1¡¢fluidity
Different thermoplastics have different process properties, shrinkage properties and physical properties. For crystalline plastics, when the processing temperature is higher than the melting point, the fluidity is better, the cavity can be filled quickly, and the injection pressure required is smaller. For amorphous plastics, its fluidity is poor, so the injection speed is slow and the pressure required is also large.
Therefore, when the mold is designed, the fluidity of the plastic can be used as the basis for calculating the size of the runner system. This can not only avoid waste of materials, extend the injection molding cycle, but also avoid the runner system from filling difficulties due to too small size. Difficult to hold pressure.
2¡¢Contractility
When the thermoplastic is transformed from the molten state to the solidified state, its volume will shrink to varying degrees. Under normal circumstances, the shrinkage rate of crystalline plastics is in the range of 1.0%-3.0%, and the shrinkage rate of amorphous plastics is in the range of 0.4%-0.8%. Therefore, the shrinkage rate and shrinkage range of crystalline plastics are larger than those of amorphous plastics, and more It is easily affected by the molding process. After demolding, the crystalline plastic can crystallize at room temperature and continue to shrink. The shrinkage depends on the thickness and temperature of the product. The thicker the product, the greater the shrinkage.
3¡¢Rheology
Rheology refers to the relationship between stress, deformation, deformation rate and viscosity during processing. According to the rheological properties of plastics, plastics can be divided into two types: shear-sensitive plastics and heat-sensitive plastics. Common shear-sensitive plastics include ABS, PS, PE, PP, POM, etc. The viscosity of this plastic will decrease rapidly as the shear rate increases. Common heat-sensitive plastics include PC, PA, PMMA, etc. This kind of plastic has a strong dependence on temperature. The higher the temperature, the faster its viscosity decreases.
From the aspect of high molecular polymers, the shear rate has an impact on the viscosity of the above two plastics. The increase of the shear rate can reduce the viscosity of the melt to varying degrees, causing the melt to produce "shear thinning" phenomenon Therefore, the rheology of injection molded products will also change.
4¡¢Orientation effect
Plastic melt will have an orientation effect during the flow process, and the orientation effect can affect the performance of injection molded products. The macromolecules of the plastic melt are stretched under the action of external force, and then are arranged parallel to each other along the flow direction. This arrangement will freeze in the solid product before the plastic is cooled and solidified, forming an orientation effect. The orientation effect will weaken the integrity of the product, resulting in uneven shrinkage of the injection molded product and warpage deformation.
Causes of deformation of injection molded products and treatment methods
1) Causes of deformation
1. The shape of the product, especially the residual stress caused by the relationship between the forming shrinkage and the thickness of the product.
2. Residual stress due to molding conditions
3. Residual stress generated during demolding
4. Deformation due to insufficient cooling time
2) Solution
1. Instant: Fully cool and solidify in the mold (extend the cooling time), increase the temperature of the barrel and reduce the injection pressure.
2. Short-term: make the mold cooling uniform.
3. Long-term: avoid the difference in product thickness, set up where the product thickness is large
The gate is placed, because the straight line is easy to cause warpage, it is made into a large R curve, the product is reversibly bent, and the number of ejector rods is increased, and the demolding angle is increased.
4. In the mold design stage, the product deformation trend is analyzed through mold flow, and the design is optimized to facilitate subsequent process adjustments.
Ways to deal with insufficient brightness of injection molded products
1. If the mold cavity processing is not good enough, such as scars, micro-holes, abrasion, roughness and other defects, it will definitely affect the plastic parts and produce a large degree of undesirable consequences, resulting in insufficient gloss of the plastic parts good. Therefore, the mold should be processed carefully to greatly reduce the roughness on the surface of the cavity. Moreover, polishing and chrome plating can be selected when necessary to achieve the best results.
2. If there are some untreated oil or water stains on the surface of the mold cavity, or if the mold release agent is used too much, the surface of the plastic part will appear dull and dull. When encountering this situation, the oil and water stains should be removed in time, and the amount of release agent should be limited at the same time.
3. If the demolding angle of the plastic part is too small, it is difficult to demold, or the demolding force is too large, which causes the surface of the plastic part to be too smooth, then the demolding angle should be increased to ensure that the plastic The smoothness of the piece.
4. If the mold exhaust performance is not very good, then too much gas will stay in the model, which will also cause the final product to appear poor gloss after molding. Therefore, it should be checked and corrected to ensure that the mold exhaust system has sufficient exhaust performance to avoid the presence of trapped gas.
5. If the cross-sectional area of the gate or runner is too small, or the change fluctuates greatly, it will lead to excessive shear force acting on the melt during the flow of the melt, which will lead to the turbulent dynamic flow of the melt. Will result in poor gloss of the final product. Therefore, the cross-sectional area of the runner and gate should be appropriately increased to reduce the shear force to avoid turbulence and improve the gloss of the product.
