Thick-walled light guide is shown in Figure 1. Maximum outline size is 272 mm * 211 mm * 163 mm, average wall thickness is 12 mm, the thickest part of wall is 29 mm, single piece weight is 380 g, and shrinkage rate is 0.6% , injection molded from optical grade PC (polycarbonate) material. Since plastic parts are optical structural functional parts, surface requires light-input surface, light-output surface and visible surface to reach mirror polishing level A0. Residual marks of splicing lines and gates are not allowed. Mounting feet and buckles are polished with 800# sandpaper. The overall appearance of thick-walled light guide must be crystal clear. Defects such as flow marks, shrinkage dents, and weld marks are not allowed during molding. Four buckles are undercuts in mold opening direction, and a side core-pulling mechanism needs to be designed for molding.

 

Figure 1 Thick-walled light guide plastic parts

(1) Molding cycle is long. In cooling stage of thermoplastic injection molding, cooling time is mainly determined by wall thickness of plastic part and maximum flow channel diameter. Cooling system equation is as follows:

 

Among them, Tc is cooling time, s; Thw is maximum wall thickness of plastic part, mm; Dr is maximum diameter of runner, mm; k is thermal conductivity of molten plastic, W/(m·K); p is density of molten plastic , kg/m³; Cr shows specific heat of constant volume of molten plastic, J·(kg·K)-1.
It can be seen from equation (1) that cooling time is proportional to square of maximum wall thickness of plastic part. If wall thickness increases by 2 times, cooling time will increase by 4 times. In addition, since thick-walled light guides are molded using PC, PC material has poor fluidity and its viscosity is highly sensitive to temperature. This requires improving fluidity of melt, and method of increasing injection temperature is now used. In actual injection process, melting temperature of material is set as high as about 300℃. In order to achieve a high-gloss and transparent effect on the surface of plastic part, mold temperature needs to be set to about 100℃, which prolongs cooling time. Therefore, how to appropriately reduce wall thickness of plastic parts has become key to shortening molding cycle and improving production efficiency. Thick-walled light guide of this model is currently developed using a single-color mold integral injection process. Molding cycle during mass production is 288 s. Comparison with molding cycle of thick-walled light guides using layered injection process is shown in Table 1. Use of layered injection process shortens molding cycle of thick-walled light guides, so plastic part was changed to layered injection process.

Table 1 Full cycle timing of different thick-walled light guides with different molding processes
(2) Molding window is narrow. Due to thick wall thickness of plastic part, injection marks will be produced if filling speed is too fast. However, if filling speed is reduced and melt frozen layer cools too quickly, flow marks will be formed. During cooling stage of plastic part, a high pressure state needs to be maintained for a long time. If pressure is too small or pressure holding time is short, feeding effect will be small, defects such as vacuum holes or surface depressions will appear inside plastic part due to condensation shrinkage, which will affect appearance quality and light distribution of plastic part.

Thick-walled light guide layered injection is shown in Figure 2. In order to reduce wall thickness of plastic part in a single injection, wall thickness of thick-walled light guide is divided into 4 equal parts, split into the first color molded part and second color molded part according to shape. Since second color molded part is appearance area of thick-walled light guide, mold temperature requirements are higher. During injection, cavity part is the first color molded part, thermal conductivity of plastic is poor, so wall thickness of second color molded part is designed to be thinner, as shown in Figure 3, local maximum wall thickness is 5 mm. The first color molded part is inner core of thick-walled part. Since there are no appearance requirements on the surface, a lower mold temperature is set during molding to quickly cool plastic parts, so proportion of wall thickness layering is increased, and local maximum wall thickness is designed to be 10 mm. Conduct mold flow simulation analysis on the overall molded parts, the first color molded parts and the second color molded parts with cooling water channels respectively. As shown in Figure 4, time when 80% of volume of plastic part reaches frozen state is set as molding cycle, molding cycle of the overall molded part is 230.7 s, molding cycle of the first color molded part is 69.52 s, and molding cycle of second color molded part is 37.11s, so the total molding cycle using layered two-color injection process is 106.63s, which is 124.07 s shorter than the overall molding process.

