Plastic Injection Molding
IE 3122 Material Processing Lab
Section (Tuesday, 1p.m.)
Lab Group #6
This paper was submitted on this day,____________, for consideration in partial fulfillment of
1. From the ANOVA table that was produced from the collected data of the experiment it
was observed that the flow rate and the injection pressure were the most impacting factors on the
outcome. This conclusion can be made by comparing the F value of those two factors to the
corresponding F critical values. For the injection pressure, it had an F value of 121.93 and an F
critical of less than 0.0001, which makes it very significant. The F value for the flow rate was
129.74 with an F critical value of less than 0.0001, which also makes this parameter the most
significant. The other parameters for this experiment also had an impact on the final result but
none of them were as significant as the flow rate and the injection pressure. The next most
significant factor was the dwell time, having an F value of 8.53 and an F critical value of 0.0056.
the least critical factor in this process turned out to be the nozzle temperature. It had an F value
of 2.10 and an F critical value of 0.1544.
Another thing that will prove that the two most impacting factors were the injection pressure
and the flow rate is the sum of squares that was calculated in the ANOVA table. The sum of
squares describes the variance of the factors of the process. The sum of squares for the injection
pressure is .68 which indicates that it has a great effect on the outcome of the product. The sum
of squares for the flow rate is .72 which is greater than the sum of squares value for the injection
pressure. Therefore making this the factor that impacts the part the most. The sum of squares
value for the least impacting factor is for the nozzle temp which had a value of 0.012.
Injection pressure and the flow rate had the most impact because they determined how much
material was being put into the mold for the given time that the injection process was to last.
Once the material was melted, the pressure that it was forced into the mold. The flow rate, which
we judged by the number of turns of the knob on the injection molding machine (1/2 or 2 turns),
was the second most impacting factor. That is because the more turns on the flow rate of the
machine, the more material would be allowed to be injected into the mold. This factor also is
impacted by the time of the injection and the pressure at which the injection occurs.
The observed error in this data could be described by the Residual sum of squares. This
describes the value of the extra/ unneeded material/ variance in the data from the predicted
values. The residual sum of squares for the data that we calculated was 0.23, which is not very
high, which indicates that our values were fairly close to the predicted/ expected values for this
experiment. Another thing that would indicate that we have an error in our data is the R2 value
that the program calculated for a fitted line for our data. It had a value of 0.8635 which is
indicates that our data has a fairly linear relationship, but also indicates that our data has error in
it as well.
2. Plastic injection molding consists of two main components: injecting and clamping.
Starting with the clamping end of the process, a mold must be made for the plastic injector to
push hot liquid plastic into. The mold is usually made of a machine-able metal, usually
aluminum or steel. The mold consists of a closed cavity for the material to flow into. A sprue is a
hollow tube leading up from the mold for the material to flow into and into the mold. A well is
located at the bottom of the sprue to build up material and push it into the mold. Another
important part of the mold cavity is the vents; these help push ambient air in the mold so the
mold can be completely filled. The injection part of the process includes a hopper for raw plastic
material to be fed into. This is usually fed into the barrel by a screw pushing the material towards
the nozzle. The barrel heats up the material till it is soft and turned into a liquid. The nozzle
pushes the material through down into the mold by pressure. The barrel and nozzle temperatures
can be controlled separately. These temperatures control the viscosity of the material. The
pressure of the injector controls the rate of flow of the injection into the die cavity.
Polyethylene is a synthetic fiber used in making various products like beverage and
containers. It is a good barrier against gas, moisture, alcohol, and solvents. The chemical
composition for polyethylene is (C10H8O4) and is a thermoplastic. Has a crystalline density of
1.455 g/cm3, a tensile strength of 55-75 MPa, a Young’s Modulus (E) of 2800-3100 MPa, and a
melting temperature of 260oC. It is strong and impact resistant. It has a semi-crystalline structure
and is transparent or opaque depending on the structure and particle size. Polyethylene is
commonly referred to as polyester for packaging materials. Before the material can be processed
it must be dried to bring down its moisture content as far as possible. Polyethylene can be formed
through plastic molding injection or blow molding. After a product has reached its end of life
cycle the polyethylene can be recycled back into the system and used again once it has been
process and chipped back up to be used again in molding.
Polypropylene (PP) is a thermoplastic polymer with whose mer chemical formula is
C3H6. PP’s density ranges from 0.855 g/cm3 to 0.946 g/cm3. Its melting point ranges from 130 to
171 degrees Celsius, however in most application it is assumed to be 160 degrees Celsius. PP is
commonly used to produce both fiber and molding. Therefore, the two main manufacturing
processes are extrusion, and injection molding, respectively for fiber and plastic molds. PP is
commonly used for Tupperware due to its higher melting point, which allows it to heat foods
without melting. It is also highly resistive to fatigue and stress, allowing it to be used in high
wear application. Its main weakness is degradation due to UV radiation.
In conclusion this lab was useful in exploring the different factors associated with the plastic
injection lab. The main factors that affected the final weight of the part were pressure and flow
rate. These two factors made the biggest difference in the final part weight. One of these factors
had a bigger impact over the nozzle temperature, barrel temperature and dwell time combined.
Even though this experiment was a half factorial, a lot of useful data was collected about the
different factors and how they interact with each other. For example factors A (Barrel Temp.), B
(Nozzle Temp.), C (Dwell Time) had almost no affect on the parts final weight. Factors D
(Injection Pressure), E (Flow Rate) had a major effect on the final part weight. This was shown
by the sum of squares factor in the ANOVA table. The ANOVA tables were very helpful in
showing the interactions and affects of the different factors on the part weight.