[Abstract] Function and technical requirements of automotive sensor plastic parts with multiple metal terminals and metal inserts are analyzed, pre-installation and positioning mechanism of metal terminals and metal inserts in precision injection mold for automotive sensors are introduced, convenient operation, compact and ingenious mold overall structure design, safe and reliable action, saving metal insert installation time, improving production efficiency, reducing defective product rate, quality of plastic parts meets customer quality requirements.

Automobile industry is a pillar industry of an industrial country. It occupies an important position in national economy and represents country's industrial level. After more than 30 years of development, my country has now become the world's largest automobile producer and consumer. With rapid growth of domestic automobile production, domestic automobile industry's demand for sensors, their supporting transmissions and meters will also greatly increase. In order to ensure quality and performance of automotive sensors, requirements for precision injection molds for automotive sensors are particularly strict. At present, domestic sensors have a huge gap with international level in high-end aspects such as high-precision, high-sensitivity analysis, component analysis and special applications. Plastic parts of mid-to-high-end sensors and their production molds are almost completely imported from abroad. Therefore, it is imperative to break foreign monopoly, realize localization of automotive sensors and high-end molds. A system platform must be established to systematically drive development of sensors. In order to ensure quality of sensor, injection mold used to produce sensor must have high accuracy requirements.

Automotive plastic parts include exterior parts, interior parts and functional parts. Figure 1 shows a certain type of automotive sensor. Plastic part is a functional part of automobile. Main feature of plastic part is that it has a variety of inserts inside. As shown in Figure 2, it includes: 3 kinds of metal terminal bracket blocks (outer bracket protective shell) and an insert metal ring. In order to ensure precise sensitivity of car sensor, it is necessary to ensure accuracy of position of metal insert, strength and quality requirements of fusion of metal and plastic during mold design and injection molding process. In order to ensure position accuracy of metal inserts, on the one hand, high-precision machine tools are required to process high-precision mold fitting parts, and on the other hand, a reasonable insert positioning mechanism needs to be designed.
Car sensors are distributed in various parts of car structure like nervous system of animals. Many car sensors are equipped with special locations and have a harsh working environment. In order to ensure that car sensor products do not crack during use, strength of fusion of insert and plastic, strength of plastic body are very important. Therefore, material of plastic part is selected PPA+GF35, among which polyphthalamide resin (referred to as PPA) is a semi-aromatic polyamide made of terephthalic acid or phthalic acid. Material is characterized by super tensile strength Strength and high flexural modulus, high hardness, resistance to long-term tensile creep, low water absorption, strong resistance to gasoline, grease and coolant, resistance to continuous high temperature of 200℃, have good plastic strength, toughness and strength, good gloss on injection molding surface, maintain good dimensional stability. Addition of 35% glass fiber to PPA greatly improves strength and rigidity of plastic, increases its service life. Market price of this material is 37 yuan/kg, which is affordable from perspective of manufacturing cost of plastic parts.

 

Figure 1 3D display of plastic parts

 

Figure 2 Plastic part insert

For molding process parameters of high-precision molds for automotive sensors made of PPA+GF35, generally considered from aspects of process conditions and molding equipment. Since PPA+GF35 plastic material has relatively good fluidity in molten state, flashing is easy to occur in mold injection molding process, so size of nozzle outlet aperture of injection molding machine and mold interface aperture must be appropriate. In addition, raw materials solidify fast, gate design of large nozzle should be considered [1]. In order to prevent water absorption from affecting dimensional changes of plastic parts, supply conditions must be strictly controlled.

Using 3D software to calculate volume of injection molding area (not including metal insert) of plastic part is 6.8312cm 3, density of PPA+GF35 is about 1.42, and unit weight of plastic part is about 9.698g. As shown in Figure 3, through analysis, 3D plastic part generally has a wall thickness of 1.3mm, maximum wall thickness is 5.9mm. Although part is too thick, it can be achieved by adjusting injection molding process and increasing gate.

