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Explaining Injection Molding With Physics Knowledge
Translated By Jasmine HL
There are 5 branches of pure physics: 1. mechanics, 2. electromagnetics, 3. thermodynamics, 4.
relativity, and 5. quantum mechanics.
There are 7 branches of multidisciplinary physics: 1. Biophysics, 2. Chemical Physics, 3. Medical
Physics, 4. Astrophysics, 5. Geophysics, 6. Economic Physics, 7. Atmospheric Physics.
So what does the injection molding process involve?
What is internal stress?
We all know that all objects are made up of molecules, and molecules are made up of atoms, and
atoms are not the most basic particles of the matter! There are nuclei, protons, neutrons,
electrons, Higgs bosons ... and so on. However, these basic particles are not close to each other,
and there are many holes between them.
Therefore, according to the theory of quantum mechanics, all objects can be compressed, so
some materials are easily compressed, and some materials are not easily compressed, which is
related to the molecular structure. After the molecules are compressed, when this external
pressure (such as injection pressure, mold cavity) disappears, the structure and order of the
squeezed molecules must be restored to their original state (like a spring), which forms internal
stress.
Speaking of which, I inserted a short story: I remember a stove once in winter when I was a kid.
There is an air inlet and a lid under the stove. The air will burn out when the cover is removed,
and the stove will burn out. Cover it before going to bed. But the lid was rusty and rotten. Dad
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cut the bottom of a plastic bottle. However, because the diameter of the bottle was too thick and
inappropriate, the dad roasted it on fire and the bottle shrank.
Why is it smaller? Shouldn't the thermal expansion and contraction become bigger after baking?
The structure of the bottle is hollow, the inside is large and the mouth is small. It cannot be
produced by injection molding, because it cannot be demoulded, so it can only be blow molded.
Air can flow, there is no solid, what shape the container is, what it is.
After the preform is heated to a temperature that can be shaped, it is put into the mold and
blown into it. After the mold cavity is blown, the preform material contacts the mold, the heat is
absorbed by the mold, and it is cooled and shaped, so after the bottle is produced There is
internal stress that shrinks inward. When we heat it with high temperature to make the material
soft, and then the internal stress is released, the bottle becomes smaller.
Second, the impact of internal stress on plastic products
1. Some products will crack after being stored for a period of time.
2. There will be stress marks on the product surface.
3. Warping deformation occurs after the product is produced.
Now let's focus on product deformation: the internal stress was introduced earlier. That is to say,
when the pressure is large enough, the material with higher shrinkage can also generate greater
internal stress (sometimes the size of the product even exceeds the size of the mold cavity, in this
state, the product will be ejected Cannot be demolded while wearing).
The larger the internal stress, the smaller the shrinkage, which can even be negative. There are
two causes of deformation. One is that the material thickness of the product and the material
thickness of the ribs are different, and the shrinkage rate is also different, as shown in the figure.
The thicker the material, the longer the cooling time, so the ribs are generally thinner, and the
shrinkage time is relatively short. Therefore, when the product material thickness continues to
shrink, there will be a pulling force inward, and the ribs have stopped shrinking At this time, the
ribs will have a force to support outwards, which will cause the two ends of the product to
deform.
So how to explain the phenomenon of deformation of the sheet-shaped product without the ribs?
This is caused by internal stress. The fluidity of plastic fluid depends on temperature, and the
heat conduction and time are proportional. If the filling speed is too slow, it will easily cause the
plastic fluid to lose temperature far from the gate. As a result, the fluidity is reduced, so greater
pressure is necessary to ensure that the mold cavity is filled.
In this way, the internal stress is too large near the gate, and the internal stress is small or no far
away from the gate. The internal stress is different, so the shrinkage rate is different.
Finally, add one more point: the adjustment of deformation must be judged in conjunction with
the position of the gate.
There is no doubt that thermodynamics is absolutely used in injection molding, and
thermodynamics and fluid mechanics are inseparable in injection molding. But we don't have to
delve into it.
We will not talk about the heat conduction formula here, because the solution of the heat
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equation has the property of smoothing the initial temperature, which means that heat is
transmitted from high temperature to low temperature. In general, many different initial states
will tend to the same steady-state (thermal equilibrium).
It is therefore difficult to reverse the initial state from the existing heat distribution, even for
extremely short time intervals. The heat equation is also the simplest example of a parabolic
partial differential equation. Calculus is the knowledge in advanced mathematics. I haven’t read it
in college, or I can’t do bad math in college. But it doesn’t matter. We don’t need to calculate it.
We just need to know the heat conduction and the thermal conductivity of the material. ,
Contact area, time, temperature difference and other conditions, so you can quickly estimate it.
In order to ensure that there is no burr phenomenon when adjusting the shrinkage of the
product, we generally give a shot stop time, for example, 4 seconds can be shot to fill 95% to 99%
of the mold cavity to complete the shot, It will be set to more than 4 seconds. According to the
size of the gate, a pause time is provided. Under the premise that the gate is not solidified, the
plastic fluid injected into the mold cavity is cooled to reduce the fluidity. Then, Fill it with holding
pressure to 100%, so that the burr inside the parting surface of the mold is not easy (you can also
use multiple pieces of holding pressure from small to large).
But to be able to use this skill to the extent that products of any structure are effective, it is
necessary to understand whether the thermal conductivity of the material is high or low, the size
of the gate, and the thickness of the product. ), The temperature of the mold (the colder the
mold, the faster the heat absorption), and the pause time and so on. These conditions are exactly
the conditions in the heat conduction formula.
1. When plastic fluid fills the mold cavity, in order to ensure that two-cavity products with small
gates will not run out of glue or shrink, the gates are relatively easy to burr (the closer to the gate,
the burr will be Big).
2. The reason is that the plastic fluid will not freeze when it is flowing fast. Once it stops flowing,
the cold setting time can be calculated according to the conditions mentioned above.
When using shot-stopping or hourglass-type processes, it will appear that the plastic fluid at the
gate position stops flowing because the mold cavity of the gate has been filled, and then this time
is added to the shot-stopping time, resulting in a relatively short gate. The large mold cavity
pauses for a long time to shoot the glue, and the gate is solidified, so the back pressure cannot be
maintained.
The essence of the hourglass-type process is pressure, location, holding pressure, and holding
time (depending on the size of the gate).
