the mold surface roughness is a very important technical parameter in mold making, because the surface finish of the molding area directly affects the surface roughness of the plastic part. A range of polishing tools is used to meet a different surface finish requirement. The society of plastics industry classifies mold surface finishes into 12 grades in 4 categories:
A—mirror image, it is typically polished with diamond paste, it’s surface finish Ra is 0—0.0001mm, applicable for plastic parts whose surface should have high luster. In general, mirror image is further classified into A1, A2, and A3.
B—smooth, it is usually polished with sand paper. It’s surface finish is between 0.0001—0.0002mm, applicable for plastic parts with a typical surface requirement. This category can be further classified into B1, B2, and B3.
C—Semi-smooth.it is polished with whetstone. It’s surface finish is between 0.0002—0.0008mm, applicable for plastic parts without a specific surface finish requirement. This category is also further classified into C1, C2, and C3.
D—General. Sand blasting is commonly used. This category is also further classified into D1, D2, and D3.
Surface finish is one of the most important considerations when creating a mold. Key factors include mold material and customer-required tolerances.
Delivery of polymer melts into the mold cavity is the most important stage of the injection molding process. I want to say the influence of cavity surface roughness, polymer material (with different flow properties) and technological parameters on the flow length of polymers into mold cavity. Application of the measurement results may have significant influence on the production of shaping parts of the injection molds especially in changing the so far used processes and substituting them by less costly production processes which might increase the competitiveness of the tool producers and shorten the time between product plan and its implementation. Because the finishing operations of machining are very time and money consuming leading to high costs of the tool production.
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The Society of Plastics Industry (SPI) standardized surface finishes on injection molding tools. The process used to achieve the finish determines the following grades:
Grade A – Diamond Buffing: Diamond buffing paste has a very fine grain, so finishes are generally very glossy and product surfaces remain flat. Grade A finishes are considered the highest standard.
Grade B – Sandpaper: This semi-gloss finish is applied using sanding grit and is ideal for hiding visible machining marks. The sandpaper grit number provides surface finish control — 600 for the finest finish, 400 for mid-range, and 320 for the most textured finish.
Grade C – Grit Stone: Grit sanding stones generally follow the grit number and finish textures as sandpaper. However, grit stones cut fairly aggressively, which creates a rougher texture compared to B grades.
Grade D – Pressure Blasting: Pressure blasting plastic surfaces with abrasives — typically sand or glass beads — results in non-directional patterns and dull or flat finishes.
Micro injection molding presents many challenges in the injection-molding community. When the dimensions of the part (and thus the cavity of the mold) are small, micro-scale factors such as mold surface roughness may play an important role in the filling of polymer melt.
Mold polishing usually determines the quality of the surface finish of the injection mold, and this manual process can quickly consume profits. The injection mold may be made very well, and the mold making process may be effective, but if the surface finish is wrong, the part is unqualified.
If unnecessary time is wasted during milling, grinding or EDM to over-polish or compensate for poor surface finish, mold polishing can quickly eliminate profits. In order to avoid this problem, each processing operation must have a surface finish requirement.
Micro-injection faces many challenges in the injection molding industry. When the size of the part (and thus the cavity of the mold) is small, microscopic scale factors such as mold surface roughness may play an important role in polymer melt filling. The effect of mold surface roughness on the cavity filling of polymer melt in micro-injection molding was studied. A disc insert with different surface roughness but the same average line was studied. The significance of the surface roughness effect was evaluated by the ratio of the flow area of the rough half to the total flow area of the molded part. The experimental results show that in the micro-injection molding, the surface roughness of the mold has a certain inhibitory effect on the cavity filling of the polymer melt. The effect on surface roughness is not significant over a limited range of injection rates. An increase in mold temperature reduces the effect of surface roughness. Within the range allowed by the process, changes in melt temperature have little effect on surface roughness.
Now in the mold industry, we have had special equipments or instruments to test the surface roughness of the mold
The surface roughness of the mold is very important, we have special tool to check.