The Utilization of Back Pressure in Injection Molding
Back pressure is a critical process parameter in injection molding. It refers to the reverse pressure exerted by the injection unit on the melt during screw retraction. By adjusting the back pressure, the plasticization quality of the melt can be effectively improved, enhancing product quality and stability.
Functions of Back Pressure
Improved Melt Plasticization:
Increased Shear Force: As back pressure increases, the resistance to screw retraction grows, extending the shear time of the melt between the screw and the barrel. This increased shear force facilitates the melting and uniform mixing of the plastic.
Expulsion of Volatiles: Back pressure can expel volatiles, moisture, and other impurities from the melt, improving its purity.
Enhanced Fill Uniformity: Back pressure can increase the melt viscosity and fluidity, aiding in filling thin-walled and complex mold cavities.
Reduced Melt Temperature: Back pressure can increase the compression ratio of the melt, reducing its temperature and preventing plastic degradation caused by excessive heat.
Improved Product Appearance: Back pressure can reduce the amount of bubbles in the melt, decreasing shrinkage and warpage, and enhancing the appearance of the product.
Setting Back Pressure
Too Low: Insufficient melt plasticization can lead to defects such as bubbles and flow marks.
Too High: The melt temperature will be too low, and the viscosity too high, resulting in poor flowability and defects such as short shots and insufficient material.
Optimal Back Pressure: Typically determined through experimentation, it is usually 5-15% of the injection pressure.
Factors Affecting Back Pressure
Screw Design: The compression ratio, thread depth, and thread angle of the screw can all affect the effectiveness of back pressure.
Material Properties: Different materials have different melting characteristics and varying sensitivities to back pressure.
Process Parameters: Injection temperature, injection pressure, and screw speed can all influence the effect of back pressure.