 

Figure 2 Thick-walled light guide layered injection

 

Figure 3 Schematic cross-section of layered thick-walled light guide

 

Figure 4 Molding cycle simulation analysis

In the early stage of design of two-color injection mold, the first and second color molded parts need to be determined based on appearance requirements of plastic part, injection material and two-color overlapping structure of plastic part. According to layered design of thick-walled light guide, if shell is used as the first color molded part, mold needs to adopt a flip-chip design, which has a complicated structure and will increase cost accordingly. Therefore, core component is selected as the first color molded part. Two-color injection mold is shown in Figure 5. The first color mold core and cavity form the first color molded part, second color mold core, cavity and first color molded part form second color molded part, left and right thick-walled light guides are formed in one molding cycle. The overall dimensions of mold are 1 410 mm * 1 150 mm * 856 mm, and the total weight is about 8 303 kg. It is a large two-color injection mold.

 

Figure 5 Cross-sectional structure of two-color injection mold
(a) Schematic cross-sectional view of two-color injection mold (b) Schematic cross-sectional view of the first color core and cavity (c) Schematic cross-sectional view of second color core and cavity; 1. First color cavity plate 2. First color molded part 3. First color core 4. Second color cavity plate 5. Second color molded part 6. Second color core

Generally, a two-color injection mold is produced using a two-color injection molding machine with two injection systems. Since plastic part is injected with same plastic twice, a single-color injection molding machine is used, and a turntable mechanism is added to movable platen for injection molding. Thick-walled light guide two-color injection mold adopts a 4-cavity layout, as shown in Figure 6. Gating systems of the first and second-color molded parts are designed using a single-point needle valve type hot runner to a U-shaped runner. End of channel acts as a cold material well to prevent cold material from filling cavity at the front end of hot runner nozzle and causing appearance defects of plastic parts. According to gate position simulation analysis results, plastic part assembly and appearance requirements, gates of the first and second color molded parts are at same position, U-shaped flow channel and gate of second color molded part are stacked above runner and gate of first color molded part. Runner and gate of first color molded part are designed on movable mold side, runner and gate of second color molded part are designed on fixed mold side. First color molded part is filled with a side gate of 10 mm * 5 mm, so that thick-walled light guide has a long enough holding time to compensate for shrinkage and avoid defects such as voids and sink marks; wall thickness of second color molded part is thinner, and side gate size is designed to be 8 mm * 2.5 mm.

 

Figure 6 Hot runner pouring system
(a) Gating system (b) U-shaped runner (c) Gate location

Optical pattern of thick-walled light guide is carved with a φ0.3 mm milling cutter, which requires long processing time and high precision requirements. In second color cavity structure of mold, cavity is composed of inner cavity plate inserts, optical pattern inserts, and outer cavity plate inserts, as shown in Figure 7. Since optical pattern area is located at the bottom of groove of second color cavity, it is too deep to be processed. In order to meet requirements of pattern processing, pattern area is processed separately in the form of inserts, and the overall mold manufacturing cycle can be shortened.

 

Figure 7 Second color cavity plate inlay structure

When molding, most of molding surface of plastic part is located on the side of movable mold cavity and has a deep cavity shape. In addition, injecting thick-walled light guides requires use of larger injection pressure and holding pressure, which results in a larger tightening force between plastic part and cavity insert. When mold is opened, plastic part will tightly wrap cavity insert and break weak part on core side of moving mold of plastic part, resulting in failure of normal production. In order to solve problem of plastic parts tightly wrapping cavity inserts, realize molding and demoulding of movable mold core side, mold is designed with a slider demoulding mechanism. One slider is designed on each side of the first and second-color molded parts. The entire mold has a total of 8 sliders.
Slider is pressed against flange of the first color molded part during two-color injection process, as shown in Figure 8 (a) and (b). It plays the role of fixing plastic part and forming undercut structure during mold opening process. Assembly relationship of slider demoulding mechanism in mold is shown in Figure 8(c). Slider is installed on the side of movable mold through guide sliding block, and is positioned using a spring positioning mechanism. Inclined guide pillar fixed to fixed mold provides mechanical driving force to move slider back and forth.

