Effect of Polyvinylpyrrolidone Content on Pure Titanium Injection Molding

Abstract

In water-soluble binder systems, polyethylene glycol (PEG) and polymethylmethacrylate (PMMA) are often used as primary and secondary components. The PEG/PMMA binder system is clean and environmentally friendly, but the discrepancy between the crystallization temperature of PEG and the glass transition temperature of PMMA leads to the generation of pores in the feedstock. The solidification pores have an adverse impact on the final mechanical properties of the samples. Polyvinylpyrrolidone (PVP), as a crystallization inhibitor, can inhibit the formation of porosity. In this study, spherical titanium powder with a diameter of less than 45 μm was used as metal powder; the binder system consisted of PEG, PMMA and SA. Different increments of PVP (0, 10%, 20%, 30 wt.%) were added to the PEG/PMMA binder system. The uniformity of the feedstock and the open channels generated after debinding were observed using SEM. The pores’ condition before and after debinding was studied using Micro CT, and the mechanical properties of the samples were also detected. By comparing the macroscopic and microscopic morphologies of the injected samples and mechanical properties of the sintered samples, it was found that a PVP content of 20 wt.% resulted in the best properties.

1. Introduction

Metal injection molding (MIM) is a near-net-shape-forming technology that combines traditional powder metallurgy with plastic injection molding technology [1,2,3]. Its advantages, including low cost, the ability to process complex-shaped parts, high production efficiency and high material utilization [4,5], make it an ideal candidate to manufacture expensive metal (such as titanium) parts [6,7].

The binder system consists of four steps: feedstock preparation, injection molding, debinding and sintering [8,9,10]. The powder and binder are weighed in a certain proportion and then mixed at a specific temperature to prepare feedstock; the specific temperature depends on the choice of binder system [11]. An injection molding machine is used to obtain the injection molding samples. The temperature and pressure holding time of injection are very important parameters, which ensure the integrity of the samples. It is worth noting that avoiding contamination from the machine is obviously vital if there is a change in the feedstock [12]. Any materials for cleaning that remain in the feedstock will generate pores after sintering. Debinding usually has two steps: solvent debinding and thermal debinding. The aim of solvent debinding is to remove the primary component and form open channels, which provide the way to escape for the second component in thermal debinding [13]. Sintering can achieve good mechanical properties. However, the oxygen pick-up needs extra attention, especially for active metals [14].

There are four binder systems: wax-based, polyoxymethylene-based, aromatics-based and water-soluble binder systems [15]. Thavanayagam et al. [16] adopted the water-soluble binder system to injection mold TC4 and used PVB as the backbone polymer. The different binder compositions and powder loadings were investigated, and the effects of the debinding time and temperature were discussed. Additionally, the porosity and microstructure of the molded samples were observed. Finally, the binder system of 80:15:5 vol% PEG: PVB: SA with a powder loading of 60% was selected, and debinding for 4 h at 35 °C or 3 h at 45 °C achieved a reduction in cost. Momeni et al. [17] injection molded 4605 low-alloy steel powder and discussed the effects of the powder loading and the binder system on the final properties. The powder loadings of 55, 60 and 65 vol% were compared and discussed. The density, strength and hardness increased with the increase in powder loading. Nevertheless, the binder system needed to be adjusted to have a high powder loading. A high powder loading was beneficial for obtaining samples with excellent mechanical properties, but the increase in viscosity caused by a higher powder loading was detrimental to the injection of samples. The 65 vol% of powder loading was selected finally. Momeni et al. [18] also studied the effect of polypropylene (PP) as a second component on the properties of 4605 low-alloy steel compacts. A wax-based binder system was used and different contents of PP (20, 24, 28 mass %) were added. The results of the tests (mechanical properties, XRD, SEM, density and rheological properties) revealed that the increase in PP improved the mechanical and physical properties and reduced the rheological properties. The tensile strength increased from 446 MPa to 658 MPa as the content of PP increased from 20% to 28%, but as the elongation decreased from 10.8% to 5.7%, the total porosity decreased from 1.15% to 0.42%. In fact, the content of PP influenced the residual carbon content (from 0.46% to 1.1% with an increase in PP), which affected the grain size, total porosity and phase constituents. Also, Momeni et al. [19] changed the contents of SA. Different feedstocks were prepared with different percentages of SA. The distribution of powder particles improved, and the strength and density of the sintered samples increased as the SA content increased from 1 to 9%. With a further increase in SA content from 9 to 17%, the content of carbon was reduced, causing the density and strength to significantly decrease. In the process of injection molding stainless steel 316L, Setasuwon et al. [20] replaced 10% wax with oil. The feedstock with 10% palm oil had a faster rate of binder removal during the second and third hour. The tensile strength and elongation increased to 500 MPa and 47% compared to not replacing the wax (490 MPa and 37%).