 

Figure 3 Wall thickness analysis of plastic parts

This mold uses a mold with two cavities [2], considering injection pressure, amount of glue and size of mold, and angle that facilitates installation of terminals, 85t vertical injection molding machine is selected, and screw diameter is ϕ 32mm. Injection pressure is 200MPa, nozzle diameter is ϕ 2.5mm, maximum mold opening is 545mm, and minimum mold thickness is 310mm.

By analyzing parameters of physical properties of raw materials, taking into account uneven wall thickness of plastic parts, partial wall thickness of plastic parts, heating capacity of injection molding machine barrel, melting temperature is set to 325℃. As shown in Figure 4, CAE analysis injection time is 0.3779s, injection pressure is 75MPa, clamping force is 32t, cooling time is expected to be 7s, initial set mold temperature is 120℃, it can be seen that initial setting of process parameters is reasonable.

 

Figure 4 Analysis of injection pressure and clamping force during filling time

Figure 5 shows the best gate position for CAE analysis, but considering influence of molten resin flow during injection on terminal positioning and position of maximum wall thickness of plastic part, gate position shown in Figure 6 is finally selected. In order to meet injection quality at position of the largest wall thickness, size of gate is determined to be ϕ 2*2mm.

 

Figure 5 CAE analysis of the best gate location Figure 6 Final gate location

This injection mold cavity layout uses one mold and two cavities. Through analysis of shape of plastic part of automobile sensor and distribution position of insert, it is established that parting surface is a plane composed of apex position of arc of plastic part, 3 metal insert positioning insert parts and 1 metal iron ring insert positioning part are designed as a lateral core-pulling slider mechanism[3]. As shown in Figure 7, fixed mold main body inserts and movable mold main inserts, fixed mold plate and movable mold plate are designed as independent circulating water system [4]. Since temperature of molten material flow can reach 325℃ during injection molding process, fixed mold bottom plate and movable mold bottom plate are respectively provided with heat insulation panels.

Since inserts need to be fitted in mold cavity before injection molding, in order to solve technical problems of small assembly space of inserts and difficulty of precise positioning of inserts, ejector slider assembly structure [5] was developed and designed, as shown in figure 8 and Figure 9. In mold opening state, slider body 8 and slider insert 3 are ejected by ejector rod 18. Insert mounting surface is higher than mold forming parting surface. Metal ring insert 28 is fitted on the head of slider insert 9. When resetting, slider body 8 is manually pushed back along guide groove to make hanging table surface contact slider seat 7 to realize reset; insert 29 is a bracket with terminals, insert 30 is a terminal bracket protective shell. After assembling insert 29 and insert 30, they are put into slot of movable mold insert 20 and positioned as a whole. Movable mold insert 19 limits end position of insert 30 at the same time.

 

Figure 7 Schematic diagram of the overall structure of the mold
1. Fixed mold base plate 2. Fixed mold plate 3. Slider insert 4. Oblique wedge 5. Guide pin 6. Ball head screw Ⅰ 7. Slider seat 8. Slider body 9. Slide block insert Ⅰ 10. Ball head jack Ⅱ 11. Movable mold plate 12. Movable mold insert Ⅰ 13. Mandrel Ⅰ 14. Square iron 15. Mandrel fixing plate 16. Ejector backing plate 17. Movable mold seat plate 18. Ejector Ⅱ 19. Movable mold insert Ⅲ 20. Movable mold insert Ⅱ 21. Movable mold insert 22. Limit nail 23. Slider insert Ⅱ 24. Slider insert Ⅲ 25. Guide pin baffle 26. Fixed mold insert 27. Fixed mold insert 28. Insert Ⅰ 29. Insert Ⅱ 30. Insert Ⅲ 31. Pressure plate 32. Spring
Slider insert 24 and slider insert 23 are in surface contact and are connected by a hanger. Boss set on slider insert 3 matches groove set on slider body 8 and is highly positioned, is locked and fixed by bolts; slider base 7 and slider body 8 are provided with a V-shaped groove. Slider body 8 and slider base 7 are connected and limited by a guide groove. Through hanger connection, slider body 8 can slide up and down along guide groove of slider base 7, guide pin 5 and slider base 7 are surface-matched, through hanger connection, spring 32 is placed in counterbore of sliding table of slider seat 7 to make surface contact with guide pin 5, guide pin baffle 25 and spring 32 are in surface contact and limit. Guide pin baffle 25 and slider seat 7 are surface matched, are locked and fixed by bolts. Movable mold insert 12 is provided with a tapered hole. When mold is closed, tapered surface of head of slider insert 9 is in surface contact with, fits with tapered hole of movable mold insert 12; ball head screw 6 is threadedly connected with slider seat 7, head ball head of ball head screw 6 is in surface contact with side surface of slider body 8. In clamping state, ball head of ball head jack wire 6 is in a compressed state, ball head screw 10 is threadedly connected with movable mold plate 11, ball head of ball head screw 10 is in surface contact with bottom surface of slider seat. In mold clamping state, ball head of ball head screw 10 is compressed, slider seat 7 is in surface contact with pressure plate 31 and is limited in position. Inclined wedge 4 is in close contact with inclined surface of slider seat 7 to act as a wedging force. Ejector rod 13 and movable mold plate 11 are in surface fit, ejector pin 13 and ejector rod 18 are in clearance fit with ejector rod fixing plate 15, are connected by a hanging platform. Ejector rod 13 and ejector rod 18 are in surface contact with ejector rod backing plate 16, bolts are locked and fixed to top rod fixing plate 15 through top rod backing plate 16, top surface of top rod 13 is in surface contact with bottom of slider base 7.

