Once the powder has been formed it is considered to be in the “green state.” The next step in the process is to fire the parts. This is one of the most critical steps in the ferrite manufacturing process since this is where the part obtains its magnetic and mechanical characteristics. The temperature profile needs to be carefully controlled in order to achieve electrically and geometrically good parts.
To do this, the green ferrite is first exposed to a gradual temperature increase to temperatures of up to 800°C in order to burn out the binders, lubricants, and any other impurities. Once these impurities are removed, the ferrite is then exposed to temperatures of 1000°C and higher (up to 1450°C) to cure the material. Finally, cooling of the parts is carefully controlled to prevent cracking and ensure the electrical properties of the parts are properly formed. During this process the part can shrink 10% – 20% depending on the powder and the pressing method (up to 70% volume reduction).
Specialized kilns are used to achieve these extremely high temperatures and atmosphere control. For Fair-Rite’s processes, there are two types of kilns: box and tunnel. Box kilns allow for greater control and more flexibility over the firing profile. Tunnel kilns provide a higher throughput, but in some cases may not be able to achieve the appropriate temperature or atmosphere profile.
The type of material used determines the type of firing – either air or atmosphere. Air firing is similar to your oven at home – just a lot hotter! On the other hand, atmosphere firing involves evacuating oxygen at certain points in the firing process to control the atmospheric profile and prevent oxidation and ensure proper composition of the parts.
|Nickel Zinc (NiZn)||Air|
|Manganese Zinc (MnZn)||Atmosphere|
|Magnesium Zinc (MgZn)||Air|
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