When acrylic products are processed by injection molding, defects such as warping, deformation, bubbles, cracks, wrinkles, pitting, craters, flashovers, weld marks, burns, silver threads, and spray lines are common. What kind of solution should we adopt when warping occurs?

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PMMA acrylic cracking solution during injection molding
Written By Jasmine HL
When acrylic products are processed by injection molding, defects such as warping, deformation,
bubbles, cracks, wrinkles, pitting, craters, flashovers, weld marks, burns, silver threads, and spray
lines are common. What kind of solution should we adopt when warping occurs?
Cracking is a more common defect of plastic products, and the main reason is due to stress and
deformation. There are mainly residual stress, external stress and stress deformation caused by
the external environment.
Cracks caused by residual stress
Residual stress is mainly caused by the following three conditions, namely overfilling, ejection
from the mold and metal inserts. As the cracks generated in the case of overfilling, the solutions
can be started in the following aspects:
Because the pressure loss of the direct gate is the smallest, if the crack is mainly generated near
the direct gate, you can consider using the multi-point distributed gate, side gate, and shank gate
method.
Under the premise of ensuring that the resin does not decompose and not deteriorate,
appropriately increasing the temperature of the resin can reduce the melt viscosity and improve
the fluidity, and also reduce the injection pressure to reduce the stress.
In general, stress is easy to occur when the mold temperature is low, and the temperature should
be appropriately increased. However, when the injection speed is high, even if the mold
temperature is lower, the generation of stress can be reduced.
If the injection and dwell time is too long, stress will also be generated. It is better to shorten it
appropriately or perform th dwell pressure switch.
Non-crystalline resins, such as AS resins, ABS resins, and PMMA resins, are more likely to cause

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residual stress than crystalline resins, such as polyethylene and polyoxymethylene.
When ejecting from the mold, the ejection force is too large and the stress is generated due to
the small mold release gradient, rough mold glue and convex mold, and sometimes even
whitening or cracking occurs around the ejection rod. Just look closely at the location of the crack
to determine the cause.
When inserting metal parts at the same time as injection molding, stress is most likely to occur,
and cracks easily occur after a period of time, which is extremely harmful. This is mainly due to
the large difference in thermal expansion coefficient between metal and resin, which generates
stress, and with time, the stress exceeds the strength of the resin material that is gradually
degraded and cracks occur.
In order to prevent the resulting cracking, as a rule of thumb, the general-purpose polystyrene
with a wall thickness of 7 ”and the outer diameter of the embedded metal part is basically not
suitable for inserts, and the impact of the inserts on nylon is minimal. Because of glass fiber The
coefficient of thermal expansion of the resin material is small, which is more suitable for inserts.
In addition, pre-heating the metal inserts before molding has a better effect.
Cracks caused by external stress
The external stress here is mainly caused by stress concentration due to irrational design,
especially at sharp corners. It is preferable to use R / 7 "-0.5 to 0.7.
Cracks caused by external environment
Chemicals, water degradation caused by moisture absorption, and excessive use of recycled
materials will degrade physical properties and cause cracks.