Technical conditions for high-gloss, seamless injection molding
★High-gloss, seamless injection molding technology
★Defects and improvement examples in high-gloss, seamless injection molding
The principle of high-gloss, seamless injection molding technology
High-gloss, mark-free injection molding (rapid heat cycle molding, RHCM), also known as rapid heating and cooling injection molding, variable temperature injection molding, or high-gloss paint-free injection molding (referred to as high-gloss, mark-free injection molding in this book), is a method that uses high-temperature superheated water (steam) to heat the mold and low-temperature cold water to cool the mold. The mold surface is rapidly heated to above the glass transition temperature of the plastic using 180°C superheated water (steam), and then the molten plastic is quickly injected into the mold cavity.
After the mold cavity is filled, cold water is immediately used as a cooling medium to rapidly cool the mold, causing a rapid decrease in the mold surface temperature, thereby changing the surface characteristics of the plastic part and resulting in plastic parts with extremely high surface quality. The process flow of high-gloss, mark-free injection molding is shown in the figure.
High-gloss, seamless injection molded plastic parts are mostly "paint-free." "Paint-free" means that the plastic parts do not require painting after molding; a single injection molding process directly achieves the "glossy" appearance required for painted surfaces. This is a truly environmentally friendly, low-VOC (volatile organic compounds) product, and is therefore widely used in products such as air conditioner casings, television decorative parts, refrigerator handles, car grilles, and automotive interior and exterior trim.
Compared to traditional injection molding, high-gloss injection molding offers many advantages, including:
Eliminates weld lines on the product surface;
01
Eliminates silver streaks and flow marks; even with glass fiber-reinforced materials, it largely prevents the appearance of exposed fibers;
02
The process is environmentally friendly, and the surface finish of the product can reach a mirror-like level, avoiding the environmental pollution caused by post-molding painting processes;
03
Due to good melt flowability, it significantly improves the molding quality of thin walls and ribs, thereby increasing the product's pass rate.
04
Steel selection for high-gloss, seamless injection molds
The main factors affecting high-gloss, blemish-free injection molds include the selection and processing of mold materials, the design of the mold's cooling channels, and temperature control. Among these, the selection of mold materials is a crucial part of achieving the rapid heating and cooling process. High-gloss, blemish-free injection molds have the following requirements for steel:
Firstly, the cooling water channels in high-gloss, seamless injection molds are very close to the mold cavity, making them particularly susceptible to cracking. Therefore, the steel must have sufficiently high toughness.
Firstly, the cooling water channels in high-gloss, seamless injection molds are very close to the mold cavity, making them particularly susceptible to cracking. Therefore, the steel must have sufficiently high toughness.
Thirdly, because the mold is subjected to alternating thermal stress from rapid cooling and heating during injection molding, the mold parts are prone to cracking. In most cases, the cooling water channels are corroded first, producing microcracks, which then expand due to production stress, ultimately leading to cracking of the mold parts.
Therefore, high-gloss, seamless injection molds require mold steel with high toughness, corrosion resistance, thermal fatigue resistance, good thermal conductivity, and a suitable coefficient of thermal expansion.
To meet the requirements of high-gloss, seamless injection mold steel, we have also summarized the experience of several experienced technicians, which is presented below.
S136 mold steel from ASSAB (Swedish Steel)
This grade of mold steel is refined by the electroslag remelting (ESR) method, resulting in a pure and fine microstructure. It possesses excellent corrosion resistance, excellent polishability, excellent wear resistance, excellent hardenability, and excellent toughness and ductility. The mass fractions of the elements in this grade of mold steel are: C (0.38%), Si (0.8%), Mn (0.5%), Cr (13.6%), P (<0.03%), S (<0.03%), etc. Its high chromium content gives it stainless steel characteristics, making it a corrosion-resistant mirror mold steel. It can be used in high-gloss, mark-free injection molds, components for high-demand food industry machinery, and for molding plastics such as PVC, PP, and PE. The mold cooling channels do not corrode as easily as those of ordinary mold steel; its heat conduction characteristics and cooling efficiency remain stable throughout the mold's lifespan, ensuring consistent performance over time.
NAK80, a high-mirror-finish plastic mold steel from Daido Steel Co., Ltd. of Japan.
The NAK brand name originates from the addition of nickel (Ni), aluminum (Al), and copper (Cu, from the German word Kupfer) to the steel. The first letters of these three alloying elements were combined to form "NAK." Since NAK80 was developed in 1980, it was named "NAK80" after its year of origin. For over forty years, this steel grade has been a best-selling product in the industry. The mass fractions of the elements in NAK80 steel are: C (0.15%), Ni (3.0%), Al (1.00%), Cu (1.00%), Si (0.30%), Mn (1.50%), Mo (0.30%), and Cr (0.30%). NAK80 steel possesses excellent mirror-finish machinability (easy grinding and polishing), is pre-hardened (no heat treatment required), has high wear resistance (high hardness), good machinability (easy cutting), good weldability, and is suitable for aesthetic design (easy etching). It has consistently been one of the preferred mold steels for high-gloss, seamless injection molding.
In recent years, Japan's Daido Steel Co., Ltd. has developed PAT868S, a special high-performance plastic mold steel. The elemental composition is kept confidential, but it is an improved version of the original SKD61. Through special smelting, it combines the advantages of 42 types of stainless steel and H13 hot-work steel, while also possessing high toughness and excellent mirror polishing properties, achieving a mirror finish of up to 10000 grit (#10000). It is rust-resistant and easy to maintain; it has the same toughness as SKD61, effectively preventing cracking; and compared to PAT868, it achieves a higher hardness (53 HRC), with a supplied hardness of ≤ 229 HB, and better wear resistance and hardenability. Therefore, PAT868S is suitable for long-term production of complex structures and high-surface-finish plastic injection molds, inner mold parts, inserts, angled ejector pins, and sliding mechanisms (side core pulling mechanisms), as well as high-gloss, mark-free injection molds and electrically heated molds.