0 Preface
In recent years, with demand of consumers for personalized appearance, market has increasingly higher requirements for quality and personalized appearance of plastic products. As a technology that can simultaneously mold plastic products of different materials and colors, two-color injection molding is increasingly used in production of plastic products. Compared with traditional injection molding process, two-color injection mold can not only ensure quality of product based on appearance requirements of product, molded surface pattern is beautiful and not easy to fall off, but can also mold products with certain special functional requirements, such as one part that is transparent and the other part that is opaque, one part that requires electroplating and the other that does not, etc.
Air-conditioning panel is core component of air-conditioning exterior decoration parts and has strict appearance quality requirements, such as small product deformation, high assembly accuracy and exquisite appearance. Use of two-color injection technology can not only form air-conditioning panels with beautiful appearance, high precision, small fitting gaps, and stable quality, but also improve production efficiency and reduce production costs. Since two-color panel is injection molded using two different materials, during cooling and shrinkage process, difference in volume shrinkage of the two materials can easily cause air conditioning panel to have a large amount of deformation, which cannot meet assembly requirements. Taking split machine panel as an example, we use Moldflow analysis software to simulate and analyze deformation of air-conditioning panel, predict deformation amount of panel, then set pre-deformation amount based on simulation analysis results and combined with actual production experience, analyzed and verified through simulation software to determine final pre-deformation amount to guide product structure design and mold design, correct panel size and mold part size, so that formed panel can meet actual production requirements, achieve goals of reducing number of mold repairs, increasing production efficiency and improving product quality. It can also provide a theoretical basis for promotion and application of two-color injection molds in production of air-conditioning panels.

Appearance of a certain split machine panel is shown in Figure 1. It is a flat rectangular panel structure with an overall size of 915mm * 275mm * 56mm. Panel is composed of a white base at the bottom and a transparent covering layer at the top. White base serves as supporting skeleton, and transparent covering layer provides appearance of panel. The two are combined through arc-shaped textures and partially smooth planes. Textures can increase three-dimensional effect of panels and make them more personalized. White substrate at the bottom is the first injection. In order to prevent substrate from being melted during second injection, substrate material requires good high temperature resistance and high melt temperature. Selected substrate material is PC, with a melt temperature of about 300℃, a heat deformation temperature of 105℃, and low price. Table 1 shows molding process parameters of PC. Top transparent layer is second injection. Material requires strong permeability, good wear resistance, and melt temperature should be lower than melt temperature of the first injection material. Choose PMMA material, whose melt temperature is about 250℃, suitable for use in panel coverings. Specific molding process parameters of PMMA are shown in Table 2. In addition, since appearance of two-color panel is required to be high-brightness white, and PC is a transparent material, high-brightness white needs to be developed for PC.

 

Figure 1 Three-dimensional structure of panel
Table 1 Molding process parameters of PC materials

Table 2 Molding process parameters of PMMA materials

Appearance requirements of panel: there should be no sharp corners or flash edges at the corners, and transition of rounded corners should be gentle; there should be no large arches and collapses on plane, arch deformation is required to be ≤2mm, and collapse deformation should be ≤0.5mm; appearance surface must be smooth and transparent, without trapped air, bright spots, black spots and welding lines.
Analysis of base material thickness is shown in Figure 2. Partially thinned display window is located on the first injection panel. Wall thickness of main body of the first injection is 2.0mm. Main wall thickness of display window is 1.0mm, size is 107mm*56mm, and distance from edge of product is 92mm. A gradual transition from thinned area to main wall thickness area is required.

 

Cover layer material thickness analysis is shown in Figure 3. Wall thickness of main body of the second injection is 1.9mm, width direction is 35mm away from edge gate, and wall thickness is 2.5mm, so as to guide melt of second injection and prevent the first injection molded product from melting.

 

In Moldfolw analysis software, for double-color injection, filling method of overlapping injection is generally selected, and analysis sequence of filling + holding pressure + overlapping injection filling + overlapping injection holding pressure + warpage is selected. Grid is generally divided into 3D grids. After panel model is imported, since it is composed of two parts, base and covering layer, two parts are meshed separately. 3D mesh of panel is shown in Figure 4. After meshing is completed, set material properties of the model, set base model to PC (grade: Makrolon2405) material, and set the covering layer to PMMA (grade: ACRYLITEH15-012) material. After gate position and feeding method are determined, set base to the first injection and cover layer to the second injection. Mold part surface temperature, melt temperature and other process parameters adopt default settings.

 

(1) Determine panel base pouring system. Base serves as assembly structure and supporting frame of panel, and design of gate location is shown in Figure 5. Since visible surface of base is all exterior surface, feeding method of hot runner and point gate is adopted. Diameter of point gate is φ1.0mm, the total length of panel is 915mm, flow length between every two gates in does not exceed 200mm, space between G1 and G2 gates is panel display area. This area is partially thinned. In order to reduce maximum injection pressure of substrate, it is necessary to add one more gate in thinning area. It was finally decided to use 6 gates for simultaneous pouring. Mold flow analysis is shown in Figure 6. There will be trapped air between three gates G1, G2 and G3 during filling and cannot be exhausted through parting surface. When designing mold, an exhaust insert is designed on the side of movable mold at this position. Simultaneous pouring of gate will produce 5 welding lines, as shown in Figure 7. Due to second injection of PM⁃MA material, melt temperature is 250℃, it directly acts on the first injection molded product, which can dilute weld line without affecting appearance.

