Research on Modern Packaging Mechanics (I)

[Abstract] The modernization of the packaging industry puts forward many problems related to various branches of mechanics. From the aspects of buffer packaging theory, product crispness and breakage boundary theory, packaging materials, packaging containers and buffer packaging design methods, etc., the relevant mechanics problems and the current research progress were elaborated, and the discipline system of modern packaging mechanics was preliminarily constructed.

Keywords: packaging mechanics; packaging dynamics; buffer packaging; product crispness; breakage boundary theory; packaging materials; packaging containers

1 Development history of packaging

Packaging is gradually developed with the evolution of mankind, social changes, the development of production, and the advancement of science and technology. In prehistoric times, humans have learned to pack, store, and transport food using natural packaging materials such as stems and leaves of plants, gueta, vitex, bark, and hides. According to archeological excavations and historical records, human beings invented the Weaving and Burning Ceramics as early as 8,000 years ago. It can be seen in the Neolithic Chinese Yangshao cultural site excavated in Banpo Village, Xi’an City. Many years ago, they will use various cup-shaped tip-bottomed bottles, gourd-shaped bottles, garlic-shaped pots, round-bottomed pots, shallow abdomen-bottomed pots, folding basins, chord-lined sand jars, and rope-pattern porcelain, etc. Transport different food and domestic water. Humans carefully carry these fragile containers filled with food by using heads, shoulder bars, and human lifts. This primitive handling method is slow and the transport distance is short and it is easy to damage the container. After long-term labor practices, humans have come to realize that the cause of damage to transport containers is the vibration and impact of the environment. In order to effectively protect the goods and improve the efficiency of transportation, the Shih-Is appeared in the last years of the 11th century BC. This was the earliest shock-absorbing tool that was invented by mankind.

The history of modern packaging should be traced back to the industrial revolution of the eighteenth century. The development of the capitalist commodity economy has promoted the development of world-wide trade. The packaging has attracted people's attention and has become an important link in the commodity economy. The global economic crisis that took place in 1930 prompted producers to not only consider the function of protecting the contents when designing product packaging, but also increase the advertising function to promote sales, thus creating a modern packaging. The globalization of economy and technology and its rapid development have effectively promoted the development of the world packaging industry. Packaging, as a service industry for the national economy, has now formed a paper, plastic, metal, glass, printing, and machinery as its main components. Modern technology and equipment, more complete modern industrial system. During the "Ninth Five-Year Plan" period, China's packaging industry increased by an average of 12% annually. In 2000, the packaging industry output value reached 220 billion yuan. During the "10th Five-Year Plan" period, China's packaging industry will continue to increase at a rate of 8%. It is expected that in 2005 the output value of the packaging industry will reach 320 billion yuan.

2 Research progress of modern packaging mechanics

2.1 buffer packaging theory

In 1945, Mindlin, the American Bell Telephone Laboratory, established the classic theory of packaging dynamics, discussed the movement law of packaging products in the process of drop impact, and mainly discussed the dynamic response of single-degree-of-freedom packaging products, and also involved the dynamic problem of infinite freedom. Due to the simplicity and practicality of single-freedom handling of packaging systems, the single-degree-of-freedom buffer packaging theory has been widely used in practice. However, there are many kinds of actual product packaging, and the transportation environment is complex. In many cases, it must be analyzed and dealt with in multiple degrees of freedom in order to obtain more reasonable design results. Such as the washing machine and other home appliance product packaging, multi-layer stacking package vibration characteristics of the product, flat glass, corrugated cardboard, honeycomb paperboard, foamed plastics and other dynamic analysis, must be based on multiple degrees of freedom or continuous body treatment. For this reason, Schell considered the multi-degree-of-freedom packaging problem, thinking that the mass ratio and stiffness ratio would affect the product's response. Urbanik studied the vibration transmissibility of stacking corrugated cartons, and discussed the shock-buffering problems of double-layered goods packaging and products with elastic components. [Wang Zhiwei processed the packaging of washing machines into a two-degree-of-freedom system and studied under typical impulse excitation The impact spectrum of the box and barrel indicates the importance of a proper mass ratio and stiffness ratio.

