Factors that negatively affect high-gloss, seamless injection molding
In the actual production of high-gloss, defect-free injection molding, several factors can affect the appearance and performance of the finished product
Common undesirable factors include:
① Foreign matter in the mold gate: such as gate residue, oil stains, carbides, dust, etc.
② Mold cavity surface damage: Improper handling during injection molding, film application, and packaging processes can lead to damage to the mold cavity surface, which directly affects the appearance quality of the product.
③ Silver streaks on the product surface: caused by insufficient drying of plastic raw materials, overheating of the barrel, foreign matter contamination, or clogged mold vents.
④ Surface contamination of the product: During the process of printing patterns on the plastic part surface, insufficient operator skill, improper paint or screen selection can lead to surface contamination of the plastic part.
The image shows a set of comparative photographs illustrating the phenomenon of mold surface damage caused by foreign objects in the gate. In Figure a), the area around the mold gate is in a normal state, while in Figure b), numerous scratches are visible around the gate. These scratches were caused by fragments of the gate material falling onto the mold cavity surface during the gate cutting process and not being removed promptly. When the mold closed, these fragments were pressed directly into the mold cavity surface.
Examples of undesirable practices and improvements made
The front panel of an air conditioner indoor unit (abbreviated as "panel") is a primary functional and decorative component of the air conditioner. To achieve an aesthetically pleasing effect, most panels are required to have a high-gloss, mark-free finish. The panel injection-molded by a certain air conditioner manufacturer is shown in Figure 4-10. The product has overall dimensions of 890 mm × 230 mm × 68 mm and is made of ABS material. The main body area has a wall thickness of 2.6 mm, but the display area (used for installing a small LCD screen) has a thinner wall thickness of 1.4–1.6 mm. The outer surface of the product is required to be high-gloss and mark-free, with no other obvious defects.
The front panel of the indoor unit of an air conditioner (referred to as the panel) is a key functional and decorative component of the air conditioner. To achieve an aesthetically pleasing appearance, most panels require a high-gloss, blemish-free finish. The panel manufactured by a certain air conditioner factory through injection molding is shown in the figure. The product dimensions are 890mm × 230mm × 68mm, and the material is ABS. The main body has a wall thickness of 2.6mm, but the display area (for installing a small LCD screen) has a thinner wall thickness of 1.4–1.6mm. The outer surface of the product is required to be high-gloss and blemish-free, without any other obvious defects.
The filling of the molten material is related to the wall thickness of the molded part. Within the conventional wall thickness range, the thinner the wall thickness, the thinner the flow layer, the greater the flow resistance, and the higher the injection pressure required. Therefore, for similar regions with different wall thickness distributions, the molten material is more likely to fill the thicker-walled areas, while the flow slows down or even stagnates in the thinner-walled areas. According to this flow pattern, the display area of the panel is a thin-walled area and is also a challenging aspect of the injection molding of this part. Efforts should be made to mitigate the effects of flow stagnation in this area to avoid the appearance of weld lines.
If the display area of the panel were designed in the center, it would facilitate balanced melt filling. However, the display area of this panel is located on the right side. When the melt flows and fills from the center to the sides, a significant deceleration or even stagnation of melt flow occurs in the thin-walled display area, ultimately leading to appearance defects such as flow marks and noticeable color differences in the final product. Therefore, placing the thin-walled display area on the right side increases the molding difficulty and requires optimizing the gate position. This mold has two gates, one on the left and one on the right, symmetrically distributed. Clearly, the position of the right gate needs to be adjusted.
To address the injection molding defects observed in the panel, the gate location of the mold was modified, effectively eliminating the weld line defects in the thin-walled display area of the panel. Furthermore, the modified gate location resulted in a more consistent distance to the melt front, leading to better pressure transmission during the holding phase. This was beneficial in eliminating issues such as shrinkage marks and color variations on the product surface, ultimately achieving a high-gloss, defect-free finish. Based on the above analysis, the mold and injection molding process were optimized and validated through trial molding. The results showed that the panel's surface had no significant weld lines or shrinkage marks, and the overall appearance was bright and glossy, meeting the quality requirements. The actual trial sample is shown in the figure.