Release time：2020-01-16 18:44 Browse：
How Are Ultra-Long Screws Treated with Dacromet?
Excellent, this is a very specific technical question. The Dacromet treatment of ultra-long screws (e.g., over 1 meter or even longer) indeed requires special process considerations and equipment support, but the core principles are consistent with standard parts.
Overall, the key to Dacromet treatment for ultra-long screws lies in solving the challenges of "how to achieve uniform immersion, centrifuging, and efficient sintering." The process flow is the same as for standard screws, but the equipment must be specially designed.
The following details the process steps, challenges, and solutions for Dacromet treatment of ultra-long screws:
Pre-treatment
Degreasing: Removes oil stains from the screw surface. Usually done by spraying or immersion.
Shot Blasting/Sand Blasting: Removes rust and scale, and creates a certain surface roughness to enhance coating adhesion. This is crucial for ultra-long screws.
Coating
Immersion: The screws are completely immersed in the Dacromet solution.
Centrifuging: After removal from the solution, the screws are quickly placed into a centrifuge. High-speed rotation removes excess coating liquid, ensuring a thin and uniform coating, particularly preventing pooling at the root and crest of the threads.
Baking/Curing
Preheating: Evaporates most moisture and some organics at a relatively low temperature.
High-Temperature Sintering: At a specific high temperature (approx. 300°C for ordinary Dacromet, 200-250°C for environmental-friendly types), the organics in the coating decompose, and the metal flakes (zinc, aluminum) sinter with chromates or chromium-free passivators to form a dense coating.
Cooling and Inspection
Natural cooling or forced cooling after exiting the oven.
Thickness measurement, sampling for corrosion resistance testing (e.g., salt spray test), adhesion inspection, etc.
| Process Step | Challenges | Solutions & Special Equipment |
|---|---|---|
| Loading & Conveying | Difficult manual handling, low efficiency, prone to damage. | Use automated overhead cranes or special lifting tools. Screws are typically neatly hung on special through-type racks. The entire rack is automatically transported by a conveyor chain. |
| Immersion & Centrifuging | 1. Length exceeds standard centrifuge dimensions. 2. During centrifuging, excessive length can cause bending, collision, or vibration, leading to uneven coating. | Use extra-long specialized centrifuges. Their drums are designed to be very long with a relatively small diameter to fully accommodate ultra-long screws. The centrifuging process must be smoothly started, rotated at constant speed, and smoothly stopped, controlled by precise programs to avoid severe shaking. |
| Baking/Curing | 1. Requires an extra-long oven. 2. Temperature uniformity inside the oven is extremely difficult to control; ends and middle sections prone to temperature differences, causing inconsistent curing. 3. May sag and bend under their own weight when hung. | Use through-type (continuous) sintering ovens. Screws along with the rack move uniformly through the long preheating, sintering, and cooling zones on a conveyor chain. The oven is designed with multi-zone temperature control systems and efficient hot air circulation systems to ensure highly uniform temperature throughout the oven (temperature difference within ±5°C). Rack design must have sufficient support points to prevent screws from sagging due to heat softening. |
Coating Uniformity: This is the biggest challenge. Must ensure consistent coating thickness on thread crests and roots, bolt head, and shank. Precisely controlling immersion time, centrifuge speed, and time is the core method.
Thread Accuracy: The Dacromet coating adds several microns of thickness. For ultra-long, high-precision screws, thread gauges (go/no-go) must be used for full inspection after coating to ensure thread fit. If necessary, thickness allowance for the coating must be reserved with customer agreement or in the process.
Corrosion Resistance: Ultra-long screws are often used in large structures (e.g., wind turbine towers, large bridges), requiring extremely high anti-corrosion life. Regular sampling for Neutral Salt Spray Test (NSS Test) is essential to ensure the required hours are met (e.g., 720h, 1000h without red rust).
Hydrogen Embrittlement Risk: The Dacromet process itself is hydrogen embrittlement-free (no pickling, no electrolytic hydrogen evolution). This is a major advantage over zinc electroplating, especially for high-strength bolts.
Ultra-long screws can definitely be treated with Dacromet, but not all surface treatment facilities can do it. You need to find a specialized manufacturer with the following capabilities:
Possesses specialized equipment for extra-long specifications: Including extra-long centrifuges and extra-long through-type sintering ovens.
Rich experience: Particularly a database of process parameters for handling similar long components (e.g., centrifuge speed, time, oven temperature profile).
Strict quality control system: Capable of effectively monitoring coating uniformity and thread accuracy.
