Weight of conjoined base is 1.6 kg, and the overall dimensions are approximately 818 mm * 265 mm * 256 mm. Plastic parts must have good strength and rigidity, and materials are PS or ABS plastic. Plastic part consists of an air duct, an air outlet, a drainage channel, a drainage outlet, an electric control box, an air deflector bracket, a wall-mounted buckle, etc. Structure is complex, as shown in Figure 1.

 

Figure 1 Plastic part structure
One-piece base is very important in air conditioner. All air conditioner spare parts are attached to it. Therefore, it is required to have small deformation and high related dimensional accuracy. It is a plastic part that integrates function and support. One-piece base has two main functions.
(1) Functional role. Geometric tolerance requirements of plastic parts are relatively high, such as in air duct area. Since wind generated by air conditioner flows through air duct quickly, direction of wind will be affected when arc shape difference is large. When molding plastic parts, Haff parting lines require high precision. Otherwise, when wind passes through air duct at high speed, noise of air conditioner will increase, causing air conditioner to be unqualified. In addition, air duct also has a drainage function. Air conditioner will produce condensation when it alternates between hot and cold. Accumulated water generated during long-term use needs to be discharged. Therefore, a drainage channel is provided, which merges with drain outlet to lead out of air conditioner.
(2) Main support function. Relevant plastic parts in air conditioner are assembled with it, such as panels, middle frames, air deflectors, etc. Other components are fixed on this plastic part, such as motors, control switches, etc. Air conditioner needs to bear its own gravity when hung on wall. Therefore, there are high requirements for accuracy of assembly dimensions and strength of plastic parts.

Drainage outlet is in an oblique downward demoulding direction and is formed by a tunnel slider; air outlet is in an oblique upward demoulding direction and is formed by an upward oblique slider; drainage groove ribs at air duct are undercut using a fixed mold oblique push structure; support ribs of middle frame are formed with large sliders, as shown in Figure 2(a); motor installation locations at both ends of plastic part are formed with 2 large sliders, as shown in Figure 2(b).

 

Figure 2 Forming slider layout

Plastic parts require small deformation and high dimensional accuracy. According to mold flow analysis, 5-point hot runner is used for high injection pressure. Clamping force exceeds injection molding machine pressure required by customer, defects such as excess material and weld marks appeared on appearance surface above air outlet, and deformation exceeded required tolerance. When a 7-point hot runner is used, injection pressure and clamping force meet requirements, and deformation also meets requirements. However, three hot nozzles must be installed above air outlet, which results in deterioration of strength of fixed mold insert. It is difficult to set up insert and hot runner cooling water path. It is estimated that molding cycle cannot meet customer's requirements. A 6-point hot runner is now used for analysis, combining advantages of 5-point and 7-point hot runner, avoiding their shortcomings, as shown in Figure 3(a). Taking into account manufacturing cost, insert strength, injection molding machine pressure and other factors, 6-point 45 needle valve hot runner was finally adopted to facilitate control of valve needle sequence, ensure dimensional accuracy of molded plastic parts and control amount of deformation. Gate layout is shown in Figure 3(b).
In order to reduce deformation of molded plastic parts, valve pins are opened in sequence: first open valve pins 1, 2, and 3, then open valve pins 4, finally open valve pins 5 and 6.

 

Figure 3 Gating system

When mold has a 1-cavity structure, the overall dimensions are 1 300 mm * 800 mm * 990 mm, as shown in Figure 4, and local three-dimensional structure of mold is shown in Figure 5. Inclined large slider 1 and tunnel-type large slider 3 both use inclined guide pillars to drive core; two tunnel-type small sliders (one is small tunnel slider in large tunnel slider 3, and the other is small tunnel slider at drainage outlet) uses tunnel large slider 3 to drive its core pulling; fixed mold large inclined push block 4 and 1 small inclined push block use nitrogen springs to synchronously drive their core pulling when opening mold; among four oblique push structures of movable mold, 2 are used for molding core pulls for support holes of air guide plate, 2 are used for molding core pulls for middle frame support rib buckle.
Figure 4 1 cavity mold structure

 

1. Large inclined slide block 2. Movable mold positioning ring 3. Tunnel type large slide block 4. Fixed mold large inclined push block 5. Fixed mold positioning ring

 

