What should be paid attention to in the injection molding of transparent lace salad bowl?

As a daily injection-molded product with practicality and aesthetics, the core requirements of transparent lace salad bowl are high transparency, no appearance defects, complete lace molding and up-to-standard structural strength. Its injection processing needs to be controlled around the whole process of raw material characteristics, mold design, process parameters, molding operation and post-treatment, to avoid the common problems of air lines, shrinkage marks and color difference of transparent parts, and to ensure the molding accuracy of lace parts. The following are the specific processing precautions, and the whole process needs to follow the injection production specifications, giving consideration to product quality and production.

Selection and pretreatment of raw materials are the basis of transparent parts processing. Food-grade raw materials with high transparency, adaptability to fluidity and low temperature resistance should be selected first. Common raw materials are PET, PP, PS and PMMA. Among them, PET and PP are more suitable for salad bowls because of food contact safety and fall resistance, while acrylic is more transparent but slightly brittle, so it should be selected according to product positioning. The raw materials must be brand new, and it is forbidden to mix back materials or miscellaneous materials, otherwise it will directly affect the transparency and produce mottled and crystalline spots; Before processing, strict drying should be carried out. The drying temperature of PET should be controlled at 120-140℃ for 4-6 hours, the drying temperature of PP at 80-90℃ for 2-3 hours, and the drying temperature of acrylic at 70-80℃ for 3-4 hours. The moisture content should be controlled below 0.02%. If it is not fully dried, the products will appear silver lines, bubbles and water lines, which will be serious.

Mold design and debugging directly determine the molding effect of products, especially the accuracy of lace parts. The mold cavity should be polished with high finish, with surface roughness Ra≤0.02μm, and the polishing direction should be consistent with the melt flow direction to avoid flow marks caused by the roughness of the cavity, which will affect the transparency and surface feel. Gate design should match the flow characteristics of molten material, and needle gate or latent gate should be preferred. Gate position should avoid lace area and visible surface of products to prevent the appearance from being affected by gate traces, and at the same time ensure that molten material fills the cavity evenly, so as to avoid scorch marks caused by excessive shearing of molten material due to too small gate, while shrinkage marks are easy to occur due to too large gate. The mold core and cavity of lace should be closely matched, the gap should be controlled at 0.01-0.02mm, the pattern carving should be clear and uniform in depth, and the mold should be provided with a reasonable vent groove, the depth of which should be controlled at 0.03-0.05mm and the width should be 2-4mm, especially at the last filling position of the molten material such as the end of lace and the dead angle of cavity. Poor venting will lead to lack of lace, incomplete molding and air lines. In addition, the mold should be equipped with a temperature control system, and the temperature of the cavity and the core should be uniform to avoid warping and uneven shrinkage of the product due to temperature difference.

Precise control of injection molding process parameters is the core of processing, which needs to be set around the principle of "uniform plasticization of melt, smooth filling and slow cooling" to reduce melt shear and temperature fluctuation. Plasticizing temperature should be accurately controlled according to the characteristics of raw materials. The processing temperature of PET is 260-290℃, PP is 200-230℃, and acrylic is 220-250℃. The temperature of each section of the barrel is gradually increased from back to front, and the temperature of the head is slightly lower than that of the previous section to prevent the melt from overheating and decomposition, which will produce yellowing and black spots and destroy transparency. The screw speed is controlled at 30-60r/min, and the back pressure is 0.5-1.5MPa. The low speed and back pressure can reduce the shear heating of the melt and avoid the internal stress caused by excessive shear, which will lead to the later cracking of the product. The injection process is controlled by stages. The injection speed is mainly low and medium speed, and the feed is slow in the early stage, so as to prevent the molten material from impacting the cavity to produce air marks. In the middle stage, the molten material is filled at a uniform speed, so as to ensure that the molten material can smoothly fill the cavity. In the later stage, the speed is slowed down appropriately to avoid lace flash and lack of material. The injection pressure is adjusted according to the wall thickness of the product, generally 80-120MPa, so as to ensure that the molten material can fully fill the delicate parts of the lace and prevent the internal stress of the product from increasing due to excessive pressure. Holding pressure is very important. The holding pressure is 60-80% of the injection pressure, and the holding time is 5-10s, which needs to be adjusted according to the shrinkage of the product. Adequate holding pressure can make up for the cooling shrinkage of the melt, avoid shrinkage marks and depressions in the bowl and lace, and slowly release the pressure after holding pressure to prevent the product from being deformed due to sudden pressure change when demoulding. The cooling temperature and time should match the characteristics of raw materials, and the mold temperature should be controlled at 80-100℃ for PET, 40-60℃ for PP, and 60-80℃ for acrylic. Higher mold temperature can reduce the difference of melt cooling speed and avoid internal stress and white fog. The cooling time is based on the complete solidification of the product, generally 10-20s. Insufficient cooling will lead to the product demoulding deformation and lace sticking to the mold, while excessive cooling will reduce the production efficiency.

