About The First Step Of The Rotational Molding Process: Grinding Raw Materials
The first step in rotational molding is grinding, so what are the requirements for grinding?
The quality of powder is related to the pinholes on the surface of the product, the bubbles in the product wall, and of course production efficiency.
The fine powder is on the outside and the coarse powder is on the inside. When heated in this way, the fine powder close to the mold melts first to form the outer skin. If the powder is very rough, there will be gaps between the powder and air inside, which will lead to pinholes after demoulding. In addition, rough powder will trap air in it. Too much air cannot be completely melted during the heating process, and the final manifestation is bubbles in the wall.
When experts encounter problems such as bubbles and pinholes, the first thing they look at is the quality of the powder. If the observed powder particles have a lot of tails, or there are a lot of large particles, then it is necessary to adjust the settings of the grinder.
So, how do we describe the quality of powder? There are three main attributes:
Dry Flow: Describes the flow rate of powder. Generally speaking, the faster, the better;
Bulk Density: Describes the accumulation of powder. Generally speaking, the bigger the better;
Particle Distribution: Describes the size distribution of powder. Generally, the finer the better.
For the results, the dry flow rate is generally between 25 seconds and 35 seconds. The higher the particle fullness, the better the fluidity; the more tails, the worse the fluidity.
Okay, so how do we control the quality of powder? Needless to say, it is a better grinder! The number of teeth of the grinding mill disc, the gap, and the working temperature of the grinding disc will all affect the above three indicators.
Regarding grinding, there are newer cutting-edge developments.
In foreign countries, many people have begun to study the use of microbeads to replace powder for rotational molding. Some people in China have also started relevant research.
So what are the advantages of microbeads?
One of the main advantages is that all materials can be made into microbeads, but there are many materials that are difficult to grind into powder, such as polypropylene, nylon, EVA, etc. The dry flow rate of microbeads is lower than that of powder, but the bulk density is higher.
This eliminates the grinding step, reduces costs, and reduces the requirements for machinery and equipment. However, relatively large particles will prolong the melting time and relatively reduce the efficiency.