Introduction
To management in the manufacturing industry and procurement/purchasing managers: What image comes to mind when you hear “aluminum die casting”? Often, the focus tends to be on the casting process itself—”high-speed, high-volume production,” “complex shapes,” and “low cost.” However, it is none other than the “secondary processing” after casting that determines the final quality, functionality, and total cost of aluminum die-cast products. Casting is merely the process of creating the material (blank); to make it function as a product, machining (cutting) to achieve dimensional accuracy, tapping to fasten parts, and surface treatment to provide corrosion resistance or aesthetic appeal are indispensable.
Unfortunately, cases are rampant where selecting a supplier with poor know-how in this secondary processing leads to problems such as, “‘The casting is cheap, but defects frequently occur in post-processing, causing costs to skyrocket,'” or “‘The tapping quality is unstable, causing frequent stops on the assembly line.'”
In this article, we will focus on the three secondary processes essential for maximizing the added value of aluminum die-cast products—”machining,” “tapping,” and “surface treatment.” We will thoroughly explain the practical know-how for each and the procurement strategies for optimizing costs while ensuring quality, all from an expert perspective.
Why is secondary processing essential for aluminum die casting?
Aluminum die casting is a manufacturing method that involves injecting molten aluminum alloy (typically ADC12) into a mold at high speed and high pressure. This characteristic is directly linked to the reason why secondary processing is mandatory.
The reason it doesn’t end with just “as-cast”
Die-cast products have an “as-cast surface (ihada)” and boast high dimensional accuracy, but this accuracy has its limits.
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Limits of Dimensional Accuracy: The dimensional tolerance for general die casting is typically around ±0.1mm to ±0.3mm. However, for areas requiring precision on the micron level (e.g., ±0.01mm), such as the mating parts of a motor or bearing press-fit areas, it is impossible to achieve this through casting alone. These high-precision areas always require machining (a cutting process) as a post-process.
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Adding Functionality: Products are often fastened with bolts to other parts. The “threaded holes” for this purpose cannot be formed with practical strength by casting, even if the shape can be created. Tapping (thread cutting) is essential. Furthermore, while aluminum is a metal that resists rust, it will corrode under specific environments (especially galvanic corrosion from contact with dissimilar metals). Moreover, it is conductive as is; if insulating properties are needed, if wear resistance needs to be increased, or if a specific color is desired for aesthetic parts, functional additions through surface treatment (anodizing, painting, etc.) are required.
The decisive impact of secondary processing on cost and quality
Secondary processing has an extremely large impact on the product’s total cost and quality stability. For example, “process diversification”—requesting casting from a manufacturer specializing in die casting, machining from a different factory, and surface treatment from yet another—appears cheap at first glance.
However, in reality, enormous “hidden costs” arise, such as “transportation costs between factories,” “management man-hours for incoming and shipping inspections at each process,” and “ambiguity of responsibility when a defect occurs.” In one case, by performing die casting, precision machining, and inspection in an integrated manner, porosity (internal defects) after machining was improved, and the defect rate was dramatically reduced from 10% to 1% (Source: OEM/EMS Partners).
The know-how for secondary processing is not just “work”; it is a “technology” that involves designing the processing sequence and jigs (fixtures to hold the product) based on a thorough understanding of the casting process characteristics (where porosity is likely to occur, how it tends to warp).
【Machining】Practical Know-How to Conquer the Biggest Enemy: “Built-up Edge (BUE)”
The biggest challenge in the machining of aluminum die casting (especially ADC12) is the “Built-up Edge (BUE).”
A built-up edge is a phenomenon where friction heat during cutting causes the aluminum to soften, melt, and adhere to the cutting edge of the tool (end mill, drill, etc.), which then repeatedly grows and breaks off. This causes critical damage to product quality.
Four Quality Defects Caused by BUE
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Deterioration of surface roughness: When the BUE breaks off, it tears off the machined surface, making the surface rough.
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Unstable dimensional accuracy: As aluminum adheres to the cutting edge, the effective cutting diameter changes, causing hole diameters to become smaller or groove widths to vary.
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Shortened tool life: When the BUE detaches, it can also cause the tool’s main cutting edge (carbide, etc.) to chip.
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Decreased machining accuracy: As the BUE grows, cutting resistance increases. In thin-walled products, this causes chatter (vibration), leading to a significant deterioration in accuracy (Source: THK Co., Ltd. OMNIedge).
