the PBA settings gives the poorest quality cellular structure from all the experimental settings. The CBA gives some improved cellular structure but there are areas of no foaming within the part (excluding the skin structure). Meanwhile, the hybrid foamed parts gave the best cellular structure, this being confirmed in Table 4 as the smallest cell diameter and highest cell density is seen in the hybrid foaming of the unfilled PP. It is well known that unfilled PP is a poor choice of polymers for the foaming process due to the lamella in the semi-crystalline structure restricting the diffusion and also, the nucleation of the foaming process [11]. This would explain why the PBA parts in this polymer have exhibited poor cellular structure. The large cells are formed due to high shear stress applied to the polymer from the nucleation, whereby cell coalescence is present and causes many smaller cells to form larger ones [41]. CaCO3 has previously been added to PP in batch processing foaming method and shown to decrease the average cell diameter, increase the cell density and show a more homogenous cell structure [42][43][44]. With the composition of the CBA being mainly CaCO3, then this has proven to show good cellular structure with the foaming process with FIM.

Unfilled PP Analysis
As seen in Figure 5, the PBA settings gives the poorest quality cellular structure from all the experimental settings. The CBA gives some improved cellular structure but there are areas of no foaming within the part (excluding the skin structure). Meanwhile, the hybrid foamed parts gave the best cellular structure, this being confirmed in Table 4 as the smallest cell diameter and highest cell density is seen in the hybrid foaming of the unfilled PP. It is well known that unfilled PP is a poor choice of polymers for the foaming process due to the lamella in the semi-crystalline structure restricting the diffusion and also, the nucleation of the foaming process [11]. This would explain why the PBA parts in this polymer have exhibited poor cellular structure. The large cells are formed due to high shear stress applied to the polymer from the nucleation, whereby cell coalescence is present and causes many smaller cells to form larger ones [41]. CaCO 3 has previously been added to PP in batch processing foaming method and shown to decrease the average cell diameter, increase the cell density and show a more homogenous cell structure [42][43][44]. With the composition of the CBA being mainly CaCO 3 , then this has proven to show good cellular structure with the foaming process with FIM.

Talc-Filled PP Analysis
In comparison to the images for unfilled PP ( Figure 5), it can clearly be identified that talc-filled PP shows enhanced properties for the foaming process ( Figure 6), except for the previously explained hybrid foaming of the unfilled PP. The CBA still gives an improved cellular structure to that of the PBA, but the PBA is significantly better with the addition of talc. Like the unfilled PP, the hybrid foaming method gives superior microcellular structure for the talc-filled PP also, with the increased foaming producing smaller, finer cellular structure. Table 4 confirms that the smallest cell diameters and highest cell density are indeed seen in the hybrid foaming of the talc-filled PP.

Talc-Filled PP Analysis
In comparison to the images for unfilled PP (Figure 5), it can clearly be identified that talc-filled PP shows enhanced properties for the foaming process ( Figure 6), except for the previously explained hybrid foaming of the unfilled PP. The CBA still gives an improved cellular structure to that of the PBA, but the PBA is significantly better with the addition of talc. Like the unfilled PP, the hybrid foaming method gives superior microcellular structure for the talc-filled PP also, with the increased foaming producing smaller, finer cellular structure.

Talc-Filled PP Analysis
In comparison to the images for unfilled PP ( Figure 5), it can clearly be identified that talc-filled PP shows enhanced properties for the foaming process ( Figure 6), except for the previously explained hybrid foaming of the unfilled PP. The CBA still gives an improved cellular structure to that of the PBA, but the PBA is significantly better with the addition of talc. Like the unfilled PP, the hybrid foaming method gives superior microcellular structure for the talc-filled PP also, with the increased foaming producing smaller, finer cellular structure. Table 4 confirms that the smallest cell diameters and highest cell density are indeed seen in the hybrid foaming of the talc-filled PP.   Table 4 confirms that the smallest cell diameters and highest cell density are indeed seen in the hybrid foaming of the talc-filled PP.

