pet bottle moldWhat is the effect of the injection speed of the preform mold on molding?
一. Introduction
二. The effect of injection speed on the flow of plastic melt
(I) Melt filling behavior
1. Filling under high-speed injection molding
2. Filling under low-speed injection molding
(II) Melt pressure distribution
1. High-speed injection molding and pressure changes
2. Low-speed injection molding and pressure characteristics
三. The impact of injection molding speed on preform quality
(I) Appearance quality
1. Surface quality
2. Dimensional accuracy
(II) Internal quality
1. Internal stress
2. Microstructure
四. The impact of injection molding speed on injection molding production efficiency
(I) Molding cycle
1. High-speed injection molding and cycle shortening
2. Low-speed injection molding and cycle extension
(II) Energy consumption
1. Energy characteristics of high-speed injection molding
2. Energy consumption of low-speed injection molding
五. Conclusion
The injection speed will affect the molding efficiency and quality stability of the preform mold. We are professional in this regard. Welcome to consult us!
一. Introduction
In the injection molding process of plastic preforms, the injection speed is a crucial process parameter. It directly affects the flow behavior of the plastic melt in the mold cavity, the quality of the preform, and the efficiency of the entire injection molding production. Reasonable control of the injection speed has far-reaching significance for obtaining high-quality preform products and optimizing the injection molding process.
二. The effect of injection speed on the flow of plastic melt
(I) Melt filling behavior
1. Filling under high-speed injection molding
When the injection speed is high, the plastic melt presents a fast flow state in the cavity. For the preform mold, this fast flow enables the melt to fill the entire cavity in a short time. For example, when producing thin-walled preforms, a higher injection speed can prevent the melt from cooling prematurely during the filling process and ensure that the melt can smoothly reach every corner of the cavity. Because the cooling speed of the thin-walled part is relatively fast, if the injection speed is too slow, the melt may solidify before it completely fills the cavity, resulting in short shots in the preform.
At the same time, high-speed injection molding can also improve the front shape of the melt. The melt can maintain a relatively neat front at high speed, reducing the fluctuation and instability of the melt front. This is very important for the quality of the preform, because an unstable melt front may cause weld marks on the surface of the preform. For example, in a multi-cavity preform mold, high-speed injection molding can make the melt fronts of each cavity advance synchronously and improve the uniformity of filling.
2. Filling under low-speed injection molding
During low-speed injection molding, the plastic melt flows slowly in the cavity. In this case, the melt has enough time to exchange heat with the mold cavity wall during the filling process, which easily causes the melt temperature to drop too quickly. For some high-viscosity plastic materials, low-speed injection molding may further reduce the fluidity of the melt and increase the difficulty of filling. For example, when using high-viscosity materials such as polycarbonate (PC) to produce preforms, low-speed injection molding may cause the melt to stagnate in the cavity, making it impossible to evenly fill the cavity, thus affecting the shape accuracy of the preform.
In addition, the stability of the melt front is poor during low-speed injection molding, and the melt is prone to splitting and turbulence. This may cause defects such as weld marks and bubbles inside the preform. Because the splitting of the melt front will cause different parts of the melt to reunite in the cavity, forming weld marks; and the turbulence of the melt may involve air and produce bubbles.
(II) Melt pressure distribution
1. High-speed injection molding and pressure changes
High-speed injection molding will cause the plastic melt to generate higher pressure in the cavity. This pressure change has an important impact on the structure and molding process of the preform mold. In the early stage of high-speed injection molding, the melt is quickly injected into the cavity, which will form a higher pressure peak in the cavity. This pressure peak is very beneficial for promoting the melt to fill thin-walled parts and complex-shaped areas. For example, for the bottle mouth part of the preform, which usually has a complex thread structure, the pressure generated by high-speed injection molding can make the melt better fill these fine structures.
