First, the shrinkage in the form of thermoplastic molding shrinkage and calculations mentioned above, thermoplastic molding shrinkage factors are as follows:
1.1 plastic varieties thermoplastic molding process due to the volume change there crystallized form from the inside should be strong, frozen in a plastic residual stress, molecular orientation and strong factor, as compared with the contraction rate than thermoset Large, wide range of shrinkage, clear direction, while shrinkage contraction after molding, an annealing process or the humidity than the thermosetting plastic generally large.
1.2 The properties of the molded plastic parts in contact with the cavity surface melt immediately cooled to form the outer layer of low density solid shell. Due to the poor thermal conductivity of plastic, the inner plastic parts shrink slowly cooled to form large, high-density solid layer. So the wall thickness, cooling slowly, the thickness of the high density shrink large. In addition, the presence or absence of the insert and insert the layout, the number of directly affect the material flow direction, density, size distribution and shrinkage resistance, so characteristic of plastic parts shrink the size, direction greater impact.
1.3 feed inlet form, size, distribution of these factors directly affect the material flow direction, density, pressure and molding time feeding action. Direct feed, feed opening large cross-section (especially thick cross-section) is small but directional shrinkage large feed opening width and a short length directivity small. From the feed or the material flow direction is parallel port near large shrinkage.
1.4 molding conditions Mold temperature is high, the melt is cooled slowly, high density, shrink large, especially for crystalline materials due to high crystallinity, volume changes, so the greater the contraction. Mold plastic parts inside and outside temperature distribution and cooling and density uniformity is also related directly affect the size of each part shrinkage and directivity. The holding pressure and time of contraction greater impact, pressure, long time, small contraction but large directivity. Injection pressure is high, the melt viscosity difference is small, small interlaminar shear stress, elastic rebound after stripping large, it also may be appropriate to reduce the shrinkage, high temperature materials, large shrinkage, but the direction is small. Therefore, the adjustment factors at the time of molding temperature, pressure, injection speed and cooling time may be appropriate to change the plastic shrinkage. Mold design according to shrink the scope of various plastics, plastic wall thickness, shape, size and form feed inlet distribution, empirically determined plastic parts various parts of the shrinkage cavity size calculated again. For high-precision plastic parts and difficult to grasp shrinkage, generally preferable to mold design follows:
① plastic shrinkage take a smaller outer diameter, inner diameter shrinkage rate, whichever is greater, in order to leave room for a revised tryout.
② mold casting system to determine the form, size and molding conditions.
③ after handling plastic parts after treatment to determine the size of the change (measured after 24 hours must stripping).
④ the actual mold shrinkage correction.
⑤ try molding process conditions can be suitably changed slightly amended shrinkage values to meet the requirements of plastic parts.
2.1 thermoplastics mobility size, generally from molecular size, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length / plastic wall thickness) and a series of index analysis. Small molecular weight, broad molecular weight distribution, poor regularity molecular structure, melt index, spiral flow length is long, the performance of low viscosity, the flow ratio of the fluidity like, name of the same plastic must check the specification determines its flowability applicability in injection molding. Press mold design requirements can be broadly used plastic liquidity into three categories:
① good liquidity PA, PE, PS, PP, CA, poly (4) methyl pentene;
② liquid medium polystyrene series resin (e.g., ABS, AS), PMMA, POM, polyphenylene ether;
③ illiquid PC, hard PVC, polyphenylene oxide, polysulfone, aromatic sulfone, fluorine plastic.
2.2 Liquidity various plastics also differ depending on the forming factors change, the factors that primarily affects the following points:
① high temperature material temperature is increased mobility, but also vary in different plastic, PS (in particular, the impact resistance and higher MFR values), PP, PA, PMMA, modified polystyrene (such as ABS, AS) , PC, CA and other plastics with large temperature changes liquidity. For PE, POM, the temperature increase or decrease their liquidity less affected. Therefore, the former should adjust the temperature in the molding to control liquidity.
② to increase the pressure of the melt injection pressure by the shearing effect, and also increases fluidity, especially more sensitive PE, POM, it should be adjusted to control the flow of the injection pressure during molding.
③ casting mold structure in the form of system, the size, layout, design of the cooling system, the melt flow resistance (e.g. type surface roughness, forehearth section thickness, shape of the cavity, exhaust systems), and other factors have a direct impact to the melt in the cavity The actual flow inside, where lower melt temperatures cause increased resistance to the flow of liquidity is reduced. Mold design should be based on the use of plastic flow, choose a reasonable structure. It can also control the material temperature during molding, mold temperature and injection pressure, injection speed and other factors to regulate the situation properly filled molding needs to meet.