For a long time, in order to improve the strength and temperature resistance of products, it has become a very good choice to use glass fiber to reinforce and modify plastics. A large number of facts also prove the good performance brought by glass fiber. However, glass fiber and plastic are two different materials, which naturally leads to the compatibility between them.
The exposed glass fiber (or floating fiber) is a direct manifestation of the compatibility between them. The exposure of glass fiber is also a problem that often encounters and puzzles many friends in the injection molding process of fiber-reinforced materials.
So how did the exposure of glass fiber come about? In short, from the form of glass fiber, glass fiber can be divided into long fiber and short fiber, which are bundled and white. Fiber additive is made by blending glass fiber and resin.
When injecting glue, the flow of material is similar to that of liquid. Everyone should have seen the river. When there are some branches and other sundries in the river, there are often some of them attached to the river bank and stay along the coast. If in injection molding, the glass fiber is exposed.
This is because the fluidity of glass fiber is much worse than that of plastic, and the flow of plastic in the mold is from the middle of the interlayer to the outside, so the best fluidity must be running to the front, while the poor fluidity will stay on the mold surface.
At the same time, glass fiber can promote crystallization, while PP and PA are crystalline materials. Crystallization is quick and cooling is quick; When the cooling is fast, the glass fiber is difficult to be bound and covered by the resin, so it is easy to expose the glass fiber.
At present, the usual solutions mainly include the following:
I. Materials:
1. Considering the compatibility between glass fiber and matrix, the surface of glass fiber is treated, such as adding some coupling agents and grafts;
2. Lubricants are added, which mainly consider the dispersion of glass fiber and some lubricants have external lubrication function, which easily run to the surface of products to form a smooth layer during molding.
3. There are other fillers that have improved effects, so I won't list them here.
Second, the injection molding processing:
1, improve the material temperature and mold temperature;
2. High pressure and high speed;
3. Rapid cold and hot forming technology (RHCM) is adopted.
Because if the molten plastic solidifies too fast when contacting the mold wall, the glass fiber can not be completely coated, that is, the phenomenon of floating fiber on the surface will occur. For RHCM molding, because of the high mold temperature, the glass fiber at the interface of the cavity can be completely covered by the plastic melt, and because this part is in a molten state, the orientation of the glass fiber tends to be consistent, which ensures the shrinkage uniformity and further ensures the molding quality.
Third, the mold
The appearance surface of the product is deliberately made into matte surface or etched surface to reduce the visual reaction of exposed glass fiber.
At present, nylon fiber-adding materials are the most widely used in the market. Due to the exposure of glass fiber, the application of this kind of products is limited, and it is mainly used in some high-strength structural parts at present. However, the appearance parts made of fiber-added materials are basically matte or etched (such as power tools), because it is difficult for ordinary fiber-added materials to achieve bright appearance.