Countermeasure 1: Tool Selection (Positive Rake Angle, Diamond/CBN Tools)
The basic principle to prevent BUE is “how to cut cleanly.” For soft metals like aluminum, it is a cardinal rule to select a sharp cutting tool with a positive (sharp) “rake angle” (Source: HILLTOP Corporation). If the rake angle is negative (obtuse), the metal is “pushed and crushed” rather than “cut,” which increases frictional heat and makes BUE more likely to occur.
Furthermore, since ADC12 contains a high amount of silicon (Si), 9.6% to 12.0%, tool wear is also severe. For this reason, in mass production, the use of PCD (Polycrystalline Diamond) tools or CBN (Cubic Boron Nitride) tools—which are much harder than carbide tools and have a low affinity (tendency to stick) for aluminum—is key to long-term tool life and stable machining quality.
Countermeasure 2: Cutting Conditions (“Not Too Slow” Speed and Appropriate Cutting Fluid)
It may seem surprising, but in aluminum machining, the cutting speed must not be too slow. The medium-to-low speed range of about 50–150 m/min is actually where BUE is most likely to occur. If the speed is too slow, the time chips are in friction with the cutting edge becomes longer, making adhesion more likely.
The countermeasure is to set the cutting speed in the high-speed range (e.g., 300 m/min or higher; in some cases, over 1,000 m/min with PCD tools). By cutting at high speed, chips are quickly ejected, and heat is dissipated before it can accumulate on the cutting edge. At the same time, the role of cutting fluid (coolant) is crucial. It requires not only cooling but also “lubricity” to create an oil film between the cutting edge and the aluminum to prevent adhesion, and “flushing properties (discharge pressure)” to powerfully wash away chips.
【Tapping】Techniques to Prevent Adhesion and Achieve Stable Threads
Similar to machining, the biggest challenge in tapping (thread cutting) is “adhesion.” Especially with small-diameter taps like M3 or M4, there is a constant risk that adhesion will cause the tap to break and the product to become scrap.
Challenge: Adhesion and Chip Clogging in Cutting Taps
When using conventional “cutting taps,” which cut threads while producing chips, the following problems occur:
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Breakage due to adhesion: Aluminum adheres to the tap’s cutting edge, causing a sharp increase in cutting resistance, and the tap breaks.
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Chip clogging: Aluminum chips tend to be long and stringy, so they clog at the bottom of blind holes (non-through holes), breaking the tap.
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Deterioration of thread accuracy: The adhered aluminum tears the threads, causing dimensional defects or insufficient strength.
Countermeasure 1: Optimization of Cutting Speed (Increase Speed to Prevent Adhesion)
Cutting speed is also important in tapping. In the case of aluminum, if the speed is too slow, frictional heat makes adhesion more likely. Therefore, assuming proper coolant supply, increasing the cutting speed (increasing the tap’s rotational speed) is effective in preventing adhesion (Source: Shinshin Co., Ltd.). Furthermore, for coolant, concentration management of “water-soluble cutting fluids” is important. If the dilution ratio is too high (too thin), lubricity decreases, causing adhesion.
Countermeasure 2: Differentiated Tool Use (Considering Plastic Forming with Roll Taps)
A very effective option for tapping aluminum die casting is the “roll tap (forming tap).” A roll tap is a tool that forms threads not by cutting with a blade and producing chips, but by pressing the tap against the material and “raising” it (plastic forming).
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Merits:
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Since no chips are produced at all, breakage troubles due to chip clogging are eliminated.
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The grain structure of the thread is strengthened (work hardening) by plastic forming, creating threads with higher strength than those made by cutting taps.
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Demerits:
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Hole diameter management before tapping is extremely critical (stricter than for cutting taps).
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It requires higher torque than cutting taps.
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While not usable in all locations, the adoption of roll taps is know-how that can dramatically improve quality and productivity, especially for blind holes or products requiring a clean environment that avoids chips (e.g., hydraulic components) (Source: MISUMI-VONA).
【Surface Treatment】The Optimal Solution for “ADC12” Selected by Purpose
Aluminum die casting (ADC12) contains a large amount of silicon (Si) to improve castability. This characteristic of “high Si content” is the biggest constraint when selecting a surface treatment.
Purpose 1: Corrosion Resistance & Paint Base (Chemical Conversion Coating: Trivalent Chromium)
The most common and low-cost option is “chemical conversion coating.” This involves immersing the product in a chemical solution to form a thin film (a few microns or less) on the surface through a chemical reaction.
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Characteristics: The main purposes are “improvement of corrosion resistance” and “improvement of paint adhesion (as a primer).”
