Handout Version L
BOTTLE CAP CASE (Rev 2_20)
A. Understanding the Case.
(This case was developed for educational purposes. While it was reviewed by an injection molding
some aspects of the case were introduced in the case entirely for educational purposes, and may not
represent actual industry practice.)
You are a member of a quality control team for a small injection molding company. Your company
produces specialty plastic bottle caps used for wide mouth PET bottles. Your company has received
complaints about the consistency of the caps from the bottling company.
Proper was thickness is important for a bottle cap because a tight seal with the bottle is extremely
important. If the diameter is too large, the product will leak from the bottle. If the diameter is too small,
the cap will not screw onto the threads of the bottle properly. Company engineers have determined that
ideal diameter for the bottle caps is 0.375 mm. However, they have allowed a +/- .004 tolerance.
The caps are made using a Cincinnati Milacron, 250 ton, high pressure injection molding machine. The
injection press runs 24 hours per day, with three shifts of eight hours each. There is a different operator
The mold used in the press is made of aluminum and can make up to nine caps in one cycle. Since the
machine is dedicated to making this part, the mold stays in the machine. The resin (plastic pellets) used
make the caps is high density polyethylene (HDPE). This is a very common resin and is relatively
inexpensive. The pellets are poured into a hopper above the machine, and then melted so the fluid can
injected into the mold.
One cycle consists of 4 basic operations: closing the mold, injection of the resin, cooling of the part, and
opening the mold. The equipment is run in semi-automatic mode where the operator closes the mold to
start the cycle, but the mold is automatically opened at the end of a cycle. The operator presses a
and the parts are ejected into a temporary bin for inspection.
In the current process, after every 10 cycles, the operator will select a sample of 5 parts at random for
inspection with a go/no-go gage. If all 5 parts are good, the temporary bin is dumped into the crate for
shipping to the customer at the end of the shift. If any of the 5 are not good, the parts are dumped into
Proper temperature control of the part is essential to producing a quality part. The mold is cooled by a
chiller, which pumps cold water through small channels in the mold in order to keep the mold cool.
Depending on the cycle time, or even the weather, the chiller can sometimes become overheated, and
result it will not be able to keep the mold temperature stable. When this happens, the diameter of the
caps may vary according to temperature changes in the mold.
If you are not familiar with plastic injection molding, you may want to learn a bit about it before
you start the case. There are many videos on YouTube that portray the process. You may find
these videos helpful to understand the case.
Injection molding basics: https://www.youtube.com/watch?v=RMjtmsr3CqA
Plastic crate with pellet loading: https://www.youtube.com/watch?v=YQRT-KIiv3g
C. Initial Capability
Run a capability analysis using the overall data. (Run both the capability analysis and the sixpack.)
C.1. Explain the purpose of a capability analysis and why you would want to run the analysis for
C.2. Paste the initial capability analysis here – both Sixpack and Process Capability Report.
C3. Discuss assumptions required to perform a capability analysis and explain whether the assumptions
been met or not. Discuss why capability analysis information may still be useful even if an assumption
had not been met.
C.4. Based on the results of the overall capability analysis, what is the capability of the current
process to hold the diameter? (discuss Cp, Cpu, Cpl and Cpk and describe what each tells about the
C.5. Assuming the process remains in its current state, what would you predict the percentages out
of specification (expected overall) to be in the future? Describe how these percentages relate to the
Purchase answer to see full