Keywords: Orthodontic Bracket; SS 17-4PH; Metal Injection Molding; Solvent Debinding. Abstract. Brackets fabrication should be done by a suitable process to produce great result. Processes commonly used are investment casting, machining, and metal injection molding. Investment Casting has a drawback in which the surface roughness is quite high for the standard of brackets and require further processing. Machining is done by removing unwanted part to get desire shape, whereas bracket shape requires a high accuracy and is quite complicated. In Metal Injection Molding, feedstock is injected into a mold where complicated shapes can be achieved with a better surface roughness. The weakness is the stages within the process are quite long. One of the problem is the efficiency of debinding stage. We conducted an experiment to enhance binder removal rate through solvent debinding with stirring and under vacuum condition. Sample use for this experiment is a cubic shape of 0.5 x 0.5 x 0.5 cm3. Experiment is done on magnetic stirrer and in vacuum furnace. The temperature is hold at 50oC. Drying process afterward is done in the vacuum furnace for 1 hour with temperature around 50oC. Amount of binder left is confirmed by STA and the particle morphology is seen by SEM. Results showed that stirring treatment enhances binder removal rate due to stirring mechanism that causes possibility of collisions between particles increases. Binder removal rate on the vacuum treatment has a mechanism similar to stirring, but with the addition of the solvent to be done on a regular basis due to decrease of solvent boiling point under vacuum. There were no cracks found on the surface with an increased rate of debinding. Stirring is use for experiment with sample of actual bracketorthodontic form. Debinding rate of the bracket sample is faster than the cubic sample. This result is affected by the dissimilarity on the volume to surface area.
Introduction Malocclusion is an aberration of the lower jaw and the upper jaw, which causing an abnormal conditions of teeth position that are common in the dentistry world. These conditions are caused by several factors such as environment, heredity, human growth & development, and pathological condition [1]. It may result in a more serious disorder in the function of the mouth, a systematic review on the effects of a malocclusion on periodontal health suggests that subjects with a malocclusion have worse periodontal health than subjects without a malocclusion [2]. The use of orthodontic brackets aims to control and fix the position of the jaw in accordance to the appropriate condition. However, this problem becomes quite complicated when we look at Indonesia, a country with population of over 210 million people, have malocclusion related problems with a percentage above 90% [3]. The orthodontic bracket itself is still done by importing from abroad, causing another problem of relatively expensive product and design that is not appropriate for jaw structure of Indonesian people. Therefore, a mass production of a national braces is urgently needed. On the first project, we use investment casting as the manufacturing process. However, the results obtained have rough surfaces that require another processing end [4]. Therefore, further studies related to the manufacturing technique of making the orthodontic bracket is needed. On this project,
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we use metal injection molding as a more advance manufacturing process. One part to be note in this process is debinding step that acts to eliminate the binder that was used as a lubricant when the feedstock is being injected. Solvent debinding is a key step because the majority of the binder will be eliminated in this point. Enhancement rate of the binder removal is one of the important things that need to be studied further.
Methodology Feedstock used in this research is 17-4PH stainless steel powder produced by Ryer Inc. The first step is the injection process to form a cubic sample with 0.5x0.5x0.5 cm3 volume. This process is done at Department of Metallurgy & Material Engineering University of Indonesia using a buildup injection molding machine with injection parameter according to the material certificate.
The green parts from the injection process were then weight using digital scale before the solvent debinding step. Three different condition of the solution is then use to observe the best debinding rate for the brown part results. First is a normal condition where the cubic sample immersed in hexane solution with temperature of 50oC and the solvent to sample ration is 7 : 1 [5]. Second condition was done in a vacuum pressure using a Vacuum Oven (Labtech LVO-2040). Third condition was done on a magnetic stirrer to cause a stirring process inside the solution. These two processes also use the same parameter as the first condition. All three conditions were done with variable of times ranging from 30 minutes to 150 minutes, so the amount of binder loss after each time could be checked to calculate the debinding rate.
The brown parts were then characterized using test equipment to determine the best condition of the solvent for debinding process. Perkin Elmer STA 6000 machine was used to check the amount and existence of the binder inside the sample through heat flow (DSC) and weight loss (TGA). Density test was also done to confirm the binder removal. The surface condition of the brown part was check using macroscopic and microscopic observation to see a possibility of any crack form due to a different solvent condition. SEM-EDS observations was also done to check the particle morphology after the debinding process that should be change without the appearance of the primary binder.