Modulus of Elasticity (E)
For each experiment observed in Tables 2 and 3, 10 parts were tested and the mean value of E and S u were calculated. The effects of the selected process factors on the E of the parts were investigated together with the weight of the parts produced.
The mean value E plot for unfilled PP (Figure 7) shows that parts produced with chemical foaming have the highest ability to withstand changes in length when under tension, 949.7 MPa, this is then followed by the combination of the two foaming methods of 825.4 MPa. Physical foaming is the lowest with 817.5 MPa. For the parts made with chemical and the hybrid foaming the parts with a higher mass have an increased ability to withstand a change in length, both have a reduction of E with a decrease in the part weight. Interestingly, there is an increased deviation in E for the physical foam test parts with the lowest mass, which can be explained by the poor foaming ability of unfilled PP. Physical foaming results show a different stress strain behavior to chemical and the combination sample in that the E increases as the mass of the parts.

Ultimate Tensile Strength (Su)
Like the modulus of elasticity, the Su of the parts were investigated together with the weight of the parts produced. The addition of the predicted results was also added to Figure 8.
The mean value Su plot for unfilled PP (Figure 8a) shows that parts produced with chemical foaming have the highest ability to withstand loads that elongate the test part (19.2 MPa) this is then is then followed by the combination of the two foaming methods (17.1 MPa). Physical foaming is shown to be generally lower Su and this is constant regardless of the mass of the test parts; as with the previous section, the poor results from the physical foaming of unfilled PP is well-known. All three foam processes demonstrate a tensile strength which is significantly higher than the simulated results. For the parts made with chemical and the combination of chemical and physical foaming the parts with the higher mass have an increased ability to withstand tensile loads. Like the E results, the hybrid foam methods have a much wider distribution of results. The deviation for the hybrid foaming methods is caused by the nucleation rate being too high, with two foaming methods combined, the nucleation energy is increased substantially and therefore causing large variations in morphology of the final part.
For the test parts produced with talc-filled PP ( Figure 8b) the overall behavior is like the E results: in that the Su reduces with reduced mass. The lower Su values compared to the unfilled PP, can be explained as the ductility is hampered with talc addition due to stress concentrations and poor shear slipping ability of the polymer matrix [33]. Su is a product of force per unit area and the nonhomogeneous cross sections and bubble content that reduces mass clearly changes the test part unit area. Chemical and physical foaming methods have a higher Su than the hybrid foaming method and all three are higher than the simulation results. The inclusion of a talc filler together with physical foaming has a notable influence on Su. There is a Su decrease with a reduction in part mass; explained Like the unfilled PP, the parts produced through the hybrid foaming method, have a much larger deviation than the other two methods and thus make a more unpredictable part. The ratio of E and mass is also the highest for the hybrid foaming, coupled with the large deviation in these results identifies a significant material and foaming behavior in terms of the parts ability to withstand changes in length when under tension. The process control of physical foaming and PP talc results are more predictable with the inclusion of the talc filler when compare to the unfilled PP results. This can be justified that talc is a well-known nucleating agent for polyolefin materials which here, has shown to produce foamed parts with which have higher E values and are more consistent.

Ultimate Tensile Strength (S u )
Like the modulus of elasticity, the S u of the parts were investigated together with the weight of the parts produced. The addition of the predicted results was also added to Figure 8.