However, too high an injection speed may also cause excessive pressure in the cavity, which will have a greater impact on the cavity wall and core of the mold. This may cause deformation of the mold, especially in the case of long-term high-speed injection molding, the precision of the mold will be affected. For example, for a high-precision preform mold, long-term exposure to excessive melt pressure may cause changes in the cavity size, which in turn affects the dimensional accuracy of the preform.
2. Low-speed injection molding and pressure characteristics
During low-speed injection molding, the pressure of the melt in the cavity is relatively low. This lower pressure may not allow the melt to fully fill some thin walls of the cavity or areas far from the gate. For example, in thicker parts such as the bottom or shoulder of the preform, low-speed injection molding may cause insufficient melt filling in these areas due to insufficient pressure, causing the preform to have local depressions or uneven thickness.
At the same time, the pressure change under low-speed injection molding is relatively gentle, which may be beneficial for some mold structures and plastic materials that are sensitive to pressure. For example, for some preforms that are prone to warping, lower pressure changes can reduce the generation of internal stress, thereby reducing the degree of warping of the preforms.
三. The impact of injection molding speed on preform quality
(I) Appearance quality
1. Surface quality
High-speed injection molding helps to improve the surface quality of the preform. Since the melt can quickly and evenly fill the cavity at high speed, the surface of the preform can obtain better gloss and flatness. For example, when producing cosmetic preforms, a higher injection molding speed can make the surface of the preform as smooth as a mirror, meeting the high requirements of the cosmetics industry for packaging appearance. Moreover, high-speed injection molding can reduce the appearance of weld marks and make the surface of the preform more beautiful.
On the contrary, low-speed injection molding may cause various defects on the surface of the preform. As mentioned above, the instability of the melt front during low-speed injection molding will produce weld marks, which will form obvious marks on the surface of the preform, affecting the appearance of the preform. In addition, low-speed injection molding may also cause flow marks on the surface of the preform, because the friction and temperature changes between the melt and the cavity wall during the slow flow process will cause the surface quality to deteriorate.
2. Dimensional accuracy
The injection speed has an important influence on the dimensional accuracy of the bottle preform. During high-speed injection molding, if the injection speed is not properly controlled, the mold may be deformed due to excessive melt pressure, thus affecting the dimensional accuracy of the bottle preform. However, within a reasonable range, high-speed injection molding can ensure the stability of the bottle preform size through rapid filling and pressure holding. For example, for some pharmaceutical bottle preforms with high dimensional accuracy requirements, the dimensional deviation of the bottle preform can be controlled within a very small range by accurately controlling the speed and pressure of high-speed injection molding.
Low-speed injection molding may directly affect the dimensional accuracy of the bottle preform due to the possibility of incomplete or uneven filling. Key dimensions such as the wall thickness and diameter of the bottle preform may deviate due to insufficient filling. For example, when producing beverage bottle preforms, low-speed injection molding may cause uneven wall thickness of the bottle preform, resulting in reduced strength and stability of the bottle.
(II) Internal quality
1. Internal stress
High-speed injection molding may generate large internal stress inside the bottle preform. This is because after the melt quickly fills the cavity at high speed, internal stress will be generated during the cooling process due to factors such as temperature changes and molecular chain orientation. If the internal stress is too large, the preform may warp or crack after demolding. For example, for some thin-walled and large-sized preforms, the internal stress generated by high-speed injection molding is more likely to cause deformation.
The internal stress generated by low-speed injection molding is relatively small, but due to problems such as uneven filling, local stress concentration areas may be formed inside the preform. These areas are prone to cracks or damage when the preform is affected by external forces or environmental factors. For example, at the junction of the thick wall and thin wall of the preform, low-speed injection molding may cause stress concentration here, reducing the quality of the preform.
2. Microstructure
The injection molding speed also affects the microstructure of the preform. Under high-speed injection molding, the molecular chains of the plastic melt will produce orientation during the rapid flow process. This orientation will affect the physical properties of the preform, such as strength and toughness. For example, for some engineering plastic preforms, high-speed injection molding orients the molecular chains along the flow direction, which may increase the strength of the preform in this direction, but it may also reduce its performance in the vertical direction.
