Defect analysis of the preform and cap products of

2022-08-15
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Defect analysis of PP large infusion bottle preform (bottle cap)

for decades, the traditional large infusion glass bottle will be replaced by the new PP plastic large infusion bottle. PP plastic infusion bottle has incomparable advantages over glass bottle: first, it is impact resistant and not easy to be damaged; Second, the weight is light, which can greatly reduce the transportation cost; Third, one-time molding will not produce secondary pollution; Fourth, it is non-toxic and will not have side effects on the human body- Fifth, the manufacturing cost is low. Calculated by a single bottle, it is only equivalent to about 40% of the cost of glass bottles. In addition, the disposable plastic bottles with high quality and low price omit the cleaning and disinfection process, save relevant energy at the same time, and simplify a considerable part of the production process

pp plastic large infusion bottle consists of five parts: bottle body (bottle blank), inner cover, outer cover, lifting ring and inner plug, as shown in Figure 1. The production is mainly injection, and the material is mainly BOPP. The defects of the above products in the production process are mainly bubbles, vacuum bubbles, black spots and black lines in the products. Now, the causes and solutions of these defects are analyzed as follows according to a certain difference between the actual test values

the production of bottle body (bottle blank) and large infusion bottle is to use the injection molding machine to inject the bottle blank first, that is, we often say PP bottle blank, and the material is BOPP. It is required to load the PP bottle blank evenly and slowly. If there is a process defect in the injection process, it will be directly reflected on the blown bottle, so sometimes the defect of the bottle, we need to find a problem on the bottle blank. The defects of PP preforms and bottles caused by preforms are mainly bubbles, which have been generated during the injection of preforms, as shown in Figure 2. This bottle blank is blown into a bottle, as shown in Figure 3. There are several kinds of bubbles in the bottle blank, and the causes are different. Generally, we judge according to the location of bubbles. As shown in Figure 2, the bubble at this position is caused by poor plasticization. In terms of technology, we can solve it by increasing back pressure, reducing not only better ball control, but also more stable football flight arc, low screw speed and increasing barrel temperature. However, the production efficiency will be reduced, which is intolerable, so professional injection molding machine manufacturers will deal with it by changing the structure of the screw. In this way, the bubble problem can be solved without reducing the production efficiency, and even improving the production efficiency

this is the case that gas is produced in the middle of the product; If bubbles are generated at the bottle mouth, as shown in Figure 4, there will generally be white spots, white lines or burning spots, which is due to the standard phenomenon of poor exhaust. It is caused by poor exhaust. Multi stage injection can be used, and the last stage is slow injection, so as to reduce the clamping force and improve the exhaust effect of the mold. As shown in Figure 5, "bubble" will also appear at this position. In fact, there is no "steam" in the "bubble" in this place, but a standard vacuum bubble, which is caused by shrinkage due to insufficient pressure holding time during injection and backflow phenomenon. Therefore, it is necessary to increase the pressure holding time and pressure

in addition, defects as shown in Figure 6 and Figure 7 occasionally appear after bottle blowing. The transverse fold in Figure 6 appears after bottle blowing, and the number of bottle blank is uncertain. The fold is out of the inner wall of the bottle, which is related to insufficient heating during bottle blowing, that is, the external temperature of bottle blank reaches the high elastic state of plastic material during bottle blowing. However, the internal temperature of the bottle blank is not reached, so the inner wall of the bottle blank is pulled hard during bottle blowing, resulting in transverse wrinkles. The blown bottle has rough surface, pits and other defects. Therefore, the defect in Figure 6 has nothing to do with the bottle blank itself, but there is a problem with the bottle blowing process

the defects in Figure 7 are related to the injection molding process and mold of the bottle blank. The longitudinal stripes in Figure 7 appear on the fixed bottle blank. After research, the longitudinal stripes are related to the damage of the bottle blank during demoulding. The bottle blank is scratched with stripes in the same direction as the demoulding direction by the mold cavity during demoulding, and these stripes will be amplified during bottle blowing to produce the above defects. Therefore, the more intelligent technologists found that if the problematic bottle blank core is polished, the problem can be solved, but it will appear again after a period of time, which shows that this is not the key to the problem. In fact, the scratch (peeling) of the bottle blank by the mold core is related to the excessive mold wrapping force of the product. The excessive mold wrapping force is related to the excessive injection pressure and holding pressure. If the injection pressure and holding pressure are too large, why don't other cavities have this problem? Later research found that the large mold wrapping force of this cavity preform is related to its gate. After measurement, the gate of the bottle blank cavity in question is larger than that of other cavities. This is the problem. With a large gate and good pressure transmission, the mold wrapping force of the bottle blank is large, so there will be demoulding scratches. Generally, the gate of PP bottle blank is 2mm (also 3mm). If the gate of a bottle blank exceeds 2.1mm, the above problems will occur

the eccentricity of the bottle blank is the main reason for the uneven thickness of the bottle wall. The main reason for the eccentricity of the bottle blank is the excessive injection pressure and holding pressure, resulting in the eccentricity of the mold core; The above problems will be caused by the inclination of the core itself or the poor strength of the core

outer cover, pp+ elastomer. Due to the existence of elastomer, the main defects of the outer cover are black spots and vacuum bubbles, as shown in Figure 8. This is a typical black spot of rubber decomposition. Because PP has better temperature resistance than elastomer, elastomer is easy to decompose and produce black spots. In the process, it can be solved by reducing the barrel temperature, back pressure and firing speed, but it is difficult to adjust and there will be some side effects. It is better to solve the problem from the screw structure of the injection molding machine, and the PP special screw of Datong machinery can effectively solve this problem. Vacuum bubbles are easy to appear at the thick edge of the outer cover, as shown in Figure 8. They are mainly made of shrinkage, and the problem can be solved by pressure maintaining and other methods

inner cover, pp+ elastomer. In addition to the black spots on the inner cover like the outer cover, the most common defect is bubbles, as shown in Figure 9, especially on the edge of the inner cover. Like other reinforcement materials, the edge of the cover is not very thick, so the probability of vacuum bubbles is not very large, usually bubbles. The reason is poor exhaust. As it is the last part of the rubber flow, and the injection molding machine used is relatively large, the poor exhaust is normal, which can generally be solved by reducing the shooting speed and clamping force. (end)

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