Result & Discussion The injection parameter of the experiment is compare to the material certificate to see any different that could cause various result after the debinding step. By using thermocouple, the temperature at each side of the machine is measure to ensure the feedstock perfectly melted and no actual different found. The only different is the use of rotating screw that is absent in our injection molding machine. This could lead to a problem where the feedstock would not melt perfectly and tangled between each other. This condition prevented by using higher temperature mold to compensate the melting problem and to reduce possibility of residual stress, cause from rapid cooling by cold mold surfaces [6]. Green part quality will be very important for the next step since a well distributed metal particles on the matrix after the injection process will results on product with better mechanical properties [7]. The weight of products is measure using a digital scale to check the mass distribution of every parts as seen on Figure 1.
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Figure 1. Mass Distribution of Green Parts
The weight variances are caused by pressure drop from the compressor when the injection process was done, this result on the decrease of the pressure use for the injection. The absence of rotation screw could also be count as a cause since it would affect the green part quality. The presence of screw will increase the shear rates while decrease the viscosity, so the melted feedstock will be easier to flow [8]. Weight loss percentage is calculated to determine amount of binder that has been removed from the green part. The weight loss percentage is increasing by times and slightly became stagnant after it went through 4% loss. This is due to high amount of primary binder inside the green part is already solve to the hexane. Based on material certificate the primary binder is 4.05% by weight, so we could compare the result of each treatment by seeing which one would get the 4.05% boundary first.
Figure 2. Debinding Rate Comparison between Each Treatments
As shown in Figure 2, the stirring treatment help give a better rate of debinding compare to the others. In just 60 minutes, this treatment already reached the 4.05% boundary and became more stagnant after that, while the other two needs around 90 minutes to pass through the boundary. We noticed that there is a debiding rate overlapping phenomena between the normal and vacuum condition. This is because the Vacuum that use as a treatment for the solvent affect the boiling point of the hexane, where a lower pressure caused a decrease in the boiling point. Therefore, with temperature around 50oC, the hexane solution is already boiled and slowly decreased in volume. Refilling was done every 60 minutes, but it also affects the temperature of the overall solution. This condition made the debinding rate of the vacuum condition less stable. The debinding rate itself is control by the diffusion process of primary binder particles from the green part to the hexane solution, influence by the diffusion coefficient that depends on certain parameter such as temperature, time, and sample thickness. This could be calculated by using a modification of second Fick’s Law [9] as written at equation 1 below [10].
(1)
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Effect of Solvent Treatment to Amount of Binder Remove The amount of binder remove is check by Simultaneous Thermal Analysis (STA) equipment. Figure 3 below shows us each result from TGA test.
Figure 3. TGA Result after Solvent Debinding
The sample for test is taken from the 90 minutes solvent debinding process where almost every treatment already passed the 4% boundary. The graphic show that the amount of weight loss from each treatment is around 2.8-2.9%. This weight decrease is caused only by the secondary binder removal while the test is done. We can confirm that primary bindery is gone by comparing the result with TGA result of the feedstock in Figure 4 below. This result show that with primary binder still in the green part the weight loss percentage would reach around 7% which also prove the 4% more from the TGA result of the green part because of the primary binder existence.
Figure 4. TGA Result Comparison between Feedstock & Brown Part
However, we could not determine and clearly distinguish the exact different from each solvent treatment. Due to overlapping, it is difficult to separate the wax contribution from the secondary binder contribution in the TGA curve [11]. Test using Differential Scanning Calorimetry was done to check the possibilities of any reaction happened through heat gain or loss. The result is in Figure 5 below.
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Figure 5. DSC Result after Solvent Debinding
As we could see, for vacuum and stirring condition there are only one peak occurred around temperature of 450oC which is the evaporation point for the secondary binder, while for normal treatment there is another peak around 250oC that confirming the presence of primary binder that is still in the green part. This result proves once again that treatments for the solvent on the debinding process could help enhance the debinding rate efficiency.
Effect of Solvent Treatment of Green Part Density
Archimedes test is use to see a change of the green part density after solvent debinding process. The test was done using a scale and density kit. The result for each solvent treatments and weight percent for each feedstock components composition are show in the Table 1 below.
Table 1. Density of Each Component & after Solvent Debinding SS 17-4PH Primary Binder Secondary Binder 7.68 g/cm3 0.90 g/cm3 0.92 g/cm3 Stirring Vacuum Normal
4.42 g/cm3 4.20 g/cm3 3.79 g/cm3
The density different of each treatment is caused by the amount of primary binder extract. The higher amount of primary binder loss will result on higher density value since the density of binder component is below the density of the feedstock and the density of the SS 17-4PH is above the feedstock.
