Trent Smith completed a mechanical engineering internship at Hewlett-Packard where he worked on several projects to improve product quality and prevent failures. His major projects included designing experiments to characterize the impact of material and process variations on laser weld strength, quantitatively analyzing residual mold stress in transparent parts using a polarimeter, and supporting a prototype manufacturing line. Through hands-on learning experiences with plastics, manufacturing, and statistical analysis, Trent gained valuable skills and knowledge that will benefit his career. His work directly helped HP optimize processes, reduce costs, and avoid delays.

1. September 3, 2014
Intern: Trent Smith
Mechanical Engineer
2nd
Internship
Mentor: Josh Cooper
541 715 3613
Josh_cooper@hp.com
Hewlett-Packard
Internship Report

2. Contents
Executive Summary ...............................................................................................................................................................................................3
Introduction.............................................................................................................................................................................................................3
A brief history and introduction to HP.............................................................................................................................................................3
Internship structure...........................................................................................................................................................................................3
Project List...............................................................................................................................................................................................................4
Laser weld development, optimization and design changes ......................................................................................................................4
Quantitative analysis of residual mold stress in transparent parts............................................................................................................5
Page-wide array zone 4 product engineering ...............................................................................................................................................6
Tooling design work ..........................................................................................................................................................................................7
Conclusion...............................................................................................................................................................................................................7
Buzz Words & Acronyms.......................................................................................................................................................................................8

3. Executive Summary
At HP, I have worked on several projects and experiments to improve product quality, knowledge
and the customer experience. As a result, I have learned invaluable experience related to plastics
engineering, molding and laser welding as well as manufacturing processes, control capabilities
and statistical analysis and modeling.
Working in a new product introduction/product engineering group for page-wide array printing
platforms, my responsibilities included characterizing and improving plastics and laser welding
capabilities, working with a proto-manufacturing line yield team and tooling/fixture design. In
order to prevent future product failures, a major project involved coordinating a design of
experiments that determined the impact of material variations on laser weld quality. Another
design of experiments was performed to define an operational envelope around optimized laser
welding parameters. One of my first projects involved learning about stress-induced
photoelasticity and how to use a polarimeter – a tool that measured stresses in transparent
parts. After becoming familiar and confident with the tool, I became the go-to engineer for
measuring residual mold stresses, stress concentrators and comparing plastic molding
parameters.
Several changes and improvements have been made as a result of my work with Hewlett-
Packard. Characterizing residual mold stress and optimal welding parameters in a previously
failing part will ensure prevention of future failures and help save on material screening costs.
With my help, costly manufacturing line delays were avoided due to line down issues and busy
engineers.
Introduction
A brief history and introduction to HP
Hewlett-Packard was founded in 1939 by Bill Hewlett and Dave Packard. It was formed in a
rented garage in Palo Alto and is considered the original Silicon Valley startup. The pair created a
series of products beginning with audio oscillators and other electrical testing equipment. As HP
grew, cameras, recorders, calculators, personal computers, printers and much more were
released. Now, HP is one of the largest technology companies in the world and a household
name that offers hardware, software and IT services to customers.
Internship structure
My work revolved around inkjet printing solutions, specifically page-wide array printers that are
designed with print heads that span the width of a page or poster, alleviating the need for
scanning back and forth. Work done by my team and I utilized and supported a prototype
manufacturing line for new page-wide array printers. I worked with engineers, technicians,

4. analysts and more to ensure confidence in the products that were being developed. I was
mentored by Josh Cooper and my manager was Carole Petersen.
Project List
Laser weld development, optimization and design changes
Laser welding involves fusing two materials together with energy from a laser. Generally, one
material is transparent to the laser beam and the other is opaque to allow for absorption. The
opaque material heats up as it absorbs laser energy and conducts heat to the transparent
material. After enough heat has been absorbed and conducted, both materials surpass their
glass transition temperature and their polymers begin to entangle, resulting in a weld. There are
many inputs to the process, including laser power, speed, cycle time, clamping force and material
properties. Outputs include weld strength and environmental stress crack resistance. It is obvious
that this is a very sensitive process that requires a solid foundation of knowledge, testing
capabilities and material characterization.
Upon arrival at HP, I was tasked with multiple projects regarding the laser welding process for a
specific assembly. First and foremost, however, I had to become familiar with the mechanics
behind laser welding, learn how to use the tool and develop a background in plastics, as none of
these concepts were taught in school. I quickly learned that laser welding in general is a very
sensitive process that must be perfected to ensure product quality. If the process is not
optimized, a weak weld with high residual stresses will form. Due to the nature of the inks that
are put into these plastics, as well as the materials’ chemical composition, failure through
cracking of the weld is not uncommon, resulting in leaking ink and inoperable components.
My first laser welding project involved designing a series of experiments that would better
characterize material and geometrical variations and their impact on weld strength and
resistance to environmental stress cracking. This can be split up into three categories: % carbon
fiber variations in laser absorbent material, laser transparent material ladder study, and laser
transparent thickness variations. For each of these categories, ranges of values were chosen that
would produce the most effective results. The studies involving carbon fiber variations in the
absorbent material and thickness variations in the transparent material required working with a
plastics supply chain manager to order the necessary materials and have them molded, as well
as designing a new part to allow molding with different thicknesses. Unfortunately, due to the
relatively short duration of my internship, materials were not available in time for the studies to
take place. Still, the other experiment was conducted, involving me personally welding over 150
parts. Next, the parts were split up to get tested in multiple ways. Because plastics can absorb
chemicals and warp, a flatness test was conducted involving our metrology department
measuring surface profiles and flatness of welded parts before and after being exposed to ink at
high temperatures for long durations. Another test involved subjecting parts to two harsh
solvents that are intended to break polymer bonds at high stress locations. Lastly, a weld
strength test was conducted to measure the force required to break a weld joint. All these tests

