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"Bring science home" all
Written by | Su Chengyu
Source: Internet
You must have noticed a design problem, many small plastic benches have a hole in the middle.
In fact, not only small benches, almost all plastic chairs, including those on buses, have holes, and some have more than one hole.
Source: Internet
This is certainly not for the convenience of your fart, but for other design reasons.
One of the reasons is that it is convenient for production casting. The industrial production of plastic chairs is mainly done through the injection molding process. It is a manufacturing process that involves injecting molten material into a mold.
The plastic pellets are first placed in the hopper of the injection molding machine and then heated through a heating drum until they are molten. The heating drum has a rotating screw inside that helps mix and propel the plastic forward.
Plastic pellets Source: YouTube
When the molten plastic is pushed to the front of the barrel, it is injected into the closed mold. The mold is precisely manufactured to the shape of the final product, and the plastic fills the cavity of the mold after entering the mold.
Molds for the production of chairs Source: YouTube
The plastic injected into the mold cools inside the mold and begins to solidify, forming into the desired shape. This step is achieved through a cooling circuit in the mold, usually circulating cooling water or oil to control the temperature.
Molded plastic chair Source: YouTube
Once the plastic has cooled and solidified completely, the mold opens and the molded plastic part is pushed out of the mold through a mechanical part such as a thimble and can then be removed.
Source: YouTube
And when the chair is removed from the mold, there will be a problem, if the mold of the chair is designed without holes, it will be particularly difficult to take it off.
Because the holes can reduce the contact area between the mold surface and the plastic, thus reducing the adsorption force between the two. This makes it easier for the plastic parts to detach from the mold surface when the mold is opened.
Plastic stool mold with a hole visible on it Source: tzchmould
Moreover, during the injection molding process, the air inside the mold needs to be expelled to prevent air from being trapped and causing defects. The holes can be used as exhaust ports to help the air inside the mold escape smoothly, reducing problems caused by air trapping, such as voids or incomplete filling.
As another point, holes are able to improve the cooling process of plastic parts, as they allow the cooling liquid or gas to pass through the thickness of the plastic part more evenly, especially in thicker sections. This helps to reduce internal stresses and warpage due to uneven cooling, making it easier for parts to be demolded.
Plastic chairs and corresponding molds Source: mold manufacturing
The point is that the production of such chair molds is relatively cheap and more material-saving, so most manufacturers of plastic chairs will use such molds. Therefore, generally this kind of plastic chair with holes is cheaper.
The next step is to transport these chairs, which must be stacked on top of each other to save space. If there are no holes in the chair, you will experience the pain of carrying a plastic chair.
Source: taobao
When multiple plastic chairs are stacked on top of each other, a tight fit between the backs of the chairs can cause air to be squeezed out, creating an area of low air pressure. Without proper vents or holes, this low air pressure state can create a slight vacuum effect that makes the chairs stick to each other and make it difficult to separate.
Source: Internet
By incorporating holes in the design of the chairs, it is possible to allow air to circulate and maintain the balance of air pressure even when the chairs are stacked very closely. In this way, when it is necessary to split the stacked chairs, the chairs can be more easily separated from each other because there is no strong vacuum adsorption effect.
Finally, some chairs have larger holes, making it easier to reach down and make them easier to reach with your hands.
Source: Internet
As for why not square or other polygonal holes, this is because the circular holes are able to distribute the stresses around the material more evenly. Square or other holes with corners tend to concentrate stresses at the corners, which can cause the material to crack or fracture more easily at these points.
Circular holes do not form stress concentration points due to the absence of sharp corners, which increases the structural strength and durability of the chair.
黑白兀鹫 图源：Wikipedia
有史以来记录到的能飞得最高的鸟是黑白兀鹫（Rüppell's vulture，学名：Gyps rueppelli）。
This vulture can fly to very high altitudes, with the highest record being at an altitude of about 11,300 meters. This record was discovered by accident during the flight of an airplane when a black and white vulture crashed into the plane, and then analyzed the altimeter on the plane to arrive at this data.
Black and white vultures are able to fly at such a high altitude mainly due to their special physiological structure in the body, which allows them to adapt to a low-oxygen environment. For example, their hemoglobin is able to effectively capture oxygen from the air at extremely low oxygen concentrations. These adaptive traits allow the black and white vultures to fly higher than most other birds in search of food.
Source: Internet
In 1995, a study conducted in the United Kingdom analyzed ear size in adults of different ages. The researchers found that the length of the ears gradually increased from young age to old age. The data they measured showed that the length of the ears grew by about 0.22 millimeters per decade. The conclusions of this study support the idea that ears grow larger with age.
In addition, other studies have shown that although the cartilage of the ear basically stops cell division in adulthood, the cartilage of the ear will gradually become thinner and sagher due to age, and the ear, especially the earlobe, will gradually sag and lengthen due to the long-term influence of gravity.
And what about the nose?
Studies have shown that changes in the nose are not entirely due to the continued growth of the cartilage in the nose, but more due to the remodeling of the cartilage and the influence of gravity. As we age, the cartilage of the nose gradually becomes thicker, heavier, and loses some of its elasticity. This results in a drooping nose tip and an enlargement of the overall nose shape. An important phenomenon is that the relaxation of the skin and cartilage of the nose makes the nose appear larger and longer.
So, while the ears and nose may seem "enlarged", they are actually the result of more relaxation and gravity than new tissue growth.