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ISSN : 1598-6721(Print)
ISSN : 2288-0771(Online)
The Korean Society of Manufacturing Process Engineers Vol.20 No.9 pp.20-27
DOI : https://doi.org/10.14775/ksmpe.2021.20.09.020

Effect of Injection Molding Conditions of Effective Surface Properties of F-theta Lens

Yong-Woo Park*, Qi Zhang**, Seong-Min Moon***, Sung-Ki Lyu***#
*Department of Convergence Mechanical Engineering, Gyeongsang National University
**R&D Department, Zhejiang Shuanghuan Driveline Co., LTD., China
***School of Mechanical & Aerospace Engineering, Gyeongsang National University
#Corresponding Author : sklyu@gnu.ac.kr Tel: +82-55-772-1632, Fax: +82-55-772-1578
20/06/2021 16/07/2021 18/07/2021

Abstract


The effective surface of lens was studied for injection molding process and to enable mass production of f-theta lens, which is the primary component of laser printers and laser scanning systems. Injection molding is an optimal method if f-theta lens is frequently used for the mass production of plastic lenses as an aspherical lens that requires ultra-precision. A uniform injection molding system should be maintained to produce high quality lenses. Additionally, to maintain these injection molding systems, various factors such as pressure, speed, temperature, mold and cooling should be considered. However, a lens with the optical characteristics of an f-theta lens can be obtained. The effects of melting and cooling of plastic resin on the effective surface of f-theta lenses and the numerous factors that affect the injection molding process were studied.



사출 성형 조건이 에프세타 렌즈의 유효면 특성에 미치는 영향

박 용우*, 장 기**, 문 성민***, 류 성기***#
*경상국립대학교 대학원 융합기계공학과
**중국 절강쌍환전동유한회사
***경상국립대학교 기계공학부

초록


    © The Korean Society of Manufacturing Process Engineers. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Laser scanning systems of general printers and las er multifunction printers convert digital data to light information when data are transferred in the form of digital signals to these laser scanning units. Core co mponents of laser scanning units include an f-theta l ens, polygon mirror, cylindrical and collimator lens, laser diode, laser control circuit board, and case. La ser scanning units are operated by transferring light dots produced from a laser diode to a drum through proper light paths; this operation constitutes the mos t essential technology of these units. A collimating l ens is used to transform a diffuse beam produced fr om a laser diode into a collimated beam by preventing diffuse beam scattering. To form an image of th e collimated beam produced from the collimating le ns on the surface of a polygon mirror, a beam prod uced from a cylindrical lens should be concentrated in a sub-scanning direction. The beam image produc ed from the cylindrical lens is formed on the surfac e of a polygon mirror through this process, and the beam is spread again and incident on the f-theta len s. The term “f-theta lens” was coined based on the principle that an angle of a polygon mirror and the tan value of focal length f are used to calculate the width of a diffuse beam on a drum. This aspheric l ens requires high precision, and injection molding is frequently used to produce these aspheric lenses in l arge quantities. An injection molding system consists of the following: a resin dryer that can dry a plasti c pellet; an injection molding machine that melts, fil ls, and compresses a certain amount of resin; a mol d that has the shape of a lens; a temperature contro l unit that maintains constant internal and external t emperatures of this mold: a self-driving robot to tak e out product; and an automatic resin supply unit. P erformance of an injection molding system should b e stably maintained to produce high-quality lenses th at have desirable optical properties. Noh(1) designed a system of a laser scanning unit based on A3, and Yoo(2) analyzed the application of injection molding to an optical system of a laser scanning unit. Rober t E(3) examined optical scanning, and Park(4) conduct ed a numerical analysis for applying injection moldi ng to an aspheric lens used in an image pick-up un it. Accordingly, numerous studies(5~17) have been con ducted to enhance the performance and properties of injection molding. This study analyzed the effects of injection molding on the properties of the effective surface of an f-theta lens.