 

Figure 8 Insert positioning and matching mechanism

 

Figure 9 Assembly diagram of slider and movable mold insert

When clamping mold, guiding angle on wedge 4 contacts chamfer of guiding pin 5, guiding pin 5 is pushed to move in direction of guiding pin baffle 25. At this time, spring 32 is compressed and mold continues to close. When pin 5 slides into chute of inclined wedge 4, guide pin 5 is ejected and reset under action of spring 32, fixed mold insert 26 and movable mold insert 21 contact, mold clamping process ends. As shown in Figure 10, when mold is opened, fixed mold plate 2 and movable mold plate 11 are separated, inclined wedge 4 is separated from inclined surface of slider seat 7, oblique groove on oblique wedge 4 is in contact with guide pin 5 to drive slider base 7 to slide in a direction away from movable mold insert 21. When slider base 7 contacts limit nail 22, head of ball head screw 10 snaps into V-shaped groove of slider base 7, slider base 7 stops moving. At this time, stop position of slider body 8 is above ejector pin 13. Under action of forming machine, ejector pin fixing plate 15 and ejector pad plate 16 move together in direction of movable mold plate 11. Under action of ejector pin 13, slider body 8 is pushed out. When head of ball-end jack wire 6 fits into V-shaped groove of slider body 8, slider body 8 stops moving (see Figure 2 and Figure 5), mold opening process is completed when plastic part is taken out.

 

Figure 10 Schematic diagram of mold opening

Practice has proved that mold structure is efficient and feasible. For sensor plastic parts in different positions of car, shape and function are different, but mold structure technology principle of insert positioning and installation is same. This technical solution is currently used in a variety of precision injection molds for automotive sensor plastic parts, such as air flow sensors, throttle position sensors, camshaft and crankshaft position sensors and other plastic parts. Through promotion and application of technical solutions, we have produced a variety of high-end automotive sensor series plastic parts, a variety of high-end automotive sensor series plastic parts are used in well-known brand cars at home and abroad, with excellent quality and stable performance, which have been praised by customers and promote localization of precision high-end molds.

Aiming at positioning and fixing cooperation between brackets of three types of metal inserts and metal ring of inserts, use of structure of slider assembly ejected by ejector rod, diagonal wedge and guide pin, realizes assembly of multiple inserts in different ways, simplifies structure of mold and installation steps of metal insert, shortens installation time of metal insert, and improves production efficiency. Mold has a compact structure, stable and reliable action, which solves problems in insert mold with multiple different positioning methods inside plastic part, reduces defect rate of plastic part, and saves a certain labor cost. Since it was put into production and used, injection molding cycle of mold and quality of insert encapsulation completely meet customer's quality requirements, which serves as a reference and promotion for development of similar metal insert sensor molds.