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SANDBLASTING & SHOT BLASTING

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1. Introduction

Sandblasting and shot blasting are two abrasive processes that create unique textures and visual effects. These methods, although based on similar principles, differ in the size of the abrasives used and the fineness of the result. They are commonly applied to various components of the Movement or the External parts (case, dial, hands) to give them a matte, satin-finished or grainy finish.

2. Manual sandblasting method

This method does not project any abrasive under pressure, unlike pneumatic sandblasting. An abrasive powder is spread on a perfectly flat glass or metal plate. The craftsman often adds a lubricant, such as oil or water. The component is then rubbed by hand against this abrasive-loaded surface. A regular movement, circular or back-and-forth, distributes the abrasion over the entire piece. This technique produces a matte or granular appearance, similar to that obtained by pneumatic sandblasting. However, it offers more direct control, but a less uniform result on large series. Corundum, aluminium oxide or diamond powder are suitable for this process. The choice of grain determines the fineness of the graining obtained. This ancient method predates the emergence of compressed air sandblasting cabinets. It remains used in artisanal watchmaking, particularly for small parts or localized touch-ups.

3. Sandblasting method (pneumatic)

Sandblasting consists of projecting an abrasive, often in the form of sand or beads, onto the surface of a component in order to create a rough or satin texture. The process can be carried out with a compressed air gun and is sometimes performed in a sandblasting cabinet to control the impact and limit the dispersion of the abrasive. The pressure, the angle of impact and the distance of the abrasive from the surface directly influence the depth and fineness of the finish obtained.

4. Shot blasting (micro bead blasting) method (pneumatic)

Shot blasting (micro bead blasting), on the other hand, uses very small metal or ceramic beads (generally between 20 and 100 microns) calibrated with greater precision, projected at high pressure onto the surface to be treated. Unlike sandblasting, this technique offers a much finer, more even and satin-finished result, ideal for subtle finishes. Shot blasting (micro bead blasting) is generally carried out using specialised machines, such as compressed air micro bead blasting cabinets, which allow precise control of the abrasive flow.

5. Abrasives

Abrasives play a crucial role in the final rendering of Sandblasting and Shot blasting (micro bead blasting) techniques. The choice of Abrasive depends on the desired texture and the Material of the component to be treated.

• Sandblasting :
• Silica sand is traditionally used, but it is increasingly being replaced by more modern and less Abrasive Materials, such as granite, corundum or Aluminium oxide. These abrasives make it possible to achieve a more uniform finish and preserve the surface of the Metal more delicately.
• Shot blasting (micro bead blasting) :
• The Glass (crystal) (in the form of glass beads) is the most commonly used Abrasive in Shot blasting (micro bead blasting) to achieve a soft, Satin-finished result. Other Materials, such as Inoxidisable (stainless) steel, Ceramics, or even hardened plastic beads, can also be used depending on the desired results.

The influence of the Abrasive, based on its size, Hardness, and calibration, on the final result is significant. For example, glass beads offer a finer, more uniform texture, while harder abrasives, such as aluminium oxide, can leave a rougher, grainier surface, perfect for a more raw effect.

6. Tools and machines

The tools used for Sandblasting and Shot blasting (micro bead blasting) vary depending on the scale of production and the complexity of the component to be decorated.

• Sandblasting cabinet: Basic equipment for Sandblasting, consisting of an enclosed chamber into which the operator places the parts to be sandblasted. The cabinet is equipped with a gun that projects the Abrasive at high pressure.
• Shot blasting (micro bead blasting) machine: Although similar to sandblasting installations, shot blasting cabins are often more sophisticated, including systems for regulating air pressure and Temperature, as well as systems for recovering the abrasive beads. The machines can be manually controlled or automated, in order to ensure a constant and homogeneous flow. The most complex among them automatically regulate the projection angle, pressure, and exposure durations according to the material and design of the component.

7. Components that can be sandblasted or shot blasted

Many elements of a watch Movement can be decorated by Sandblasting or by shot blasting. Here are some typical components:

• The bridges: These essential parts of the Movement benefit from a Satin-finished or granular finish, offering not only visual beauty, but also better scratch resistance.
• The main plates: The main plates of the Movement, which support the other components, can be shot blasted (micro bead blasted) in order to create a homogeneous and aesthetic surface.
• The dials: Certain surfaces of a Dial can be Sandblasted or Shot blasted (micro bead blasted).
• The cases and the Metal bracelets: Various elements of the Case (middle cases, bezels, case backs, pushers) or of the bracelets are sometimes Sandblasted or Shot blasted (micro bead blasted).

8. Effects obtained and results

• Matte or Satin-finished finish: depending on the nature and grading of the Abrasive, as well as the projection pressure.
• Unique aesthetic: these finishes create subtle plays of Light and interesting reflections, bringing a crafted and refined look to the Movement and horological External parts (case, dial, hands).
• Increased resistance: these processes also allow the treated surfaces to be slightly hardened, offering better protection against wear, corrosion and scratches through the Cold working phenomenon.
• Visual effect: the contrast between polished, sandblasted or shot-blasted surfaces on the same component can highlight the details of the Movement, giving a more intricate and elegant look.

9. Influence of the Abrasive

The influence of the Abrasive on the final result is essential to achieve the desired texture. The Abrasive determines not only the fineness of the finish, but also the overall visual appearance of the piece:

• Finer abrasives (such as glass beads in shot blasting) provide a soft rendering, with no visible roughness, ideal for a subtle and refined finish.
• Coarser abrasives (such as sand or aluminium oxide in sandblasting) generate more pronounced textures, creating a raw and robust effect, perfect for exposed parts or movements with an industrial design.

The use of these techniques requires precise mastery to guarantee an aesthetic finish that meets the industry’s high quality standards, while preserving the artisanal soul of the craft. Excessive pressure, an incorrect projection angle, or overly long exposure will distort the contours of the piece and break the sharp edges.