Because of its clean and non-polluted traits, water-soluble binder has attracted widespread attention in recent years. Polyethylene glycol (PEG) is commonly used as the major component in a water-soluble binder system due to its advantages such as easy water solubility, non-toxicity and commercial availability [21,22]. Polymethyl methacrylate (PMMA) has good compatibility with PEG and high tensile strength, and is often selected as the backbone polymer of PEG-based binder [23]. In addition, PMMA is a very clean polymer, leaving very little residue during decomposition. Therefore, the PEG/PMMA system can be a good choice for pure titanium MIM given the reactive nature of titanium [24].

Despite its clean nature, the use of the PEG/PMMA binder system in MIM industry remains rather limited. The void problem caused by the difference between the crystallization temperature of PEG and the glass transition temperature of PMMA remains a big concern. A few studies have been conducted on how to address this problem. For instance, Hayat et al. [25] adopted a crystallization inhibitor–polyvinyl acetate (PVAc) to suppress the void formation. Different contents of PVAc were added to the PEG/PMMA system to study the effectiveness of this approach. They concluded that with the addition of PVAc, void-free samples with good mechanical properties can be obtained. Wen et al. [26] explored the reduction in binder-induced defects by incorporating PVP. The results showed that the incorporation of PVP enhanced the uniformity of the feedstock effectively, and the decrease in pores meant better densification, which resulted in better mechanical properties.

In this work, PEG, PMMA and SA were chosen as binder components and mixed with spherical pure titanium powder, and different proportions of binder components were tested. Due to the temperature difference between PEG and PMMA, and thus the pores usually generated, a crystallization inhibitor, PVP, was added to this system to reduce the voids caused in the cooling process and to improve the mechanical properties of the samples. The best molecular weight of PVP was selected after various attempts, which is the key contribution of this work.

2. Materials and Methods

2.1. Experimental Materials

Atomized spherical titanium powder [27,28] from Shandong Lianhong New Material Technology Co., Ltd., Shandong, China was used in this research. The mean particle size was 34.2 μm. The particles’ morphology and chemical composition are presented in

Table 1. The composition of spherical titanium powder.

Element (%)	Ti	N	O	H	Fe	C
Content (wt.%)	Bal.	0.007	0.095	0.002	0.18	0.01

and Figure 1, respectively. The binder system was composed of PEG (Beijing Solarbio Technology Co., Ltd., density of 1.2 g/cm³), PMMA (Taiwan Chimei, Shandong, China density of 1.19 g/cm³), SA (Dongguan Hongxing New Material Co., Ltd., Shandong, China, density of 0.94g/cm³) and PVP (Aladdin reagent, Shandong, China, density of 1.144 g/cm³).

2.2. Experimental Methods

The binder system consisted of 76 wt.% PEG, 22 wt.% PMMA and 2 wt.% SA. When the powder loading approached the critical point, the binder was not enough to cover the metal powder; generated voids reduce the density, so 66 vol.% was selected according to the deviation between the theoretical density and experimental density. To explore the effects of adding PVP, the experiments were divided into four groups: A, B, C and D, according to their contents of PVP (0, 10, 20 and 30 wt.%), respectively. Titanium powder and all binder components were weighed according to their mass fraction, density value, powder loading, etc. The mixture was then compounded at 160 °C for 90 min. The steps of the whole process are shown in Figure 2.

For injection molding, a small desktop pneumatic injection molding machine was used. The injection molding parameters were as follows: the barrel temperature was 160 °C; the nozzle temperature was 165 °C; the mold temperature was 50 °C; the injection pressure was 0.8 MPa; and the holding time was selected as 5 s. Figure 3 presents the shape of the molded sample.

Water debinding was carried out at 50 °C. After debinding, the samples were dried and weighed. To obtain the debinding curve, the injected samples were debonded for 6 h and dried for 12 h initially. Then, the samples were debonded up to the 28 h point, with removals conducted at 2 h intervals and drying after each removal. The debinding rate was calculated according to the mass change before and after water debinding, and the process was repeated until there was no large fluctuation in sample weight after drying.

