金屬射出成形脫脂製程中缺陷產生的原因及其解決方法

Abstract

金屬射出成形製程具有極佳的成形能力，適用於製作具有複雜形狀的小零件，但在脫脂過程中，製程參數不當的控制常常會使得試片尺寸穩定性差，甚至產生裂縫、魚鱗紋、發泡等缺陷，導致成本之增加甚至造成成品機械性質的降低。由於這些缺陷的產生大部分來自於脫脂製程，因此本研究的目的在於瞭解缺陷產生之原因及如何有效地予以改善。 本論文第一部份探討溶劑脫脂時降低試片膨脹量的方法。由雷射膨脹儀的結果可以知道，降低試片膨脹量可藉由減少黏結劑含量、於射料中加入偶合劑、於溶劑中添加膨脹抑制劑以及降低脫脂溫度來達到，若考慮改變製程參數的困難度以及脫脂的效率時，則以在射料中添加偶合劑或是在溶劑中加入10 vol%的膨脹抑制劑為較有效率的方法。 第二部分探討溶劑脫脂時，試片內連通孔形成現象。利用Fick擴散理論計算在不同溶劑脫脂時間下，試片內可溶性黏結劑的分佈情形，再利用銑床和索氏萃取器來準確得知試片內可溶性黏結劑的實際分佈，結果發現這些分佈數據與擴散理論所預測的數值相符合。實驗中亦利用氣泡測試和螢光顯微鏡觀察心部何時形成連通孔，結果發現對於8.7、6.0及3.0mm厚的方塊試片，心部產生連通孔所需的最低溶劑脫脂率均約為62.0%，心部的孔道所佔的比例為9.2 vol%，與一般燒結過程中連通孔轉變為封閉孔所需達到的孔隙率(8 vol%)相近。 第三部份探討熱脫脂時，溶劑脫脂率對於後續熱脫脂時缺陷產生的影響。實驗結果發現溶劑脫脂時所形成的連通孔道，在加熱過程中，會由於毛細力作用，而使得熔融的可溶性黏結劑(石蠟及硬脂酸)及骨架黏結劑(聚乙烯)在試片內發生重分配的現象，造成溶劑脫脂後所形成的連通孔道再度被封閉。由於熱裂解時所產生的氣體分子無法快速逸出至胚體外，使得較厚的試片(>4.0 mm)，即使其溶劑脫脂後具有連通孔，試片仍無法以較快的升溫速率(5℃/min)進行熱脫脂。最後藉由調整溶劑脫脂率及熱脫脂時的升溫速率，可找到對於不同厚度試片的安全脫脂條件。

Unsatisfactory dimensional control and defects are frequently seen in metal injection molded (MIM) parts, particularly after solvent debinding and thermal debinding. Most of these problems are related to the high amount of swelling of the binder components during solvent debinding and the cracking or blistering during the subsequent thermal debinding. The objective of this study was thus to eliminate the defect formation in metal injection molded parts during solvent and thermal debinding. In the first part of this work, the laser dilatometer analyses indicate that the amount of swelling during solvent debinding could be reduced by decreasing the binder content, adding coupling agent into the feedstock, adding swelling inhibitor into the solvent, and lowering the debinding temperature. Considering the productivity and the ease of implementation, adding coupling agents and swelling inhibitors are the two most effective methods. In the second part, a diffusion model was established to predict the binder distribution in the compact and the minimum amount of binder removal that is needed to create interconnected pore channels in the middle of the MIM specimen. The model agreed with the experimental data that were obtained using the soxhlet extraction method. The distributions of pore channels were also examined using bubble and dye penetration tests. The results show that when 62.0% of the soluble binder is removed, interconnected pores are formed in the middle of the 8.7, 6.0, and 3.0mm thick specimens. The total amount of pore in the core region equals to about 9.2%, which is close to the criterion when a sintered compact enters the final stage sintering and starts to close off the cylindrical pore channels. In the third part, the connected pores, which are formed after solvent debinding, are closed again due to the redistribution of soluble binders and backbone binders in the heating process. When decomposed gas molecules cannot escape easily to the ambient through the interconnected pores during thermal debinding, the pressure builds up at the core section and then causes cracking. Therefore, although the thicker specimen (>4.0 mm thickness) has higher than the minimum amount of binder removal, it can’t use normal heating rate (5℃/min) to do following thermal debinding process. Finally, the safe conditions of the two-stage debinding process for specimens with different thickness are determined by adjusting the solvent debinding completion and the heating rate of thermal debinding.