 

Figure 8 Slider demoulding mechanism
(a) Slider acts on the first color molded part (b) Slider acts on second color molded part (c) Assembly of slider demoulding mechanism

Form of ejection mechanism is related to shape, structure and plastic properties of plastic part. Ejection mechanism is designed according to appearance and functional requirements of plastic part and demoulding resistance. Ejection force balance ensures that plastic parts will not be deformed or damaged, and mold must be pushed out smoothly, steadily and reliably. Since most of plastic part has separated from mold cavity when mold is opened, demoulding resistance is small. Optical pattern at the bottom of plastic part serves as a functional structure and does not allow for traces of push-out. Push-out mechanism is designed in non-appearance area around the plastic part, as shown in Figure 9. Runner directly below hot runner nozzle is designed with a φ8 mm round push rod. A 6 mm plane extending from edge of optical pattern surface is used to evenly arrange direct push blocks and φ5 mm round push rods. A φ6 mm round push rod is designed at rounded corner of step where mounting foot is well stressed, and a push tube is arranged at installation hole. Mold uses direct push block, push tube, and round push rod to push out plastic part, push distance is 30 mm.

 

Figure 9 Launch mechanism

One of design principles of cooling system is that distance between cooling water path and cavity wall is roughly equal to achieve a roughly balanced temperature throughout cavity. In order to avoid uneven cooling of plastic parts leading to extended molding cycle or warping deformation, cooling system of mold adopts a combination of "straight-through water pipe + inclined water pipe + water well". As shown in Figure 10, cooling system is designed with a conformal water path along shape of plastic part as a whole, water well cooling is designed in areas with uneven local cooling. Distance between water path and surface of plastic part is 20 to 25 mm to ensure uniform cooling of plastic part.

 

Figure 10 Cooling system
The first and second color cores and cavities are temperature controlled through independent water channels. The first color cavity molds non-appearance surface of the first color molded part. When injecting second color molded part, high-temperature melt will melt non-appearance surface of the first color molded part, then cool and solidify again. Therefore, the first color cavity can be set to a relatively low cooling temperature to shorten cooling time. A separate cooling water channel is set up at hot runner nozzle and is not connected in series with other water channels, which is conducive to heat dissipation in hot nozzle area. Diameter of all cooling water channels is φ11.5 mm, diameter of water well is φ18 mm, and distance between water channels is 45~50 mm.

Structure of thick-walled light guide two-color injection mold is shown in Figure 11. It is produced by a single-color injection molding machine with an injection motorized template and a turntable. During the first injection, needle valve of first color hot runner nozzle is opened, and needle valve of second color hot runner nozzle is closed; when injection of first color molded part is completed and mold opening movement begins, plastic part is pressed by slider and fixed on the side of first color movable mold. After slider completes its stroke driven by fixed mold inclined guide pillar, it rotates 180° together with movable mold and plastic parts under positioning of spring positioning mechanism, then mold is closed, realizing cavity change action of two-color mold.

 

Figure 11 Two-color injection mold structure
1. Injection molding machine fixed mold plate 2. First color fixed mold 3. First color movable mold 4. Injection molding motorized mold plate 5. Moving mold turntable 6. Second color movable mold 7. Second color fixed mold
During second injection, the first and second color hot runner nozzle needle valves are opened at the same time. The second color cavity completes two-color molding of first and second color molded parts. At the same time, the first color cavity is injected into first color molded part to prepare for injection molding of next two-color plastic part. In mold closing stage when second color molded part starts to be injected, slider moves to initial position under action of fixed mold inclined guide pillar to press plastic part. When second injection is completed and mold is opened, injection molding motorized mold plate drives two-color mold movable mold to retreat, slider mechanism realizes demoulding of two-color plastic part with undercut structure on movable mold side, and at the same time fixes two-color plastic part on second color movable mold core. At this time, first color movable mold is not pushed out, and second color movable mold is pushed out. After two-color plastic parts are pushed out, robot picks up the parts. Finally, two-color mold movable mold rotates 180° counterclockwise and then closes, starting two-color injection cycle.