5. allow comparative analysis between transparent material types to help drive decisions for future
products and prevent failure through cracking or deformation.
Another design of experiments was focused on the physical laser welding parameters and
optimizing them. Here, a range of parameters were combined to be run on 34 different parts. In
this study, weld speed, laser power, collapse height and clamping force were varied and their
impacts on weld strength and stress were explored. After setting up custom contour files that
tell the machine where to weld and how quickly, I was able to weld each part with a unique
setting. Similar to the previous design of experiments, the parts were tested with a push tester
and a chemical solvent. The results were analyzed using JMP’s statistical capabilities to model the
reactions and predict optimal welding parameters.
Another simple project regarding the laser welder involved a re-design of a welding cover plate.
The cover plate is made out of polished acrylic, transparent to the laser, that provides a flat
surface on one side and a negative of the part to be welded on the other. This allows for a
clamping force to be applied evenly throughout the welded part. This project included designing,
drawing and ordering the new cover plates.
Quantitative analysis of residual mold stress in transparent parts
In order to characterize residual mold stress in a specific part, I was tasked with learning how to
use a polarimeter. A polarimeter is a tool that measures stress-induced birefringence, or
wavelength retardation, by passing polarized monochromatic light through a sample and then
another polarizer angled 90 degrees relative to the first. Normally, two polarizers oriented at 90
degrees from each other would block all light passing through, resulting in darkness. If, however,
the light passes through a stressed, transparent material, it refracts at a certain wavelength and
a rainbow of colors appears corresponding to different stress levels. These colors, called fringe
patterns, can quickly identify regions of high stresses and allow qualitative stress analysis. This is
very beneficial, but no quantitative data can be drawn until the tool is paired with a compensator.
A compensator is a calibrated rectangular piece of plastic with a known, linearly increasing stress
gradient. With the turn of a dial, the plastic is moved along its major axis over the sample being
analyzed. When the stress in the sample matches that of the compensator, stresses are
cancelled out and no light can pass. The reading on the compensator in turn corresponds to a
wavelength retardation that is used to calculate stress at a specific point.
Initially intended as a method of characterizing and reducing residual mold stress in a specific
part to prevent cracking after being laser welded, the polarimeter was used to compare the
effects of different molding parameters on residual stress. The usefulness of the tool was quickly
realized, and implemented in other areas. I became the expert in photoelastic analysis of stresses
in transparent parts. First, I was directed to determine the impact of knit lines on weld strength. A
knit line is an inherent flaw in any molding process that is a result of viscous plastic flowing
around and not completely fusing together when it meets on the other side of geometrical
features. The result is a thin line that reflects laser energy instead of transmits it, causing a ‘cold
weld’ region. This cold weld, in turn, creates a stress concentrator in the weld joint that can
hasten cracking. In order to determine if knit lines negatively impacted specific parts, cross