Flexural Modulus Data
Tensile mechanical properties are important for polymer part design. In addition to E and Su the mechanical measure of maximum load bearing capability of a material without undergoing any permanent deformation is considered. Maximum Flexural strength (σfM) results are shown for the two PP materials (Figure 9).
The mean value σfM plot for unfilled PP (Figure 9a) shows that parts produced with chemical foaming have the highest ability to resist breakage when bent (30.8 MPa). This is then followed by the combination of the two foaming methods (29.8 MPa). For the parts made with chemical and the combination of chemical and physical foaming the parts with a higher mass can resist the highest stress. The values are 30.5 MPa and 28.9 MPa respectively, thus identifying chemical foaming not only as the highest σfM but also more consistent over a range of part masses. There is a wide range of deviation in σfM for the hybrid foam test parts and this is consistent for the tensile tests in the previous section. The physical foaming results show different σfM behavior when compared to the other foaming methods and this outcome is like the tensile results also.
The results for the test parts produced with talc-filled PP (Figure 9b) show a different behavior to the unfilled PP behavior. All foaming combinations have a lower σfM than unfilled PP. They also have a similar σfM (27.5-27.3 MPa) at high mass and this σfM decreases as the mass decreases due to more foaming in the parts. Again, chemical foaming is more consistent in terms of mass and mechanical properties. In terms of the parts ability to withstand highest stress at its moment of yield the process control of combination foaming with talc-filled PP shows that there is a wide σfM distribution particularly with parts with low mass.
All the mechanical test data presented in this research, except foaming of unfilled PP, follows a clear trend, with reduced weight and increased foams, the mechanical properties decrease. The experimental data with generally the strongest mechanical properties, the chemically foamed parts, does not have superior cell density but, in the case of both PPs tested, they do have the thickest skin thicknesses. The current research trend is to obtain foamed polymer samples with the highest cell density but in this research, it can be observed that cell density has little influence on mechanical strength, in comparison to skin thickness; which has a major weighting on the tensile and flexural properties. Shish-kebabs show highly superior strength and modulus, but are only able to form on the skin layer due to the weak shear and fast solidification [31]. However, when the skin begins to reduce in size, then this can lead to reduced mechanical strength.
The simple model results in Figure 9 are again, under predictions of the actual results, apart from the highest weight-saving for the talc-filled PP using the hybrid foaming method. The conclusion for this is the same as previously mentioned for the tensile tests; the skin thickness used The mean value S u plot for unfilled PP (Figure 8a) shows that parts produced with chemical foaming have the highest ability to withstand loads that elongate the test part (19.2 MPa) this is then is then followed by the combination of the two foaming methods (17.1 MPa). Physical foaming is shown to be generally lower S u and this is constant regardless of the mass of the test parts; as with the previous section, the poor results from the physical foaming of unfilled PP is well-known. All three foam processes demonstrate a tensile strength which is significantly higher than the simulated results. For the parts made with chemical and the combination of chemical and physical foaming the parts with the higher mass have an increased ability to withstand tensile loads. Like the E results, the hybrid foam methods have a much wider distribution of results. The deviation for the hybrid foaming methods is caused by the nucleation rate being too high, with two foaming methods combined, the nucleation energy is increased substantially and therefore causing large variations in morphology of the final part.
For the test parts produced with talc-filled PP ( Figure 8b) the overall behavior is like the E results: in that the S u reduces with reduced mass. The lower S u values compared to the unfilled PP, can be explained as the ductility is hampered with talc addition due to stress concentrations and poor shear slipping ability of the polymer matrix [33]. S u is a product of force per unit area and the non-homogeneous cross sections and bubble content that reduces mass clearly changes the test part unit area. Chemical and physical foaming methods have a higher S u than the hybrid foaming method and all three are higher than the simulation results. The inclusion of a talc filler together with physical foaming has a notable influence on S u . There is a S u decrease with a reduction in part mass; explained by the reduction in polymer across the sample's cross-sectional area. However, for all combinations, the S u is lower with the talc when compared to the unfilled PP. For both materials, the simply modeled results underestimate the S u . The likely cause to this is that the equations use a generic skin thickness and as seen in the microscopical characterization section from this research, this is not the case.

Flexural Modulus Data
Tensile mechanical properties are important for polymer part design. In addition to E and S u the mechanical measure of maximum load bearing capability of a material without undergoing any permanent deformation is considered. Maximum Flexural strength (σ fM ) results are shown for the two PP materials (Figure 9).
Polymers 2019, 11, 1896 13 of 16 in the equations is inaccurate and it is recommended for future research that this is formulated to achieve more accurate results.

