Surface treatment methods for commonly used plastic molds

The commonly used surface treatment methods for plastic molds include nitriding, electroplating, sun marking, and sandblasting. Nitriding and electroplating are methods to improve the lifespan of molds, while sun marking and sandblasting are decorative methods for mold surfaces.
1、 Nitridation
Chlorination is divided into nitriding and nitrocarburizing. The biggest advantage of this process is the low heat treatment temperature (usually 500-600), small deformation after heat treatment, and the formation of a hard nitride layer, which improves the wear resistance and bite resistance of the mold. The corrosion resistance, heat resistance, and fatigue strength of the mold have been greatly improved.
1. Nitriding: The methods of nitriding include gas nitriding, liquid nitriding, solid nitriding, ion chlorination, etc. Our current common method is gas nitriding, which involves introducing nitrogen (NH3) into approximately 550 ° C? In the furnace, nitrogen obtained from ammonia decomposition is infiltrated into the steel. The nitriding time is relatively long, generally around 0.015-0.02mm per hour for shallow layers, and 0.0050.015mm per hour for deep layers. In high alloy steel, due to the high content of alloying elements, the diffusion rate of nitrogen is low, and the nitriding rate will be lower than the above data. The time for gas nitriding (when the workpiece is less than 300X300X50mm) is generally 8-9 hours, and the depth of the nitriding layer is between 0.1-0.2mm. The surface hardness after nitriding is between HV850-1200 (HRC65-72), and the surface color is bright.
2. Nitrocarburizing: This is what we call soft nitriding, also known as liquid nitrogen. The nitrocarburizing temperature is slightly higher than the nitriding temperature, which does not have a significant impact on the hardness of the nitrided layer. It will not increase the brittleness of the infiltration layer, but it can increase the diffusion rate. Nitrocarburizing is generally recommended at around 570, while low-carbon steel can undergo nitrocarburizing at temperatures above 600 to obtain a thicker compound layer. The depth of the nitriding layer increases the fastest within the first 3 hours of nitrocarburization, and after more than 6 hours, the depth of the nitriding layer does not increase significantly. Therefore, the duration of nitrocarburization generally does not exceed 6 hours. The depth of the nitriding layer is generally 0.05-0.100mm, with a surface hardness of HV1000 (RC68 or above) and a surface color of 3. Some requirements for nitriding materials: dark gray.
(1) At the nitriding temperature, materials that do not undergo annealing can be nitrided.
(2) Metals with high chromium content (such as 420, S136, 2083, M300) cannot be subjected to gas nitrogen (as gas with high chromium content is difficult to penetrate into the steel).
4. Some phenomena after nitriding
(1) After nitriding, there will be some "swelling" phenomenon on the surface of the workpiece, which is the formation of a thin (0.02-0.03mm) white bright layer on the surface of the workpiece, which is relatively soft,
This layer must be polished off before the workpiece can recover to its original size, and the hardness after removing this layer is also the hardest.
(2) For some thin-walled, sharp corners, and threaded areas, appropriate protection should be provided during nitriding to prevent cracking.
5. Relationship between Nitriding and Welding
(1) If the workpiece has been burned or welded during the processing, it is necessary to inform the heat treatment plant when sending it for nitriding to facilitate local tempering treatment. Otherwise, the hardness of the workpiece after nitriding is uneven and it is easy to crack or collapse. (2) When the workpiece needs to be welded due to improper use or other reasons after nitriding, if it has a large area, it must be sent back to the heat treatment plant for denitrification treatment (heating to above 800), then welded, and then nitrided after processing (note: it may cause changes in the hardness of the entire workpiece). If it belongs to local welding, there are two methods: one is to polish off the nitriding layer for welding, and the other is to heat it locally, burn it red, and wait for nitrogen removal before welding.
2、 Electroplating
The purpose of electroplating is to prevent corrosion, improve the surface hardness and wear resistance of the mold, resist scratches and bite, facilitate demolding, and increase the lifespan of the mold. The commonly used method currently is nickel plating. The coating is approximately 0.025mm. Especially useful for plastic materials with acidic gas decomposition, such as PVC, POM, etc. Electroplated coatings are relatively sensitive to impact, and if they are subjected to impact for a day, they will fall off.
The difference between electroplating and nitriding: 1. Electroplating changes the size of the mold cavity surface, while nitriding does not change the surface size of the mold cavity; 2. The electroplating layer requires continuous maintenance, while the nitriding layer does not require maintenance.
3、 Sunburn and sandblasting
Sunburn pattern is a decorative surface formed by using a photographic substrate principle and using chemical etching to etch different patterns onto the surface of the workpiece. And sandblasting is a mechanical method that sprays sand particles uniformly at high speed and pressure onto the surface of the workpiece, forming a surface decoration method.
The relationship between the two and nitridation:
The commonly used method is to first perform sun drying or sandblasting treatment before nitriding, so as not to cause uneven depth of the material surface texture. If nitriding is performed first, followed by sun marking or sandblasting. It will cause different textures to form on the surface of the same product.