What is surface finish? Injection molding is widely used to create plastic products. This is a ubiquitous process, as it is quite cheap and fast and allows the production of goods in large quantities. One of this method's main advantages is the molded parts' original surface quality. Even without additional processing, the produced parts have a smooth surface and can be used in the next stages.
Sometimes molded parts do not need additional coloring or tinting after production. This is achieved by simply pre-coloring the plastic granules. However, since injection molded products are used in a wide range of applications, post-processing can still be used.
Some applications require very smooth plastic surfaces for aesthetic or other reasons. In other areas, on the contrary, a rough surface is needed to extend the life of the product. In this article, we will review the main surface treatment methods for injection molded parts.
Molds for injection blow molding are made of aluminum and steel. Moulds have certain defects that are transferred to the manufactured parts. Milling cutters leave traces of processing. The molds are processed and polished to a certain extent, but this increases the cost of tooling the molds and prolongs the mold completion time. In some cases, traces remain on the products' inner sides, which is acceptable in principle. However, sometimes tool marks affect the functionality and appearance of parts. In this case, additional finishing is required.
Primarily, manufacturers opt for additional texturing for aesthetic reasons. A matte or smooth surface gives the part a finished look and also hides imperfections such as seam lines, dye smearing, streaks, and other injection molding effects. Also, from a commercial point of view, customers prefer perfectly finished products.
Adding surface texture improves the adhesion of labels and paint. In particular, the matte textured finish holds dye longer and is also well suited for labeling. Plastic defects such as flashing and short shots can also be avoided by adding texture to the mold. A properly designed and textured shape will also help trapped gases escape.
The processing of products made by injection molding is different from the surface treatment of products made on a CNC machine or using 3D printing. In the last two cases, surface finishing is an additional step. However, in the case of injection molding, the surface of the mold itself is prepared (this is called texturing) so that the part comes out finished with the desired type of finish. For example, if you need a matte finish, the inside of the mold is treated accordingly.
There are several surface finish options for injection molded parts. Surface treatments for injection molding can increase or decrease the roughness of parts. For example, a glossy plastic surface is preferred for children's toys. A rough surface, on the contrary, is required for mechanical parts or for technical products for which low cost is important.
What is SPI? It stands for the Society of the Plastics Industries that has established industry standards for finishes and surface features of products created by plastic injection molding techniques. According to the SPI classification, there are 12 standard types of surface finishes for plastic products. Shapes can be textured by machine or by hand.
According to the SPI standard, all coatings are divided into four categories, from smooth coatings to rough ones. Here they are:
Another finishing standard is VDI 3400, more commonly referred to simply as VDI. This is the mold texture standard set by Verein Deutscher Ingenieure – that’s the VDI meaning. This standard implies electrical discharge machining (EDM). Despite the fact that the Society of German Engineers introduced this standard, it is actively used by tool manufacturers on all continents.
