Plastic Injection Molding

The laboratory exercise consisted of molding a variety of plastic tensile test specimens under different operating conditions. The nominal molding conditions from the manufacturer required a barrel temperature of 475°F, and a nozzle temperature of 500°F.

The standard time interval was set at 10 seconds, and a standard pressure of 115 psi.

Once the machine was functioning properly, we first set the cycle time, pressure, and nozzle temperature, and varied the barrel temperature. The specimen list is shown below.

Speciman---Temperature-°F---Pressure-psi---Time-sec.

--------------Barrel--Nozzle

Set cycle time and pressure, varying barrel temperature

   #9     450    500       115         11      Should have been  
                                               10s               

  #14     475    500       115         10      nominal temps.    

  #19     500    500       115         10                        

The specimens 9, 14, and 19 all turned out to be good quality parts. This data indicates that the barrel temperature can vary some, while good parts can still be produced. This makes sense to me because the plastic last travels through the nozzle, where the temperature was held constant. Therefore, it is possible that all the plastic left the nozzle at a temperature close to the nozzle temperature of 500°F.

Secondly, the cycle time, pressure, and barrel temperature was set, and the nozzle temperature was varied. The specimen data was:

Speciman---Temperature-°F---Pressure-psi---Time-sec.

--------------Barrel--Nozzle

Set cycle time and pressure, varying nozzle temperature

  #15     475    475       115         10                     

  #14     475    500       115         10      nominal        
                                               temps.         

  #18     475    525       115         10                     

Specimens 14 and 15 were good parts, however specimen 18 was not. This was very likely because the nozzle temperature was 525°F. Therefore, the plastic became too hot, and caused the part to misformed.

Lastly, the cycle time, barrel temperature, and nozzle temperature were set at nominal values, and the pressure was varied. The data collected for this portion is shown below.

Speciman---Temperature-°F---Pressure-psi---Time-sec.

--------------Barrel--Nozzle

Set cycle time and nominal temperature, varying pressure

  #14     475    500       115         10      

  #16     475    500       100         10      

  #17     475    500       80          10      

Specimens 14 and 16 both turned out to be good parts under the molding conditions. However, specimen 17 was short shot. This was a result of the reduced pressure of 80 psi. This pressure was not sufficient to force the plastic into the mold, because it allowed the plastic to cool before the mold was full, and therefore the part was short shot.

The good quality specimens which were obtained were tested in the tensile test machine. The results of the tests are shown below.

Specimen------Ult. Load----Elong.-----Failure Load-----Elong.------Width-------Height

#7---------------275.5 lb.----0.48 in.-----122.73 lb.-------5.01 in.----.1775 in.-----.476 in.

#9---------------240.9 lb.----0.51 in.-----121.43 lb.-------5.62 in.----.1445 in.-----.479 in.

#12--------------325.1 lb.----0.52 in.-----143.62 lb.-------8.05 in.----.203 in.------.471 in.

#13--------------245.0 lb.----0.51 in.-------122 lb.---------5.4 in.-----.142 in.------.4785 in.

#14--------------242.9 lb.----0.57 in.-----112.94 lb.-------3.89 in.----.1555 in.-----.474 in.

#15--------------253.3 lb.----0.54 in.-----116.86 lb.-------2.10 in.----.1510 in.-----.478 in.

#19--------------265.1 lb.----0.39 in.-----101.90 lb.-------1.12 in.----.1790 in.-----.4755 in.

-A plot of the Failure Load vs. Elongation can be seen here.

-A plot of the Width of the specimen vs. the Elongation can be seen here.

These charts show very little correlation at all between the two variables in each case. One observation is that the specimen with the largest cross sectional area had the largest elongation and failure load. This is what would be expected for plastic.

It is very likely that the strengths of the different sections was a result of density variation between the specimens. Parts that had been shot at lower pressures would have a resultant density which is lower than the others. Also temperature variations could cause density fluctuations. Parts that were shot at lower temperatures could have had a lower density, and therefore a lower tensile strength. This accounts for the radical points on the plots above.

VACUUM FORMING

1. Plots of the temperature vs. cross section height measured at the eyebrow, and temperature vs. cross section height at the nose can be seen here.

Both plots look very similar, which implies that the plastic heated very uniformly in the oven. The heights of the specimens increased as the temperature increased until about 270°F. The heights then tailed off some. This implies that the optimal oven temperature is about 270°F for a time of 20 seconds that the plastic is in the oven.

2-4 The widths of the vacuum formed plastic specimens were not measured or recorded in laboratory.