Relationship between injection process conditions and PP molding shrinkage
With the continuous improvement of injection molding processing precision requirements, in addition to correct material selection and modification of selected plastics, how to correctly select the molding process conditions, and reasonably analyze the relationship between the comprehensive shrinkage rate of the products and the parameters of the processing technology, And the various causes of shrinkage and the influence of processing factors on it are an important issue. 1 Experimental part 2 Results and discussion women Jeans Jeans women jeans GUANGDONG MERICAN INDUSTRIAL CO.,LTD , https://www.gdmerican.com
1.1 Experimental materials
PP: 1390 type, injection grade copolymerized PP, Beijing Yanshan Petrochemical Company.
1.2 Equipment injection machine: SZ-100/160NB, Ningbo Plastic Machinery Factory;
Mold: Steel injection flat plate mold.
1.3 Specimen specifications
200 mm × 100 mm × 4 mm flat plate.
1.4 Process conditions Table 1 lists the injection molding process conditions for PP.
Table 1 PP injection molding process conditions project injection process parameters
Barrel temperature / °C 180 200 210 220
Injection pressure / Mpa 60 80 100 120
Holding pressure / Mpa 40 50 60
Injection temperature / ° C 200 210 220 230
Injection time / s 3 4 5 6
Holding time / s 30 40 50 60
Mold temperature / ° C 50 60 70 80
Cooling time / s 20 30 40 50
2.1 Shrinkage process and mechanism Careful observation of the PP sample under the combination of the various process conditions listed in Table 1 can be found that the shrinkage of the entire product is 1% -3%, and this shrinkage consists of three parts, namely the gate Shrinkage before solidification, cooling shrinkage, and free shrinkage after demolding.
2.1.1 shrinkage of the product before the gate solidifies
The shrinkage of the PP article prior to solidification of the gate is highly dependent on the ability to compensate for the melt. As soon as the material enters the mold, the melt temperature drops rapidly as the mold temperature is lower, and the density and viscosity of the melt are continuously increased. The ability to compensate for the melt depends on the amount of holding pressure and the entire filling time at which the melt continues to transfer into the mold. This process continues until the gate is solidified and closed. Therefore, it can be considered that the holding pressure and the dwell time have a great relationship with the shrinkage at this stage. If the holding pressure is large and the dwell time is long, the shrinkage rate of the product is relatively low.
2.1.2 The shrinkage of the product during cooling is solidified from the gate until the entire stage of demolding is the cooling phase. At this stage, no other new melt enters the cavity, and the weight and density of the PP product no longer change. During the cooling process, the shrinkage of the product is related to the mold temperature and cooling rate, cooling time, the cooling rate is fast, the product is not too loose, and the shrinkage rate may be small; if the mold temperature is high, the cooling rate is slow, the molecules have sufficient relaxation time, shrinkage The rate may increase.
2.1.3 Free shrinkage of the product after demolding The PP product after demolding needs to be allowed to stand at room temperature for a period of time because the product after demolding has a free shrinking process. If the process conditions are not set properly, some deformation after natural stress release may occur during this period.
Therefore, the total shrinkage we measured from the product is the sum of the above three parts. The shrinkage of plastic materials is normal, but how to control and master the relationship between product shrinkage and process factors is a more complicated research topic, and must be analyzed from the process factors that affect the shrinkage rate of products.
2.2 Process factors of the size of the film shrinkage rate
2.2.1 Effect of the average pressure of the cavity on the shrinkage and stability During the PP injection process, the melt is filled into the cavity at a specific temperature and pressure and continues until the gate is sealed. This pressure can also be called the sealing pressure. . The sealing pressure plays a decisive role in the shrinkage of the product. The higher the sealing pressure, the smaller the shrinkage of the product; if the process conditions and equipment at the time of injection cannot guarantee a stable sealing pressure, the product will cause shrinkage fluctuations and affect the dimensional stability of the product.
The cavity pressure before sealing depends on the holding pressure. The holding pressure can actually be regarded as the injection pressure to maintain the holding pressure. If the gate sealing time is certain, then the holding pressure must be increased to the sealing pressure of the gate. Increasing, the effect of holding pressure on the gate sealing pressure actually reflects the effect on shrinkage, and the holding pressure is actually a direct reflection of the injection pressure.