一、 Process procedures
The requirements for injection molding are different between "recordable disc" (CD-R) and "high-density recordable disc" (DVD+R/DVD-R). DVDR requires a double-sided bonding process, so there are high requirements for warping and thickness uniformity.
1. Production process of recordable optical discs
Incoming inspection → Raw material safety inventory → Plastic drying → Injection molding white film → Production line (dye spin coating, metal splashing, spin coating protective adhesive, hardening, online testing) → Off line testing machine (quality inspection) → DVDR adhesive → DVD-R pre engraving → Printing → Visual inspection → Shipping
2. Basic composition of injection molding system
Clean plastic pouring into the dryer → Plastic drying → Drying plastic conveying → Melting → Injection molding into the mold → Solidification → Punching → Removal from the mold → Cooling and shaping
Figure 1: Illustration of Disk Base Production
The general injection molding system only has two types of machines: "PC Dryer" and "Injection Molding Machine (IMM). If a mixing system is needed, additional machines such as" Screw Type Resin Mixer "will be added to mix the two different plastics. The purpose of mixing plastics is to perform coloring or plastic recycling operations, Apply various colors to the originally colorless and transparent disc base, but it is important to choose colored plastic and the appropriate amount added. Adding too much proportion will reduce reflectivity and affect the laser head's ability to read holes in the disc. Before use, it is necessary to carefully read the instruction manual of the plastic supplier. Depending on the target, the mixing and coloring operation should be carried out on the disc base of CD-R or DVDR, as the laser recording and reading wavelengths of the two are different.
There are two common practices in the industry regarding the use of recycled materials: directly purchasing processed and cut recycled materials, commonly referred to as sub brand materials, for drying production, or using a screw mixer to directly cut and mix the nozzle materials online for reuse. The order of difficulty based on production accuracy and process level is from DVD-R, CD-R, CD-ROM to V-CD. Based on experience, the recycling nozzle (Sprue) of a "recordable disc" can be cut and mixed again for the production of a "pre recorded disc" for recycling, while the remaining nozzle of the CD-ROM in the production of pre recorded discs can be used for the production of "cake boxes" or other plastic products for recycling.
Figure 2. Basic structure diagram of injection molding machine
二、 Introduction to Injection Molding Machine Equipment
1. Basic structure of injection molding machine
As shown in the figure, the injection molding machine has a "screw heating and pressure system" in the right half and a "forming mold and guide rail system" in the left half. Plastic particles are injected into the screw of the gun barrel from the hopper, gradually advancing towards the nozzle in sequence and melting under the heating of the heating belt. After the screw completes the backward filling, the melted plastic is injected into the mold according to the progress of the screw, and the plastic is quantitatively cooled and formed, and the water outlet is cut off, At this point, the mold is opened and the robotic arm is used to take out the formed disc base, which is then cooled and shaped appropriately in the subsequent cooling disc.
Figure 3. Nozzles in the front area of the screw
Figure 4. Appearance and components of hydraulic injection molding machine
Figure 5. Appearance and components of ToyoST50discPRO-S fully electric injection molding machine
2. Injection screw
The solid plastic raw material "Polycarbonate" enters the Screw Channel through the self feeding funnel, and through high-speed rotation of the screw, it produces a shear effect with the glue melting agent (Barrel), causing the plastic to be mixed and transported along the screw groove. The solid plastic is heated by the heating band on the outer side of the glue melting cylinder and the shear heat generated by the screw, causing the glue to melt into a molten state.
The functions of the injection screw include feeding, conveying, compression, melting, exhaust, homogenization and plasticization. The screw can be divided into four zones according to its function:
(1) Solid Conveying Zone
This section is connected to the feeding hopper, where plastic particles (PC rest) are tightly compressed to form a solid bed, which is subjected to the shear effect of the melt tube. In addition to heating the plastic and pushing it forward for transportation, this section also generates a solid bed.
2) Delay of Melting
This section is the area from the end of the solid conveying area to the formation of the Melt Pool. Its main feature is the formation of a thin film layer composed of molten glue on the surface of the solid bed and the melt cylinder. The molten glue in the thin film layer accumulates in the front screw groove due to the drag force of the melt cylinder. The pressure formed at the end of this area is large enough to deform the solid bed, and gradually forms a melting pool in the front screw groove. This section is the groove depth of the fixed screw groove, which is responsible for preheating, conveying and pushing plastic solids. It is necessary to ensure that the plastic begins to melt at the end of the feeding section, that is, to preheat to the melting point.
(3) Melting Zone
This section is a gradually decreasing screw groove with deep teeth, and its functions include melting, mixing, shear compression, and pressurized exhaust of plastic raw materials. Plastic will completely melt in this section and its volume will decrease, so the design of compression degree (compression ratio) is important. The solid melting zone is divided into two parts, one is "plasticizing", and the plasticizing mainly comes from the shearing effect of the melt tube; The other is "Melt Conveying", which transports the melted adhesive to the melting pool.
(4) Melt conveying zone
When all plastic is completely melted, it begins to enter this section and ends at the end of the plasticizing screw. This area aims to maintain a uniform temperature of the melt and stabilize the pressure and flow rate of the melted plastic. This section of the screw thread has a fixed groove depth, and its main functions include mixing, glue conveying, and metering. In addition, it must also provide sufficient pressure to maintain a uniform temperature of the glue and stabilize the flow rate of the molten plastic.
3. Mold
There are two main parts in the mold: static mold and moving mold. The mirror surface of the mold is embedded with a layer of reinforced metal (usually referred to as "Diamond Like Coating, DLC" in CD-R/DVD-R).
Carbon related materials, including diamond, graphite, glassy carbon, and recently highly anticipated C60, have various forms and are widely used in the industry. Carbon has various chemical binding forms, which create diverse mechanical, electrical, and optical properties. DLC is a carbon element material with new characteristics developed through the vacuum plasma thin film process developed in conjunction with semiconductor manufacturing technology. The arc discharge plasma in high fidelity air decomposes hydrogen carbide gas, causing ions and excited molecules in the plasma to accelerate and impact the mirror surface, forming a thin film. The DLC film has a fine and dense structure, with a very smooth black surface without crystalline particles. Its physical characteristics are: smoothness, wear resistance, impact resistance, and anti adhesion. It has a friction coefficient of 0.1, a heat resistance temperature of 450oC, a hardness of 4000 (HV), and wear resistance.
Table 1: Comparison between Diamond Film (DLC) and Other Metal Molds
Figure 6. Axxicon Mold Installing the Pressing Mold on the Moving Mold
4．压模位置对注塑条件的影响
对于压模置放的位置因模具设计而异，有的模具设计是将“压货(Stamper)装在静模，也有的模具设计将压模装在动模，例如Technoplas与Axxicon的差别，两者相对的关系与影响列于表中。
Figure 8. Technoplas mold with cylindrical bearings
The Technoplas mold has a designed cylindrical bearing next to the mold, which improves the alignment accuracy of the mold.
Table 2 Comparison between AXXicon Mold and Technopas Mold
5. Mold temperature machine
There is a heater inside the mold temperature machine that will heat the circulating water to the working temperature.
Mold temperature settings: CD50~55oC, DVD-ROM90~95oC, CD-R110oC, DVD-R120~125oC.
6. Differences between hydraulic and electric injection molding machines
The precision reproduction of injection molding machines is the technical pursuit goal of injection molding machine manufacturing factories, and it is also a key consideration for injection molding industry when purchasing equipment. Machine precision reproduction can ensure stable quality of formed products, standardized dimensions, and determined physical properties, thereby reducing assembly errors and improving productivity and yield; Moreover, the formed products can be quickly replaced for production, improving the yield rate, reducing unnecessary downtime, and meeting the requirements of the market and timely production; In addition, it can easily control the forming conditions, avoid excessive waste of filling materials, and also meet the needs of using automated equipment. In recent years, the development trend of injection molding machines should be high precision, high circulation, energy saving, low cost, high operability, space saving, and compliance with environmental requirements; And models such as fully electric injection molding machines are in line with the trend of future injection molding machines.
The fully electric precision plastic injection molding machine mainly replaces the hydraulic system with a servo motor drive system; The injection molding machine mainly has six action units, including locking, ejection, injection, metering, mold adjustment, and injection seat forward and backward. The fully electric injection molding machine uses servo motors for position, speed, and torque control of the injection, metering, locking, and ejection units. The mold adjustment and injection seat forward and backward can be completed using a general electric motor.
The fully electric injection molding machine needs to have a transmission mechanism that converts the rotational motion of the motor into linear motion, including a timing belt, ball screw, and toggle component combination. In the pressure control of injection molding and locking, a "high-resolution pressure sensor" (Load cell) is used for pressure feedback, Cooperating with the encoder to perform precise position measurement, the feedback current information is converted into voltage in the interface circuit module and handed over to the calculator for operation. The obtained compensation signal is then transmitted to the current amplifier to correct the action of the servo motor, thus achieving "closed loop control", which has stable and precise performance.
Due to the fact that the driving source of all electric injection molding machines is different from the injection machine of hydraulic actuators, traditional injection molding machine controllers are not suitable. Therefore, manufacturers mostly develop their own controllers and servo control laws, which not only need to perform dead cycle control of position, but also perform dead cycle control of speed and stroke simultaneously, greatly improving control stability and accuracy
For example, the Toyo fully electric injection molding machine uses a "toggle" to drive the "Moving Side" for "Mold Closing" and "Direct Clamping Force" work, while the fully hydraulic injection molding machine is directly driven by cylinder pressure to drive the mold closing and locking pressure work. As shown in the figure, the cylinder stroke of the hydraulic injection molding machine is directly proportional to the direct pressure stroke of the mold, However, due to the buffering of the elbow bar, the servo motor stroke and direct compression stroke of the fully electric injection molding machine exhibit nonlinear responses. Therefore, the original control experience on the hydraulic injection molding machine cannot be transferred to the fully electric injection molding machine, and the operator needs to have a new learning experience on the fully electric injection molding machine.
3、 Process Technology of Recordable Optical Disk Base
1. Differences between pre recorded and recordable discs
The difference in pit shape between general pre recorded discs (V-CD, CD-ROM) and high-density pre recorded discs (DVD-ROM) lies in the pit width, while the difference in pit shape between pre recorded discs and recordable discs lies in the pit and groove.
Figure 14. Basic structural differences between different generations of optical discs
When molten plastic flows in the mold, the resistance generated by the recordable disc track is greater than the pre recorded pit resistance. Therefore, the injection molding of recordable discs is based on the principle of using injection pressure method and increasing mold temperature. But when using a higher mold temperature, the durability level of the mirror coating should also be relatively improved, and the self high temperature cooling time should also be suitable to avoid warping. The so-called process window refers to the process adjustment area that meets the conditions of warping, birefringence, and replication index (intersection).
Figure 17. SEM shows the geometric data of the CD-R groove and the concavity of the top layer of the dye (dabs)
The typical geometry of the CD-R groove is as follows:
Plate base thickness: 1.16-1.18mm
Trench depth: 180nrn~200nrn, ± 5um
Trench width: 450nrn~550nrn, ± 5um
Slot wall inclination angle: 45o~55o, ± 5o
2. Characteristics and types of plastics
The melting and viscosity of PC materials are high, and the injection pressure resistance is high. However, when the temperature inside the pipe is too high or the retention time is long, it is easy to cause thermal decomposition, discoloration, and reduce physical properties. It is necessary to pay attention to the mold temperature ranging from 85oC to 120oC. For thicker finished products, it is not easy to form and is prone to residual stress, which can cause cracks in the future.
3. Drying
There are two types of plastic dryers: closed air circulation and convection. There are models that require cooling water and those that do not, with the focus on effectively drying plastic particles.
4. Transportation
The difference between using stainless steel pipes and PVC pipes lies in the friction and powder generated by plastic particles passing through the pipeline. The corner curvature of the feeding pipe and the internal polishing cleanliness are very important, and the pipeline should not be too long. Attention should be paid to the issue of air leakage. The plastic that has been dried for four hours should pass through the leaking pipeline. It should not take two minutes for the plastic to become damp by the moisture in the environment.
5. Plasticizing
The faster the screw rotation speed, the greater the shearing action. If the rotation speed is too fast, the degree of plasticization is insufficient, and it is easy to produce 'Color Patches'. The general operation starts with setting a lower speed and gradually increases to the target speed.
