INDUSTRY NEWS

How to improve the appearance finish of castings?

Mar. 03,2022

I Common reasons for affecting the surface finish of castings:

casting parts surface finishing

1. Raw materials, such as the shape of molding sand, are divided into round, square and triangle. The worst is triangle, and the gap is particularly large (if it is resin sand molding, the amount of resin added will also increase, and of course, the amount of gas will be increased at the same time. If It is easy to form pores if the exhaust is not good), the best is round sand. In the case of pulverized coal sand, the proportion of sand (strength and humidity of the sand) also has a great influence on the appearance. If it is hardened sand of carbon dioxide, it mainly depends on the coating.

2. Material, if the chemical composition ratio of the casting is out of balance, such as low manganese, it is easy to produce loose, rough surface material
 
3. Casting system, if the casting system is unreasonable, it is easy to cause loose castings. In serious cases, the castings cannot be poured, and even complete castings cannot be made.
 
 
Unreasonable slag holding system will cause slag to enter the cavity and cause slag eyes
 
4. Slag making, if the slag in the molten iron is not cleaned up or the slag is not blocked during casting, so that the slag runs into the cavity, slag holes will inevitably appear
 
5. Artificial, due to carelessness, the sand is not cleaned up or falls into the sand when the box is closed, the shape is that the sand is not firm, or the sand ratio is unreasonable, the sand strength is not enough,,,, and the casting will produce sand holes.
 
6. Exceeding the standard of sulfur and phosphorus will cause cracks in the castings. This is something that must be paid attention to when producing or guiding production to ensure the quality of the castings.
 
The reasons mentioned above are only a small part of them. Due to the ever-changing and esoteric production of casting, problems encountered during production will often occur. Sometimes a problem occurs for a long time and the cause cannot be found.

II. Three factors affecting the roughness of gray cast iron
 
     As an important measure of gray cast iron surface quality, surface roughness not only directly determines the appearance of gray cast iron parts, but also has a great influence on the equipment quality of the machine and the service life of gray cast iron parts. This paper focuses on analyzing how to improve the surface roughness of gray cast iron parts from three aspects of machine tool, tool and cutting parameters.
 
1. The influence of machine tools on the surface roughness of gray cast iron parts
     The poor rigidity of the machine tool, the poor accuracy of the spindle, the insecure machine tool fixation, and the large matching clearance of the machine tool components will affect the surface roughness of the gray cast iron parts.
 
     For example: if the runout accuracy of the machine tool spindle is 0.002mm, which is 2 microns, it is theoretically impossible to process workpieces with a roughness lower than 0.002mm. Generally, workpieces with a surface roughness of Ra1.0 can be processed. processed. And gray cast iron itself is a casting, so it will not be as easy to process a high surface roughness as a steel part. In addition, the poor conditions of the machine tool itself make it more difficult to ensure the surface roughness.
 
     The rigidity of the machine tool is generally set at the factory and cannot be modified. In addition to the rigidity of the machine tool, the spindle clearance can be adjusted, the bearing accuracy can be improved, etc., so as to make the machine tool clearance smaller, so as to obtain higher surface roughness for gray cast iron parts during processing. degree is guaranteed.
 
2. The effect of cutting tools on the surface roughness of gray cast iron parts
Choice of tool material
 
     When the tool material has a high affinity with the metal molecules of the material to be processed, the material to be processed is easy to bond with the tool to form built-up edge and scale spurs. Therefore, if the bonding is serious and the friction is serious, the surface roughness will be large, otherwise, the surface roughness will be small. . In the same processing of gray cast iron parts, it is difficult for carbide inserts to reach the surface roughness of Ra1.6. Even if it is achieved, the tool life will be greatly reduced, while the CBN tool made of BNK30 has a low friction coefficient of the tool material and excellent high temperature heat. Stability and wear resistance, the surface roughness of Ra1.6 can be easily machined under the condition that the cutting speed is several times higher than that of cemented carbide, and the tool life is dozens of times that of cemented carbide tools, and the surface brightness is improved by one. Magnitude. twenty two
 
Selection of tool geometry parameters
 
Among the geometrical parameters of the tool, the main declination angle Kr, the secondary declination angle Kr' and the radius re of the tool nose arc have a great influence on the surface roughness. When the main and auxiliary declination angles are small, the height of the residual area of ​​the machined surface is also small, so the surface roughness can be reduced; The secondary declination angle depends on the rigidity of the machine tool. The influence of the radius re of the tool nose arc on the surface roughness: when the rigidity allows re to increase, the surface roughness will decrease. Increasing re is a good way to reduce the surface roughness. Therefore, reducing the main declination angle Kr, the secondary declination angle Kr' and increasing the radius r of the tool nose arc can reduce the height of the residual area, thereby reducing the surface roughness.
 
