μ-MIM® Technical Newsletter Vol. 34: Quality assurance of μ-MIM
Compared to other metal component manufacturing processes, the metal injection moulding (MIM) process is fairly complicated and each process step is interactive. Furthermore, our μ-MIM technology normally deals with very complex designed small components in serial production. Thus, extra high-standard quality control is required both in and around the production line to satisfy the customers’ requirements. This time, we will introduce our quality assurance.
MIM process and quality control
MIM processes can be categorised into five processes, namely 1) feedstock manufacturing, 2) injection moulding, 3) debinding and sintering, 4) additional post-processing and 5) measurement & inspection.
In process 1), the material control in both metal powder and binder materials will be listed on top. In metal powder, incoming inspection, not only the chemical composition of the powder but also the powder size distribution and the shapes are measured. The storage control of the metal powder and the binder material is also important. If the storage condition is not appropriate, the degradation of the materials progresses faster, and it leads to poor mechanical properties in the sintered components. The MIM feedstock pellet is produced using the kneading and pelletiser machines. During the kneading process, enough shear stress should be applied to the elevated temperature feedstock to gain a uniform distribution of metal powder in the feedstock. At the same time, careful temperature observation of the feedstock is required. If there are overheated areas, the thermal degradation of the binder is progressed and it will suppress the productivity significantly. Additionally, our μ-MIM components are mostly quite small and the volume of the component can be smaller than a pellet. Therefore, the uniformity of the feedstock control is one of the important quality control points so that we take extra good care of each single pellet quality.
In 2), the injection moulding process, the highest stress is applied to the feedstock. Due to the high stress in elevated temperatures, unexpected binder degradation can be seen. Since we recycle the runner or miss shot green parts, the close observation of feedstock during the injection moulding is also held. We gain a stable volume of the green parts after the injection moulding process, thus, precise weight measurement is deployed for the quality evaluation of green parts in general. Also, the appearance inspection is conducted to improve the material yield. The final component quality will be determined by 3) the debinding and sintering process. The debinding process can be held in 2 steps, 1st and 2nd debinding, in the different furnaces. With the 1st debinding, the majority of the binder is decomposed, thus, the organic contamination in the furnace is likely to occur. In the 2nd debinding, a very little binder to support the metal powder particle in shape is remaining and that little amount of the binder will be decomposed in higher temperature than the 1st debinding. Generally, this 2nd debinding is held in the sintering furnace.
There are many MIM manufacturers that operate the 1st and 2nd debinding in separated furnaces but we, μ-MIM, conduct all in one process. The 1-furnace operation requires good atmospheric control during the debinding process, compared to the separated furnace operation. Therefore, the furnace price will be much higher than the separated type. However, our system can avoid a loss by the handling in between the debinding and sintering process handling, since the post 1st debinded parts are very fragile. In the sintering process, more than 10% in linear shrinking by thermal diffusion among the metal powder is observed. So, it requires the appropriate selection of the ceramic plate selection, e.g., the material, porosity, design, operation condition, etc. for realising stable quality serial production.
In 4) secondary processes, the surface finishing (sandblasting, tumbling, plating), the machining (tapping, coining), the thermal treatment (ageing, precipitation hardening) can be listed. Powder metallurgy components have a certain level of roughness because of the metal powder, however, the same finishing level as machined components is achieved in secondary processing. As the last process, 5) delivery inspection includes the appearance visual inspection, tolerance inspection by micrometre, 3D optical measurement system, X-ray CT, and some chemical composition measurement by SEM-EDX, combustion test.
ISO13485 certified quality assurance
μ-MIM quality assurance system is certified by ISO 13485 since the demand from the medical devices manufacturing industry keeps increasing. As mentioned above, there are many control values and points according to the process step and that will be multiplied by the component’s design. We keep improving our quality assurance system as the measurement technology develops.
Column
Hello, I am Sakai who works in the production department. I am taking responsibility for injection moulding and internal system management. I have been working for 7 years and I feel there has been a big development of information technology via my system control work. Also in injection moulding work, the injection machines are the same as before but the injection parts became more complex.
On weekends, I enjoy watching sports. I used to visit the competition on-site to cheer for my favourite team, but I watch those matches on TV at home, nowadays. Recently, I visited the Ise shrine and Atsuta shrine to pray for a return to normality in the near future. I also enjoyed the local dishes on the way back home from the shrines.
Conclusion
Cracking, slumping or blistering is commonly observed due to the binder swelling and/or residual stress difference between the surface and interior. It is possible to avoid these problems by selecting appropriate organic solvents and temperature control.
The catalytic deboning uses sublimation of binder, thus it is possible to decompose the binder in relatively short processing time with minimizing the deformation. However, with a strong acidic atmosphere, theapplicable metal material option is limited.
In the sintering process, the necking, which is bridging the metal powders by thermal diffusion, starts at the sintering temperature. Once the necking starts, the shrinking is observed and increases the density, thus before starting the necking reaction, all the organic components should be decomposed and gas between the powders should be removed. In the sintering process, the unwanted chemical reaction, such as oxidation or carbonization, leads to mechanical property loss, therefore a precise atmosphere control with low heating rate at the beginning of the process is required. Also, when it reaches the sintering temperature, some add it vegetal can be evaporated.