Plastics Process Analysis, Instrumentation, and Control. Группа авторов

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СКАЧАТЬ decreased slightly to a minimum at the core layer. Then it increased slowly until reaching the skin layer next to the heated stationary half, at which point the degree of lamellae orientation was less than that of the opposite skin layer. The variation of lamellae orientation was attributed to the different flow and temperature histories of the RHCM and CIM molding processes (26).

       1.6.3 Injection Molding: Heating

       1.6.3.1 Modeling a Fast Mold Temperature

      The modulation of the mold temperature during injection molding is a strategic issue since it allows modulating and calibrating interesting properties of the moldings (27).

      Thin heating devices (28) were layered on the cavity surface allowing its fast temperature evolution between injection and cooling channels temperatures.

      The heating device was made up of four layers (27): an 80 µm thick electrically conductive layer of carbon black loaded poly(amide imide), which induces the cavity surface heating by Joule effect, two Kapton electrically insulating layers, one on each side of the electrically conductive layer. A steel layer, 100 µm thick, protects the heating device from the incoming melt. The heating power was chosen such that, when the polymer reaches the cavity, the cavity surface temperature was intermediate between the mold temperature, as held by the cooling channel, and the injection temperature.

      The heating devices were made by a conductive layer between two insulating layers with thicknesses selected in order to realize a heating/cooling cycle as fast as possible.

      Several tests were performed, injecting iPP, using different heating powers and heating times to analyze the effect of the fast cavity surface temperature evolution on the molding morphology and properties. The heat transfer through the mold was modeled, accounting for the Joule effect in the conductive layer of the heating devices (27).

The injection molding process coupled with the system designed to achieve a fast evolution of the cavity surface temperature can be considered a multiphase process (because during the process the polymer in the cavity changes from the molten to the solid state) during which there is a transient heat transfer in the mold (including the multilayer heating device) that has to be considered simultaneously and combined with the transient heat transfer in the flowing polymer. Commercial software can be adopted to analyze this combination of transient processes but the solution can be obtained only adopting a serial combination of the analysis of the two different processes (27):

      1 The injection molding process (the transient temperature distribution inside the flowing polymer for a given mold temperature), and

      2 The evolution of the mold temperature distribution for a given polymer temperature distribution.

      The solution of one of the two processes can be used as a boundary condition for the other process and therefore the solution of the whole process can be obtained by alternatively considering the transient of one of the two processes, while the other is held at steady state, and vice versa.

      To validate the proposed modeling of the heat exchange during the process, the simulated temperature evolutions at the polymer-cavity and the heating device-mold interfaces were satisfactorily compared with the experimental ones recorded during the tests conducted adopting different mold temperature evolutions. Furthermore, pressure evolutions during the process recorded at different positions along the flow path were satisfactorily compared with the simulated ones to validate the predictions of the thermomechanical histories experienced by the polymer (27).

       1.6.3.2 Hot Air Heating

      Simulations and experiments were conducted with gas temperatures of 200°C to 400°C to investigate the impact of external gas-assisted mold temperature control on the quality of the weld line of molding products (29).

The general process consisted of six steps. In Step 1, as soon as the molding cycle was completed and the product was ejected, the hot gas source was transported to the heating area at which the cavity temperature was still low. In step 2, the hot gas sprayed directly into the heating area. Due to heat convection, the thermal energy was transferred to the cavity area, heating that area to the target temperature, after which the heating process stopped. At this time, the cavity remained at a high temperature. In Step 3, the gas drier was removed from the molding area and the two half molds were closed before a new molding cycle began by melt filling into the cavity in Step 4. In this step, the hot cavity allowed the melt to easily flow into the thin-walled area. As soon as the entire cavity filled, the cooling process started with the heat transfer from the hot melt to the mold plate in Step 5. The process stopped when the part temperature reached the ejection temperature. In Step 6, the two half molds opened to eject the part, and the next molding cycle occurred.

      In the heating step, the heating rate was 19.6 ^◦Cs^–1 from 30°C to 128.5°C in the first 5 s in a 400°C gas environment (29).

      When applied to heating the weld line area of an injection mold, the external gas-assisted mold temperature control improved the appearance of the weld line when the cavity temperature was preheated to 150°C.

      For the tensile strength test, a melt flow simulation comparing the packing pressure of different mesh thicknesses revealed that external gas-assisted mold temperature control helped maintain a high pressure in the weld line area in different packing periods.

      This was verified by an experiment where the external gas-assisted mold temperature control was applied with 400°C gas to change the mesh area temperature. The result indicated that an increase in the weld line area temperature from 60°C to 180°C improves the tensile strength of all mesh thicknesses, which was more pronounced with thinner parts, especially at 0.4 mm. The simulations revealed that high temperature is concentrated in the weld line area of the cavity surface, thus reducing the energy wasted during heating (29).

       1.6.3.3 Momentary Mold Surface Heating

      The momentary mold surface heating process heats the mold surface over 400°C in a few seconds with a gas flame, and then the surface is cooled down very quickly (30). A shiny surface with 98% light reflection was obtained using the momentary mold surface heating process for an injection molded part consisting of 30% glass fiber-reinforced poly(carbonate) (PC).

An additional piece of equipment to supply the gas fuel and air as well as the specially designed momentary mold surface heating mold is the only difference compared to a CIM system. The details of this injection molding approach using the momentary mold surface heating process have been reviewed (30).

       1.6.3.4 Infrared Heating

      Micro-injection molding is an important method for the fabrication of a microneedle array. Here, the mold temperature is one of the important factors that affect the mold-filling quality of the polymer melt during the micro-injection molding. An infrared heating method has been adopted to raise the mold temperature rapidly for improving molding quality of microneedle array (31).

      According to the simulation of the reflector type, which has an important effect on the efficiency of the infrared heating system, an infrared heating system with high efficiency was developed and used in the infrared-heating-assisted micro-injection molding system. A series of verification experiments were carried out to verify the feasibility and the heating effect of the developed system.

      The experimental results showed that the developed infrared heating system can achieve СКАЧАТЬ