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Cautions: In the past, “hexavalent chromium” was mainstream, but due to environmental regulations (like the RoHS Directive), “trivalent chromium chemical conversion coating” (such as Alodine or Palcoat), which has a lower impact on the human body and environment, or “non-chrome (chrome-free) treatment” are now mainstream. Since the film itself is very thin, wear resistance cannot be expected.
Purpose 2: Wear Resistance & Insulation (Anodizing: Cautions for ADC12)
Anodizing (anodic oxide coating) is a treatment that artificially generates a thick, strong oxide film (Al₂O₃) on the aluminum surface.
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Merits: The film is extremely hard (in the case of hard anodizing), dramatically improving “wear resistance.” Also, since the film does not conduct electricity, it can provide “insulating properties.” By impregnating dyes into the microscopic pores of the film, coloring, such as “black anodizing,” is also possible.
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Cautions for ADC12: Anodizing is known to have very poor compatibility with ADC12 (high-Si die-cast material). Si inhibits the formation of the anodic film, so even if the treatment is applied, “uneven coloring,” “stains,” or “roughness” are likely to occur, making it difficult to obtain a uniform and beautiful appearance (Source: OGANE Co., Ltd.). If anodizing ADC12, it is necessary to select a specialized vendor with dedicated chemical solutions or special know-how.
Purpose 3: Aesthetic Appeal & High Corrosion Resistance (Painting: Cationic Electrodeposition & Powder Coating)
When high corrosion resistance and a beautiful appearance are required simultaneously, painting is the optimal solution. The following two methods are frequently used for industrial products:
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Cationic electrodeposition coating: A method where the product is immersed in a pool filled with paint, and electricity is applied to adhere the paint. A uniform paint film can be formed even in the corners of complex shapes, and it excels in adhesion and corrosion resistance. It is widely used as a primer for automotive parts.
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Powder coating: After adhering powder paint to the product using static electricity, it is baked at high temperatures to form the film. The film is thick (e.g., 30–100μm), excels in impact resistance and chemical resistance, and the paint can be reused, making it a low-environmental-impact method.
The “One-Stop Procurement” Perspective for Achieving Cost Reduction in Secondary Processing
As we have seen, the secondary processing of aluminum die casting is a collection of know-how that is only viable based on an understanding of the casting characteristics.
The “Hidden Costs” Caused by Process Diversification
As a procurement manager, you might adopt a strategy of “process diversification” by placing separate orders: “Casting to Company A, Machining to Company B, Surface Treatment to Company C.” However, this increases the following “hidden costs” and “quality risks”:
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Logistics costs: Transportation costs and lead times are incurred three or more times (A → B → C → Your company).
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Management costs: Man-hours for placing orders, managing delivery times, and conducting receiving inspections for each company are tripled.
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Division of quality responsibility: If porosity is found during machining, the locus of responsibility becomes ambiguous—is it a “casting defect” or is the “machining method bad”?
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Division of know-how: Company B (machining) does not know the quirks of Company A’s (casting) mold (e.g., where it tends to warp), so they cannot design the optimal jig or machining sequence.
The Advantage of an Integrated Production System in Vietnam
The solution to this problem is “one-stop (integrated production) procurement,” where “Casting → Machining → Tapping → Surface Treatment → Inspection” are all completed within a single company.
Especially in Vietnam, which offers an excellent balance of labor costs and quality, suppliers who have established this integrated production system can be powerful partners for Japanese procurement managers. By having the casting and machining departments routinely provide feedback, improvements are made daily “at the casting stage to facilitate subsequent processes (machining).”
This not only reduces the “hidden costs” mentioned earlier but also achieves a reduction in defect rates, shortening of lead times, and clarification of quality responsibility, making it possible to significantly reduce the product’s Total Cost of Ownership (TCO).
Conclusion
The value of an aluminum die-cast product is not determined by the casting process alone. Rather, it is the technical capabilities in secondary processing—”machining,” “tapping,” and “surface treatment”—that determine the product’s final dimensional accuracy, functionality, and durability. The know-how to overcome challenges specific to aluminum, such as “built-up edge” and “adhesion,” and to select the optimal surface treatment based on an understanding of ADC12’s material constraints (high Si content), is what determines the supplier’s quality.
For all procurement and purchasing managers, we strongly recommend that when selecting suppliers, you maintain the perspective not only of casting costs and equipment but also, “How much secondary processing know-how do they possess?”, “When quality problems occur, do they have a system to investigate the root cause from both casting and processing aspects?”, and “To what extent have they internalized processes in a one-stop manner?”
Reviewing suppliers based on this total cost, including these secondary processes, will be a certain move that directly leads to strengthening your product competitiveness and reducing costs.