    2. Injection molding

    Injection molding is a process in which resin is melted at a high temperature in a mold and cooled to form a specific shape. As high temperature and pressure generate stress in a lens during injection molding, these conditions are regarded as factors that hamper optical properties. In particular, f-theta lenses are thicker than general injected objects, and the thicknesses of these lenses are irregular. For this reason, hydraulic injection molding machines cannot overcome packing issues, thus frequently generating short shots. To solve this problem, this study used an electric hydraulic injection molding machine (sodic), which is shown in Fig. 1. Injection molding was also performed with a sufficient amount of packing time to prevent a short shot, and supportive devices such as an injection material dryer were utilized to minimize variables that might occur in the injection process.

    Table 1 shows values established for basic conditions for an injection molding machine used to perform injection molding of an f-theta lens. These include the specifications of the machine, cavities, materials, and a filling time. First, values for the injection molding machine were established for the first cycle of injection molding. Injection molding was then conducted based on these established values, and the molding results were measured. In addition, values for the injection molding machine were adjusted for the second through fifth cycles. Injection molding was performed based on these adjusted values, and the results were measured to assess the effects of injection molding on the properties of the significant surface of the f-theta lens.

    Table 2 lists the established values for the injection molding machine. The upper-limit pressure was fixed as 1500 kg/㎠, upper-limit time as 40 s, rotation rate for measurement as 50 %, amount of suck back as 2 mm, velocity ratio for suck back as 30 %, delay times for injection and measurement each as 0 s, purging velocity as 30 mm/s, number of purging iterations as 5, retreat time required for an injection unit as 0 s and overall delay time as 0 s. Values related to pressure , cooling time , and back pressure were adjusted based on the injection molding properties of the f-theta lens , and experiments were conducted based on existing and adjusted values for the first to fifth cycles.

    Table 3 shows the set values associated with the change in mensuration. The reference values for the first, second through fourth, and fifth measurements were set as 46 mm, 65 mm, and 66 mm, respectively . As distance values for S1 to S8 differed in terms of measurement criteria, it was estimated that these differences in values may affect the quality of the f-theta lens.

    Table 4 lists velocity-related values established according to value adjustments. These values were calculated according to the criteria for measurement and adjusted values. Tables 5 and 6 list packing pressure contents. As resin is injected, melted, and filled in a mold under high pressure, pressure is applied in a reverse direction. For this reason, great pressure is applied until melted resin becomes solidified. In this regard, packing pressure has significant effects on the injections of lenses used in optical systems. In this study, the reference value for packing velocity was set as 8 mm/s.

    Tables 5 and 6 list values related to packing pressure time and packing pressure, respectively. Regarding the opening and closing of a mold, the capacity of the injection molding machine was established as 100 tons. In addition, clamping forces for the first to third and fourth to fifth cycles of injection molding were set as 50 % and 60 % of the entire ratio(100 %), respectively. Conditions for opening and closing of a mold were determined based on these set values, as shown in Table 7. To protect a mold, the upper-limit torque was set as 15 %, protection time as 5 s, comparative numerical value as 0.15 mm, and standard numerical value as 0.30 mm.

    3. Results and Discussions

    Research on injection molding is inevitably required to produce aspheric f-theta lenses in large quantities. Although injection molding is the optimal process for mass production, it can have negative effects on optical performance. The focus of this study was to analyze root mean square and peak to valley and the properties of a significant surface of an f-theta lens. Accordingly, it should be noted that the optimal values of root mean square and peak to valley for the core of a mold as derived from previous studies were applied as reference values in this study.

    Figs. 2 and 3 show the first and second correction values for the incident surface, respectively, with Figs. 2(a)(b) and 3(a)(b) indicating the corresponding correction values for the flat surface and sides. Figs. 4 and 5 show the first and second correction values for the exit surface, respectively, Figs. 4(a)(b) indicating the corresponding correction values for the flat surface and sides. These correction values were reflected in injection molding.

