What are the pre-plating considerations for zinc alloy die castings?-企业官网

Pre-Plating Considerations for Zinc Alloy Die Castings

The success of electroplating zinc alloy die castings largely depends on the effectiveness of pre-plating treatments. Incorrect or neglected pre-treatment steps can lead to high rework or rejection rates, given the difficulty in reworking zinc alloy die castings. This special material requires meticulous attention to its unique characteristics. Below are the key considerations:

Pre-Plating Treatment Considerations for Zinc Alloy Die Castings

Understanding the Structural Characteristics: Zinc alloy die castings have a surface layer that is smooth and dense, measuring about 0.02 to 0.10 mm thick. Beneath this layer is a porous and loose structure. During mechanical polishing, it is crucial to avoid breaking through the dense layer to prevent exposure of the porous inner material, which can lead to defects such as bubbling and peeling of the plating.
Mastering the Chemical Properties: Zinc alloy die castings may contain areas rich in zinc and aluminum due to the casting process. Strong alkalis tend to dissolve the aluminum-rich phases, while strong acids dissolve the zinc-rich phases. Using strong acids or alkalis for degreasing or etching can cause bubbling and peeling of the plating. Therefore, mild chemicals should be used to avoid these issues.
Accelerating the Transition from Pre-Treatment to Plating: The pre-treatment aims to remove oils and oxidation from the surface, ensuring that metal ions in the plating solution deposit directly onto a clean surface. The parts should be plated immediately after pre-treatment to prevent re-oxidation or formation of hydrates on the surface, which can impair adhesion. If mild oxidation or hydration occurs, a dip in a 10g/L boric acid solution followed by activation in a NaCN solution can restore the surface purity.

Plating Process for Zinc Alloy Die Castings

Dewaxing: This step removes polishing wax and oils, ensuring a clean surface for plating. Ultrasonic dewaxing should leave no residues, and water tanks should be clean to prevent contamination. Trichloroethylene dewaxing must also leave no residues or traces of the solvent.
Degreasing: Molds used in casting leave behind oil residues that must be removed to ensure good adhesion of the plating. Zinc alloys should not be degreased with strong alkaline chemicals but rather with weak alkaline compounds. Electrochemical degreasing, combining cathodic and anodic methods, is often used to improve the cleaning of recessed areas.
Activation: Post-degreasing, a thin oxide layer remains on the surface. Acid activation removes this layer to enhance plating adhesion. Common activation solutions include a mix of hydrofluoric acid and sometimes sulfuric acid. Pure water should be used to prepare the activation solution, and thorough rinsing is essential to remove any residual acids.
Rinsing: Pure water is used for rinsing to remove harmful particles and solutions, preventing issues like bubbling during plating. A final ultrasonic rinse ensures the thorough removal of contaminants.
Pre-Dipping: A solution of 2-8% NaCN neutralizes acid residues and purifies the surface. Parts are then directly pre-plated without further rinsing, reducing contamination risks.
Pre-Plating: A thin layer of cyanide copper is applied quickly to ensure full coverage and strong adhesion. The copper layer must be at least 7 microns thick to prevent rapid diffusion and subsequent issues during subsequent plating stages.
Bright Cyanide Copper Plating: Directly follows pre-plating without rinsing, focusing on achieving a smooth, bright finish. Adequate thickness ensures robustness against activation failures.

Summary

The pre-plating process is critical for zinc alloy die castings. It is essential to maintain the concentration and purity of cleaning and activation solutions, replace them regularly, and ensure thorough rinsing. Any delays in plating after pre-treatment can lead to oxidation and poor adhesion, and attention to the pre-plating thickness is crucial for successful subsequent plating processes. Careful handling and immediate response to any defects can significantly improve overall quality and reduce losses.