Anodising
Anodising is a process commonly used for corrosion/abrasion. It is an electrochemical process wherein durable and decorative surface finishes are formed on a component made of aluminium or aluminium-based alloys.
Aluminium has the tendency to form an oxide layer when exposed to atmosphere. This oxide is a very thin layer but has inherent protective properties.
Anodising process
The process consists of passing an electric current through an acid solution in which the immersed aluminium component act as an anode and aluminium or lead typically provides the cathode. During the process electrolysis breaks down the water into oxygen and hydrogen. The oxygen combines with the aluminium to produce aluminium oxide and hydrogen is released.
During this process, aluminium is converted to aluminium oxide which occupies about 2 times the volume of raw aluminium.
Chromatizing
During chromatizing a thin chemical coating layer is formed on the surface of the aluminium part which enhances:
- Corrosion resistance
- Bonding ability
- Electrical conductivity
Chromatizing as a process is generally quicker and easier compared to anodizing, as it does not involve Electrical Process Control.
Vacuum Impregnation
Minor leakages of gases, air or liquid can cause an entire batch of production to be rejected. Today industry accepts impregnation as remedy for minor porosity and leakages. Impregnation can be performed on the casting before or after machining. Rather than rejecting the entire casting/ machined product, impregnation has proven to be the most cost effective solution. The main function of impregnation is to seal the casting in the leaking zones. Vacuum impregnation will seal voids with stable yet flexible material, the sealant providing resistance to attack of heat, oil, chemicals and other external influences.
Impregnation process
Dry Vacuum
The basket consisting of components to be impregnated is lowered into the autoclave. The basket is then subjected to high-speed vacuum.
Resin Transfer
Reservoir is subjected to similar vacuum as that of auto clave. The sealant is transferred from reservoir to the auto clave. Impregnate the vacuum in the auto clave & is maintained until the components are thoroughly impregnated. After the specified time gap, the valve is opened to return to auto clave at atmospheric pressure and further forcing the sealant to fill into the porous zones in the component.
Resin return
Autoclave lead opens, component basket is removed and transferred to the next station. Vacuum is maintained in the reservoir and the sealant is transferred from the auto clave back to the reservoir. The components are washed by agitation. Room temperature water is used to remove the residual sealant from the component surface.
Cure
The component basket is lowered into water maintained at 90° C for 10 minutes allowing the sealant to polymerize.
Vacuum impregnation does not affect the dimensions of the castings. The micro porosities, if any, are blocked. The entire operation takes around 30 min and components can be leak tested immediately. Leak Testing
Porosity/micro porosity leads to leakage (for air, gas and liquid/fluid). When the component is used in a sealed application, the amount of leakage or drain is the important factor, permissibility for which is defined by the component manufacturer’s specifications. This leakage can be found out by the differential pressure decay method. It works on a reference volume to test a particular component. The reference component is made with precision and accuracy & is used to calibrate the machine. A comparison is drawn between the desired component and the reference component and which can compensate for any ambient pressure and temperature variation, which can happen simultaneously on both parts. A specific time is defined as a stabilization time wherein all exogenous factors can stabilize and a uniform effect on both the components will be applicable. Hereafter the leak, if there is any, will be because of porosity or defect of the desired component. A Transducer can detect this. The resolution is defined in decimal Pascal and will meet almost all automotive manufacturers’ requirement by this method.
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