Thermal debinding and sintering were conducted in a vacuum sintering furnace. The samples were sintered at 1300 °C for 2 h. The holding time of 120 min at 380 °C is due to the reaction of titanium with carbon, oxygen and nitrogen at about 400 °C, ensuring the decomposing gas has enough time to escape and thus avoiding the increase in impurity content in the final sintered samples. The parameters were also set according to the TG curves and the details are displayed in Figure 4. The relative density of the sintered samples was calculated after measuring the density.

2.3. Characterization

The influence of PVP on the crystallization of PEG was analyzed using differential scanning calorimetry (DSC, TA Q2000, USA). The temperature was first increased to 160 °C and then cooled to −40 °C. The heating and cooling rates were both 10 °C/min. Thermogravimetric analysis (TGA, NETZSCH STA 449F1 STA449F 1A-0292-M, Germany) was employed to analyze the thermal decomposition behaviors of the feedstock. The temperature was increased to 600 °C at a rate of 10 °C/min. Both the DSC and TGA were carried out in a high-purity argon atmosphere.

The rheological test was conducted on a rheometer (Haake Mars60, Germany) at the temperature of 160 °C and the shear rate in the range of 0 to 1000 s⁻¹ to observe whether the shear viscosity of the feedstock was suitable for injection molding. The die used for measuring rheology was a 20 mm parallel plate, repeating once.

The morphologies of the samples before and after water debinding, and the fracture surfaces of the samples after the tensile test, were observed using a tungsten filament scanning electron microscope (VEGA3 TESCAN, China). Micro CT was carried out using a ZEISS Xradia520 (Germany) to obtain the three-dimensional structures and morphologies of the samples.

The mechanical properties of the sintered samples were determined using an MTS universal tester. The impurity content was determined using inductively coupled plasma emission spectrometry (ICP-OES, USA).

3. Results and Discussion

3.1. Rheological Properties

The rheological properties of the feedstocks were first determined in order to confirm their suitability for injection molding. Figure 5 presents the shear viscosity versus the shear rate plot. The measured values showed apparent viscosity, measured in a single experiment.

As can be seen from Figure 5, the shear viscosities of all four feedstocks were below 1000 Pa·s in the shear rate range of 0 to 1000 s⁻¹ at 160 °C. Overall, Feedstock D had the highest viscosity. It can be inferred from the graph that the viscosities of the feedstocks depend on the added amount of PVP, i.e., it is directly proportional to PVP content. Nevertheless, the incorporation of PVP was not harmful to the feedstocks’ flowability; the viscosities remained in a feasible range.

3.2. Inhibition of PEG Crystallization by PVP

Figure 6 presents the SEM micrographs of injection-molded samples with different contents of PVP. For the feedstock with zero PVP, many large pores can be seen in the sample, displayed in Figure 6a. When the PVP content was 10 wt.%, the number and size of pores both decreased, as shown in Figure 6b. For Feedstock C, barely any pores are present on the surface of the sample; the titanium powder and binder are evenly mixed. However, tiny pores appear inside Feedstock D, indicated in Figure 6d.

The crystallization temperature of PEG is below 50 °C while the glass transition temperature of PMMA is about 120 °C. The difference between the two temperatures causes inconsistent volume shrinkage during the cooling process, leading to defects such as pores during the injection molding process [29]. When PVP was added to the feedstock, its large molecular weight compressed the crystallization space of PEG molecules and inhibited the crystallization of PEG molecules.

When the content of PVP was increased to 10 wt.%, the decrease in pore size indicated that the addition of PVP had an inhibition effect on PEG crystallization and the phenomenon of inconsistent volume shrinkage in the cooling process was alleviated. However, there were still pores, as can be seen Figure 6b and which can be attributed to the inadequate content of PVP. It is fair to assume that increasing the PVP content in the binder will increase this interaction, resulting in pore-free samples. However, when the PVP contents were increased to 30 wt.%, tiny pores re-appeared, as can be seen in Figure 6d. The appearance of these tiny pores could be due to the cross-linking between excessive PVP molecules [26], which reduced the compression of PEG molecules by PVP, resulting in the formation of tiny pores.