    Figs. 6(a) to 10(a) and Figs. 6(b) to 10(b) show measured values for the incident and exit surfaces, respectively. Injection molding was conducted based on different conditions for injection molding of an f-theta lens. In the first cycle of injection molding, root mean square and peak to valley for the incident and exit surfaces were measured as 0.2155 ㎛ and 3.5988 ㎛ and as 0.2633 ㎛ and 4.1952 ㎛, respectively. The second to fifth cycles of injection molding were conducted based on the correction values. In the second cycle of injection molding, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1895 ㎛ and 2.9156 ㎛ and as 0.2189 ㎛ and 3.9419 ㎛, respectively. In the third cycle of injection molding, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1988 ㎛ and 2.5163 ㎛ and as 0.2774 ㎛ and 3.9748 ㎛, respectively. In the fourth cycle of injection molding, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1855 ㎛ and 2.0495 ㎛ and as 0.2633 ㎛ and 4.1955 ㎛, respectively. In the fifth cycle of injection molding, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1902 ㎛ and 2.6711 ㎛ and as 0.2165 ㎛ and 3.1953 ㎛, respectively. Through these measurement results, this study verified that root mean square and peak to valley for f-theta lenses can vary under different injection molding conditions. It is expected that the injection data obtained in this study can be effectively used for mass production of f-theta lenses.

    4. Conclusions

    This study analyzed the effects of injection molding conditions on root mean square and peak to valley to examine the optical properties of the significant surface of an f-theta lens used in a laser scanning unit. It is anticipated that data of injection molding for f-theta lenses can be used as a basis for enhancing the optical performance and mass production of f-theta lenses.

    • 1. In the first cycle of injection molding for an f-theta lens, root mean square and peak to valley for the incident and exit surfaces were measured as 0.2155 ㎛ and 3.5988 ㎛ and as 0.2633 ㎛ and 4.1952 ㎛, respectively.

    • 2. In the second cycle of injection molding for an ftheta lens, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1895 ㎛ and 2.9156 ㎛ and as 0.2189 ㎛ and 3.9419 ㎛, respectively.

    • 3. In the third cycle of injection molding for an f-theta lens, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1988 ㎛ and 2.5163 ㎛ and as 0.2774 ㎛ and 3.9748 ㎛, respectively.

    • 4. In the fourth cycle of injection molding for an f-theta lens, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1855 ㎛ and 2.0495 ㎛ and as 0.2633 ㎛ and 4.1955 ㎛, respectively.

    • 5. In the fifth cycle of injection molding for an f-theta lens, root mean square and peak to valley for the incident and exit surfaces were measured as 0.1902 ㎛ and 2.6711 ㎛ and as 0.2165 ㎛ and 3.1953 ㎛, respectively.

    Acknowledgments

    This study was supported by the Basic Science Research Program through the NRF of Korea (NRF) funded by the MEST (NRF-2020R1A2C1011958).

    Figure

    KSMPE-20-9-20_F1.gif
    Injection molding process
    KSMPE-20-9-20_F2.gif
    1st correction of the incident surface
    KSMPE-20-9-20_F3.gif
    2nd correction of the incident surface
    KSMPE-20-9-20_F4.gif
    1st correction of the exit surface
    KSMPE-20-9-20_F5.gif
    2nd correction of the exit surface
    KSMPE-20-9-20_F6.gif
    1st result injection of f-theta lens
    KSMPE-20-9-20_F7.gif
    2nd result injection of f-theta lens
    KSMPE-20-9-20_F8.gif
    3rd result injection of f-theta lens
    KSMPE-20-9-20_F9.gif
    4th result injection of f-theta lens
    KSMPE-20-9-20_F10.gif
    5th result injection of f-theta lens

    Table

    Injection molding conditions
    Temperature change during injection molding
    Setting according to change in mensuration
    Setting according to speed change
    Setting according to holding pressure time
    Setting according to holding pressure
    Setting according to mold opening&closing

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