Conclusions
Two grades of Copolymer PP, unfilled and talc-filled, were foamed through various techniques to characterize their mechanical properties and link the existing modeling technique to the cellular structures. The novel use of combining two discreet foaming mechanisms of chemical and physical foaming to create a hybrid foaming process was used to compare conventional IM against different process variations of low-pressure FIM techniques. The additives within the chemical foaming agent, C6H7NaO7 and CaCO3, along with the talc-filled PP, have shown improved cellular structures. The modulus of elasticity (E) and ultimate tensile strength (Su) for both PP blends, exhibited the strongest values when foamed through CBAs alone. Meanwhile the lowest mechanical tensile strength was exhibited always through the hybrid method of both the chemical and physical blowing agents. Flexural strength follows a similar pattern to that of tensile data; that the CBA produces the strongest foamed parts while the hybrid methods are always the weakest. The smallest cell diameters, along with high cell density and superior homogenous cell structure, was found in the PP parts produced in combination with the hybrid foaming configuration. Part consistency (part weight and mechanical strength consistency) followed a clear trend that with increased foaming (higher weight savings), the less consistent the parts were. This was clear with all the foaming methods but prominent when the hybrid foaming configuration is used.
The results presented in this paper have been linked with previously published skin theory models to show how the mechanical properties of FIM parts are proportional to the skin thickness of cellular structure. The predicted results showed similar trends to that of the experimental results, but future work involves improving the accuracy of the model.   The mean value σ fM plot for unfilled PP (Figure 9a) shows that parts produced with chemical foaming have the highest ability to resist breakage when bent (30.8 MPa). This is then followed by the combination of the two foaming methods (29.8 MPa). For the parts made with chemical and the combination of chemical and physical foaming the parts with a higher mass can resist the highest stress. The values are 30.5 MPa and 28.9 MPa respectively, thus identifying chemical foaming not only as the highest σ fM but also more consistent over a range of part masses. There is a wide range of deviation in σ fM for the hybrid foam test parts and this is consistent for the tensile tests in the previous section. The physical foaming results show different σ fM behavior when compared to the other foaming methods and this outcome is like the tensile results also.
The results for the test parts produced with talc-filled PP (Figure 9b) show a different behavior to the unfilled PP behavior. All foaming combinations have a lower σ fM than unfilled PP. They also have a similar σ fM (27.5-27.3 MPa) at high mass and this σ fM decreases as the mass decreases due to more foaming in the parts. Again, chemical foaming is more consistent in terms of mass and mechanical properties. In terms of the parts ability to withstand highest stress at its moment of yield the process control of combination foaming with talc-filled PP shows that there is a wide σ fM distribution particularly with parts with low mass.
All the mechanical test data presented in this research, except foaming of unfilled PP, follows a clear trend, with reduced weight and increased foams, the mechanical properties decrease. The experimental data with generally the strongest mechanical properties, the chemically foamed parts, does not have superior cell density but, in the case of both PPs tested, they do have the thickest skin thicknesses. The current research trend is to obtain foamed polymer samples with the highest cell density but in this research, it can be observed that cell density has little influence on mechanical strength, in comparison to skin thickness; which has a major weighting on the tensile and flexural properties. Shish-kebabs show highly superior strength and modulus, but are only able to form on the skin layer due to the weak shear and fast solidification [31]. However, when the skin begins to reduce in size, then this can lead to reduced mechanical strength.
The simple model results in Figure 9 are again, under predictions of the actual results, apart from the highest weight-saving for the talc-filled PP using the hybrid foaming method. The conclusion for this is the same as previously mentioned for the tensile tests; the skin thickness used in the equations is inaccurate and it is recommended for future research that this is formulated to achieve more accurate results.

Conclusions
Two grades of Copolymer PP, unfilled and talc-filled, were foamed through various techniques to characterize their mechanical properties and link the existing modeling technique to the cellular structures. The novel use of combining two discreet foaming mechanisms of chemical and physical foaming to create a hybrid foaming process was used to compare conventional IM against different process variations of low-pressure FIM techniques. The additives within the chemical foaming agent, C 6 H 7 NaO 7 and CaCO 3 , along with the talc-filled PP, have shown improved cellular structures. The modulus of elasticity (E) and ultimate tensile strength (S u ) for both PP blends, exhibited the strongest values when foamed through CBAs alone. Meanwhile the lowest mechanical tensile strength was exhibited always through the hybrid method of both the chemical and physical blowing agents. Flexural strength follows a similar pattern to that of tensile data; that the CBA produces the strongest foamed parts while the hybrid methods are always the weakest. The smallest cell diameters, along with high cell density and superior homogenous cell structure, was found in the PP parts produced in combination with the hybrid foaming configuration. Part consistency (part weight and mechanical strength consistency) followed a clear trend that with increased foaming (higher weight savings), the less consistent the parts were. This was clear with all the foaming methods but prominent when the hybrid foaming configuration is used.
The results presented in this paper have been linked with previously published skin theory models to show how the mechanical properties of FIM parts are proportional to the skin thickness of cellular structure. The predicted results showed similar trends to that of the experimental results, but future work involves improving the accuracy of the model.