Surface Treatment Methods of Common Plastic Moulds

Surface Treatment Methods of Common Plastic Moulds

At present, the commonly used surface treatment methods for plastic molds include nitriding, electroplating, sunblocking and sandblasting. Among them, nitriding and electroplating are methods to improve the life of molds, while sunblocking and sandblasting are methods to decorate mold surfaces.

1、 Nitriding

Chlorination is divided into nitriding and nitrocarburizing. The biggest advantage of this process is that the heat treatment temperature is low (generally 500-600), the deformation after heat treatment is small, and the nitride layer generated is very hard, which makes the die wear resistant

And improved anti occlusion. The corrosion resistance, heat resistance and fatigue strength of the die have been greatly improved.

1. Nitriding: Nitriding methods include gas nitriding, liquid nitriding, solid nitriding, ion chlorination, etc. At present, we usually use gas nitriding to introduce nitrogen (NH3) into about 550? The nitrogen obtained from the decomposition of ammonia gas penetrates into the steel. The nitriding time is long, generally about 0.015-0.02mm per hour for shallow layer, and about 0.0050.015mm per hour for deep layer nitriding. In the high alloy steel, the nitriding speed will be lower than the above data due to the high content of alloy elements and the low diffusion rate of nitrogen. The time of gas nitriding (the workpiece is less than 300X300X50mm) is generally 8-9 hours, the depth of nitriding layer is 0.1-0.2mm, the surface hardness after nitriding is HV850-1200 (HRC65-72), and the surface color is bright.

2. Nitrocarburizing: that is what we call soft nitriding, also known as liquid nitrogen. The nitrocarburizing temperature is slightly higher than the nitriding temperature, which will not have a great impact on the hardness of the carburized layer. It will not increase the brittleness of the infiltration layer, but it can increase the diffusion rate. Nitrocarburizing generally adopts 570 or so, and low-carbon steel can be nitrocarburized above 600 to obtain a thicker compound layer. The depth of nitrocarburizing layer increases fastest in the first three hours. After six hours, the depth of carburizing layer does not increase significantly, so the time of nitrocarburizing generally does not exceed six hours. The depth of nitriding layer is generally

0.05-0.100mm, the surface hardness is HV1000 (above RC68), and the surface color is 3. Some requirements of nitriding on materials: dark gray.

(1) At 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 gassed (it is difficult for gases with high chromium content to be pumped into steel).

4. Some phenomena after nitriding

(1) After nitriding, the surface of the workpiece will appear some "swelling" phenomenon, which is a very thin (0.02-0.03mm) white bright layer on the surface of the workpiece, and it is relatively soft,

This layer must be polished before the workpiece can be restored to its original size. The hardness after removing this layer is also the hardest.

(2) Some thin walls, sharp corners and threads shall be properly protected during nitriding to prevent cracking.

5. Relationship between nitriding and welding

(1) If the workpiece has been burned and welded during processing, the heat treatment plant must be informed when it is sent for nitriding to facilitate local tempering, otherwise the hardness of the workpiece is uneven after nitriding and it is easy to crack or collapse. (2) When the workpiece needs to be welded due to edge collapse or other reasons due to improper use after nitriding, if the area is large, it must be sent back to the heat treatment plant for nitrogen removal treatment (heating to more than 800), then welded, and then nitrided after processing (note: it may cause the hardness of the whole workpiece to change). In case of partial welding

There are two methods: one is to grind off the nitriding layer for welding, the other is to locally heat and burn red, and then conduct welding after nitrogen removal.

2、 Electroplating

The purpose of electroplating is to prevent corrosion, improve the surface hardness and wear resistance of the mold, resist scratches and bites, facilitate demoulding, and increase the service life of the mold. At present, the common method is nickel plating. The coating is about 0.025mm. It is especially useful for some plastic materials (such as PVC, POM, etc.) with acid gas decomposition. Electroplated coating is sensitive to impact, and will fall off if impacted one day.

The differences between electroplating and nitriding are as follows: 1. Electroplating changes the size of the mold cavity surface, while nitriding does not change the size of the mold cavity surface; 2. The electroplated coating needs continuous maintenance, while the nitrided coating does not need maintenance.

3、 Shaping and sandblasting

The sunburn pattern is a decorative pattern formed by etching different patterns onto the surface of the workpiece by chemical etching based on the principle of a photographic plate. Sand blasting is a mechanical method, which is a surface decoration method formed by uniformly spraying sand particles on the surface of the workpiece at high speed and high pressure.

Relationship between the two and nitriding:

The commonly used method is: sunburn or sand blasting treatment shall be carried out before nitriding, so as not to cause different grain depths on the material surface. If nitriding is carried out first, then drying

Grain or sand blasting. It will cause different textures to be formed on the surface of the same product.