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Injection molding processing conditions

with the continuous improvement of the requirements of injection molding processing accuracy, in addition to the correct material selection and modification of the selected plastic, how to correctly select the molding process conditions, reasonably analyze the relationship between the comprehensive shrinkage of the product and the processing parameters, as well as the various reasons for the shrinkage and the influence of processing factors on it is a very important topic

1 experimental part

1.1 experimental raw material

pp: 1390 type, injection grade copolymer PP, Beijing Yanshan Petrochemical Company

1.2 equipment

injection machine: sz-100/160nb, Ningbo Plastic Machinery Factory

mold: steel injection plate mold

1.3 sample specification

200mm × 100mm × 4mm Flat plate

1.4 process conditions

Table 1 lists the injection molding process conditions of PP

Table 1 injection molding process conditions of PP

Project injection process parameters

barrel temperature/℃ 180 200 210 220

injection pressure/mpa 60 80 100 120

holding pressure/mpa 40 50 60

injection temperature/℃ 200 210 220 230

injection time/s 3456

holding time/s 30 40 50 60

mold temperature/℃ 50 6070 80

cooling time/s 2030 40 50

2 results and discussion

2.1 contraction process and Mechanism

by carefully observing the injection of PP samples under the combination of process conditions listed in Table 1, it can be found that the shrinkage of the whole product is 1%-3%, and this shrinkage is composed of three parts, namely, shrinkage before gate solidification, cooling shrinkage and free shrinkage after demoulding

2.1.1 shrinkage of products before gate solidification

shrinkage of PP products before gate solidification largely depends on the compensation ability of the melt. As soon as the material enters the mold, because the mold temperature is lower than that of the large tonnage hydraulic universal testing machine, the melt temperature drops rapidly, and the density and viscosity of the melt are constantly improving. The correct and good protection and maintenance method of melt compensation can not only improve its use efficiency, but also depend on the magnitude of the holding pressure and the whole filling time of the melt continuing to transfer into the mold under this pressure. To the surprise of the industry, this process should continue until the gate solidifies and closes. Therefore, it can be considered that the holding pressure and holding time have a great relationship with the shrinkage at this stage. If the holding pressure is large and the holding time is long, the shrinkage of the product is relatively low

2.1.2 product shrinkage during cooling

the whole stage from gate solidification to demoulding is the cooling stage. At this stage, no other new melt enters the mold cavity, and the weight and density of PP products will not change. During the cooling process, the shrinkage of the product is related to the mold temperature, cooling rate and cooling time. The cooling speed is fast, and the product has no time to relax, so the shrinkage rate may be small; If the mold temperature is high, the cooling speed is slow, and the molecules have sufficient relaxation time, the shrinkage may increase

2.1.3 free shrinkage of products after demoulding

PP products after demoulding need to stand at room temperature for a period of time, because the products after demoulding also have a free shrinkage process. If the process conditions are not set properly, some deformation after natural stress release may occur during this period

therefore, the total shrinkage measured from the product is the sum of the above three parts. It is normal for plastic materials to shrink, but how to control and master the relationship between product shrinkage and process factors is a more complex research topic, which must be analyzed and studied from the process factors that affect the shrinkage rate of products

2.2 technological factors that affect the shrinkage rate

2.2.1 the influence of the average pressure of the mold cavity on the shrinkage rate and stability

in the process of PP injection, the melt is filled into the mold cavity at a specific temperature and pressure and continues until the gate is sealed. This pressure can also be called sealing pressure. Sealing pressure plays a decisive role in product shrinkage. The higher the sealing pressure is, the smaller the product shrinkage is; If the process conditions and equipment at the time of injection cannot ensure a stable sealing pressure, the product will shrink and fluctuate, affecting the dimensional stability of the product

the mold cavity pressure before sealing depends on the holding pressure. The holding pressure can actually be regarded as the injection pressure during the holding stage. If the gate freeze sealing time is certain, then increasing the holding pressure must increase the sealing pressure of the gate. The influence of the holding pressure on the gate sealing pressure actually reflects the influence on the shrinkage rate, and the holding pressure is also a direct reflection of the injection pressure, Figure 1 shows the relationship between injection pressure and product shrinkage

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