Introduction: Thermoplastics are widely used in daily life. They are made into plastics with the main components of thermoplastic resins and various additives. Common thermoplastics mainly include PP, PE, PVC, PC, etc. This article shares the factors affecting the shrinkage of thermoplastics molding, and the content is for your reference:
Molding shrinkage refers to the percentage of the shrinkage of the size at room temperature to the original cooling size after the plastic part is taken out of the mold and cooled to room temperature. However, because shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to the reasons for each molding, the shrinkage rate of the plastic part after molding is called the molding shrinkage rate.
The reasons that affect the shrinkage of thermoplastic molding are as follows:
In the molding process of plastics, thermoplastics, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors, so compared with thermoset plastics, the shrinkage rate is larger. The shrinkage has a wide range and obvious directionality. In addition, the shrinkage after molding, annealing or humidity treatment is generally larger than that of thermosetting plastics.
Feed port form, size, distribution
Factors such as the form, size, and distribution of the feed inlet directly affect the direction of material flow, density distribution, pressure maintaining and shrinking effect and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have less shrinkage but greater directivity, and shorter feed ports with shorter width and length have less directivity. Those that are close to the feed inlet or parallel to the direction of the material flow will shrink more.
Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystalline material, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the inner and outer cooling and density uniformity of the plastic part, which directly affects the shrinkage and directionality of each part.
In addition, holding pressure and time also have a greater impact on contraction, and if the pressure is high and the time is long, the contraction is small but the directionality is large. The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound is large after demolding, so the shrinkage can also be reduced by an appropriate amount. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.
Mold design method
When designing the mold, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the inlet form, the shrinkage rate of each part of the plastic part is determined according to experience, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, the following methods should generally be used to design the mold:
① Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter to leave room for correction after the test.
②The mold trial determines the form, size and molding conditions of the gating system.
③The plastic parts to be post-treated shall be subjected to post-treatment to determine the size change (measurement must be 24 hours after demolding).
④ Correct the mold according to the actual shrinkage.
⑤ Retry the mold and appropriately change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.
Factors affecting the fluidity of various plastics
The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:
①Higher material temperature increases fluidity, but different plastics have their own differences, such as PS (especially those with high impact resistance and higher MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS) The fluidity of, PC, CA and other plastics varies greatly with temperature. For PE and POM, the temperature increase or decrease has little effect on their fluidity. Therefore, the former should adjust the temperature during molding to control fluidity.
② As the pressure of injection molding increases, the molten material is subjected to greater shear and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control fluidity during molding.
③The form, size, layout, cooling system design, flow resistance of the molten material (such as the surface finish, the thickness of the channel section, the shape of the cavity, the exhaust system) and other factors directly affect the molten material in the cavity. The actual fluidity inside, if the molten material is promoted to lower the temperature and increase the fluidity resistance, the fluidity will decrease. When designing the mold, a reasonable structure should be selected according to the fluidity of the plastic used. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust the filling condition to meet the molding needs.