A lamp holder component in a lighting fixture is composed of PC plastic parts and tempered glass lenses. Plastic parts are both appearance parts and functional parts. Injection molding technology has high requirements. It must meet appearance standards and functional requirements of drawings. In particular, joints with tempered glass must meet technical requirements of sealing, shading, reliable connection, and accurate positioning with lamp holder light source.
Appearance of plastic part is EDM texture VDI24, material is PCMakrolon2458, black, the overall dimensions are about 44mm*35mm, and thickness of tempered glass is 2mm, as shown in Figure 1. Performance parameters of material and injection process parameters are shown in Table 1.

 

1. Tempered glass 2. Plastic parts
Figure 1 Product structure
Table 1 PC Makrolon 2485 performance parameters and injection process parameters

Main raw material of product tempered glass lens is optical glass, which has excellent optical properties, high hardness, high refractive index, good water resistance, corrosion resistance and good thermal stability, and is suitable for medical, mining, construction and other environments.

Plastic part is analyzed by demolding angle. Parting line of mold is at intersection of positive and negative demolding angles, as shown in Figure 2. Positive angle part above is cavity, and negative angle part below is core. Plastic part mold is designed and manufactured according to this mold opening scheme. While analyzing mold opening direction of plastic part, it is necessary to consider whether there are texture requirements on appearance of plastic part, which will affect demolding quality of appearance of plastic part. Texture of appearance of plastic part is VDI24, and demolding angle is required to be greater than 2°. If it is less than 2°, texture will be scratched when plastic part is demolded. From demolding angle analysis, it is known that demolding angle of plastic part is 3°, which meets texture demolding requirements.

 

For easy installation, add a buckle of a lateral countersunk square hole on two lugs in the middle of plastic part to snap together assembly parts, as shown in Figure 3. Buckle is inverted in normal mold opening direction and is not easy to demold. A slider needs to be designed on mold to demold sideways before pushing out molded plastic part.

 

Process of placing tempered glass as an insert into mold for injection is same as principle of traditional insert injection process. Tempered glass is regarded as an insert and placed in mold. Then, molten plastic is filled into cavity, cooled and solidified to make glass and plastic part a whole.
Insert (tempered glass) needs to be positioned in mold so that it does not move during injection molding. Product is an optical component. There should be no dust, dirt, etc. on tempered glass. When it is produced in a clean workshop, a robot is used for automated production to avoid contact with pollution sources such as personnel. After being made by glass enterprise, tempered glass is placed in a transparent anti-static tray for sealed packaging. After being transported to workshop, it is uniformly placed on a special stainless steel frame. Steel frame is placed next to injection molding machine manipulator. A positioning fixture for tempered glass is placed on machine table, as shown in Figure 4. Tempered glass is similar to a plate. It is positioned in fixture by its shape. Manipulator takes a piece of glass from tray and puts it into positioning fixture first. Positioning fixture and mold have fixed spatial positions. Through programming and calibration, manipulator moves back and forth between the two to achieve precise placement of tempered glass into mold for injection.

 

Fixing mechanism of tempered glass needs to be designed in mold, as shown in Figure 5. Its working principle: when tempered glass is placed in mold by manipulator, tempered glass is attached to fixed model cavity surface, and mold gas circuit is evacuated to form a negative pressure. Suction cup deforms and sucks tempered glass to fix it in mold cavity to form a seal. After waiting for mold closing injection, gas circuit is changed to blowing to make glass and plastic part leave fixed model cavity together. Sealing of fixture is a key factor in injection molding. Once there is air leakage, it will fail and cause tempered glass to fall under its own weight. For this purpose, a sealing ring is installed, raw tape should be wrapped around thread at the end of brass pipe so that air can only run in air path and tempered glass can be fixed in mold cavity. Vacuum detection is judgment condition for start of insert injection. When tempered glass forms a negative pressure in fixed air path, sensor will send a signal to injection molding machine system to notify mold that insert has been installed and mold can be closed for injection instructions to start next step. If there is no negative pressure in air path, sensor will not send a signal that there is an insert, and injection molding machine will not proceed to next step, which plays a protective role. Otherwise, mold closing injection will inject molten plastic into air path, causing air path to be blocked, so that mold cannot work normally and can only be stopped for cleaning, and cleaning work is cumbersome.

