Polishing Treatment for Plastic Molds
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Why Adding Glass Fiber to the Plastic Parts ?
About the floating fiber
Plastic parts have many shortcomings in the using process. To make plastic parts more suitable for other fields (replace steel with plastic ) and change the properties of resin like wear resistance, heat resistance, strength, fire resistance, etc., it’s a common way to mix the glass fiber and additives on the original basis.
However, the most common defect is plastic parts surface has “floating fiber” after adding glass fiber. So the plastic parts with glass fiber are mainly suitable for the internal structure of the product. And we should consider the problem of floating fiber when using the plastic parts in appearance. The surface of the mold is usually etched or the plastic part is coated on its surface to cover the “floating fiber”.
Advantages of resin mixed with glass fiber
- The overall shrinkage of material becomes smaller, and the dimensional stability of the plastic parts is good.
- The melting temperature of glass fiber is higher than that of resin, and it is non-flammable. After addingit, the plastic parts have the effect of flame retardant and self-extinguishing.
- The mechanical strength of glass fiber is high. And the overall strength of plastic parts becomes better after mixing. Compared with steel, it also has the advantages of plastic.
- Compared with resin, glass fiber is cheaper.
Disadvantages of resin mixed with glass fiber
- The brightness of the surface obviously reduces, and the surface is not smooth and rough.
- There are problems in the compatibility of glass fiber and resin, which affect the appearance quality of the product.
- The fluidity of the melt becomes poor. So we should increasethe injection speed, injection pressure, mold problems, and material temperature of the injection molding machineduring processing.
- The material of mold needs to be hardened with corrosion-resistant quenching material (S136, 8407). And it is necessary to use aspecial screw (chrome plated) in the injection molding machine.
- The transparent resin will become opaque after mixing with glass fiber. The “floating fiber” of the black plastic is particularly obvious.
Reasons and solutions for “floating fibers” on plastic parts surface:
The main reason is that the fluidity of glass fiber is slow during injection molding, and the resin and glass fiber do not merge and separate. When the product has floating fiber, the simplest solution is to replace it with the good material (quality). But in actual processing, it is not allowed to replace the material. It is still necessary to adjust the machine to find a solution from the process.
1. The quality of glass fiber.(Material)
Some of the purchased materials are fully mixed by the manufacturer. But some have never been processed. For the materials having been mixed by manufacturers, the prices of the materials can reflect the quality. While for the self-mixing, we need to consider the ratio and length of the glass fiber. The long glass fiber is prone to floating fiber than the short glass fiber. But the tensile strength of the long glass fiber is better than short glass fiber.
2. Mold/material temperature. (Process)
The density of the glass fiber is smaller than that of the resin and the fluidity of glass fiber is poor. When the glass fiber touches the cool mold surface during injection molding, the glass fiber will first condense, and the resin in the interior will condense afterwards. It cannot wrap the glass fiber and causes the glass fiber to be exposed. Therefore, we should increase the mold temperature. Delay the crystallization rate of glass fiber. Besides, increase the material temperature and enhance the fluidity of the material.
3. Injection speed/pressure.(Process aspect )
The fast resin will flow in the core of the melt, while the slower will float on the surface of the melt. Increasing the injection speed can alleviate the difference in fluidity between glass fiber and resin. When the fluidity of resin and glass fiber is very poor, while increasing the injection speed has no effect. At this time, we should increase the injection pressure and speed to alleviate the gap in fluidity.
4. Friction and shear force.(Mould aspect)
When the screw rotates the plasticizing material and the melt is injecting and filling, where they have passed by like the gate the sharp notch of the mold core, and the flow of the melt will generate frictional shear force. It’s a factor of surface “floating fiber”. The gate size on the mold is at best large. The runners should be short and thick and so the flow can reduce. Reduce the speed of the screw to reduce the possibility of separating the material and the melt because of shear force.
According to the use requirements of the productn, we should add 5%-55% of the glass fiber in the processing. Of course, the higher the ratio, the more difficult the technical requirements. The difficulty in the debugging of the molding process is with the same parameters, it may not help if following the prescribed order. Changing the process parameters based on the mold structure, product shape, and material characteristics is related to the stability of the four elements of material, mold, equipment, and process conditions.
In actual processing, high pressure, high speed, high mold temperature, low screw speed are indeed helpful to improve the problem of “floating fiber”. But the cooling time of plastic parts with high mold temperature is longer. And the cycle time of plastic parts with low screw speed is longer. It finally affects the molding rate of the product. Therefore, it is not that the higher the parameter, the better. We should debug the product reasonably. Then analyze and summarize the reasons based on the product quality to avoid the same defect in the product next time.
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