Assembly EndUser: Using Design of Experiments to Trim Plastic Bonding Times

September 1, 2005
/ Print / Reprints /
ShareMore
/ Text Size+
Although three seconds isn't long, if it's 33% of a discreet manufacturing process's assembly time, dropping the cycling period from nine seconds to six presents a significant savings. This article addresses application pressure vs. bonding time when adhering clear plastic viewing-lens panes to the back of hand-held device housings during assembly. Plastics adhesion problems are widespread throughout the industry because many plastics have chemically inert, nonporous surfaces exhibiting low surface tensions. This property is usually advantageous to the design engineer, but it often results in assembly difficulties when bonding plastic to other components.

To learn if assembly throughput times could be shortened, researchers recently conducted a series of design of experiments (DOE) studies at a cell phone design and prototype manufacturing facility. In this DOE, experimenters analyzed a cell phone's design, in which a clear viewing lens, treated with pressure-sensitive adhesive (PSA), was machine-pressed into the back of a housing. Restraining pins and the PSA are designed to keep the lens in place during repeated drop tests.

The existing assembly procedure called for 30 psi to be applied for nine seconds to affix the lens to its housing. The DOEs were designed to learn whether the nine-second pressure time could be reduced to achieve the following improvements.

  • Increased throughput
  • Reduced cycle time
  • Increased profit


Figure 1. The red and black “I-beams” are ‘least significant difference’ bars set at 95% confidence. The vertical overlaps indicate there is 95% confidence that Factor B (pressure) has no effect at 2 or 4 seconds. Thus, Factor A (time) does not depend on Factor B, meaning an interaction is not occurring. (Courtesy of Stat-Ease Inc.)

Screening Experiment

Researchers first created a two-factor, general-factorial DOE - one factor being pressure (pounds per square inch, or psi), and the other being time (seconds) the pressure is applied. Three cell phone batches, each containing 40 phones, were available. The statistical team used the first batch of 40 phones to conduct a screening experiment to find any significant effects. The subsequent two batches of 40 cell phones were later used, after the significant effects were identified.

Within the DOE screening experiment, four bonding times (2, 4, 6 and 8 seconds [Factor A]) and two pressure settings (30 and 40 psi [Factor B]) were addressed. Because there were eight factor combinations (four ‘Factor A' levels, two ‘Factor B' levels) and 40 available phones, five phones were tested at each factor combination. All 40 were tested in random order by dropping each unit a designated number of times from a height of five feet, on all six sides of the phone. The six-side drop sequence was identical for all tests. If a lens dislodged on any drop, it was considered a failed unit. Responses were recorded as pass=1, fail=0 (see Table). The underlying response distribution was not normally distributed, so all responses were logic-transformed before the Analysis of Variance (ANOVA) was used to analyze the experiment. The table shows the screening experiment data, including factors, factor levels and actual and transformed factor-combination responses. ANOVA was performed on the transformed responses using Design-Expert® software.

Figure 2. With Factor A (time) set to six seconds, the probability of success was 100% at either pressure setting (Factor B). (Courtesy of Stat-Ease Inc.)

Sequential Experiment

With the screening experiment, researchers learned that the amount of applied pressure (Factor B) was not statistically significant. By viewing the interaction graph (see Figure 1), they observed that Factor A (time), when set to six or eight seconds, increased the probability of passing the test (a secured lens), regardless of applied pressure.

Using these screening experiment results, the statisticians designed a follow-up experiment by reducing times to five and six seconds, respectively, but keeping pressure to the two levels of 30 and 40 psi as before. Their objective was to determine if time could be reduced to five seconds while retaining quality. They used a randomized complete block design for the two-level factorial experiment. As before, Factor A was time and Factor B was pressure. Each block was represented by a batch of 40 phones. The phones were tested as before and the responses recorded. Each batch of 40 cell phones underwent testing at four factor combinations (two levels of A, two levels of B). Ten cell phones were tested at each factor combination. All 40 in each batch were tested in random order.

Recommendation

Investigating the interaction graph for the experiment in Figure 2, the researchers observed that pressure could be either 30 or 40 psi at six seconds. At five seconds, however, the required pressure was 40 psi, with the probability of passing the test being 90%.

The experimenters' recommendation was 30 psi for six seconds to guarantee that the phones satisfied the quality requirement. Based in part on this DOE, the prototype cell phone design was subsequently transferred to a high-production manufacturing facility. The findings have since increased the viewing-lens assembly speed by 33%.

For more information, contact Heidi Hansel, Stat-Ease Inc., 2021 E. Hennepin Ave., Suite 480, Minneapolis, MN, 55413; phone (612) 378-9449; e-mail heidi@statease.com; or visit http://www.statease.com.

Did you enjoy this article? Click here to subscribe to Adhesives & Sealants Industry Magazine.

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

ASI April 2014 Photo Gallery

Our April 2014 issue is now available!

Podcasts

ExxonMobil Tackifier Expansion

Dwight Tozer, vice president of ExxonMobil’s Adhesion Industry business, discusses the company’s latest tackifier expansion project with Editor-in-Chief Susan Sutton.

More Podcasts

Adhesives & Sealants Industry Magazine

ASI December 2014 cover

2014 December

The annual Buyers' Guide is here! You won't want to miss this.

Table Of Contents Subscribe

Bonding Plastics

Are you involved in bonding plastics or other difficult-to-bond substrates?
View Results Poll Archive

THE ADHESIVES STORE

handbook-sealant-tech.gif
Handbook of Sealant Technology

The Handbook of Sealant Technology provides an in-depth examination of sealants, reviewing their historical developments and fundamentals, adhesion theories and properties, and today’s wide range of applications.

More Products

ASI 2014 Buyers GuideASI's Buyers' Guide

Annual purchasing resource for equipment used in the manufacture/formulation of adhesives, sealants, pressure sensitives, tapes and labels and for application of finished adhesives. 

STAY CONNECTED

facebook_40px twitter_40  youtube_40pxlinkedin_40 google+ icon ASI 30px

Clear Seas Research

With access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.