The molding operation and site control should pay attention to details to reduce the defects caused by human factors. High-precision equipment should be selected for injection molding machine, the clearance between screw and barrel should be reasonable, and there should be no abrasion or material leakage. Before the equipment runs, empty machine debugging should be carried out to check the running accuracy of each part. During the production process, operators should regularly check the drying conditions of raw materials, the mold temperature and the stability of process parameters, and randomly check the products every half hour, focusing on the transparency, lace molding integrity, air marks, shrinkage marks, flash and other defects, and adjust them in time when problems are found. A special demoulding device is required in the demoulding process, and the demoulding thimbles are evenly distributed, and the ejection speed is slow and synchronous, so as to prevent the thimbles from damaging the surface of the product or causing the lace to deform, especially for the thin lace, which is easy to be damaged due to excessive ejection force. The mold should be cleaned regularly, and the molten residue and oil stain in the cavity, gate and exhaust slot should be cleaned in time. The residue will cause black spots and scratches on the product, which will affect the appearance and transparency. Use special soft tools when cleaning to avoid scratching the high-finish surface of the mold.

Post-treatment and quality inspection are the keys to ensure the final quality of products. Transparent injection parts are prone to cracking and whitening due to internal stress, so proper annealing treatment is needed. The molded products are put into a constant temperature oven, with the annealing temperature of 60-80℃ for PET, 70-90℃ for acrylic and 1-2 hours for slow heating and natural cooling, which can effectively eliminate internal stress and improve products. In the post-treatment process, the product should be handled with care to avoid collision and scraping, and to prevent scratches on the surface, which will affect the transparency and aesthetics. The quality inspection should follow the standards of food-grade daily-use products, with no air marks, water marks, shrinkage marks, yellowing, black spots and scratches on the appearance, clear lace molding, no material, flash and deformation, even bowl wall thickness and no obvious thickness difference; Transparency should be up to standard, and there is no white fog and turbidity under natural light; In terms of structural strength, the fall resistance and pressure resistance tests are carried out, and there is no cracking or deformation. At the same time, the food contact safety test is needed to ensure that the raw materials are free of harmful substances.

In addition, the production environment should be kept clean and dry, and the temperature should be controlled at 20-30℃ and the humidity should be 40-60% to prevent dust and impurities from entering the mold cavity, resulting in black spots and impurities on the products; Use sealed pipes in the process of raw material transportation to prevent raw material pollution; Non-conforming products after production should be stored separately, and it is forbidden to mix them into raw materials for reprocessing at will. If recycling is needed, a small amount should be mixed into brand-new materials in proportion, which is only applicable to non-food contact products, and it is forbidden to use recycled materials for food-grade transparent lace salad bowls.

To sum up, the injection molding of transparent lace salad bowl should be carefully controlled from the whole process of raw materials, molds, technology, operation and post-treatment. The core is to control the stability of melt plasticization, filling and cooling, reduce internal stress and appearance defects, and ensure the molding accuracy of lace parts. Only by closely cooperating with each other can we produce qualified products that meet food-grade standards, have high transparency, beautiful appearance and stable structure.