Plasticizing and injection molding processing tech

2022-10-19
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Plasticization and injection molding processing technology of CD

I. raw materials and injection molding process of CD

in order to obtain a good injection molding process, we first need to strictly control the raw materials for molding, and different raw materials use different molding processing processes. At present, the raw materials used for optical discs generally fall into two categories: Polycarbonate (PC) and methyl methacrylate (PMMA). Polycarbonate is often used

polycarbonate is generally obtained by polycondensation of bisphenol A and phosgene. Polymers with different degrees of polymerization can be obtained by controlling acidity or feeding ratio. Polycarbonate with different uses can be obtained by controlling different molecular weights and adding different materials such as fillers and release agents. The materials used for optical discs require good transparency, high dimensional stability, high replication rate and low birefringence, good mechanical and processing properties, and sufficient toughness to limit microscopic cracking, but they still need to maintain good fluidity and high purity. In this way, the water absorption, light transmittance, refractive index, tensile strength, glass transition temperature, melt index Molding shrinkage and impurity content are controlled; In order to get better molded products and higher yield, the particle size must also be controlled

the main chain of bisphenol a type PC resin we used contains aromatic heterocycles, which improves its strength and modulus. However, there are the following kinds of micro bubbles caused by insufficient plasticization in the molding process: some impurities are often mixed in the process, and it is more difficult to avoid that during the molding process, due to the different cooling rates in the substrate surface, the surface materials contact the mold wall with low temperature, and quickly cool and solidify into a hard shell, However, the material inside the workpiece is still in the molten state. With its cooling and shrinkage, internal stress will be generated inside the substrate, and then small crazes and even cracks will be formed. Cracks will appear on the surface of the substrate. Although these defects are very small, some of them can't even be seen by the naked eye, they have become the fatal weakness to reduce the mechanical strength of the optical disc. Due to the existence of appearance defects, they also affect the internal and external quality of the product. Therefore, It is very necessary to choose a better molding process to minimize the generation of internal stress

it is worth noting that the physical parameters of PC resins of different brands (produced by different manufacturers) are also different, so corresponding adjustments should be made when determining the injection molding process

II. Optical disk injection molding machine and injection molding process parameters

optical disk injection molding machine is a precision injection molding system, including plasticizing injection unit, mold closing unit, take out manipulator and monitoring display unit, and electro-hydraulic control system

the plasticizing injection unit is composed of cylinder, screw, injection unit, nozzle, heating system, water cooling and power system (hydraulic or servo motor). After hours of drying, PC pellets enter the plasticizing cylinder from the feed hopper and are heated to the molten state in the cylinder. The molten material is extruded by the screw rotation and injected into the mold cavity from the nozzle according to the set amount. The barrel temperature is controlled by PID (proportional integral differential) controller, and the actions and parameters in the injection process are controlled by programmable hierarchical control

the mold closing unit is composed of static and dynamic mold with precise mold temperature control, mold opening and closing mechanism (hydraulic or servo motor) and nozzle cutting and ejection system. In general CD injection molding machines, the static mold part is on the side of the injection molding unit, equipped with a mirror module, and the dynamic mold part is equipped with a stamper (Note: some machines are the opposite). After mold closing, there is a mold cavity between the dynamic and static mold. The molten PC material is injected into the mold cavity and formed by pressure maintaining and cooling. Then the cutting unit cuts and punches the central hole to form a cd/dvd disc base. The mold temperature of CD is ℃, and that of CD-R is ℃. The mold temperature is controlled by two thermocouples clamped on the mold and the mold temperature controller

the disc removal manipulator is controlled by two axis servo motors. After the mold is opened, the disc is quickly absorbed and sent to the next process

the monitoring and display unit displays all injection molding process parameters, such as the starting position of packing, speed/pressure switching position, pressure maintaining end position, pressure maintaining curve, injection speed curve, mold locking force and time, screw position, packing time, plasticizing temperature, mold opening and closing position and speed, mold opening and closing time and other data. Closed loop program is used to control quality parameters. Monitor the abnormal state of the equipment, guide the machine to stop safely, display parameter modification records, and archive different documents for different process settings. The screen display also includes maintenance guidance and other functions

the main technical parameters that affect the optical disk injection molding machine are:

How do we use computer and network technology in our physical testing laboratory? Connecting influence parameters indirect influence parameters

-- mold locking force -- raw material

-- injection speed -- each injection volume

-- holding pressure -- melting temperature

-- speed/pressure switching point -- melting uniformity

-- mold temperature

-- barrel temperature

-- hydraulic oil temperature

-- time interference variable of single process step

-- screw revolution -- friction -- wear condition

-- screw back pressure ambient temperature - required by surface properties More scientific research institutions

a forming cycle of optical disc substrate is shown in Figure 1. Generally, there are seven procedures, including mold opening and closing, filling, pressure maintaining, plasticizing, cooling, mold opening, and disk base removal. It can be seen in Figure 1 that the molten PC material fills the mold cavity from the beginning of mold closing. In order to obtain good reproduction quality, the extruder Market in China has developed rapidly, and the method of using different injection speeds at different positions, that is, the method of segmented injection, is adopted; Pressure maintaining adopts subsection pressure maintaining by adjusting the pressure maintaining time and pressure; Plasticization also adopts the method of segmentation; Cooling starts from plasticizing delay to mold opening

Figure 1: a molding cycle

(to be continued)

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