Blow molded thermoplastics are characterized by expanding when heated

Author: MULAN - Plastic Molding Manufacturer

In the injection molding process, the molten plastic is first injected into the mold cavity, and after filling, the molten material cools and solidifies, and shrinks when the plastic part is taken out of the mold, which is called molding shrinkage. During the period of time when the plastic part is taken out of the mold and stabilized, there will still be slight changes in size. One kind of change is to continue to shrink, and this shrinkage is called post-shrinkage. Another variation is that some hygroscopic plastics swell due to moisture absorption. For example, when the water content of nylon 610 is 3%, the size increase is 2%; when the water content of glass fiber reinforced nylon 66 is 40%, the size increase is 0.3%. But it is the forming shrinkage that plays a major role. At present, the method of determining the shrinkage rate of various plastics (forming shrinkage + post-shrinkage) generally recommends the provisions of DIN16901 in the German national standard. at 23°C±Dimensions of the mold cavity at 0.1°C and place it for 24 hours after forming, at a temperature of 23°C and a relative humidity of 50±The difference between the dimensions of the corresponding plastic parts measured under the condition of 5% is calculated. The shrinkage rate S is expressed by the following formula: S={(D－M)/D}×100%(1) Among them: S- shrinkage rate; D- mold size; M- plastic part size. If the mold cavity is calculated according to the known plastic part size and material shrinkage rate, it is D=M/(1-S). In order to simplify the calculation in mold design, the following formula is generally used to find the mold size: D=M+MS(2) If a more precise calculation is required, the following formula should be applied: D=M+MS+MS2(3)However, when determining the shrinkage rate, since the actual shrinkage rate is affected by many factors, only approximate values ​​can be used, so the calculation of the cavity size by formula (2) basically meets the requirements. When manufacturing the mold, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so that it can be properly trimmed if necessary.