What drawing details must specify anodizing on cosmetic aluminum surfaces?

A drawing for cosmetic anodized aluminum should specify the substrate and prefinish condition, governing anodizing requirement, Type and class or color, applicable film requirement, sealed condition, cosmetic zones, approved appearance standard, defect boundaries, masked and rack-contact areas, finished dimensions and inspection method. It should not say only "anodize" or promise a paint-code match. For die castings, the drawing must also acknowledge flow marks, porosity and machined/as-cast contrast through explicit acceptance criteria.

Identify the substrate and condition before finish

Call out the exact aluminum alloy and product form. The same anodizing instruction can produce different appearances on an extrusion, machined plate and high-pressure die casting. For a casting, identify the controlled material specification and temper or condition where relevant. Define whether each cosmetic surface is as-cast, machined, polished, tumbled, brushed or blasted before anodizing.

Surface preparation belongs in the controlled manufacturing definition because anodizing follows the substrate. If the drawing requires a texture, state the intended process or an approved appearance result and identify where it applies. Avoid copying an arbitrary roughness value into a cosmetic note unless the measurement location, method and correlation to appearance have been established. Roughness alone cannot limit flow pattern, porosity or color. The machining route should be identified wherever toolpaths or exposed subsurface metal will remain visible.

Use an unambiguous anodizing callout

Reference the contractually applicable specification and revision, then state the required Type, class or coloring route and sealing condition as that specification defines them. Do not mix labels from unrelated standards. Type III hardcoat is not arc anodizing or plasma electrolytic oxidation. If the customer's internal specification governs, provide it with the RFQ and resolve conflicts before quotation.

State thickness or coating requirements only where they serve function or inspection. The processor should confirm feasibility on the selected alloy and geometry. A nominal value without tolerance, measurement location and test method invites disagreement. On a die casting, readings may be sensitive to curvature, roughness and local microstructure; witness coupons and finished-part measurements may serve different purposes and should be named accordingly.

Map cosmetic zones and process witness areas

Mark A, B and non-cosmetic zones directly on views or a referenced surface map. Define which surfaces are visible in normal assembly and the orientation in which they are judged. Put permissible gates, ejector marks, parting-line vestige, rack contacts and plug marks in controlled locations. If a rack point must remain electrically bare, treat it as both a process need and a corrosion/appearance boundary.

For a die casting, state how pits, exposed porosity, laps, stains, flow lines and machined-to-cast shade transitions are assessed. "Free from all defects" is not measurable and may be physically inconsistent with the selected alloy and route. Use approved limit samples where numerical metrology cannot represent appearance. Those samples should be tied to part number, revision, surface route and controlled viewing conditions.

Define color by a production reference

A paint code or screen value can communicate design intent but does not by itself define translucent anodized color. Substrate chemistry, texture, film, dye, sealing and illumination all contribute. Establish a physical master produced from the intended alloy and prefinish route. When variation is expected, approve light/dark and gloss/texture boundaries rather than one ideal sample that no production lot can repeatedly equal.

State light source, viewing distance, orientation, comparison method and any instrument method that has been correlated to visual acceptance. If cast and extruded parts are assembled together, identify whether they are compared side by side. Metamerism can make a match under one light separate under another, so use the product's relevant lighting conditions when appearance carries buyer value.

Call out interfaces, masking and finished dimensions

Specify performance tests as complete acceptance rules

When corrosion, wear, seal quality, electrical isolation or color stability matters, identify the test method and revision, sample type, preparation, locations, quantity, conditioning, duration or cycles and pass/fail criterion. Do not write an isolated salt-spray duration or hardness value copied from another part. The requirement must connect to the chosen alloy, environment and failure mode.

Distinguish process-control coupons from finished-part qualification. A coupon may help monitor an anodizing bath, while the casting contains pores, edges, machined areas and rack locations absent from that coupon. State which one releases the product. Where destructive testing is required, define the sampling plan and whether the tested items come from the production lot.

Keep control-plan details out of the drawing when appropriate

The drawing should contain durable product requirements. Supplier-specific tank settings, proprietary pretreatment chemistry and day-to-day inspection frequency normally belong in approved process documentation or the control plan unless the customer specifically controls them. This distinction lets the drawing define what must be achieved without freezing an unsupported recipe.

Before release, review the callout with the casting source, machinist and qualified anodizing processor. Confirm capability through production-intent samples and document any allowed exceptions. The drawing, approved samples and control plan should agree; when they conflict, the contract must identify which requirement governs.

A practical release checklist

Send the controlled drawing, 3D model, alloy requirement, cosmetic map, assembly context, mating-material information, environmental exposure and annual quantity with the RFQ. Ask the supplier to return a marked-up feasibility review covering racking, masking, expected visual variation, measurement locations, tests and change triggers. Resolve these points before color approval and before a tolerance is based on an assumed coating allowance.

The objective is not a longer finish note. It is a requirement that different people can manufacture and inspect the same way. A concise drawing supported by a controlled appearance standard and approved finish plan is stronger than a page of universal numbers. The exact controls remain project-specific, especially on silicon-rich die-cast aluminum.