Determining the Probability of the Visual Detection of Sink Marks on Differently Textured Injection Molded Products

Customer demands for high-quality, defect-free injection-molded plastic products are continually increasing. Additionally, consumers are expecting a greater variety of options in products, with increasingly reduced delivery times. In order to satisfy these expectations, product developers use computer software to sim ulate the processing of the plastic as w ell as the final appearance of the product. Much effort is expended to ensure that computer-generated renderings of sim ulated prototypes accurately represent the size, shape, color and surface texture of the final product. At the same time, the effects of attribute defects such as sink marks or splay are often difficult to sim ulate in renderings. Injection molding simulation software can predict the occurrence of these defects, but the severity of their appearance in the final product can be questionable. While customers routinely reject products because of these attribute defects, it benefits the manufacturer to understand the effects of these defects on the appearance of the product. In the case of the defect known as sink marks, it is well understood in the industry that surface textures can be used to hide minor occurrences of the defect. However the exact nature of this relationship has not been completely explored. It is known that the surface texture scatters the light and interferes which the observer’s ability to see the sink mark. However it is not understood how deep (or coarse or aggressive) the texture m ust be in order to hide sink marks of given depths. Figure 1 in Appendix C shows the appearance of sink marks on a textured surface. PURPOSE/BACKGROUND INFORMATION This research is intended to study how surface texture effects the visual detection of an injection molding defect called a sink mark. A sink mark is shallow depression or dimple on the surface of a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. finished plastic part due to shrinkage or improper filling of the mold cavity. Human observers will evaluate a series of injectionmolded parts that exhibit various depths of sink marks and have various levels of texture. The project w ill take place in the Plastics Processing Laboratory at Western Michigan University’s Parkview Campus. Human observers w ill view pairs of parts in an inspection fixture illum inated by a moving fluorescent inspection lamp. The length of involvement for each observer w ill be two sessions on separate days each lasting approximately sixty minutes. SUBJECT RECUITMENT Recruitment signs and pamphlets w ill be posted at the College of Engineering and Applied Sciences and at volunteer plastics molding companies. Volunteer companies w ill be solicited through existing relationships between the W estern Michigan manufacturing community and the Plastics Processing Laboratory at W estern Michigan University. A solicitation letter (shown in Appendix D) w ill be sent to contact personnel at target companies that have an interest in robust injection molding. Access to subject from the employees of the companies will be solicited through the company’s contact personnel and will be documented by the signing of an access granting letter. The informed consent process and form w ill emphasize non-coercive methods and no reprisal for non-participation. Subjects w ill be selected based upon their response to signs and pamphlets that invite them to participate in the research (Appendix E). The invitation information shall explain the purpose and benefits of the research to potential subjects and efforts w ill be made to present subject participation as noncompulsory, coercion w ill be avoided. The Student Investigator’s office phone number w ill be provided on the sign and/or pamphlet. Potential subjects w ill be asked to contact the investigator about the research. Once potential subjects have expressed an interest in learning more about participating, an appointment w ill be set-up. During this appointment, they will be informed as to the purpose of the research, the activities involved and potential risks. In order to increase the sensitivity of the research instrument, subjects m ust possess visual acuity correctable to 20/20 vision. Visual acuity and contrast sensitivity w ill be assessed with the Freiburg Visual Acuity & Contrast Test (‘FrACT’) (Bach, 2006; Wesemann, 2002). The subjects will wear any necessary corrective appliance (eyeglasses or contacts) during the viewing session. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Additionally, laser corrective surgery is not an exclusionary criterion (Dennis et al., 2004). Individuals with who have undergone Laser-Assisted In Situ Keratomileusis (Lasik) or other corrective surgeries are not excluded from this research. Subjects younger than 18 and older than 40 years old w ill not participate in this research. The exclusionary criterion is for two purposes. First, there is no need in this research to involve protected populations under the age of legal consent, as they have limited purchasing power in the high-end consumer markets where many plastic products are found. Secondly, research has shown that the quality of vision deteriorates with age (Dollinger, 1996! Madden and Plude, 1996; Plude and Hover, 1985). Generally the age of forty is accepted as a standard threshold for the onset of considerable changes in vision. INFORMED CONSENT PROCESS The process and background of the research w ill be explained to the potential subject prior to signing the informed consent form. The aspects of the experiment, such as sink marks and textures will be explained to the subject. Sink marks w ill be described and sam ples w ill be shown to the subject. Various stipples w ill be shown to the subject and the effects of stipple on the visibility of sink marks w ill be explained. This information w ill be given as part of the informed consent process so that potential subjects will be aware of what they w ill be asked to identify. The researcher w ill use the scripts contained in Appendix F to inform and educate potential subjects. The potential subject w ill be shown the inspection fixture and its operation w ill be explained and demonstrated. The potential subject w ill be informed that the researcher w ill present the pairs of parts to be evaluated, while the potential subject w ill make decisions about the sample parts based upon visual inspection. During the inspection process, the subject w ill not be able to handle the part but w ill be allowed to tilt the part by tilting the table of the fixture. Illustrations of the design and the motion of the table are shown in Appendix C. Potential subjects w ill be told that each session will last approximately one hour and rest periods w ill be provided. Potential subjects w ill be further informed of the potential for discomfort to their eyes. Most observers working in a plastics company evaluate parts at the molding press for eight hours. They usually take two breaks and then a lunch period to help provide Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the rest they need. The viewing of parts w ill only take about forty minutes; therefore the risk of discomfort to the observer’s eyes will be a minim al risk (Horton, 1997; Ebaraj et al., 1999). The potential subject will be informed that they will be allowed rest periods as required if they begin feeling eye or head discomfort. The rest period w ill last until active subjects feel they can continue. If time becomes a factor in completing the survey in the allotted amount of time, the subject w ill be released from the study. The rest periods w ill be available when active subject requests them. The coding of the data and the subject identity w ill be explained to the potential subject. The subject will be informed that the survey w ill comprise of two (2) sessions performed on different days. The potential subject w ill be told that the research is a component of Kurt Hayden’s dissertation research and the results w ill be published in the dissertation and in professional journals. Once the potential subject has been made familiar with the study through the scripts and examples, they w ill be asked to sign the informed consent form (Appendix G). Two copies of the form will be give to the potential subject, one to be signed and returned to the researcher and one for the subject to keep.