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In the injection molding industry, the plastic shell has a certain shrinkage after molding, which will be analyzed from four aspects: molding process, plastic shell structure, mold structure and plastic performance.

1. Influence of molding process on shrinkage of plastic shell

(1) When the molding temperature is constant, the injection pressure increases and the shrinkage decreases;

(2) The shrinkage decreases with the increase of holding pressure;

(3) The shrinkage decreases with the increase of melt temperature;

(4) The higher the mold temperature, the greater the shrinkage;

(5) The longer the holding time is, the smaller the shrinkage is, but the shrinkage is not affected after the gate is closed;

(6) The shrinkage decreases with the increase of cooling time in the mold;

(7) When the injection speed is high, the shrinkage tends to increase slightly, with little effect;

(8) The molding shrinkage is large, and the post shrinkage is small. The posterior contraction was very large in the first two days and stable in about a week. Plunger injection machine has large molding shrinkage.

2. Effect of plastic structure on shrinkage of plastic shell

(1) The shrinkage of thick wall plastic parts is greater than that of thin-wall plastic parts (but the shrinkage of most 1mm thin-wall plastic parts is greater than 2mm, which is due to the increased resistance of the melt in the mold cavity);

(2) The shrinkage of plastic parts with inserts is smaller than that of plastic parts without inserts;

(3) The shrinkage of plastic parts with complex shape is smaller than that of plastic parts with simple shape;

(4) The shrinkage rate of plastic parts in the height direction is generally less than that in the horizontal direction;

(5) The shrinkage of slender plastic parts in the length direction is small;

(6) The shrinkage rate of plastic parts in the length direction is less than that in the thickness direction;

(7) The shrinkage of inner hole is large, and the shrinkage of shape is small.

3. Influence of mold structure on shrinkage of plastic shell

(1) The shrinkage decreases with the increase of gate size;

(2) The shrinkage in the vertical gate direction decreases, while the shrinkage in the parallel gate direction increases;

(3) The shrinkage away from the gate is smaller than that close to the gate;

(4) The shrinkage of plastic parts with mold restrictions is small, and the shrinkage of plastic parts without restrictions is large.

4. Effect of plastic properties on shrinkage of plastic shell

(1) The shrinkage of crystalline plastic is higher than that of amorphous plastic;

(2) Plastic with good fluidity has small molding shrinkage;

(3) After adding filler into the plastic, the molding shrinkage is significantly reduced;

(4) The molding shrinkage of the same plastic is different in different batches.