 

 

Gate position design of panel covering layer is shown in Figure 8, and filling analysis results are shown in Figure 9. Since covering layer is appearance of panel, defects such as gate breakpoints, bright spots, trapped air, and welding lines should not appear on the surface. Feeding method of hot runner turning side gate is used, and 7 gates are poured in sequentially under timing control (G8 gate is opened first). In addition, due to large width of panel, single-side injection will cause excessive injection pressure. In order to reduce injection pressure and prevent the first injection molding base from being melted by high-pressure melt, side gate needs to be designed as a long strip gate. After melt injection is completed, long gate condensate needs to be removed by machining.

 

(2) Analysis of filling process parameters. Filling analysis was performed on panel base and covering layer respectively, and relevant process parameters were obtained as shown in Table 3. It can be seen from Table 3 that filling time of panel base and covering layer is 1.6s and 3.5s respectively, and filling time is short; due to limitation of molding two-color products, maximum injection pressure of base is 104.5MPa, and cavity pressure after removing gate pressure is 57.81MPa, injection pressure is lost in runner; the second injection pressure is 99.22MPa, cavity pressure is 85.02MPa after removing gate pressure, pressure of two cavities is within 100MPa, flow front temperature is lower than material decomposition temperature. Design of gating system and setting of injection process parameters can meet production requirements.

Table 3 Filling process parameters of panel base and covering layer

(3) Analysis of panel deformation results. Since base of air conditioning panel is assembled with cover through buckles, it has strict deformation requirements. Arch deformation in demoulding direction is ≤2mm and collapse deformation is ≤0.5mm. Figure 10 shows analysis results of deformation of panel in demoulding direction. It can be seen from Figure 10 that panel shows an overall arching deformation trend in demoulding direction, and deformation amount on product is about -4.10~2.19mm, totaling 6.29mm, which exceeds assembly size requirements.

 

Mold pre-deformation is to design cavity in opposite direction of deformation of molded product, which is equivalent to compensating deformation so that deformation of product after demoulding meets quality requirements. Traditional molds rely on experience design, but mold needs to be modified repeatedly or even scrapped. When designing pre-deformation, choose opposite direction. Pre-deformation proportion factor needs to be combined with structural characteristics of product. Specifically, arch deformation in length direction of product has the greatest impact on assembly, so pre-deformation design is performed in length direction of product.
After mold design is pre-deformed, product structure changes. Under same process parameters, shrinkage of product in mold cavity is inconsistent with that before, and amount of deformation will also change. Therefore, pre-deformation of product cannot be designed according to 1:1 proportion factor. According to traditional design experience, pre-deformation design is carried out according to 60% of deformation result. Molded product can meet deformation requirements. Specific pre-deformation amount is 6.28*0.6=3.768mm. Pre-deformation amount is designed to be 4mm after rounding, as shown in Figure 11. Import pre-deformed model into Moldflow software to conduct mold flow analysis. Pre-deformation analysis results are shown in Figure 12. After pre-deformation, deformation of product is about -2.67~1.60mm, totaling 4.27mm. After pre-deformation, deformation is still large. Since a reverse pre-deformation of 4mm is designed, flatness deformation of final product is predicted to be within 2mm. We try to manufacture mold according to pre-deformation amount of 4mm. Mold structure of the first and second injections is shown in Figure 13 and Figure 14.

 

 

 

Sructure of two-color panel injection mold is shown in Figure 15. Working process of mold: the first mold closing injection, product material is white PC, hot runner is turned into a latent gate for feeding. Second injection material is transparent PM⁃MA. Hot runner turns to side gate to feed material. After designed pre-deformed first injection melt fills mold cavity, semi-finished product is placed in movable mold and mold is opened immediately. Semi-finished product is not pushed out at this time. Movable mold is rotated 180° and then closed. Transparent PMMA material is injected twice to cover semi-finished product of the first injection. After product cools and shrinks, mold is opened and pushed out. Robot picks up parts on the side of movable mold of second injection to realize automated production. After mold trial verification, molding of product is qualified. Cooling time is controlled at 30 seconds, and molding efficiency is improved. Actual molded product is shown in Figure 16.

 

 

This article introduces method of Moldflow simulation software to simulate and analyze two-color panel molding of air-conditioning split machine. Through simulation analysis, mold pre-deformation design is guided, pre-deformed model is iteratively analyzed to predict final deformation results, providing technical support for shortening mold development cycle, reducing mold manufacturing costs, solving product deformation, improving production efficiency and product quality.