To verify the point that the addition of PVP has a visible inhibition effect on PEG crystallization, DSC was carried out. Figure 7 shows the inhibition of PEG crystallization caused by different amounts of PVP, and

Table 2. The heat absorption and enthalpy of crystallization of four feedstocks.

	Heat Absorption (J/g)	Enthalpy of Crystallization (J/g)
Feedstock A	−28.47	25.32
Feedstock B	−20.58	16.04
Feedstock C	−16.35	11.17
Feedstock D	−15.62	8.85

records the heat absorption and enthalpy of crystallization of the four feedstocks where the data are from DSC curves. It can be seen from Figure 7 and Table 2 that the addition of PVP can effectively reduce the crystallization energy of PEG. This can be attributed to the interactions between negatively charged oxygen ions in the carbonyl group of PVP and positively charged hydrogen ions at the end of the PEG chain [30]. When PVP of 10 wt.% was added, there was a distinct reduction in the energy of crystallization; the energy decreased continuously with the increase in PVP. However, the decline was much more gradual as the content was increased to 30 wt.%.

The TG curves of the four different feedstocks are presented in Figure 8. As can be seen, the initial and final decomposition temperatures increase with the increase in the contents of PVP. The mass loss of samples is greatest in the range of 250–450 °C. A low heating rate should be set in this range during the thermal debinding process so that the binder components have enough time to escape from the sample [31].

3.3. Effect of PVP Content on Surface Quality and Water Removal of Samples

The surface morphologies of the injection-molded samples of different feedstocks are shown in Figure 9. As can be seen from Figure 9, samples from Feedstocks B, C and D have better appearance than the sample from Feedstock A. It seems that the addition of PVP can improve the appearance of samples to a certain extent.

After the injection molding, water debinding was performed. The main purpose of water debinding is to remove PEG and PVP, thereby forming an interconnected open pore network within the feedstock to provide a channel for gaseous monomers during the subsequent thermal debinding [32,33]. The PEG and PVP molecules begin to dissolve as water permeates to the interior of the sample through capillary action. The difference in concentration between the sample and the surrounding water bath causes these molecules to leach out into the bath until the two parts reach a state of equilibrium [34,35]. The process of debinding can be expressed by the model below:

$$\ln \left(\frac{1}{F}\right)=\raisebox{1ex}{$D_{e}t{\pi }^2$}\!\left/ \!\raisebox{-1ex}{${\left(2L\right)}^2$}\right.+\mathrm{K}$$

(1)

In Equation (1), F represents the proportion of the residual polymer, t represents time, 2L is the thickness of the sample and K represents the change in the mechanism of debinding. De is the diffusion coefficient of the polymer. Equation (1) can also be written as

$$\ln \left(\frac{1}{100\%-F}\right)=\raisebox{1ex}{$D_{e}t{\pi }^2$}\!\left/ \!\raisebox{-1ex}{${\left(2L\right)}^2$}\right.+\mathrm{K}$$

(2)

where (100% − F) represents the proportion of polymer removal.

The appearance of the four groups of samples after debinding is displayed in Figure 10, and the subsequent interconnected pores generated after water debinding are shown in Figure 11.

By comparing Figure 6 and Figure 11, it can be concluded that most PEG and PVP molecules had been removed during the water debinding, forming open channels. Residual PEG and PVP molecules were removed along with PMMA in the subsequent thermal debinding. As can be seen from Figure 10, sample D has distinct cracks on its surface. This was further verified by separately carrying out water debinding of sample D (Figure 12). As can be seen from Figure 12, there were cracks in samples of group D. The molecular weight of PVP is relatively high. When the added amount of PVP is large enough, it can result in stresses within the sample during the leaching process. When these stresses are greater than the strength provided by the backbone polymer, swelling, cracks and other defects appear on the surface of the sample. Hence, the PVP content of 30 wt.% was not suitable, as it caused samples to be destroyed.

The debinding process of PEG and PVP molecules is mainly driven by the concentration gradient. The injection-molded samples with different PVP contents were debonded at 40 °C for 24 h. The formula for calculating the debinding rate is as follows:

$$\mathsf{\phi }=\frac{{\mathrm{m}}_0-{\mathrm{m}}_1}{{\mathrm{m}}_0\mathrm{p}}\times 100\mathrm{\%}$$

(3)

where m₀ is the original weight of the injected sample, m₁ is the weight measured after the sample was debonded for 24 h and air-dried and p is the mass fraction of PEG and PVP (calculated from the binder formulation).

