Heat pump body is a supporting product of a certain medical device. Product picture is shown in Figure 1. Maximum size of plastic part is 156.67mm*113.85mm*200mm, average wall thickness is 4.0mm, and weight of plastic part is 332 grams. Material is PA66+15GF, shrinkage rate is 0.45%. Nylon has excellent wear resistance and self-lubricating properties, its wear resistance is higher than that of copper. It also has high mechanical strength and toughness, resistance to weak alkalis and general organic solvents, use temperature is generally-40 ℃ ~ + 100 ℃. Disadvantage is that its water absorption is large, which affects dimensional stability. Addition of glass fiber filler to nylon resin can effectively improve impact strength. Nylon materials are widely used in instrument parts (coils, switches, connectors, washers, housings), mechanical parts (gears, bearings, cams, bushings), etc. Heat pump body has very high requirements for plastic parts, and needs to withstand high internal pressure during use. In addition to high mechanical strength and toughness of plastic part, plastic part itself must not have weld marks, bubbles, burrs and other injection defects.
It can be seen from product drawing that structure of plastic part is complex, installation plane is required to be flat, and demolding slope cannot be designed. 3 U-shaped notches on the edge of installation plane are used to assemble fastening bolts. There is a rib in the middle of installation plane, and there are two small bosses on it. Therefore, installation plane needs to design slider core. There is a liquid outlet pipe on the shoulder of container, which directly leads to inside of container. Opposite side of liquid outlet pipe also needs to be designed with lateral core pulling. Therefore, plastic parts need to design 3 directions of lateral parting and core pulling mechanism, as shown in Figure 1. Position of 3 sliders is shown in Figure 3. There is a threaded inner hole on the top of plastic part, thread length is 16mm, thread pitch is 2mm, this thread is a non-standard thread, and an automatic threading mechanism needs to be designed. Comprehensive analysis of a full set of mold design has two difficulties, one is design of three slider core pulling mechanisms, selection of parting surface between sliders and and cavity, and the second is design of fixed mold unthreading mechanism. 

 

Figure 1 Product map of heat pump body

 

There is an essential difference between unthreading of large-scale casing plastic parts and unthreading of round plastic parts of bottle caps. Way of unthreading round plastic parts is often related to sequence of mold opening and complexity of the mold structure. Way of unthreading of casing plastic parts is often relatively simple, and most of them use oil cylinders or pneumatic components to unthread. Therefore, most of fixed mold unthreading uses an oil cylinder to drive rack to drive gear to unthread thread. Difference between fixed mold unthreading and moving mold unthreading is that mold opening action cannot be used to unthread thread with aid of mold opening force. Therefore, it is necessary to design a separate threading mechanism, complete threading action before opening mold, otherwise it will damage product and even cause an accident.
Plastic part has a large size and a mass of 332 grams. In addition, structure is more complex, with three-direction core pulling and fixed mold unthreading. Injection molding machine needs a large tonnage, so only one cavity can be designed. Mold design is shown in Figure 2. Mold base is a non-standard mold base with a specification of 5340. In order to ensure processing accuracy, mold base is customized by professional mold base manufacturer Longji. Selected injection molding machine is a 300-ton injection molding machine. After calculation, plasticizing capacity and clamping force meet requirements.
Periphery of plastic part is surrounded by 3 sliders, glue at the bottom of slider is a bad mold design and should be avoided. Inner hole of hollow column at the top is threaded, threading mechanism needs to be designed, and there is no position to design gate. Therefore, in accordance with requirements of mold design specifications agreed with customer, a hot nozzle is used for single point injection of glue on curved surface of top of plastic part. Hot nozzle brand is YUDO. Gate needs to be cut off manually. Material of plastic parts is PA66+15GF. Due to abrasiveness of glass fiber, it is not suitable for needle valve hot nozzles. For plastic parts with strength requirements, this kind of large nozzle can make plastic parts compact and increase mechanical strength significantly.
Mold needs to design 3 sliders, of which inner hole depth of slider 1 is 60mm, plus safety distance of 3mm, and core pulling distance is 63mm. This slider is designed with a cylinder drive, and remaining two sliders still use inclined guide pillar core pulling mechanism. Inclined guide column core pulling has characteristics of stability and reliability. In order to increase life of mold, facilitate adjustment and replacement, inclined surface and bottom of all sliders are designed with wear-resistant plates, made of quenched steel, and an oil groove is opened on the surface.

 

Installation plane of molded plastic part of sliding block 25 is relatively large and lateral force received is also large. For this reason, an anti-locking mechanism is designed for inclined wedge 28 to prevent its deviation. There is a travel switch on oil cylinder of slider, which is convenient to control core pulling distance. Bead of all sliders is made of graphite copper bead to ensure good lubrication and guidance.
Unthreading mechanism is driven by an oil cylinder, which drives rack, rack drives gear to rotate threaded core through a two-stage transmission to realize fixed mold unthreading. Parameters of cylinder are 50 bore, 300 stroke, 80 rack teeth, rack and all gears have a modulus of 1.5. Two-stage transmission can shorten length of rack. After injection molding, pinion with Z1=24 teeth is driven by rack and pinion meshing. This pinion then drives large gear with its coaxial Z2=90. This large gear meshes to drive pinion with Z3=24. This small gear drives coaxial thread core to rotate, thereby realizing automatic threading. Transmission ratio is 90/24*90/24. Therefore, unthreading of rack and pinion is a speed increasing mechanism, stroke of cylinder and length of rack can be shortened by increasing speed.
There are 6 blind holes on the bottom edge of plastic part. 6 cylinders are designed in this part, and there is a hollow column in the center of plastic part, which is also convenient for designing cylinder. In addition, a thimble is designed on inner top to realize demolding of plastic part under action of cylinder and thimble.
In order to ensure injection cycle, mold is designed with a good cooling system. Temperature of mold for producing nylon is relatively high. In order to avoid heat loss, movable mold base plate and fixed mold base plate are respectively designed with heat insulation boards.
Both sides of movable and fixed mold are designed with oil cylinders, and oil cylinder is on sky side. Therefore, mold needs to be designed with a lifting device. Lifting mold beam 45 is fixed on mold through support column 46.
Export mold customers generally require mold flow analysis, using MPI 6.0 mold flow analysis, simulation results are as follows:
1. Filling and pressure demand model shows that it can be filled correctly.
2. We expect to maintain uniform pressure distribution on parts. In 5.9mm wall thickness, there is a slight risk of voids in the area near top edge and bottom port of container.
3. Warpage behavior, maximum warpage deflection value of research model illustrates various variants.
4. Draw fiber direction diagram. Colors in these figures represent intensity of direction. Red indicates a high degree of fiber orientation, and blue indicates a low degree of fiber orientation. Generally speaking, following statement is correct: the more uniform orientation of fiber, the smaller warpage.
5. Shows roundness and uniformity of part. We investigated that displacement along path in two-dimensional graph from position where applied energy starts requires high tolerances. Displacement at path is displayed as an absolute value.

 

 

 

 

Figure 2 Mould diagram of the heat pump body

 

Figure 3 3D mold diagram

 

 

 

 

 

 

 

 

 

Figure 4 Mold flow analysis diagram