How to adjust the machine for injection molded parts that need to be sprayed with silver paint
Injection molded parts that require silver paint have extremely high surface quality requirements. Any minor imperfections (such as sink marks, weld marks, and scratches) will be magnified after painting, affecting the final product’s appearance. Therefore, during the injection molding process, precise machine adjustments must be coordinated with the painting process to control the surface quality of the plastic parts from the source, laying a good foundation for the subsequent painting process. This coordinated machine adjustment not only ensures the dimensional accuracy of the plastic parts, but also pays attention to the surface flatness, gloss, and defect-freeness. It is a critical link between the injection molding and painting processes. During the machine adjustment process, comprehensive adjustments to parameters such as temperature, pressure, and speed are required, while optimizing the mold’s exhaust and cooling systems to produce injection molded parts that meet painting requirements.
Controlling sink marks on the surface of plastic parts is a primary task during machine setup. Sink marks are often caused by insufficient melt filling or uneven cooling shrinkage, and can be addressed by adjusting the injection pressure and holding pressure parameters. When concave sink marks appear on the part surface, the holding pressure and holding time should be appropriately increased to ensure adequate melt shrinkage during cooling and shrinkage. For example, for ABS parts, the holding pressure can be increased from 60% to 70%-80% of the injection pressure, and the holding time can be extended by 0.5-1 second to ensure that the melt fully fills every corner of the cavity. Also, the injection speed should be checked for appropriateness. If the speed is too slow, the melt cools too quickly during filling, which can easily form sink marks in thick areas. In this case, the injection speed in the mid-range can be appropriately increased to minimize premature cooling of the melt. Furthermore, low barrel temperatures can increase melt viscosity and reduce fluidity, which can also cause sink marks. Therefore, the barrel temperature should be gradually increased based on the material’s characteristics. For example, for ABS, the barrel temperature can be increased from 200°C to 220°C. However, be aware that excessively high temperatures may cause material decomposition and affect surface quality.
Eliminating weld marks is crucial for ensuring surface uniformity after painting. The density and glossiness at weld marks differ from other areas, creating noticeable marks after painting. During machine adjustment, the melt and mold temperatures can be increased to enhance melt fusion at the weld. For example, for PP parts requiring silver paint, the barrel temperature can be raised by 10-15°C and the mold temperature to 50-60°C. This allows the two melt streams to maintain a high temperature upon converging, reducing intermolecular resistance. Furthermore, optimizing the injection speed and employing a “slow-fast-slow” staged injection method should be employed. Increasing the injection speed at the weld mark increases the impact kinetic energy of the melt and promotes melt fusion. Furthermore, poor mold venting can lead to gas entrapment at the weld mark, resulting in bubbles or loose particles. Check for unobstructed venting. If necessary, add venting slots with a width of 0.03mm and a depth of 0.02mm at the weld mark to ensure smooth gas discharge and improve weld quality.
To reduce scratches and impurities on the surface of plastic parts, it is necessary to coordinate the machine adjustment operation from two aspects: raw material pretreatment and mold cleaning. Impurities in the raw materials or particles that are not fully melted will form scratches or protrusions on the surface of the plastic part. Therefore, before adjusting the machine, it is necessary to ensure that the raw materials are fully dried and screened. In the case of recycled materials, the recycled material ratio should not exceed 20%, and impurities must be removed through crushing and screening. During the machine adjustment process, it is necessary to check whether there are foreign objects in the nozzle and runner, and regularly clean the accumulated materials in the barrel to prevent aging and decomposition of materials from mixing into the melt. At the same time, adjust the parameters of the ejection mechanism to avoid scratches on the surface of the plastic part due to excessive ejection speed or unbalanced ejection. For example, the ejection speed can be reduced by 10%-15%, and the ejection force of each ejector can be ensured to be uniform. If necessary, wear-resistant gaskets can be added at the contact point between the ejector and the plastic part to reduce friction marks. In addition, the surface finish of the mold cavity directly affects the surface quality of the plastic part. The surface roughness Ra of the cavity must be no greater than 0.8μm. The cavity must be thoroughly cleaned before adjusting the machine to remove residual mold release agent or oil stains to avoid the formation of contamination points on the surface of the plastic part.
Controlling the warpage and dimensional stability of plastic parts ensures assembly accuracy after painting. Warpage can lead to inaccurate positioning of plastic parts during the painting process, affecting the uniformity of the paint film and, in severe cases, causing assembly interference. The internal stress of plastic parts can be reduced by optimizing the parameters of the cooling system during machine adjustment. For example, for plastic parts with uneven wall thickness, the cooling time can be extended to ensure uniform cooling of all parts of the plastic part. At the same time, the cooling water temperature in various areas of the mold can be adjusted, using a lower water temperature (such as 5-10°C) in thick wall areas to speed up cooling; and a higher water temperature (such as 20-25°C) in thin wall areas to reduce cooling differences. In addition, excessive injection pressure can generate greater internal stress within the plastic part, leading to warping after demolding. While ensuring full filling, the injection pressure should be appropriately reduced, usually by 5%-10%, and the mold temperature should be increased to reduce the generation of internal stress. For plastic parts with high dimensional accuracy requirements, it is necessary to adjust the holding time and cooling time through multiple mold trials to establish a corresponding relationship between parameters and dimensional changes, ensuring that the dimensional fluctuation of the plastic parts is controlled within ±0.05mm to meet the assembly requirements after painting.
