Plastic Injection Molding Experiment

Submitted to: Dr. John Davis

Feb. 20, 2014

ETME 216 Section 003

Plastic Injection Molding

Lab Summary

Jake TeSelle

Jarek Jensen

Nathan McDonald

Michael Lundstrom

Daniel Murphy

Martin Reaves

Introduction

Purpose of Experiment:

In this lab, a hydraulic plastic injection molding machine was used to fabricate two separate

parts. Aluminum molds sprayed with silicone mold release were injected with two types of

thermoplastic. A thermoplastic is a polymer that can be remelted and reformed. The two

plastics used were polypropylene and polystyrene. The task was to create 3 good parts in a

row after determining the optimal pressure and temperature for each mold with each type of

plastic. The lab allowed the relationships between material temperature and injection pressure

to be explored in the aim of dialing in a repeatable process.

Test & Evaluation

Apparatus used:

● Aluminium Mold: Screw driver and Checker pieces

● Plastics: Polypropylene, Polystyrene (our group used polypropylene for both parts)

● Screw driver tip

● Plastic mallet

● Plastic injection Molder

● Camera

● Metal plates

● Crank handle

● Temperature gauge

● Small metal spoon

Process/Procedure:

For this lab a hydraulic press was used to inject molten polypropylene plastic into two separate

machined aluminium molds. One mold was to manufacture a screwdriver handle and the

second was to manufacture a checker and square part. Once the mold had been sprayed with

a small amount of silicon mold release to make the part easier to remove from the mold, the

mold was centered and clamped below the injection port. Plastic feedstock pellets were

inserted into the heater to liquify them. An initial pressure was then set and the mold was

activated. The nozzle came down, made contact with the mold, and injected it with plastic until

it was full. Overfilling the mold resulted in not succeed in making the right part. After letting the

mold cool, it was unclamped and the mold was opened to reveal the parts. They were then

inspected for quality. Then the process was repeated to try to successfully make the same part

three times in a row for both molds.

Findings

Data Gathered:

Screw Driver Polypropylene

Run No. Temp(F) Pressure (Psi) Comments Injection Pressure(PSI)

1 375 40 Need more flow 785.398

2 500 40 Some flashing 785.398

3 500 45 Air bubbles 883.57

4 500 45 GOOD 883.57

5 500 45 GOOD 883.57

6 500 45 GOOD 883.57

Checker/Square Polypropylene

Run No. Temp(F) Pressure (PSI) Comments Injection Pressure(PSI)

1 500 45 Lots of flashing 883.57

2 500 40 Same 785.398

3 500 35 Mold explosion 687.223

4 500 30 Did not fill 589.048

5 500 32 Mold explosion 628.318

6 500 30 GOOD 589.048

7 500 30 GOOD 589.048

8 500 30 GOOD 589.048

A desirable part has very few to no air bubbles within the part. By adjusting the pressure and

heat of the polypropylene, the number of air bubbles within the part can be lowered. When the

part cools the air bubbles cause it to shrink unevenly which causes a rough finish. The part

should not have excess plastic around the outside of the part as shown on the first two parts in

figure 4. Having excess plastic around the part can be caused by the injection pressure being

too high, this is called flashing.

Conclusions

The purpose of this lab was to familiarize students with several procedures and difficulty of

plastic injection molding. Plastic injection molding takes careful technique and calculations to

produce quality parts. In this particular experiment the plastic injection molders were set to a

starting temperature of 375 F and the pressure at 40 psi. The two part aluminum mold was put

together and clamped into place under the injection molder. Polypropylene pellets were poured

into the machine where an internal hot iron melted them. The first run produced just a small

blob of plastic within the screwdriver mold. It was understood that the result was due to the lack

of fluidity of the plastic. The temperature was increased to 500 F while the pressure was left at

40 psi. Run two created an actual screwdriver but it was determined that it contained too much

flashing. The third run was done with the temperature still at 500 F, while the pressure was

increased to 45 psi. The resulting part was good but contained air bubbles. At this point it was

determined that the plastic injection molder was set to the correct specifications, the problem

was in the aluminum mold itself. On the fourth run the aluminum mold was line up very carefully

to make a more airtight seal to help prevent the air bubbles in the part. The fourth run was a

success; the same precise procedure was repeated three more times producing a quality part

each time.

Part two of the lab was completed by testing a different mold with the same plastic

injection molding machine and plastic (polypropylene). The mold in this part of the lab was

completely different. It was a mold to make a checker game piece and a thin square part. The

instructions were to leave the machine at the final settings from the previous screwdriver mold.

This seemed suspect to not work due to the difference in thickness of the actual part size. The

purpose was to see the implications of an incorrect injection setup. At 500 F with 45 psi

settings, the part had too much flashing. The pressure was dropped to 40 psi but did not

change the results. After three more unsuccessful runs at different pressures a quality checker

was finally molded at 500 F with 30 psi of pressure. The process was repeated three more

consecutive times producing an acceptable part each time.

Plastic injection molding is a science requiring perfection to produce quality parts that do

not contain defects. Many factors have to be taken into account when producing plastic injected

parts. The proper plastic needs to be chosen depending on the type of desired part. Thickness

of the part should be considered when choosing the plastic. Different plastics mold differently

depending on mold wall thicknesses and size of molds. The results of this particular lab showed

that the polypropylene worked better for the screw driver mold due to it being a thicker mold; the

polystyrene worked better for the thinner checker mold. This is due to the characteristics of the

plastics. One of the most important factors of a successful part in this particular lab was giving

the plastic the proper amount of time to reach its maximum melting point. This allowed the

plastic to have a high fluidity going into the mold. All of these considerations need to be

considered when producing plastic parts. This lab showed the complexity of engineering plastic

injection molded parts. The lab was a successful experience for our group.

Attachments

Calculations:

❑❑❑❑❑injection pressure

Data Sheets:

Figures & Graphs:

Fig 1 Attempts 1-6 of the Screw Driver

Fig 2 Screwdriver Aluminium Mold

Fig 3-Manuel Plastic injection Molder

Fig 4-Checker/Square Part Attempts 1-8

Drawings, Sketches & Pictures:

Reference Materials:

Manufacturing Processes Lab Manual ETME 216 & 217. Dr. John R Davis ScD

Anything Else Important: