Key design considerations for high-gloss, seamless plastic parts
① The draft angle of the high-gloss surface of the product should be at least 7° on each side;
② The thickness of the reinforcing ribs at the base must be controlled to within 40% of the main wall thickness; otherwise, surface sink marks will be more noticeable under high-gloss conditions;
③ Studs should be designed on non-visible surfaces whenever possible, and the draft angle should be increased to reduce surface shrinkage.
Selection of high-gloss, seamless injection molding materials
In high-gloss injection molding technology, material selection is a crucial step. Materials suitable for this process generally possess the following characteristics:
Good material flowability, allowing for better replication of the mold surface, reduced shear stress, and improved weld lines;
01
Good scratch resistance, meaning good surface hardness; the surface hardness of the plastic should reach at least H on the pencil hardness scale;
02
Good thermal stability, minimizing the generation of volatile substances, especially in flame-retardant materials, to prevent mold corrosion and reduce surface haze on the product;
03
Good inherent gloss;
04
Good toughness and a certain degree of rigidity.
05
Suitable materials for high-gloss injection molding include ASA, ABS, PA, PC, PMMA, PC+ABS, PMMA+ABS, and PC+ABS+GF (glass fiber). Among these, ABS has the lowest hardness, while PMMA+ABS has the highest hardness.
ABS is commonly used in the production of various surface casing and decorative parts. Adding metal powder to ABS solves the problem of uneven distribution of metal powder in the plastic melt, resulting in a beautiful surface with a metallic effect. Adding 10% PC or PMMA to ABS can improve the surface gloss, surface flatness, transparency, hardness, and dimensional stability of the product, allowing it to replace high-precision metal parts. PC material, after molding, maintains its gloss and transparency without deformation, allowing for the production of transparent, curved products (such as lampshades) without dimensional shrinkage, thus not affecting the viewing angle and ensuring the focusing angle of the light. The performance parameters of commonly used materials are shown in the table below.
| Test Item | Test Standard | ABS | PMMA+ABS | PC | PC+ASA | PC+ABS |
|---|---|---|---|---|---|---|
| Melt Flow Rate (g/10min) | ASTM D 1238 | 20 | 17 | 15 | 21 | 19 |
| Vicat Softening Temperature (°C) | ISO 306 | 106 | 120 | 100 | 115 | 125 |
| Notch Impact Strength (°) | GB 8807 | 80 | 100 | 96 | 90 | 98 |
| Tensile Strength (MPa) | ISO 178 | 60 | 90 | 55 | 70 | 73 |
| Hardness | GB 2411 | B | H | HB | HB | HB |
The structure of high-gloss, seamless injection molds
As a core component in the high-gloss, seamless injection molding process, the quality of the mold has a direct and crucial impact on the quality of the molded product. The following points should be considered during the design of high-gloss, seamless molds.
① In high-gloss, seamless molds, the temperature of the front mold (stationary mold) is higher than that of the rear mold (moving mold). Therefore, the expansion of the front mold insert is greater than that of the rear mold. To prevent the front mold from being squeezed after expansion, for flat products with insignificant side pressure, the front and rear cores can be designed without locating features. If locating features are needed to prevent side pressure, they should be designed according to the principle of the high-temperature side enclosing the low-temperature side.
② Under high temperature, high pressure, and cyclic stress, thin steam hole walls can easily cause cracking of the mold insert. Therefore, the minimum thickness of the steam hole wall should be at least 1.5 times the diameter of the steam hole, as shown in Figure 4-6. At the same time, sharp corners should be avoided in high-temperature cores, and all groove bottoms should be treated with the largest possible rounded corners (R-corners).
③ The cavity inserts of high-gloss, mark-free injection molds must be insulated with heat insulation plates. The installation of the heat insulation plates is shown in the figure.
④ The A-plate (fixed mold plate) or B-plate (moving mold plate) on the high-temperature side of the high-gloss, mark-free injection mold needs to have cooling channels added near the guide pins or guide bushings to enhance cooling.
⑤ To prevent damage from thermal expansion and compression, high-temperature cores exceeding 1300mm in length should be installed using a central positioning method, meaning that the area around the core needs to be relieved to allow for expansion.
Mold heating and cooling systems
High-gloss, seamless injection molding requires high mold temperatures (generally around 80-130°C, or even as high as 180°C). Mold heating is typically achieved through two methods: steam heating (saturated or superheated steam, reaching temperatures up to 180°C) and electric heating rods (or tubes). Steam heating uses a rapid heating and cooling mold temperature controller to supply steam to the mold during the injection molding process, thus rapidly increasing the mold temperature.
After the holding pressure stage, high-gloss, seamless injection molding switches to cooling water for rapid cooling, quickly reducing the mold temperature to 50-70°C, or even lower. Holding pressure molding at higher mold temperatures helps eliminate defects such as weld lines, flow marks, and internal stresses in the product, while cooling at lower mold temperatures shortens the molding cycle. Because the mold undergoes heating and cooling cycles during operation, the mold steel must be able to withstand the fatigue caused by rapid temperature changes; to prevent heat loss from the mold, an insulation plate is usually installed on the fixed mold side.
High-gloss, seamless injection molding die trial run
① The front mold (stationary mold) should be thoroughly cleaned with a mold cleaning agent. It is strictly forbidden to use an air gun to blow on the mirror-finished parts of the front mold, or to wipe the mirror-finished parts with rags or other objects.
② For cleaning the rear mold (moving mold), first wipe off the surface rust inhibitor with a rag, then clean it with a cleaning agent, and finally wipe the parting surfaces (non-high-gloss surfaces) of the front and rear molds clean with a rag.
③ When starting mold trials, after ensuring that the injection molding machine and mold are functioning normally, first heat the barrel. After the barrel temperature reaches a certain level, turn on the hot runner (if any) and set it to a temperature suitable for the material, while preventing the plastic from overheating and decomposing.
④ Check whether the melt from the barrel (injection unit) and the hot runner is carbonized during air injection. After confirming that the plastic heating is normal, turn on the steam for the high-gloss surface to heat the mold, and then start injection molding.
⑤ After the injection molding process is completed, the surfaces of the front and rear molds must be cleaned with a special cleaning agent, and finally coated with a special rust inhibitor.
Mold temperature control units and related equipment
High-gloss, seamless injection molding requires a rapid heating and cooling mold temperature controller to directly control the mold temperature. Therefore, a cooling water tower and an air compressor are needed to assist the mold temperature controller. Additionally, the water quality of ordinary tap water is often too hard, easily causing scale buildup and clogging the water channels. Therefore, a water treatment system is also needed to convert hard water into soft water. In summary, the mold temperature controller and its peripheral equipment for high-gloss, seamless injection molding are as shown in the diagram.