The linearization of packaging dynamics is simple and practical, and it is easy to clarify the basic laws. However, the actual buffer packaging system is non-linear, and the research work has practical value to non-linear broadening. Wang Zhiwei and others systematically studied the impact and impulse response of a typical nonlinear buffer packaging system, and analyzed the impact of system parameters on the impact response. Non-linearity and randomness are two important characteristics of the buffer packaging system, but the analysis and design work of the nonlinear buffer packaging under the random environment has not yet been fully developed.

2.2 Product brittleness and breakage boundary theory

Brittleness refers to the allowable maximum impact acceleration value when the product retains its original function or does not suffer damage after it is subjected to impact. Brittleness reflects the product's ability to resist impact, is an index of product's impact strength, and has the description of the function keeping ability. Theoretical analysis, empirical estimation and direct measurement are all difficult to accurately determine product fragility. For this reason Komhauser, Pen-dered proposed the concept of damage sensitivity. In 1968, Prof. Neo-ton proposed the damage boundary theory, and used the magnitude and speed variation of the typical acceleration pulse impulse that the product can withstand to indirectly describe the fragility of the product, and to make the damage boundary curve (the abscissa is the speed change, vertical The coordinate is the acceleration amplitude). When the acceleration pulse amplitude or velocity change is less than its critical value, that is, the acceleration pulse coordinate point is located inside the damage boundary curve, the product is safe, otherwise, the product is not safe. This theory opened the door to the brittle impact test, laid the foundation for modern buffer packaging design, and had great significance in the development of packaging mechanics.

At the same time, Lansmint and MTS have successively developed impact testers that are suitable for determining product damage boundaries. Neoton's damage boundary theory was used in the 1977 American National Standard. When measuring the product damage boundary curve, Schell suggested that it is more reasonable to use the mean value of the acceleration pulse instead of the amplitude. The use of the peak-to-peak sawtooth acceleration pulse impact is closer to the actual situation than the Neoton's rectangular acceleration pulse. Goff and Pierce also suggested using half-sine acceleration. Pulse to determine product damage boundary curve.

The damage boundary curve can be measured by the product impact test, or it can be derived from the impact spectrum of the product. Thompson and Wang Ping, respectively, deduced the damage boundary curves excited by the back-peak sawtooth pulse and the finite-rising step pulse.

In addition to the brittleness of the product, the breakage boundary is also related to the characteristics of the packaging cushioning material. The above description of the damage boundary is for the linear buffer material case. However, the cushioning materials actually used for packaging have non-linear characteristics, and most of them are strong non-linear features. To this end, Wang Zhiwei, Wang Zhenlin, etc. carried out research on the damage boundary theory of nonlinear buffer packaging system products. Wang Zhenlin et al. discussed the product damage boundary curves for three times of nonlinear buffer packaging. Wang Zhiwei and others used a unified method to systematically study the linear and nonlinear systems (including tangent, hyperbolic tangent, and cubic) under the excitation of various typical acceleration pulses (rectangular, semi-sinusoidal, pre-peak sawtooth, and post-peak sawtooth). Impact spectrum and damage boundary curves, discuss their important features, and develop a corresponding software system. In 1998, Wang Zhenlin proposed the concept of displacement damage boundary and pointed out the adaptation conditions of Neoton's damaged boundary.

The damage boundary curve is of great value for the design of the buffer packaging system. This concept was developed when products were evaluated for crispness under the excitation of different acceleration pulses. However, the functional failure or damage of most packaging products during transportation was caused by a drop impact. In this case, it is very difficult to measure the impulse acceleration pulse acting on the product or package. In order to evaluate the product's drop damage or functional failure in nonlinear packaging systems, Wang Zhiwei recently selected the packaging system parameters and dimensionless drop impact velocity as two basic evaluation quantities, and proposed the concept of a drop damage boundary curve.

Vibration and random excitation are often encountered in the transportation of packaging products. The ability of the product to withstand vibration and random excitation is different from the ability to withstand impact excitation. In 1970, Rountree and Safford studied the fragility problem of missile instruments under random vibration. Song Baofeng gave the definition of product fragility under random excitation in 1992. The characterization of the product's ability to withstand vibration and random excitation remains to be further explored.