Процесс окраски пластиковых деталей слайдов
Создано 08.12
When assessing the manufacturing quality of outdoor slides, the first thing to consider is the color. The more uniform and durable the color of the playground slide, the higher the craftsmanship. Unlike surface coating technology, the color of high-quality slides is not added to the product after manufacturing, but is finished simultaneously with the base material and is permanently fused with the base material. This is based on a precise manufacturing system centered on Dry Coloring and Rotational Molding. All processes have a decisive impact on the performance and appearance of the product.
1.LLDPE Pretreatment.
Pretreatment is the precise crushing of linear low-density polyethylene (LLDPE) particles. Industrial-grade LLDPE raw materials are generally granular and cannot be directly rotated and molded. They must be ground and crushed into powder of specific sizes. In addition to controlling the general technical indicators of this process, that is, the particle size range of powder between 30-70 mesh, the key control indicators of this process are that the powder has excellent Dry Flow and uniform Bulk Density. This particle size specification selection is very important. If the particle size is too large, it may cause the color powder to be dispersed unevenly on the surface of the LLDPE powder during subsequent physical mixing, which may cause color spots or color differences in the product. If the particle size is too small, it may cause agglomeration of powder, which may affect the uniformity of the plastic melt. Excellent fluidity can ensure that the powder is freely and evenly distributed in the mold, so as not to cause uneven wall thickness or structural collapse. A stable bulk density is the prerequisite for accurate feeding and the uniformity of weight and size of each product.
2. Selection of colorant
The essence of coloring is the selection of colorants, that is, pigments. The toner used for rotational molding LLDPE must also meet a series of technical standards. The first and most important one is excellent heat resistance. In the heating stage of rotational molding, the temperature in the furnace can reach several hundred degrees Celsius. The color powder must maintain the chemical structure for a long time at this high temperature. It will not change color or decompose. The second is high dispersibility. The color powder particles need to be fine enough to be evenly adhered to the surface of LLDPE powder in the later physical mixing and prevent the appearance of "color spots" and "color stripes". For the outdoor slide, the life of the product depends on the weather resistance and light resistance of the color powder. It must be able to resist ultraviolet radiation and erosion from the environment for a long time and keep bright colors without fading. In addition, the toner must also be chemically inert. It will not have any undesirable reactions with LLDPE resin, will not affect the mechanical properties of the material, has no migration, and meets the high-level safety and environment protection.
3. Dry coloring.
When the LLDPE powder and color powder is all ready, the drying coloring and mixing process will start. Generally, it will be done in the high-speed mixer. By the shear force and frictional force generated in the mechanical stirring process, the fine color powder can be evenly physically adsorbed and partially embedded on the surface of relatively bigger LLDPE powder. It’s not chemical combination, but evenly coated in a very high degree physically. The key point of the process control is to control the mixing time, rotating speed and loading ratio properly.
When the color powder starts to contact with particles in the low-speed stirring process, and the most initial contact can be reached. In the high-speed stirring process, the color powder will be embedded in the micro-pores on the particle surface through the collision between particles. It’s easy for the color difference to be caused by insufficient mixing, and it’s easy for the powder to be agglomerated in the certain part due to frictional heat generation when the mixing time is too long, and it will affect the fluidity of the powder.
The humidity in the whole mixing process also needs to be strictly controlled. Any water in the whole process will make the bubbles emerge in the powder in the heating and molding stage, and it will result in the product defect in the later process. Whether the whole process will be successful or not depends on the uniformity of the color in the final product.
4. Rotational molding.
After the well-mixed coloring powder is accurately measured and injected into the mold, the core stage of rotational molding starts. The whole process can be divided into several stages of dynamic control. The first stage is the heating and melting stage. The mold carrying the powder material rotates around and on its own axis in the heating chamber along the two mutually perpendicular axes. The temperature curve of the heating furnace is strictly controlled so that the LLDPE powder can be gradually melted from the inside out after being heated and then evenly coated in layers on the inner wall of the mold through the action of the dual-axis rotation. The temperature control is the key to the color shaping. If the temperature is too low, the melt viscosity will be high and the color powder can’t be fully dispersed, which is likely to cause the local concentration too high. If the temperature is too high, some toners may cause thermal decomposition actions, and thus the color will become lighter or emit an unpleasant odor. Especially for organic pigments, red and yellow, the impact is more obvious. The rotational speed of the mold also needs to be controlled accurately. Only at the low-speed rotation can the melt evenly cover the mold surface. At the high-speed rotation, the bubbles are eliminated by centrifugal force and thus don’t cause color defects due to bubble occlusion.
5. Cooling and curing.
Following immediately is the cooling & setting process. The mold goes from the heating chamber to the cooling room. The mold is usually cooled by natural cooling and air cooling. We don’t stop the rotation of mold to avoid the plastic that not completely cured to flow down and sag due to the gravity, which will cause internal stress of the product. The cooling rate also shall be controlled properly. Too much cooling will cause the product’s internal stress to increase, which may cause warping deformation or decrease impact strength. If the cooling speed is too slow, the production efficiency will be reduced. After the product is completely cooled and solidified, the demolding process can be carried out finally. The complete product with the same color inside and outside can be removed.
From the particle crushing to the rotational molding, each link of the dry coloring process is based on the uniformity and stability. In terms of consumers, the plastic part of the children slide molded by the process not only has bright color and long-lasting color, but also avoids the phenomenon of safety accidents caused by the peeling of the coating, so that the product can ensure the good appearance and performance in the long-term use. This technical logic of coloring the base material is the basis for the long-term use of high-quality slide products.\
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