Figure 5 Local three-dimensional structure
When designing mold, it is necessary to consider that it can be combined for later production, and external dimensions of mold cannot exceed the requirements of injection molding machine. First, consider using a straight mold base, which can reduce width of mold. Positioning rings of moving and fixed molds adopt an asymmetric chamfer design to ensure smooth installation during later assembly. Mold push plate is pushed out mechanically, and push-out pads are designed at a position of 700 mm * 150 mm on mold and at the center of mold. When mold is combined into one mold and two cavities for production, accuracy of mold is required to be higher. When combining, ensure that thickness error of two molds is within 0.03 mm. Otherwise, clamping force of two cavities cannot be balanced during mold clamping, resulting in unqualified molded plastic parts. Position tolerance must be guaranteed to be within ±0.02 mm, otherwise there will be a risk of material leakage in detachable bridge-type hot runner system. Ensure that two sets of molds can be effectively installed on injection molding machine if they are used independently or assembled as one set of molds. Cooling system of inclined large slider 1 discharges water from bottom of slider. A water pipe slot is opened on the side of movable mold frame to install cooling water pipe of this slider to ensure a smooth appearance on this side of mold, avoid interference of cooling parts after the two sets of molds are assembled, provide guarantee for smooth assembly in later stage, as shown in Figure 6.

 

Figure 6 Cooling system of inclined large slider 1

Above-mentioned 1-cavity mold structure design plan takes into account relevant structural particularities after combination, such as discharge design of cooling water pipe of inclined slider on operating side to ensure cleanliness of operating side of mold; asymmetric chamfer design of positioning rings of movable and fixed molds facilitates installation of guide mechanism after assembly and ensures positioning accuracy; design of straight mold base ensures compactness of 2-cavity mold after assembly.
Combined mold is based on above-mentioned 1-cavity mold. Two identical molds are reassembled through related parts to form a 2-cavity mold structure that can be produced on one injection molding machine. After combination, an I-shaped mold is formed, as shown in Figure 7. In mold structure, an integral fixed mold base plate 1 and an integral movable mold base plate 2 are added as main load-bearing plates to connect two molds with one mold and one cavity. The overall dimensions of combined mold are 1 300 mm * 1 600 mm * 1 210 mm. Use movable mold positioning ring 3 and fixed mold positioning ring 4 of 1-cavity mold for positioning, lock two degrees of freedom in horizontal direction, X and Y, use 2 sets of straight positioning blocks without slope to lock rotational freedom in Z direction as rotation axis, and only use 4 parts to constrain required degree of freedom. In order to increase stability, strength and positioning reliability of combined mold, 8 sets of connecting buckles are designed between two pairs of molds so that they can pull each other after being combined. How to connect gating systems of two pairs of one-cavity molds after combination is difficult, because hot runner has thermal expansion and inaccurate positioning control, two molds may be stretched or cracked, dead corners may appear in hot runner, and some plastics may decompose, affecting molding quality of plastic parts. After analysis, a bridge plate 5 is added to hot runner manifold to connect main nozzles of two molds, two molds are connected in a flat manner to reduce influence of processing errors and thermal expansion. At the same time, runner hole on bridge plate 5 should be smaller than docking hole of main nozzle on original 1-cavity mold to prevent dead ends after thermal expansion displacement. After combination, 4 hydraulic cylinders are used to drive 2 sets of push-out mechanisms of original mold. Hydraulic oil circuit is designed in parallel in integral movable mold seat plate 2 to ensure that 2 push plates are pushed at the same time and plastic parts molded in two cavities are pushed out at the same time.

 

Figure 7 2 cavities mold structure after combination
1. Integral fixed mold base plate 2. Integral movable mold base plate 3. Single cavity mold movable mold positioning ring 4. Single cavity mold fixed mold positioning ring 5. Bridge plate

Combined mold is a large mold, various conveniences must be considered during disassembly and installation process, so following points should be noted.

(1). Single pair of molds is installed on integral movable and fixed mold base plates in the form of separate movable and fixed molds. Mold is large in size. When movable and fixed molds are installed separately, half mold is still heavy. For installation safety, first lay the overall seat plate flat, then install two movable molds on integral seat plate, use positioning ring and positioning block to lock position, then install screw 1, turn movable mold over to install screw 2, and install fixed mold in same way, as shown in Figure 8.

 

Figure 8 Fixed structure
(2) In order to balance push-out of molded plastic parts, hydraulic oil circuits of four hydraulic cylinders must be designed to be balanced. If necessary, a hydraulic oil circuit balancer can be installed, and hydraulic oil circuit is designed in mold to make mold look neater.
(3) For convenience of separation or assembly, 1-cavity mold design cannot have any external parts on operating side and outside of mold base to avoid being unable to assemble and produce.
(4) Diameter error of movable and fixed mold positioning rings of 1-cavity mold is controlled within a certain range. In order to improve positioning accuracy, straight positioning blocks without slope are preferred for positioning blocks.