Holes That Cannot Be Oxidized During Machining

Last Updated on October 12, 2023 by assistant

I don’t know if you have noticed that every time you are looking for a CNC machining service manufacturer, their salesperson will explain to you: “We can produce your parts and provide post-processing services for your parts, but after oxidation In some parts, there will be some places where the holes are not oxidized properly.”

In response to this, most people would feel that the company’s level of oxidation capabilities is lacking. Right? You are in front of the screen. In fact, this misunderstanding is caused by some service providers failing to explain clearly why this phenomenon exists. In this blog, we will take an in-depth look at redox pores in machining, giving you a comprehensive and up-close look at the most amazing part of the parts you are about to produce – the pores that cannot be oxidized.

First of all, the holes we currently see in manufacturing are divided into two types: blind holes and through holes. Here are the details.

What is a blind hole?

In engineering and machining, a blind hole is a hole drilled, bored, or machined to a specific depth but not completely through the workpiece. Blind holes are holes that connect the surface layer to the inner layer but do not penetrate the entire board. Blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth. They are used to connect surface circuitry to the underlying internal circuitry. The depth of the hole usually does not exceed a certain ratio (hole diameter). In other words, it is a hole that is open at one end and closed or blocked at the other end.


Use of blind holes
Blind vias serve different purposes in different applications and are typically used for:
Fastener installation
Blind holes are often drilled to accommodate fasteners such as screws, bolts, or rivets. Blind holes allow fasteners to be inserted and tightened from one end, providing a secure connection without protruding beyond the workpiece.
Fluid and gas channels
Blind holes can be used to create internal channels or channels for the flow of fluids or gases within a part or assembly. These holes may be part of a larger system, such as coolant passages in an engine block or hydraulic passages in a manifold.
Threaded hole
Blind holes are typically tapped to form internal threads for insertion of screws, bolts, or threaded inserts. Threads provide a way to fasten parts together or connect additional components without penetrating the entire workpiece.
Countersinks and countersunk holes
Blind holes can be countersunk or countersunk to create a recessed area to accommodate the fastener head or to create a chamfer. These features allow the fastener to be flush with the surface or provide a beveled edge for aesthetic or functional purposes.

What is a through hole?

In engineering and machining, a through hole is a hole drilled, bored, or machined completely through the workpiece, creating openings at both ends. Unlike blind holes, through holes have entry and exit points that allow objects, liquids, or gases to pass through the workpiece.


Uses of through-hole
through hole have a variety of uses, typically used for:
Fastener alignment
Through-holes are often created to align components and enable fasteners such as bolts or pins to pass through the workpiece and secure multiple parts together. This allows for safe, precise assembly.
Fluid and gas flow
Through holes can be used to create channels for the flow of liquids, gases, or air. For example, in plumbing, holes are drilled in pipes to allow water or other fluids to pass through.
Electrical wiring and connections
Through holes can be used to pass wires, cables, or connectors through a workpiece or circuit board. This facilitates the connection of electrical components or the integration of wiring within the assembly.
Alignment and positioning
Through holes can be used as reference points for alignment or positioning during machining operations. They can be used as guides or indicators for other machining operations such as drilling or milling.

The main difference between blind holes and through holes

The main differences between blind vias and through vias are the depth and the presence of exit points. Blind holes are drilled to a specific depth but do not penetrate the entire workpiece, whereas through holes extend from one surface to the other, resulting in openings at both ends.

How to choose the right holes for your design parts

The choice of blind and through holes depends on the specific requirements of the application. Blind holes are typically used when a fastener or fluid flow needs to be contained within the workpiece, while through holes are preferred when an object, fluid, or line needs to pass completely through the workpiece.

After a preliminary understanding of blind holes and through holes, I think you still don’t know why blind holes and through holes cannot be oxidized. Here’s why.

First, it is difficult to anodize blind-hole aluminum. The concentration of the internal solution easily drops, and high-concentration solutions cannot be replenished in time. Secondly, electrical shielding makes it difficult to form a thick oxide film on the inner hole (blind hole). Of course, there won’t be a certain thickness of the microlayer. The porous film makes it difficult to deposit pigments during the subsequent dyeing process, which is why it is difficult to color.

Secondly, when anodizing, involves the use of acid. If there is acid remaining in the blind hole, the acid will slowly seep out during the entire oxidation process of the product and during the final water-washing process, causing corrosion of the product and its surface. Will cause stains to corrode by acid. Such a result is something neither you nor we want to see. In order to let everyone understand the reaction process more intuitively, the following is the acetic acid etching process in anodization.

Acid etching process:
Aluminum and dilute sulfuric acid can react at room temperature to form 2Al + 3H2SO4 = Al2(SO4)3 + 3H2↑. The products are hydrogen and aluminum sulfate. The reaction is actually a bit slow and becomes violent when heated.

Besides, not only will blind holes be affected when oxidized, but when product parts need to be sandblasted when sand enters the blind holes, the sand needs to be cleaned. Otherwise, when the sand enters the oxidation tank, it will react with the solution, be corroded and rust, and finally condense in the blind holes, making it impossible to tighten the screws.

In short, the impurities remaining in the blind holes will slowly overflow from the remaining alkali etching solution in the next process, which will not only contaminate the solution but also make the area and its surroundings difficult to polish during chemical polishing or electrochemical polishing. When anodized or chemically oxidized, not only is it impossible to form an oxide film inside and around it, but it also leaves dark traces that cannot be removed.

In order to avoid the above effects, when we process blind holes, we will put a rubber plug to seal the blind hole part. This will cause the surface of the part to be oxidized, however, the blind holes will not be oxidized, or they will be oxidized to a small amount, which is normal.

As for why there are no through holes, the following is a common anodizing process. Everyone should see that when the aluminum workpiece is oxidized, a hanging device is needed. The place where the hanger is placed is the through-hole part, and the hanger will be oxidized. If the via is partially sealed, the via will not be oxidized.
General process flow:
Aluminum workpiece → Hanging device → Oil removal → Water washing → Alkali etching → Water washing → Light extraction → Water washing → Anodizing → Water washing → Deionized water washing → Dyeing or electrolytic coloring → Water washing → Deionized water washing → Hole sealing → Water washing → lower hanging device

The above is the reason why holes that cannot be oxidized appear during mechanical processing. If you need oxidation treatment or custom sheet metal processing services, you can contact us. We are a first-class parts manufacturer, providing you with the most complete parts manufacturing services.