6. sections were taken and examined at the cold weld. First, stresses were analyzed using a
polarimeter to determine the amplitude of stress concentration. Next, polymer attacking solvents
were dropped onto the cross-sectioned samples to determine if and when cracking would occur.
Other parts to be analyzed came from a product designer that was having cracking issues after
laser weld. After locating regions of high stresses and determining that they were higher than
stresses in similar parts, changes were issued to reduce overall stress levels and prevent cracking
after being laser welded.
In order to calculate stress from wavelength retardation, a material specific stress-optical
coefficient is required. Unfortunately, for the materials being used, this coefficient was not readily
known or available. In order to experimentally determine the value, I designed, machined and
built a tensile-test fixture that would apply a known (with the use of a load cell) stress in a
rectangular sample. This allowed a back-calculation of the stress-optical coefficient and thus the
ability to calculate actual stresses for all parts of the same material.
Another study involving photoelastic analysis stemmed from an attempt to reduce noise in a
solvent screening selection experiment. The experiment used rectangular coupons that were
clamped to an ellipse fixture so as to induce a known stress gradient along their length. The
coupons were submerged into various solvents in order to match the cracking pattern with that
seen after long exposure to inks. After the coupons were analyzed, the data was messy and
unexpected, there were too many sources of noise to draw any justifiable conclusions. After
examining some coupons with the polarimeter, it was determined that each coupon had a
significant difference in residual mold stress which was dependent on orientation. After this was
found, a smaller experiment was repeated with coupons that had similar stress levels and they
were loaded into the ellipse fixture the same way. While there was still some noise in the data
due to the test fixture itself, the overall results were greatly improved.
Page-wide array zone 4 product engineering
All of the material and process characterization done in other projects result in valuable
knowledge gained for implementation in the page-wide array prototype manufacturing line. The
line breaks up the assembly process into multiple zones, where engineers continuously work to
develop better processes and tools. In turn, part of my responsibilities while working at HP
included line support in a specific assembly zone as a product engineer. As a product engineer,
duties included addressing daily and reoccurring line issues, determining contamination sources,
writing and implementing temporary engineering exceptions (TEEs) for operator instruction,
specifying, buying and calibrating new equipment and reorganizing the zone layout. One major
project was a gauge repeatability and reproducibility (Gauge R&R) study. The study was
conducted on a leak testing tool that was fairly new to the zone and had not yet been validated.
For any measuring device, it is important that the measurements it makes are reproducible even
with outside variables such as operator, time of day, etc. In order to conduct an R&R, both
‘passing’ and ‘failing’ parts were collected. All parts were run in random order by two operators,
one after the other. This was repeated a total of four times throughout the day. Next, data was
collected and analyzed to determine a precision to tolerance ratio – an accurate measurement of

7. repeatability for a gauge tool. This entire study was conducted again after testing parameters
were changed in order to reduce total cycle time.
Tooling design work
Apart from planned, preliminary responsibilities, I was contacted multiple times to design tooling
and fixturing equipment. One design project involved modifying a mounting device for an inverted
microscope. Another project was to design a fixture for installing rubber fluidic interconnect plugs
into holes in a part. Before, operators had to forcefully push the plugs in with a screwdriver – this
proved to be ineffective and an issue ergonomically. A fixture was designed that housed the
plugs and guided the part into them with much less effort by the operator. A more involved
project consisted of adding barcode scanner mounts to an electronic testing (etest) fixture. Two
different etest fixtures were in use, each with different requirements for mounting a scanner. One
design was fairly straight forward, involving a clamping sheet metal mount to prevent requiring
changes to the existing fixture. The other design required much more work, as the scanner
needed to move into and out of position so that multiple parts could be tested quickly and
consecutively. A 24 volt micro switch was also required to signal when the scanner was in
position. Tight geometrical constraints proved to be a very limiting factor for the mount, so a
pivoting design was implemented that would swing the scanner into and out of position while
preventing any clashing. For all the design projects, drawings were made and reviewed then sent
out to machine shops to be fabricated.
Conclusion
As a result of this internship, I have gained invaluable knowledge that I would not have otherwise
learned from school alone. Being exposed to manufacturing processes and control has proven to
be very beneficial and interesting, which was unexpected. With this exposure, I gained insight
regarding tooling methods and design, process control, statistical modeling and test
lab/analytical methods, to name a few. More specifically, learning about and implementing
design of experiments was extremely beneficial. This knowledge will be carried throughout my
career as a method of optimizing parameters. Experience with photoelasticity and birefringence
has also proven to be very advantageous. Direct methods of stress analysis are rare, having
experience measuring and characterizing stresses will also be useful for the duration of my
career. Working with laser welding and plastics has been completely new to me. I learned about
interactions, degradation and fracture in polymers – materials that are not considered or
explored in the undergraduate mechanical engineering program at Oregon State. Lastly,
continued experience with design work and drawings has only proven to enhance my skill set.
It is extremely rewarding to know that my work has and will continue to directly and positively
impact future products at HP. My work in characterizing and analyzing stresses in transparent
parts has caused an increase in awareness of the tool’s usefulness. Future design and molding
parameters will be successfully optimized with the help from this method and the documentation
that I have left the company with. This will save vast amounts of time, money and trouble by
preventing future stress-induced cracking issues. My work in developing and implementing DOEs
for the laser welding process and material variations will also be used to implement future
product improvements, allowing cost savings in material lot screening and field failures. Design

8. work and product engineering support has helped to relieve other busy engineers and allow them
to focus on other priorities, resulting in more efficient work done by all.
Buzz Words & Acronyms
 DOE: Design of experiments
 ESCR: environmental stress crack resistance
 Etest: electrical tester for use with printers
 Gauge R&R: gauge repeatability and reproducibility study
 Laser welding: method of fusing two plastic parts together
 Polarimeter: tool used for measuring birefringence in transparent parts
 Solvent testing: method of determining stresses through exposure to polymer degrading
chemicals
 Stress-optical coefficient: material specific value that correlates wavelength retardation
and thickness to stress
 TEE: temporary engineering exception