Vertical injection molding machine shrinkage rate (5)

Date：2015-10-14 21:06:38

Shrinkage of the second chapter of vertical injection molding machine injection molding products
        2.1 vertical injection molding machine Injection Molding shrinkage mechanism analysis
        Polymer response to external stimuli and action related to the length of time, this is called the relaxation time-dependent characteristics. Characterization of relaxation characteristic time scale is the relaxation time, relaxation time can be understood as a polymer transition from one state to a new state of equilibrium with the stimulus needed to adapt to the characteristics of the time. When the temperature rises, the energy of the thermal motion of the polymer increases, the distance between the polymer also expand due to thermal expansion, which are conducive to the polymer chain segments micro Brownian motion, so the relaxation time can be shortened with increasing temperature. When the plastic is melted in a high temperature state, the relaxation time is very short, the polymer can be considered at the moment that can be achieved with external stimuli equilibrium state, so the injection machine through the nozzle, runner and gate into the cavity injection plastic melt process, the polymer is easily the direction of the pressure generated by the injection of fluid shear stress stretching, conformational entropy decrease. But not the whole polymer chains extending along the flow direction, because the polymer is a polymer formed by a plurality of aggregates, and pull around the entanglements between the polymer chain orientation stretch just so happened polymer chain Some segments.
        Since the viscosity of the plastic melt is large, the thickness of the mold cavity and far less than the length of the stream flow, so the plastic melt in the cavity of the flow is laminar flow in the thickness direction of the mold cavity, from the melt surface to melt the core layer is a layer flow, direction, orientation of the polymer chains in the inner flow levels. Due to mutual attraction and entanglement plays a role in physical crosslinking of the polymer chains, making the molecules are arranged along the flow direction are connected to each other, just as a section of the endpoint is fixed at the gates pull elongated "rubber tendons. " After the end of the packing stage, on the one hand vertical injection molding machine injection molding products due to undo various points while holding pressure volume expansion of the trend, on the other hand with the specific volume of the points will decrease in temperature and wish to shrink, so over a period of time, the pressure drop caused by a specific volume expansion and temperature drop caused by a specific volume reduced offset, no shrinkage phenomenon; When the pressure drops to equal to atmospheric pressure, shrink process begins.
        Vertical injection molding machine injection molding products as of the number of gates and hampered the mold shrinkage can be divided into four categories:
        (1) single gate, and shrink unhindered mold cavity or core within the vertical injection molding machine injection molding products.
        (2) multiple gates, and contraction is not within the vertical injection molding machine injection molding products hinder mold cavity or core, and this time will produce weld marks.
        (3) single gate, inside the vertical injection molding machine injection molding products mold cavity mold shrinkage impeded or cores. Contraction of vertical injection molding machine injection molding products will be hampered by the mold cavity, the inner vertical injection molding machine injection molding mold shrinkage products will be hampered mold core, and this time will produce weld marks.
        (4) Multi-gate, inside the vertical injection molding machine injection molded articles impeded shrinkage cavity or core. Contraction of vertical injection molding machine injection molding products will be shrink within the mold cavity hinder vertical injection molding machine injection molding products will be hampered mold cores.
        First discuss the simplest case. Vertical injection molding machine injection molding products in the shrinking process, the original orientation occurs along the flow direction of the polymer chain stretching disappear due to flow shear stress tends to conformational entropy largest natural curly state, generating orientation reply, so the establishment of injection molding shrink machine injection molding products at each point along its flow path to the gate. When there are multiple gates, it would have welding marks, single gate, if there is the presence of flowing surface of the core, will produce weld marks. Spot welding marks on because it is the fusion of two streams, so both by the shrinkage force from the flow path, but also by the contractile force of the flow path from each other, the result is the contraction direction along together; weld marks near the point, on the one hand by the contractile force from their respective flow path, we must also reduce the distance between the welding marks, the result is shrunk along the force direction. At some point along its flow path contraction impeded cavity or core, and then releasing the past, part of the shrinkage impeded displacement can not occur.
        In one example, point A Division contraction displacement to the right, but can not move downward; contraction at point B, while not directly impeded the cavity, but the point B shrink along its flow path so that A, from point B shrink, Since the intermolecular forces between molecules at a distance less than the balance from the performance of repulsion, so when later from the A, B point of equilibrium distances no longer reduced, so that the contraction point B along the flow path will also be a cavity indirect obstacles. The above discussion of the vertical injection molding machine injection molding products various points within the flow surface of the shrinkage direction. But the vertical injection molding machine injection molding products temperature layers in the thickness direction is different, so each layer contraction displacement occurs along the flow path is not the same desire. Since the surface layer temperature vertical injection molding machine injection molding products and mold wall contact drop very fast, when the core unit of vertical injection molding machine injection molding products is still in the molten state, the surface layer has been cured happen.
        Early in the process of contraction within the mold, the plastic does not exclude the crystalline surface layer due to crystallization and it may want to shrink a greater extent than the core, this time due to the high temperature of the core portion of the molecule has a strong ability to exercise, so the core portion Polymer will move lower in the surface layer of the polymer _ dagger attracted by the surface layer together with the contraction; in other cases, the degree of shrinkage of the surface layer of desire is always less than the core, but the ability of the surface layer of molecular motion due to the already With decrease in temperature decreases rapidly, thus shrinking trend of the core not only can not drive the moving surface layer of the polymer, and on the lower surface of the layer of molecules force the shrinkage of the core impeded. To do this calculation contraction in the flow face by vertical injection molding machine injection molding product surface layer polymer motion. Vertical Injection Molding Machine Injection Molding shrinkage in the thickness direction is constantly exchanged via the polymer chain segments and the gap position, the gap gradually transferred to the surface layer and release achieved, so vertical injection molding machine injection molded article thickness direction The contraction should also be moving state of the surface layer polymer decision.
        J.Greene vertical injection molding machine for polystyrene injection molded article thickness direction of the measurement results of the density distribution, support from the side of the article on the ability of molecular motion plastic surface layer of the control point of view plastic parts shrink process. J.Greener measurement data show that in the thickness direction of the article, the lowest density at the center, close to the highest density at the surface of the article, Tschung this explanation is: When packing stage at the height of the mold cavity pressure to form near the epidermis high-density cured layer, When the gate is completely solidified, there is no mass transfer, then the core is still molten product at a high temperature, the cooling process is relatively fixed in a housing (housing means contractility is much less than the internal Melt contractility) carried out, it is not dense. This can explain the vertical injection molding machine depends on the size of injection-molded articles contraction of the surface layer.
        For flat-shaped vertical injection molding machine injection products, systolic displacement of each point along the flow path does not occur on the thickness of the force has always been narrow distance between molecules, so within each layer thickness direction in the surface layer of the polymer athletic ability allowed range To move closer together. The thickness of the core layer opposite directions by two shares contraction force of contraction of the surface layer by unilateral force, so that the thickness of each layer in the direction of the center layer shrinks. For cylindrical and box-type vertical injection molding machine injection products, the core of the inner surface of the layer thickness direction have hindered contraction along the flow path, so that the outer surface layer contraction occurs along the flow path will cause within , the distance between the outer surface of the layer is reduced, shrinkage in the thickness direction, the inner, the distance between the outer surface of the layer reaches equilibrium when the distance after no longer reduced, so that the outer surface of the layer along a flow path contraction will also be the core of indirect obstacles.

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