When the molecular chains of the melt are injected at a low speed, they have more time to relax, and the microstructure is relatively uniform. However, due to problems in the filling process, pores or microscopic defects may appear inside the preform, which will affect the airtightness and mechanical properties of the preform.
四. The impact of injection molding speed on injection molding production efficiency
(I) Molding cycle
1. High-speed injection molding and cycle shortening
Higher injection molding speed can effectively shorten the molding cycle of the preform. Because the melt can quickly fill the cavity, the filling time is reduced. In the entire injection molding process, the filling time usually accounts for a large proportion of the molding cycle. For example, after increasing the injection molding speed to a certain extent, the filling time may be shortened from several seconds to less than one second, thereby significantly shortening the molding cycle. For enterprises that mass-produce preforms, this can produce more preforms per unit time and improve production efficiency.
In addition, high-speed injection molding can also speed up the pressure holding and cooling process. Since the melt can reach the pressure holding and cooling state faster after high-speed filling, the entire molding cycle can be further shortened. For example, in some preform molds that use rapid cooling technology, high-speed injection molding combined with an efficient cooling system can shorten the molding cycle by 30% - 50%.
2. Low-speed injection molding and cycle extension
Low-speed injection molding will extend the filling time, resulting in a longer molding cycle. This not only reduces production efficiency, but may also increase production costs. For example, under the same production task, low-speed injection molding may take twice as long as high-speed injection molding to complete the same number of preform production. Moreover, since low-speed injection molding may cause problems such as insufficient filling, more debugging and rework are required, further extending the production cycle.
(II) Energy consumption
1. Energy characteristics of high-speed injection molding
High-speed injection molding requires higher injection pressure during the filling stage, which will increase the energy consumption of the injection molding machine. However, due to the shortened molding cycle, the energy consumption per unit product may not increase significantly from the perspective of the entire production process. For example, although the energy consumption in the injection stage of high-speed injection molding is higher, due to the improved production efficiency, under the same output, the total energy consumption may be comparable to that of low-speed injection molding, or even lower in some optimized cases.
Moreover, high-speed injection molding can further reduce energy consumption with efficient cooling systems and energy-saving injection molding machines. For example, some new injection molding machines use energy recovery technology, which can recover part of the energy during the pressure holding and cooling stages of high-speed injection molding for the next injection molding process, thereby improving energy utilization.
2. Energy consumption of low-speed injection molding
Although low-speed injection molding has relatively low energy consumption in the injection stage, the total energy consumption may be higher due to the long molding cycle. Moreover, low-speed injection molding may require a longer heating and insulation process to ensure the fluidity of the plastic melt. For example, for some high-viscosity plastic materials, in order for the melt to smoothly fill the cavity during low-speed injection molding, the barrel temperature needs to be set slightly higher, which will also increase energy consumption.
五. Conclusion
Injection speed is a key parameter in the injection molding process of bottle preform molds. It has a profound impact on the flow behavior of plastic melt, the quality of bottle preforms and the injection molding production efficiency. In actual production, it is necessary to comprehensively consider and reasonably control the injection speed according to factors such as the characteristics of plastic materials, the shape and quality requirements of bottle preforms, and the performance of injection molding machines to obtain high-quality and efficient bottle preform production. At the same time, with the continuous development of injection molding technology, the optimization and precise control of injection molding speed will become an important research direction to improve the quality and efficiency of preform mold injection molding.
Foshan Heyan Precision Mold Technology Co., Ltd.'s main business is preform molds, blow molds, bottle cap molds and other products. It has a strong engineering, R&D, technology, and sales team, advanced processing equipment and rich project experience. If you want to know more or have special needs, please call 13318345050, WeChat ID.
