Effect of temperature and pressure on residual stress
After discussing effect of flow on residual stress, let's look at effects of temperature and pressure on residual stress respectively. The first is plastic temperature. There are three mechanisms to heat plastic in injection molding system:
Injection machine tube heater;
Injection screw rotation friction shear heat;
Shear heat during plastic filling flow.
Increasing temperature can increase mobility of polymer chains, that is, increase relaxation behavior, so residual stress at high material temperatures will be lower. Impact of material temperature on three areas is as follows:
High material temperature
Thickness of area A of directionally solidified layer decreases;
Degree of polymer orientation in zone B of oriented high shear layer decreases;
Thickness of region C of non-oriented core layer increases.
Low material temperature
Thickness of area A of directionally solidified layer increases;
Degree of polymer orientation in zone B of oriented high shear layer increases;
Thickness of area C of non-oriented core layer decreases.
Main effect of pressure on residual stress is pressure-holding effect. As more plastic is continuously added to mold cavity, pressure compression brings polymers closer to each other and space between molecular chains is smaller, so it will cause:
Another polymer chain orientation behavior occurs between gate and vicinity of gate;
Orientation of polymers near gate can easily cause cracks in this area along flow direction;
If holding pressure rate is slow and gate is small, degree of solidification of oriented polymer chains will increase.
If there is no stress state between polymer chains (distance between them is not too close or too far), then they maintain the most appropriate distance between each other (the lowest energy state). Otherwise there will be two stress phenomena:
Internal tensile stress: When distance between polymer chains is farther than optimal distance, tensile stress occurs inside plastic part.
Internal compressive stress: When distance between polymer chains is closer than optimal distance, compressive stress occurs inside plastic part.

 

Figure 1: Polymer spacing and stress state after cooling of plastic part
When holding pressure ends, cooling effect on residual stress begins to be significant, because polymers in plastic parts with different thickness layers will cool and solidify according to different P-V-T paths under different temperature and pressure conditions. Therefore, the overall plastic part will undergo uneven cooling, and plastics with different thickness layers will also cause residual stress due to different shrinkage amounts.

 

Figure 2: Effect of plastic temperature on polymer chain orientation
Compressive or tensile stress inside plastic part refers to stress between polymers due to pressure holding effect, subsequent cooling and solidification. It is related to pressure, temperature and cooling rate, so influence of temperature and pressure on distance between molecular chains needs to be comprehensively considered. If in a stress-free state, the most appropriate distance between polymers in plastic part is 1.00. Take this as an example to illustrate:
When plastic is heated to 250℃ in material tube, distance between polymers expands from 00 to 1.03;
Space between polymers will be compressed during pressure holding. Assuming that pressure holding stage is over and cooling stage has just begun, pressure distribution state in mold cavity is 500 kgf/cm2 at gate and 300 kgf/cm2 at flow end. At this time, average distance between polymers is 01.
During cooling process, distance between surface and internal polymers of plastic part changes:
Surface plastic is cooled first because it is close to mold wall, causing polymers in this area to be close to each other due to cooling, and distance is reduced from 01 to 0.99;
Pressure inside mold cavity begins to decrease, causing distance between polymers in uncured central layer area to expand, increasing from 01 to 1.02;
When plastic continues to form a solidified layer from surface layer inward, polymers in new solidified layer area will also be closer to each other due to cooling, and distance will be reduced from 02 to 1.00;
Pressure inside mold cavity continues to decrease, causing distance between polymers in uncured central layer area to continue to expand, increasing from 02 to 1.03.
Cooling continues and plastic continues to solidify from outside in:
Solidified layer continues to be generated into mold cavity, and polymers in newly generated solidified layer area are also brought closer to each other due to cooling, and distance is reduced from 03 to 1.01;
As cooling continues, pressure inside mold cavity continues to decrease, and distance between polymers in unsolidified central layer continues to expand, with distance increasing from 03 to 1.04;
When the entire plastic part is completely cooled and solidified, polymers in original central area will also be closer to each other due to cooling, and distance will be reduced from 04 to 1.02.
Because cooling and solidification of plastic parts is from outside to inside, layer-by-layer heat transfer causes motion behavior of polymer chains to be related to thickness of parts, causing molecular chain spacing and thickness to be unevenly distributed. Parts close to surface of plastic part show compressive stress, and tensile stress increases toward inside. It is also because range of tensile stress zone is much larger than compressive stress zone, so dimensions of plastic parts tend to shrink in all directions after cooling and solidification.
If residual stress at different flow lengths is considered in more detail:
Compressive stress zone: near gate > far from gate
Tensile stress zone: away from gate > near gate
Therefore, filling end of plastic part shrinks more, and size after molding will be smaller than size near gate.
Cooling process not only causes stress distribution inside plastic part, but cooling speed also affects polymer chain orientation, residual stress, and degree of shrinkage of plastic part (see Table 1).

Table 1: Effect of mold temperature on molecular chain orientation, residual stress and shrinkage degree
If cooling rate on both sides of mold is uneven, it will cause asymmetric stress distribution in thickness of plastic part, lead to larger shrinkage on one side and warping:
Mold side with slow cooling rate (high mold temperature) has smaller compressive stress;
Maximum tensile stress zone moves to the side of mold with a slow cooling rate;
Plastic part shrinks more on the side with slower cooling rate;
Plastic part bends toward side with a slower cooling rate.