Setting Skills of Back Pressure Parameters in Plastic Injection Mold Molding

In the plastic injection mold molding production process, back pressure is the reverse pressure when the screw retreats. As a core process parameter in the plasticizing link, it directly affects plasticizing quality, melt temperature stability, product density, and mold molding effect. Reasonable setting of back pressure parameters can effectively improve the appearance defects of plastic parts, enhance dimensional accuracy, and reduce the loss of injection mold cavity and exhaust system.

I. Core Impact of Back Pressure on Injection Mold Molding

Appropriate back pressure can make the plastic melt mix more uniformly, discharge air and moisture in the raw materials, avoid bubbles, air holes, sliver marks and other problems in plastic parts, and at the same time improve melt density, make the plastic parts fill more fully in the mold cavity, and reduce shrinkage marks and material shortage. When the back pressure is too low, the screw plasticization is insufficient, the melt density is insufficient, and the filling is unstable after entering the mold, resulting in loose product texture; when the back pressure is too high, it will cause overheating and degradation of the melt, which not only affects the material performance but also aggravates the wear of the mold cavity, and at the same time prolongs the plasticizing time, reduces production efficiency, and even causes material overflow and wire drawing, increasing the burden on the mold gate and parting surface.

II. Back Pressure Setting Range Corresponding to Different Raw Materials and Molds

For ordinary plastic raw materials matched with conventional injection molds, the back pressure can be set in a low range to ensure the basic plasticizing effect and avoid damage to the simple mold structure due to excessive pressure. When using engineering plastics, the melt plasticization requirements are higher, and the back pressure needs to be appropriately increased to fully melt and homogenize the raw materials, adapt to the high requirements of precision injection molds, and ensure the molding quality. For heat-sensitive materials, it is necessary to reduce the back pressure and shorten the screw residence time to prevent overheating and decomposition of materials, pollute the mold cavity, and increase the cleaning difficulty. When molding high-viscosity materials, moderately increasing the back pressure can improve fluidity, make it fill smoothly in complex structure molds, and avoid insufficient filling in dead corners.

III. Practical Skills for Adjusting Back Pressure Combined with Mold Structure

When the mold cavity is deep and the structure is complex, it is necessary to increase the back pressure to enhance the melt compactness and ensure the complete molding of the corners of the plastic parts, but it is necessary to cooperate with a reasonable injection speed to prevent mold expansion and flash on the parting surface due to excessive pressure. When using multi-cavity molds for production, the back pressure should not be too high to ensure uniform feeding of each cavity, and avoid insufficient filling of some cavities and full filling of some cavities due to uneven pressure. When producing mirror molds and high-gloss molds, the back pressure needs to be stable and moderate. A stable melt state can improve the surface finish of plastic parts and reduce the scratch and wear of the mold surface. For molds with narrow runners and small gates, the back pressure should be appropriately reduced to prevent the gate from being damaged by excessive melt pressure and prolong the mold service life.

IV. Key Points of Dynamic Adjustment of Back Pressure in Production Process

When the temperature of the mold and barrel is not stable at the initial start-up, the back pressure can be appropriately reduced. After the equipment and mold reach the process temperature, adjust to the standard parameters. If air holes and looseness are found in the plastic parts during production, the back pressure can be slightly increased, and the molding effect can be observed for gradual optimization; if melt discoloration and overflow flash occur, the back pressure should be reduced in time, and at the same time, check whether the mold exhaust is smooth. When replacing raw materials and molds, it is necessary to re-calibrate the back pressure parameters, and do not use the original settings to avoid product defects and mold damage caused by parameter mismatch. In the long-term production process, it is necessary to keep the back pressure parameters stable. Excessive fluctuation will lead to inconsistent dimensions of plastic parts and increase the loss caused by frequent changes in mold stress.

V. Core Principles of Back Pressure Setting

The back pressure setting should be based on the raw material characteristics, take the mold structure as the basis, and take into account the plasticizing quality and mold protection. On the premise of ensuring uniform and dense melt, the lowest possible back pressure should be adopted, which can not only improve the molding efficiency but also reduce the mold wear, prolong the service life of the injection mold, and achieve a stable and efficient production effect.