 

When tempered glass is injected as an insert, it must withstand a certain clamping force. Mold is produced on an 800kN injection molding machine. Minimum clamping force of machine is 200kN. Product size is not large, and clamping force required for injection is lower than minimum clamping force of machine. Considering production stability of machine, mold adopts a 250kN clamping force. After mold is closed, force will be transmitted to tempered glass. Tempered glass is incompressible, thickness will fluctuate and have tolerances during production. It is necessary to consider that force on glass should be small and can meet molding sealing requirements to avoid clamping force acting on tempered glass. Floating core insert structure is used on mold, as shown in Figure 6, which can avoid clamping force acting directly on tempered glass and solve problem of thickness fluctuation of tempered glass.

 

Figure 6 Floating core insert structure
Advantage of floating core insert is that sealing force on tempered glass during injection molding is changed to a flexible fit to avoid damage to tempered glass due to rigid force. Upper end of insert is a cylinder about 3mm long, and the lower end is a cone about 12.3mm long with a slope of 1°. This structure allows the insert to move up and down in fine-tuning.
Mold assembly structure is shown in Figure 7. The lower end of floating insert adopts a conical mold. Its advantage is that mold parts can be instantly positioned when they move to the bottom, and can be immediately disengaged when released, reducing matching surface, avoiding frequent friction causing jams and causing floating core insert to fail. In addition, thickness tolerance of tempered glass is 0~0.05mm. Insert will change by 0.05mm when it is fitted with tempered glass. Use of conical surface design solves this problem, so that floating insert will adjust slightly by itself when tempered glass is under pressure. Top surface of floating core insert has a flatness requirement of 0.01mm and is polished to A3 grade, so that its top surface has a good combination with tempered glass to avoid scratches on exterior surface of tempered glass. Floating core insert is made of pre-hardened steel 718HH with a hardness of 34~38HRC. Chemical composition and physical properties of steel are shown in Table 2. 718HH is a vacuum-melted chromium-nickel-molybdenum alloy steel with excellent polishing, photo-etching properties and good processing performance. 718HH steel can be effectively matched with tempered glass. If hardness of matched material is too high, it is easy to cause damage to tempered glass. If hardness of material is too low, it is easy to cause it to wear too quickly. Using 718HH steel and heat treating it to 34~38HRC can meet mold design requirements.
Table 2 Chemical composition and physical properties of 718HH steel (at 200℃)

Figure 7 shows injection mold structure of tempered glass insert, and its movement process is as follows.

 

1. Fixed mold base plate 2. Stripper plate 3. Fixed mold plate 4. Fixed distance pull rod 5, movable mold plate 6. Support plate 7. Pad 8. Protective column 9. Moving mold base plate 10. Push rod guide column 11. Support column 12. Ejector 13. Screw 14. Return confirmation switch 15. Push rod fixing plate 16. Lower push plate 17. Push rod fixing plate 18. Secondary ejection mechanism 19. Upper push plate 20. Dustproof plate 21. Return rod 22. Secondary guide column 23. Return hanging module 24. Core insert 25. Floating core insert 26. Damping opener and closeer 27. Screw 28. Cavity plate insert 29. Product 30. Pull rod 31. Gate sleeve 32. Positioning ring 33. Brass pipe 34. Guide column 35, Precision positioning 36. Right slider 37. Counter 38. Left slider
Figure 7 Mold assembly structure
(1) Mold opening process. After mold is closed and high-pressure clamped, injection molding machine starts injection process. After plastic particles are melted, they are injected into mold gate sleeve through injection molding machine nozzle, flow into mold runner, then pass through gate to finally fill cavity, pressure is maintained and injection is stopped. At this time, molten plastic in cavity is gradually cooled to a solidified state under action of mold cooling system. Injection molding machine ends injection process and removes high-pressure clamping force. Rear end of injection molding machine bends arm to pull machine slide to make mold movable part move backward to open mold. After moving to set position, it stops. Machine push rod pushes push rod 12 to move push plate. Push rod fixing plates 15 and 17 realize secondary movement under action of secondary ejection mechanism 18, complete core pulling of movable mold insert and push plastic part out by push rod. Robot takes out plastic part and puts a new tempered glass in cavity at the same time. Tempered glass is vacuumed by suction cup to form a vacuum fixation. Suction negative pressure is maintained until injection is completed and changed to blowing. Robot leaves mold to complete mold opening action.
(2) Mold closing process. Injection molding machine push rod pulls push rod 12 to make push rod fixing plate 17 return to its original position for a while. Under action of secondary ejection mechanism 18, it drives push rod fixing plate 15 to return to movable mold base plate 9 together. Mold completes ejection reset action. Machine sends a signal to flex arm at rear end of machine. Flex arm extends to make machine slide carry movable mold forward and gradually approach fixed mold. After reaching set position, it waits for injection. Mold completes mold closing action and starts injection molding.