Injection Molding Lab
Objective
Exposure to basic processes and parameters of injection molding using a benchtop machine.
Discussion of Results
The variation in mold geometries and properties was due to the fact that certain aspects of the mold process were controlled while others remained constant. The purpose in doing this was to observe the effects of these changing parameters to gain a better understanding of their influence in the mold cycle. By increasing the barrel and/or nozzle temperatures above the recommended values, flashing was the usual result. This is because the plastic is melted to a lower viscosity and thus fills the cavity quicker and more completely but often seeps between the mold halves as well, creating flash. By decreasing the temperatures, a short shot is usually the result. The reason for this is that the plastic melts to a lower viscosity and cools before the cavity fills. Flash is also created by lowering the clamping pressure such that the injection pressure forces the mold halves to separate. A higher clamping pressure will make a better part, but more pressure than is necessary will cause undue wear to the machine.
Injection
Molding
Results
Trial Barrel Nozzle Pressure Cycle Observations
Temp. Temp. (F) (psi) Time (s)
(F)
1 450 475 115 Unknown almost full
2 450 500 115 2 less full
3 450 500 115 31 flash
4 450 500 115 30 stem
5 450 500 115 7 half shot
6 450 500 115 11
7 450 500 115 11
8 450 500 115 11
9 450 500 115 11 rippled on
end
10 450 500 115 2.8 stem
11 450 500 115 2.8 1/4 part
12 450 500 115 30 flash
13 450 525 115 10 good
14 475 500 115 10 little
flash
15 475 475 115 10 little
flash
16 475 500 100 10 bubble;
flash
17 475 500 80 10 half shot
18 475 525 115 10 big flash
19 500 500 115 10 flash
Recommended
Temperatures-- 475 500
Three
typical
specimens
with varied
barrel
temperature
Expectation
9 450 500 115 11 short shot
14 475 500 115 10 good
19 500 500 115 10 flash
Three
typical
specimens
with varied
nozzle
temperature
15 475 475 115 10 short shot
14 475 500 115 10 good
18 475 525 115 10 flash
Three
typical
specimens
with varied
pressure
14 475 500 115 10 good
16 475 500 100 10 short shot
17 475 500 80 10 half shot
Tensile
Test
Results
Trial Ultimate Elongation Failure Elongation Initial Initial
from I.M Load at Load at Thickness Width
(lb) Ultimate (lb) Failure (in) (in)
results Load (in) Load
(in)
7 275.5 0.48 122.73 5.01 0.1775 0.476
9 240.89 0.51 121.43 5.62 0.1445 0.479
12 325.11 0.52 143.62 8.05 0.203 0.4705
13 245 0.51 122 5.4 0.142 0.4785
14 242.85 0.57 112.94 3.89 0.1555 0.474
15 242.2 0.63 101.84 4.05 0.1485 0.479
16 253.3 0.54 116.86 2.1 0.151 0.478
19 265.05 0.39 101.9 1.12 0.179 0.4755
The stress-strain diagram for plastics as they are pulled at a slow rate of increasing tension has a rapid rise in stress in the elastic region. After the yield point, the stress level remains fairly constant while the strain increases to the failure point. The properties of plastics can explain this resulting strength analysis. Plastics are made of polymer chains that can be extremely long such that a complete part could be made from one chain. This property gives the material the ability to be stretched without failure. As the plastic is put in tension, the chains are allowed to stretch because the molecules are only bonded strongly with other molecules before and after it in the chain. Thus, the chains tend to straighten out and elongate as they also get thinner. Evidence of this can be seen by examining the tensile test specimens. The strands of plastic at the frailure point are dense collections of these elongated polymer chains.
The elongations when graphed against the initial cross-sectional area of the specimens show virtually no correlation. As the area increases, the elongation should also increase, however, we also varied the pressures and temperatures when making the specimens, so a direct correlation should not be expected. By changing the pressures and temperatures in the molding process, the density and overall strength of the part is effected. As density increases, i.e. higher pressures and cycle times, the chains are compressed more and can then elongate more, thus causing the results obtained in the lab.