The tool engineer said, "The selection of the arc angle of the tool nose is based on the rigidity and roughness requirements of the workpiece. If the rigidity is good, try to choose a large arc angle, which can not only improve the processing efficiency, but also improve the surface finish. ; But when boring or cutting slender shafts or thin-walled parts, because of the poor system rigidity, a smaller tool nose arc radius is often used.” 3.3
 
Tool wear
 
The wear of the tool is divided into three stages: initial wear, normal wear and severe wear. When the tool enters the stage of severe wear, the wear rate of the tool flank rises sharply, the system tends to be unstable, the vibration increases, and the variation of surface roughness also rises sharply.
 
In the field of gray cast iron, many parts are mass-produced, and have high requirements on the consistency of product quality and production efficiency. Therefore, many machining companies choose to change the tool without waiting for the tool to reach the third stage of severe wear, which is also called compulsory When changing the tool, the machining company will repeatedly test the tool to determine a critical point, which will not affect the overall production efficiency while ensuring the surface roughness requirements and dimensional accuracy of gray cast iron.
 
3. The influence of cutting parameters on the surface roughness of gray cast iron parts Different cutting parameters have a great influence on the surface roughness, which should be paid enough attention. Finishing is an important process to ensure the surface roughness of gray cast iron parts. Therefore, when finishing, the cutting parameters should be mainly to ensure the surface roughness of gray cast iron parts, and take into account productivity and necessary tool life. The cutting depth for finishing is determined by the machining accuracy and surface roughness requirements, and is determined by the allowance left after roughing. Generally, the cutting depth is controlled within 0.5mm. At the same time, under the conditions of the rigidity of the machine tool, the cutting performance of the tool can be fully utilized, and the gray cast iron parts can be processed at high speed with a high cutting speed.
4. The influence of other factors on the surface roughness of gray cast iron parts
For example, there are some casting defects in gray cast iron parts, unreasonable selection of cutting fluid, and different processing methods will affect the roughness of gray cast iron parts.
 
The tool engineer said, "In addition to the three major factors of machine tools, tools and cutting parameters, cutting fluid, gray cast iron parts themselves, processing methods and other factors also have a certain impact on the surface roughness of gray cast iron parts, such as turning, milling, When boring gray cast iron parts, the CBN tool can also process the surface roughness of Ra0.8 under the conditions permitted by the machine tool, cutting parameters and other factors, but it will affect the tool life, which needs to be judged according to the actual processing conditions. ".
 
5. Summary
 
In view of the fact that the surface roughness has a direct impact on the performance of machine parts, and the factors affecting the surface roughness in actual production come from many aspects, it is necessary to take into account various factors in all aspects, and propose a more economical approach to the surface roughness as needed. applicable requirements.
 
 
 
III. How to improve the surface finish of castings (ductile iron castings)
sandblasting
Craft:
Gasoline (120#) scrubbing with compressed air drying → sandblasting → compressed air blowing out sand particles → hanging → weak corrosion → flowing cold water washing → electro-galvanized or hard chrome.
Weak corrosion process: w (sulfuric acid) = 5% ~ 10%, room temperature, 5 ~ 10s.
Etching and scrubbing
When the workpiece is not allowed to be sandblasted due to the special requirements of precision or surface finish, only the method of etching and washing can be used to purify the surface.
step:
①Gasoline scrubbing (120#). When the oily workpiece or gasoline is dirty, wash it again with clean 120# gasoline.
 
② Compressed air to dry.
③ Erosion. w (hydrochloric acid) = 15%, w (hydrofluoric acid) = 5%, room temperature, until the rust is removed. If the rust is too much and the oxide scale is too thick, it should be scraped off mechanically first. The etching time should not be too long, otherwise it is easy to cause hydrogen permeation of the matrix, and the surface free carbon will be exposed too much, resulting in partial or total failure to coat the coating.
 
④Brushing with lime slurry can fully expose the crystal lattice on the surface of the workpiece and obtain a coating with good bonding force.
 
⑤ Rinse and wipe. Remove lime adhering to the surface.
⑥ Install and hang. The electrical conductivity of cast iron is poor, so the contact should be firm when installing and hanging, and there should be as many contact points as possible.
 
⑦ activation. The purpose of activation is to remove the oxide film formed in the processes of wiping and hanging. Formula and process conditions: w (sulfuric acid) = 5% ~ 10%, w (hydrofluoric acid) = 5% ~ 7%, room temperature, 5 ~ 10s.
 
⑧ Rinse with running water.
 
⑨ Electro galvanized or hard chrome.