How to solve defects in injection mold processing?

For injection mold processing, we must try to avoid some defects. We provide seven methods for this problem in this article.

1. Reasonably select and dress the grinding wheel. It is better to use a white corundum grinding wheel. It is hard and brittle, and it is easy to produce new cutting edges, so the cutting force is small and the grinding heat is small.

It is better to use medium grain size, such as 46~60 mesh. The hardness of the grinding wheel is medium-soft and soft (ZR1, ZR2 and R1, R2), that is, a coarse-grained, low-hardness grinding wheel, which has good self-excitation and can reduce cutting heat. It is very important to choose the appropriate grinding wheel during fine grinding. For the high vanadium and high molybdenum conditions of mold steel, it is more suitable to use GD single crystal corundum grinding wheel.

When processing cemented carbide and materials with high quenching hardness, diamond grinding wheels with organic binders are preferred. Organic binder grinding wheels have good self-grinding properties and the roughness of the ground workpiece can reach Ra0.2μm. In recent years, with the application of new materials, CBN (cubic boron nitride) grinding wheels have shown very good processing effects.

They are better than other types of grinding wheels in fine processing on CNC forming grinders, coordinate grinders, and CNC internal and external cylindrical grinders. During grinding, attention should be paid to timely dressing of the grinding wheel to keep the grinding wheel sharp. When the grinding wheel is passivated, it will slide and squeeze on the surface of the workpiece, causing burns on the workpiece surface and reduced strength.

2. Reasonable use of cooling lubricant in injection mold processing to play the three major roles of cooling, washing, and lubrication, keep the cooling and lubrication clean, so as to control the grinding heat within the allowable range to prevent thermal deformation of the workpiece. Improve the cooling conditions during grinding, such as using oil-immersed grinding wheels or internal cooling grinding wheels. Introduce the cutting fluid into the center of the grinding wheel, and the cutting fluid can directly enter the grinding area to play an effective cooling role and prevent the surface of the workpiece from being burned.

3. Reduce the quenching stress after heat treatment to a minimum, because the quenching stress and the network carbide structure will produce phase changes under the action of grinding force, which can easily cause cracks in the workpiece. For precision injection mold processing, in order to eliminate the residual stress of grinding, low-temperature aging treatment should be carried out after grinding to improve toughness.

Temperature is important for injection mold processing

4. To eliminate grinding stress, the mold can also be immersed in a 260~315℃ salt bath for 1.5min, and then cooled in 30℃ oil. In this way, the hardness can be reduced by 1HRC and the residual stress can be reduced by 40%~65%.

5. For precision grinding of precision molds with a dimensional tolerance within 0.01mm, pay attention to the influence of ambient temperature and require constant temperature grinding. It can be calculated that for a 300mm long steel piece, when the temperature difference is 3℃, the material changes by about 10.8μm (10.8=1.2×3×3, 1.2μm/℃ deformation per 100mm), and each finishing process needs to fully consider the influence of this factor.

6. Use electrolytic grinding to improve mold manufacturing accuracy and surface quality. During electrolytic grinding, the grinding wheel scrapes off the oxide film instead of grinding the metal, so the grinding force and grinding heat are small, and there will be no grinding burrs, cracks, burns, etc.

The general surface roughness can be better than Ra0.16μm; in addition, the wear of the grinding wheel is small. For example, when grinding cemented carbide, the wear of the silicon carbide grinding wheel is about 400%~600% of the weight of the cemented carbide ground off. When using electrolytic grinding, the wear of the grinding wheel is only 50%~100% of the weight of the cemented carbide ground off.

7. Choose grinding amount reasonably, and adopt fine grinding method with smaller radial feed or even fine grinding. For example, appropriately reduce radial feed and grinding wheel speed, increase axial feed, reduce the contact area between grinding wheel and workpiece, improve heat dissipation conditions, and effectively control the increase of surface temperature.