Surface & Particle Morphology after Solvent Debinding Process Cracking is frequently occurred if the process is not well executed. Since the solvent is heated in order to increase the debinding rate, the binder components become soft or even melt. This problem is mainly caused by the swelling of polymers when solvents penetrate into the binder [12]. There was no crack found in the experiment with addition of stirring and vacuum treatment, as seen in figure 6.
Normal Stirring
Vacuum
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Figure 6. Surface Comparison After Solvent Debinding Process
Powder particles will have a change on the surface morphology after the solvent debinding process since the binder will be remove. Observation from SEM image in Figure 7 shows that the greyish thin layer around the particles is reduced significantly in the brown part after solvent debinding. A comparison between brown parts result of normal and stirring treatment is shown in Figure 8. The stirring condition clearly have a better result which could be seen by less amount of binder left. Both results of treatment also show that a slightly thin layer is still exist after the process cause by the secondary binder that could not be remove by solvent, which is confirm with high amount of carbon in the layer through EDS test. However, the formation of open pore channels from this process allows more rapid removal of the remaining binder without swelling or cracking during the thermal process [13,14].
Figure 7. SEM Comparison of Feedstock (Left) & Brown Part (Right)
Green Part Stirring
Figure 8. SEM Result After Normal (Left) and Stirring Treatment (Right)
Normal Vacuum
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Mechanism of Solvent Debinding Treatments
Stirring Treatment Stirring is an important parameter in a chemical reaction process. If the mixed-up were not stirred uniformly, the reactant concentration, hexane in this case, will be settle at the bottom of beaker glass. Therefore, an unbalance reaction result could be produced between each side. An illustration of stiring mechanism can be seen in Figure 9.
Figure 9. Normal Condition (Left) & Stirring (right)
Vacuum Treatment Reaction rate will become slower in a vacuum condition since the atmosphere pressure will decrese. This condition will increase the volume of the substance and space between particles became wider. Therefore, possibility of contact between solvent particle and binder will be minimized, as we could see in illustration at figure 10 below.
Figure 10. Effect of Pressure on Particle Volume [15]
However pressure effect could be neglected when the substances are in liquid or solid form since both have imcompressible characteristic [16]. From the experiment before, vacuum condition of the solvent result on higher debinding rate compare to normal condition. This is cause by decrease of hexane evaporation point when pressure is minimized or in vacuum condition. The evaporation of hexane result on the forming of solvent twister in the beaker glass. Acidentally, this phenomena cause stirring effect for the hexane. However, this condition is compensate by decrease of the hexane through times when the solvent debinding process is done, figure 11.
Figure 11. Vacuum Mechanism
Solvent Debinding Process of Orthodontic Bracket Stirring as the best parameter from the experiment is then used in solvent debinding process for orthodontic bracket. Figure 12 shows the Brown Part result from macroscope observation.
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Figure 12. Optical Macroscope Observation of Orthodontic Bracket
Figure 13 below shows the weight loss percentage after each times of immersion in the hexane. In just 30 minutes the amount of binder extract from the sample is already above 4% by weight. This is much faster than the cubic sample that need around 90 minutes to remove the primary binder. This is cause by different surface area of the two samples. Components with more surface area to volume ratios debind faster when compared to the shapes with lower surface area/volume ratios [17]. Moreover, compact thickness and the diffusion path length are also determine the debinding rate [18]. Further research is necessary to confirm the effect of dimension parameter on debinding rate of the braces.
Figure 13. Debinding Rate of Orthodontic Bracket Samples
Conclusion 1. Green Part produce by metal injection molding machine of DTMM UI have a variation on the
mass distribution caused by pressure drop and mold design. 2. Stirring treatment result on higher debinding rate (60 minutes) compare to the normal and
vacuum condition (90 minutes) for the cubic with 0.5cm3 volume. 3. Brown part density from stirring and vacuum treatment have a higher value compare to the
normal condition since more primary binder is dissolve at same time. 4. Particles morphology show that a thin layer of secondary binder is still exist after the solvent
process. 5. Stirring treatment enhance the debinding rate by improving intensity of particle collision
between solvent and primary binder. This is caused by the movement of solvent particle that is well distribute to every side and not settle at the glass bottom.
6. Vacuum condition supposed to decrease the debinding rate because of the wider space between particles. However, evaporation point of hexane is decrease in vacuum condition causing stirring effect during the solvent debinding process, resulting on the same mechanism as the stiring treatment.
Acknowledgement
The Author thank Ministry of Research, Technology and Higher Education of the Republic of Indonesia for their support of this work under DIKTI Reseach Program.
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