[Abstract] Taking laminated structure design of injection mold for refrigerator heat dissipation cover as a case, design points and working process of large flip-chip laminated injection mold are introduced, technical problems such as runner, load-bearing and synchronization of mold are successfully solved. Design of laminated mold greatly improves actual production efficiency and utilization rate of injection molding machine.

Laminated mold is a cutting-edge technology in development of plastic injection molds. It is suitable for molding flat, shallow-cavity shell plastic parts or small multi-cavity thin-walled plastic parts, and products with relatively large demand. For this kind of shallow cavity plastic parts, if conventional single-layer injection mold structure is used, injection volume and mold opening stroke of injection molding machine may only use 20% to 40% of rated injection value, while laminated injection mold can increase output according to number of layers on same injection molding machine with almost same energy consumption and efficiency conditions without increasing clamping force (compared to conventional mold, clamping force of laminated mold is only increased by 8% ～15%, output can be increased by 90%～95%), greatly improve production efficiency and equipment utilization, greatly reduce injection molding costs and speed up product delivery.
However, double-layer molds have higher technical requirements and precision requirements for design and manufacturing than ordinary molds, such as reasonable runner design, mold opening synchronization mechanism design, appropriate mold thickness control, etc., while double-layer designs for large products are more complicated, load-bearing design of movable part in the middle of mold is also very critical.
Refrigerator heat dissipation cover shown in Figure 1 is a common component in global brand refrigerators. Most refrigerators need to use this component. Demand is huge. It belongs to large flat, shallow cavity shell type plastic part, which is very suitable for laminated mold structure. However, due to large size of plastic part, appearance requirements determine that mold also needs to adopt an inverted structure. Weight of laminated mold reaches nearly 20t, and there is still considerable difficulty in design and manufacturing.

Size of plastic part is about 495*436*63.5mm, front side is appearance, which requires high requirements, and no traces of glue inlet are allowed. Back side is a functional bone position, connecting pillars and hooks, etc. (see Figure 1). Plastic parts are made of thermoplastic material HIPS, which has good impact resistance and rigidity, oil and water resistance, and can effectively protect internal structure of refrigerator. Plastic part has more heat dissipation windows, and the overall shape is also larger. Finished product needs to pay attention to quality of front (flash) at each window and the overall deformation tolerance control of plastic part.

 

Figure 1 Refrigerator heat dissipation cover

First of all, according to customer's requirements for appearance of plastic parts, appearance surface is not allowed to have traces of glue openings, so it is not possible to make gates from fixed mold surface of plastic parts. Secondly, if you choose to feed glue from side, you may use a side fan gate, but this gate requires manual post-finishing, which is not conducive to automated production and is directly rejected. In this way, hot runner needle valve can only be used to feed plastic on inner surface of plastic part (ie side of movable mold core), which is so-called flip-chip mold structure. In laminated mold, splitter plate and ejector mechanism of hot runner need to be made on floating sandwich layer in the middle of laminated mold, main flow channel is extended from nozzle of injection molding machine across the first layer parting surface to middle splitter plate through multi-layer needle valve structure, ensures that high temperature and high pressure molten plastic will not leak when mold is opened.
In addition, average wall thickness of plastic part is 2mm, plastic characteristics of HIPS raw material (such as melt index 3.5g/10min), and structural characteristics of plastic part. etc., according to preliminary judgment, it is more appropriate to use 4 points of glue.
Based on above analysis, preliminary design of hot runner system structure is shown in Figure 2.

 

Figure 2 Hot runner system structure
According to above-mentioned runner design model, a detailed CAE mold flow analysis is carried out. From analysis results, runner scheme is feasible, meets client's capacity, and meets customer's requirements for injection molding efficiency (customer requirements are within 75s).
Figure 3 shows distribution of pressure field. Figure 4 is approximate location where weld line may appear on plastic part, which has also been recognized by customer.

 

Fig. 3 Distribution of pressure field Fig. 4 Approximate position where weld lines may appear on plastic

Movable mold of plastic part structure has multiple undercut positions, which need to be used for sloping top, sliding block and other structures to eject mold, and combined with ejector pins, push blocks, and cylinders to eject plastic parts. Although it is a little complicated, it can be regarded as a conventional design, as shown in Figure 5.
Due to inverted structure, ejection power cannot directly rely on ejection mechanism of injection molding machine, so oil cylinder is designed as driving power of ejection mechanism (see Figure 6).

 

Figure 5 ejection system structure

 

Figure 6 Hydraulic cylinder drive power ejection mechanism

Due to large demand for plastic parts, customers have very short production cycle requirements, and tolerance requirements of plastic parts are also relatively high, resulting deformation must be strictly controlled. Therefore, cooling system design of mold must be adequate and balanced. In addition to dense and uniform water path of dynamic and fixed mold cavity and core, slider, lifter and hot nozzle (high heat area) are also designed with independent water paths, as shown in Figure 7.