Figure 13 presents the water debinding curves of the four groups of samples. The samples in group A without PVP have the highest debinding rate. The addition of PVP increases the debinding time, with the lowest debinding rate exhibited by Feedstock D, which had 30 wt.% PVP. Nevertheless, more than 88% of PVP and PEG could be removed within 24 h of water debinding.

The micro-CT test was conducted before and after water debinding of sample C to further investigate the interconnected pores. Figure 14 provides the 3D images of the internal pores of Feedstock C, which were obtained using a micro-CT test. Figure 14a presents the pores of the sample before water debinding, while (b) shows the pores after water debinding, represented by the small spheres. There are few spheres in Figure 14a, but the number of pores increased significantly after water debinding (Figure 14b). The data about the size distribution of pores are presented in Figure 15, where orange represents the sample before water debinding and green represents the sample after water debinding. It can be concluded from Figure 15 that the number of small holes in the sample increases significantly after water debinding. The main reason is that PEG and PVP in all parts of the injection-molded samples dissolve in water after water debinding, resulting in many small holes.

Through analysis of the micro-CT results, it was found that the feedstock added with 20 wt.% PVP was homogeneous; PEG and PVP distributed in all locations dissolved in water, leading to the distinct increase in small-sized pores. The comparison of pore size distribution before and after water debinding confirmed that water debinding results in the formation of open channels, which provide pathways to volatile degradation species during the subsequent thermal debinding step. This minimizes the risk of cracking and blistering during the thermal debinding or sintering steps.

3.4. Effect of PVP Content on Mechanical Properties of Samples

The same thermal debinding and sintering curve was used to test the mechanical properties of samples containing different contents of PVP. The residual PEG, PVP, PMMA and SA must be eliminated before the sintering stage. SA could escape from the samples after they were heated to 200 °C. The decomposition of PMMA was vital in this step, so a slow heating speed was set around the decomposition temperature.

The effect of different contents of PVP on sintered samples is listed in

Table 3. Mechanical properties of each group of samples.

	Ultimate Tensile Strength (MPa)	Elongation (%)	Relative Density (%)	Content of Impurity (%)
				C	O	N
Feedstock A	386.4	8.83	97.8	0.03	0.22	0.01
Feedstock B	499.0	11.49	98.6	0.05	0.25	0.02
Feedstock C	547.9	12.53	98.3	0.06	0.28	0.02
Feedstock D	355.3	7.78	97.5	0.09	0.32	0.03
Grade 3 ASTM F2989-13 [36]	495 (min)	10 (min)	~98–99	0.08 (max)	0.30 (max)	0.05 (max)

. For Feedstock D, the cracks caused by the process of water debinding made the ultimate tensile strength lower than the sample without PVP. The addition of 10 wt.% PVP increased the ultimate tensile strength from 384.6 to 499 MPa. Although the addition of PVP results in better mechanical properties, it also increases the impurity contents simultaneously. Therefore, the addition of PVP is only beneficial up to a certain limit, depending on the binder composition. As shown by Table 3, PVP with the content of 20 wt.% resulted in the best properties.

Figure 16 presents SEM micrographs of fracture surfaces of the samples (Feedstocks A and C) after the tensile test. The sample with no PVP has visible voids. However, the voids in Feedstock C are much smaller, which confirms that the addition of PVP plays an important role in inhibiting crystallization.

4. Conclusions

This study employed a crystallization inhibitor, PVP, to reduce the void problem caused by the PEG/PMMA binder system. The effects of different contents of PVP on feedstock were studied. The following conclusions can be drawn:

(1)

The addition of PVP effectively eliminated the voids caused by the inconsistent volume shrinkage, which exists due to the temperature difference. The large molecular weight of PVP compressed the crystallization space of PEG molecules and inhibited the crystallization of PEG molecules.

(2)

It was confirmed that the appropriate content of PVP is 20 wt.% PEG, ensuring defect-free samples after water debinding.

(3)

The novel binder system had good rheological properties and feedstock uniformity, resulting in overall good mechanical properties. It was revealed that the 20 wt.% PVP resulted in a density of 98.6%, ultimate tensile strength of 547.9 MPa and elongation of 12.53%, respectively, which meet ASTM F2989-13 Grade 3 quality.