Master the four factors to control plastic shrinkage
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Characteristics of thermoplastics is expanded after heating, cooling contraction. Of course, after pressing the volume will be reduced. In the injection molding process, molten plastic is first injected into the mold cavity, filling the end of the melt is cooled and solidified, remove appears contraction of plastic parts from the mold, the mold shrinkage is called contraction. Plastic removed from the mold into the stable this time, there will still be minor changes in the size, one variation is to continue to shrink, another variation is that some hygroscopic plastic due to moisture absorption arising from expansion, but which play a major role mold shrinkage.
Second, the structure and properties of plastic parts
For molded wall thickness, it is generally due to the thick-walled longer cooling time, so shrinkage is larger for general plastic parts, when the difference between the melt along the dimension perpendicular to the melt flow direction of a larger size when the difference is larger shrinkage from the melt flows from the point of view, away from the big gate portion of the pressure loss, and thus there is greater than the rate of contraction is close to the gate area. Because the ribs, the shape of holes, bosses and sculpture, with a contraction resistance, these parts and therefore less shrinkage.
(1) thick-walled plastic parts is greater than the thin-walled plastic parts shrinkage (but most plastic 1mm but larger than 2mm thin-walled parts shrinkage, which is due to the melt in the mold cavity because of increased resistance);
(2) insert the plastic band is smaller than the shrinkage without the insert;
(3) the shape of the plastic parts of complex shape than the simple shrinkage is smaller;
(4) plastic parts in the height direction is normally smaller than the shrinkage in the horizontal direction;
(5) an elongated plastic parts small shrinkage in the longitudinal direction;
(6) the size of the plastic parts in the longitudinal direction is smaller than the thickness dimension of shrinkage;
(7) bore shrinkage, small form shrinkage.
2) the nature of the impact of plastic shrinkage rate
(1) crystalline plastic shrinkage greater than amorphous plastics;
(2) good fluidity plastic molding shrinkage;
(3) plastic filler added to mold shrinkage rate decreased significantly;
(4) different batches of the same plastic mold shrinkage rate is not the same.
Third, the impact of the mold structure
Gate forms of shrinkage is also affected. With a small gate, the gate due before the end of the holding pressure that is leaving the cure shrinkage of plastic parts is increased. Injection mold cooling circuit structure is also a key mold design. Improper cooling circuit design, plastic parts due to uneven temperature throughout and shrinkage difference, with the result that the size difference or deformation of plastic parts. In the thin-walled portion, the temperature distribution on the mold shrinkage is even more obvious. The following four points:
Effect of plastic mold structure to shrink rate
(1) large gate size, reduced shrinkage;
(2) perpendicular to the direction of the gate is reduced shrinkage, shrinkage in the direction parallel to the gate increases;
(3) away from the gate near the gate is smaller than the shrinkage;
(4) there is mold limited shrinkage small plastic parts, plastic parts shrinkage portion of the unrestricted large.
Fourth, the impact on the plastics molding shrinkage
(1) cylinder temperature: The higher barrel temperature, preferably leaving the pressure transmitting contractile force decreases. But with a small gate, the gate due to solidification shrinkage rate is still early leaving large. For wall thickness of plastic parts, even if the cylinder temperature is higher, the shrinkage rate is still greater.
(2) feeding: in molding conditions, in order to minimize feeding plastic parts size remains stable. But insufficient feeding can not keep up the pressure, but also make shrinkage increases.
(3) Injection pressure: injection pressure is greater shrinkage influence factors, notably the holding pressure after the filling. In general, when a large pressure due to large density of the material, the shrinkage rate is small.
(4) injection speed: Injection speed has little effect on shrinkage. But for thin-walled plastic parts or the gate is very small, and the use of reinforcing material, the injection speed is shrinkage.
Mold temperature: At higher temperatures usually mold shrinkage also large. But for thin-walled plastic parts, mold temperature is high melt flow resistance is small, and further shrinkage smaller scale.
(5) Molding cycle: molding cycle and shrinkage no direct relationship. But take note, when to speed up the molding cycle, mold temperature, melt temperature, etc. must also change, thus affecting the change in shrinkage.
(6) Mold temperature: the higher the mold temperature, the greater the shrinkage
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