Briefly introduce: What are the surface treatment processes of tuning screws?
The surface treatment process of tuning screws should be selected in combination with its application scenarios (such as radio frequency communication, precision instruments and industrial transmission), material characteristics (brass, stainless steel and carbon steel) and functional requirements (conductivity, corrosion resistance, wear resistance and anti-loosening). The following are the mainstream surface treatment processes and technical points in the industrial field:
1. Conductive surface treatment (applicable to RF/microwave tuning screws)
Tuning screws should ensure low contact resistance and signal transmission stability in RF equipment, and this kind of technology is the first choice:
silver-plate
Core advantages: excellent conductivity, oxidation resistance and low high-frequency loss, which is the standard process of RF tuning screws.
Technological points
H65 brass is preferred as the base material, which needs degreasing → pickling → pre-copper plating to make the base, so as to enhance the adhesion of silver layer and prevent the coating from falling off.
The thickness of silver layer should be controlled at 5~15μm: too thin is easy to wear and oxidize, and too thick will affect the thread matching accuracy (especially the tiny specification of M1/M2).
After plating, anti-tarnishing treatment (such as passivation and immersion in organic protective film) should be carried out to avoid the silver layer from sulfiding and blackening in wet environment.
Application scenario: base station antenna tuning screw, microwave device tuning screw.
gild
Core advantages: conductivity and corrosion resistance far exceed that of silver plating, and there is no oxidation blackening problem, and the contact resistance is extremely low.
Technological points
The nickel layer thickness is 0.5~1μm, and the gold layer thickness is 0.5~3μm (thin gold meets the conductivity, while thick gold improves the wear resistance).
The cost is high, and it is only used for high-end precision RF equipment (such as tuning screws in aerospace and military fields).
Precautions: strictly control the composition of gold plating solution to avoid pinholes and pits in gold layer.
Second, anti-corrosion surface treatment (suitable for wet/corrosive environment)
When the tuning screw is used in outdoor, chemical equipment and other scenes, it is necessary to focus on improving corrosion resistance:
Galvanizing+passivation
Core advantages: low cost, good anti-corrosion effect, suitable for carbon steel, 40Cr and other iron-based tuning screws.
Process classification
Galvanizing: the thickness of the coating is 6~12μm, and after plating, it is passivated by color or black. The neutral salt spray test can reach 72~120h without rusting.
Hot-dip galvanizing: The coating thickness is 20~50μm, which has stronger corrosion resistance. However, the thicker coating will affect the matching accuracy of M1/M2 micro-threads, and it is only suitable for large-scale tuning screws.
Note: the passivation solution should be environmentally friendly (without hexavalent chromium), which conforms to RoHS standards.
Passivation of stainless steel
Core advantages: For the 304/316 stainless steel tuning screw, a dense oxide film is formed on the surface by chemical passivation, which improves the corrosion resistance and does not affect the thread accuracy.
Key points: Nitric acid+potassium dichromate passivation solution is used at 40~60℃ for 20~30min. After passivation, it needs to be fully washed and dried to avoid residual passivation solution from corroding the substrate.
Application scenario: tuning screws in medical instruments and food equipment.
Dacromet coating (zinc-chromium coating)
Core advantages: strong corrosion resistance (neutral salt spray test exceeds 500h), high temperature resistance (no failure at 300℃) and no risk of hydrogen embrittlement, suitable for high-strength carbon steel tuning screws.
Key points of process: the coating thickness is 4~8μm, which needs coating → baking and curing. The coating evenly covers the thread profile and does not affect the assembly torque.
Note: The hardness of the coating is low, so it should not be used in high wear scenes.
Three, wear-resistant/anti-loose surface treatment (suitable for frequently tuned screws)
hard chromium plating
Core advantages: surface hardness as high as HV800~1000, excellent wear resistance, suitable for tuning screws that need to be screwed repeatedly.
Technological points
The substrate needs to be polished first to ensure the surface roughness Ra≤0.4μm to avoid pitting on the chromium layer.
The thickness of chromium layer should be controlled at 3~10μm, too thick will easily lead to thread locking. After plating, it is necessary to dehydrogenate (keep the temperature at 200~220℃ for 2~4h) to prevent hydrogen embrittlement.
Application scenario: tuning screw and precision instrument adjusting screw in industrial transmission equipment.
Nylon coating (locking coating)
Core advantages: coating a layer of nylon resin on the surface of the thread profile, the nylon layer will be elastically deformed after tightening, realizing self-locking and locking, and reducing thread friction noise.
Technological points
The coating thickness is 5~20μm, which needs to be accurately controlled to ensure that the threads can still be smoothly screwed after coating.
Nylon coating should be resistant to high temperature (≥120℃) to avoid softening and failure in high temperature environment.
Application scenario: tuning screws in automotive electronics and household appliances, taking into account the requirements of locking and wear resistance.
Four, decoration/protection integrated processing
Electrophoretic coating
Core advantages: uniform coating, strong adhesion, color painting (black, gray, etc.), both anti-corrosion and decorative effects.
Key points: It is suitable for carbon steel and aluminum alloy tuning screws. The coating thickness is 10~20μm m. It needs to be cured at high temperature after electrophoresis to ensure the hardness of the coating.
Sandblasting+anodizing (special for aluminum alloy)
If the tuning screw is made of aluminum alloy, it can be sandblasted (to improve the surface roughness) first, and then anodized to form a hard oxide film, which has corrosion resistance, wear resistance and aesthetics.
Five, surface treatment process selection principle
RF scene: Silver plating (cost-effective) or gold plating (high-end demand) is preferred;
Corrosion environment: stainless steel is passivated, carbon steel is passivated by zinc plating or dacromet;
Frequent tuning: hard chromium plating or nylon coating is selected;
Micro-size (M1/M2): Avoid thick plating process (such as hot-dip galvanizing) and choose thin plating (silver plating and flash gold plating) first.
