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Plastic molding shrinkage

by:CSSSLD     2021-01-23
Specific volume and specific volume, is refers to the unit mass of loose plastic of the volume. Compression ratio refers to the volume of plastic raw materials and peek the ratio of the volume of injection molding processing parts, its value is constant is greater than 1. They can be used to determine the size of the mould material room. Some plastic processing will break out toxic, stimulating and corrosive gases. For example, polyoxymethylene can decompose, produce irritating gas formaldehyde; Polyvinyl chloride (PVC) and its derivatives or copolymer can decompose the hydrogen chloride gas is both stimulating and corrosive. In the plastic forming process, it must be strictly grasp the process procedures, to prevent the harmful gas corrosion harm the human body and mould and processing equipment. Plastic and inspiratory sex, sticky mould sex, plasticity, uniformity and crosslinking, etc.

the greatest influence factors in the process parameters is the molding pressure, the pressure can make the plastic melt pressure more compact, reduce shrinkage. Higher melt temperature makes the melt density increases, large shrinkage rate, but the direction is small. To improve the die temperature, usually make shrinkage rate increases, especially for crystalline plastics. Within a certain range, prolong the holding time can reduce shrinkage, but prolong the holding more than corresponding time gate closed, the influence on shrinkage rate will no longer have.

heat-sensitive refers to the sensitivity of plastics on the thermal degradation. Some plastics, such as polyvinyl chloride, polyformaldehyde is more sensitive to temperature, if the molding temperature is too high, is easy to change color, degradation. In order to prevent the heat-sensitive plastic forming process in thermal decomposition phenomenon, can take into plastic stabilizer, choose the correct control equipment, forming temperature and forming cycle, timely cleaning equipment in the decomposition, etc.

thickness increase parts, shrinkage rate increases; Complex shape parts of shrinkage rate less than the shrinkage rate of simple shape parts, reinforcement, hole, convex sets and sculpture shape with shrinkage resistance, so these areas smaller shrinkage rate. Die structure are the main factors affecting the gate location, gate cross-section size, gate number, etc. Use direct gate and large cross-section gate, can reduce the shrinkage rate, but the direction is strong; Gate wide and short, little direction; From the gate or near a part shrinkage perpendicular to the direction of material flow.

hygroscopicity is refers to the plastic material tend to absorb the moisture from the air. Long-term storage of plastic raw materials, it is easy to achieve balance of moisture absorption. Contain more moisture, stamping shape will appear on the surface of silver, internal bubble ills, such as for some plastic will produce hydrolytic degradation and so on, cause mechanical performance degradation. So the plastic must be fully dry before processing and forming, make the moisture content in 0. 5% - 0. 2% mass fraction, and continue to heat preservation in the machining process, in order to prevent moisture absorption.

in the plastic melt in the mold, cooling and curing hardens after having certain shape and out of the mould, the nature of the size change to narrow called contractility. Contractility has the size of the unit length of injection molding processing parts expressed as percentage of shrinkage, referred to as the shrinkage rate. Shrinkage rate of thermoplastics generally greater than the thermosetting plastic shrinkage rate, crystal type of plastic shrinkage rate is greater than the non-crystalline type of plastic shrinkage rate.

hygroscopicity and water absorption of plastics distinguish between the two terms. Water imbibition refers to the plastic under prescribed conditions after soaking in water for water absorption ability, with water absorption. Water absorption in 23 with plastic & plusmn; 2 ℃ under the condition of soaking in distilled water for 24 h after the increase of the quality of the ratio. Bibulous rate is large plastic is big also, moisture absorption rate from water absorption test data can be directly determine the plastic hygroscopicity.

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