 

Figure 7 Cooling system structure
a — — Moving and fixed mold cooling water circuit b — — Independent cooling of hot nozzle c — — Independent cooling of inclined top slider

In order to make plastic parts shrink uniformly, residence time of plastic parts in each cavity is equivalent, it is necessary to design a set of mechanism to ensure that two parting surfaces of laminated mold must be opened accurately and synchronously, mold opening distance must be precisely controlled. In this mold, a precision rack and pinion is used for synchronous control, as shown in Figure 8a.

 

Figure 8 Synchronization mechanism
a — —Synchronous control of rack and pinion b — Synchronous control of mold opening distance
Above synchronization mechanism is driven by opening and closing power of injection molding machine. Rack is fixed on movable mold and fixed mold part. When injection molding machine opens mold, movable mold side rack follows movable mold. At the same time, rack also drives gear to rotate. When gear rotates, it drives rack on fixed mold side. Action processes are all connected together at the same time, mold opening distance is same in two layered cavities (Figure 8b). Because mold is relatively large, in order to ensure balance, two sets of synchronous gear mechanisms are symmetrically designed on both sides of mold.

One of characteristics of laminated mold is that intermediate floating sandwich layer needs to have a supporting structure to bear its weight after mold is opened, and it needs to be able to slide freely in mold opening direction. For large-scale laminated molds, because floating part in the middle is relatively heavy, a supporting structure is usually designed to bear weight by using green column of injection molding machine or slide rail of bed, as shown in Figure 9.

 

Figure 9 Mould load-bearing support structure
However, above method is first to lock production machine. Not only is it difficult to trial mold, but injection molding manufacturer cannot deploy and replace production machine. At the same time, it also has high technical requirements for installation mold and debugging workers, and mold needs to be exported abroad for injection production. Above-mentioned load-bearing scheme is not suitable. This design scheme uses mold body as support foundation. Structural support beam and mold are designed as a whole, which separates from dependence on injection molding machine. Mold is similar to ordinary mold during installation and debugging, operation is simple and convenient. At the same time, in trial production, as long as injection molding machine meets basic capabilities such as modulus and clamping force. Structure diagram is shown in Figure 10. Middle part of mold weighs up to 12t. After mold is opened, main support depends on combination of guide column and load-bearing beam. Load-bearing beam is main stress point, so design is very important.
Load-bearing beam is designed to be fixed on the side of fixed mold and sunk into mold plate 30mm to increase load bearing surface. Material is P20, which has good toughness and rigidity. Load-bearing block is made of oil steel series with good wear resistance and rigidity. Contact surface between block and supporting beam is made as a self-lubricating wear-resistant block to reduce friction during sliding. Sliding distance of support point needs to be accurately calculated and cannot exceed calculation result range. Maximum sliding safety distance of support point is 320mm ( See Figure 10b).

 

Figure 10 Structure diagram of load-bearing block and supporting beam
a — — Bearing block and support beam position b — — Support beam installation control

According to results of mold flow analysis and equipment situation of customer's injection plant, customer finally decided to use a 1,600t injection molding machine to produce this set of molds. According to parameters of client machine, thickness of its production machine is 1,550mm. Taking into account strength requirements of mould (thickness of mold plate), ejection distance of plastic part and space of manipulator, the total hard thickness of final control mould is 1,548mm, which just meets requirement of capacity modulus. Figure 11 is a schematic diagram of the overall design.

 

Figure 11 Mold assembly structure
Due to double parting surface, picking manipulator also needs a dual-arm manipulator. In order to ensure load-bearing beam case, reduce mold opening distance as much as possible. Through optimized design of manipulator, final cavity mold opening distance is determined to be 300mm. At the same time, top of mold must have enough space for manipulator to move. Therefore, arrangement of hot runner system wiring sockets and compressed air pipelines needs to be optimized in conjunction with hanging mold beam to make full use of joint space of the two. For example, hot runner system line is connected to junction box of fixed mold from both sides of mold through a cable, which effectively avoids motion line of manipulator, hanging mold beam is also designed to avoid motion line of manipulator in the middle part. Figure 12 is a schematic diagram of combined design of wiring arrangement of hot runner system and hanging mould beam.

 

Figure 12 Wiring arrangement of hot runner system and combined design structure of hanging mould beam

Final dimension of mold is 850*2,300*1,548mm, and the total weight is about 17t. It is a large flip-chip laminated mold. After two months of manufacturing, the first mold trial was successful, and finally reached or exceeded various technical indicators. Actual production cycle was less than 65s. Realize customer's goal of high efficiency, low cost and automated production. After molds were delivered abroad, they brought good economic benefits to customers, and were highly praised by customers.