Solvent process is one of the main production methods of purple film at present. Because the solvent process can produce high-quality and multi-variety shellac products with low impurity content, it is widely used in the world. However, due to the need for a large amount of organic solvents (such as alcohol) and complex processing technology, the technical requirements for process equipment (mainly stainless steel equipment) are high, the investment is large, and the promotion is also limited.

The solvent process belongs to low-temperature processing. The process method is to clean the shellac raw materials first, dissolve them with alcohol, remove impurities through filtration or centrifugal separation, volatilize the alcohol through decompression distillation, recover the solvent in the evaporator, and finally press it into flakes. The whole production process does not need to apply high temperature and high pressure to the raw materials themselves, thus maintaining the original characteristics of the raw materials, thus ensuring that the molecular structure of the finished shellac products is not damaged, it has the advantages of strong viscosity and long thermal life. The purple film produced by this process is of good quality and fully protects the interests of customers.

The thermal filtration process has the advantages of simple process, easy operation, short production cycle, no organic solvent, low cost and so on. But the disadvantage is that the quality of purple film products is unstable, and the quality of products is not guaranteed. Now the production process has been eliminated.

The main equipment of the hot filtration method is a hot filter kettle. First, 8% sand and 0.1 oxalic acid are added to the granular glue, and the hot filter kettle is sealed. Then the steam (0.4-0.5 MPa) generated in the boiler is transported to the hot filter kettle containing shellac raw materials, and the shellac raw materials are melted through high temperature and high pressure,
Then use 0.4-0.5 MPa of compressed air pressure filtration, and through the filter to remove impurities, forced to squeeze out the glue, and finally the filtered melt into a sheet. However, under high temperature and high pressure for a long time, it will inevitably lead to aging and deterioration of shellac raw materials,
In this way, the molecular structure of shellac is inevitably destroyed, which has a great impact on the viscosity and thermal life time of the purple film. Moreover, in the production and use of finished products, the quality and life of the products will be greatly reduced, which will seriously damage the interests of customers.