Three factors to control the internal quality of injection molding products
Three factors to control the internal quality of injection molding products
In the injection molding products, the
local stress states are different, and the deformation degree of the products
will be determined by the stress distribution. If there is a temperature
gradient when the product is cooled, this kind of stress will develop, so this
kind of force is called "forming stress".
There are two kinds of internal stress in
injection molding products: one is molding stress, the other is temperature
stress. When the melt enters the mold with lower temperature, the melt entering
the mold cavity wall cools rapidly and solidifies, so the molecular chain
segment is "frozen".
Due to the poor thermal conductivity of the
solidified polymer layer, there will be a large temperature gradient in the
thickness direction of the product, while the core of the product will solidify
very slowly, so that when the gate is closed, the melt unit has not solidified.
If the injection molding machine stops feeding the cooling shrinkage, because
the internal shrinkage of the product is opposite to the action direction of
the hard skin layer, the center will be In static tension, the surface layer is
in static compression.
In the process of melt filling flow, in
addition to the stress caused by volume shrinkage effect, there are also the
stress caused by the expansion effect of runner and gate outlet; the stress
caused by the former effect is related to the direction of melt flow, and the
stress caused by the expansion effect of outlet will be perpendicular to the
direction of flow.
For semi crystalline polymers, another
effect should be paid attention to, that is, when the glass transition
temperature is exceeded, the molecular segments of some non crystalline phases
retained between the crystallization units will start to move, but they are
limited by the crystalline phase, preventing the return of the tensile chain,
thus forming the internal stress. For crystalline polymers, there is also a
deformation induced stress. When the stress applied to the crystalline polymer
melt exceeds the elastic deformation limit, the lattice will flow along the
sliding surface, resulting in the displacement of plastic deformation, instead
of a part of elastic deformation.
Under the condition of stress relaxation
with constant total deformation, the stress gradually decreases to a minimum
value which is not equal to zero, and this retention value is called
"deformation induced".
For the explanation of this situation, it
can also be assumed that there is a crystal model of crystalline polymer.
During the crystallization process, stacking displacement is formed, which
makes it difficult for the lattice to further pile up on the sliding surface,
so a reaction force is generated, which is equal to the stress required to
maintain the lattice displacement structure, and the lattice displacement
structure is formed in the non-equilibrium state without stress. This is the
explanation of the displacement mechanism of "deformation induced internal
stress", but it is not suitable for amorphous polymers.
Relationship between internal stress and
product quality
The existence of internal stress in
products will seriously affect the mechanical properties and service
performance of products; due to the existence and uneven distribution of
internal stress in products, cracks occur in the use process of products, and
irregular deformation or warpage often occur when products are used below the
glass transition temperature, which will also cause "whitening",
turbidity and deterioration of optical properties of products.
Reducing the temperature of the gate and
increasing the slow cooling time will help to improve the uneven stress in the
products and make the mechanical properties uniform. For crystalline polymers,
the tensile strength is anisotropic.
With the increase of melt temperature, the
tensile strength of both crystalline and amorphous polymers will decrease, but
the mechanism is different: the former is affected by the decrease of
crystallinity; the latter by the effect of orientation.
The impact strength of injection molding
products shows more prominent anisotropy. The impact strength is not only
related to the molecular structure of the polymer and the injection process
conditions, but also related to the product structure shape, gate and location,
number, distribution and arrangement.
This is because the impact strength is mainly
determined by the internal stress (orientation stress, temperature stress,
deformation induced stress) formed during polymer processing.
Shrinkage process
The shrinkage of injection molding products
can be divided into three stages.
The first stage is the pressure maintaining
stage before the gate solidification. The shrinkage of the product depends
largely on the compensation of the melt. Because of the low temperature of the
die, the melt temperature is decreasing and the melt density and viscosity are
increasing. Therefore, the compensation ability of melt mainly depends on the
magnitude of holding pressure and the time to maintain the transfer to the mold.
The second stage is the cooling stage from
gate solidification to demoulding. At this stage, no melt will enter the
cavity, the weight of the product will not be changed, but the density or
specific volume of the product will change.
The third stage is the shrinkage from the
beginning of demoulding to the use stage. This is a free contraction.
Control of shrinkage
Mold temperature should not be too high.
For example, for POM products. When the mold temperature is 80 ℃ 40 ℃, the
shrinkage is 5%.
The barrel temperature should not be too
high. For example, when the melt temperature of POM products is 190 ℃ 10 ℃, the
shrinkage is 2.5%.
The injection pressure can be increased
properly. For example, for POM products, when the injection pressure is 78mpa
9.8MPa, the shrinkage is 5%.
Increase the injection rate properly.
The pressure holding time should be set longer.
Increase the cooling time appropriately.
Select the material with uniform particles
to make the particles evenly heated, with uniform temperature and uniform
cooling speed.
The materials with proper molecular weight
and melt index and uniform molecular weight distribution should be selected so
that the process conditions are easy to control, the filling flow is stable and
the shrinkage is reduced.
For crystalline polymers, the conditions of
reducing crystallinity and stabilizing crystallinity should be provided, and
for non crystalline polymers, the factors of reducing solution orientation
should be created.
The shrinkage can be reduced by drying and
reducing water content.
Select the polymer with good fluidity and
low melt index.
The composite material with reinforced
filler can reduce the shrinkage.
On the mold side:
According to the shrinkage rate of the die,
the tolerance design of the die cavity is reasonable, and the die material with
small expansion coefficient is selected.
A proper increase in gate cross-sectional
area will help to reduce shrinkage.
Shortening the inner flow passage and
reducing the flow length ratio is conducive to feeding.
