ÿþ<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <title>Design Tips for Rapid Injection Moulding</title> <!--Message originally sent to: <%IF NOT ISEMPTY [email] THEN [email]%><%ELSE%><%END IF%>--> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta content="MSHTML 6.00.2900.2604" name=GENERATOR> <STYLE type=text/css> <!-- a:link { color: #9933cc; text-decoration: underline; } a:visited { color: #9933cc; text-decoration: underline; } a:hover { color: #9933cc; text-decoration: underline; } a:active { color: #9933cc; text-decoration: underline; } --> </STYLE> </head> <body style="margin:0; background-image:url('images/striped_backer.gif');"> <div id="fake_body" style="font-family:Verdana, Arial, Helvetica, sans-serif; font-size:11px; color:#000000;"> <div id="main" style="width:637px; background-color:#e0e3db; margin:7px auto 0 auto; border-top:4px solid #d2d2d0; padding:10px;"> <div id="header" style="width:637px; height:61px; font-size:10px; color:#666666; padding-left: 2px;"> <a href="/Default.aspx?s=PMDT0706" name="top"> <img src="images/proto_logo_rg.gif" width="202" height="56" border="0" id="logo" style="float:left;"></a> <img src="images/dg_title.gif" width="419" height="56"><br> </div> <div id="nav_inside" style="width:637px; margin-top:5px; height:9px; background:url('images/nav_backer_inside.gif') bottom repeat-x;"> </div> <div id="container" style="width:637px; padding:25px 0 5px 0; background-color:#ffffff;"> <div id="backer" style="background-image:url(images/backer.gif);"> <div id="left_body" style="float:left; width:368px; margin:0 5px 15px 18px; padding-right:20px; border-right:1px solid #bababa; display: inline;"> <img src="images/dg_month.gif" width="178" height="30" style="margin-bottom:25px;"> <p style="font:bold 18px Verdana, Arial, Helvetica, sans-serif; color:#669933; margin-top:0;">Good Clip, Bad Clip </p> <p>You&rsquo;ve seen the routine on TV&mdash;good cop, bad cop. Bad Cop is brittle, scary, and liable to snap at any moment. You wouldn&rsquo;t want to add to his stress level by not telling him what he wants to know. Good Cop, on the other hand, is flexible and easy-going. You could sit and chat with him all day long; eventually you just have to tell him what he wants to know because he&rsquo;s such a nice guy. </p> <p>Moulded-in clips on plastic parts cover much the same range. Their job subjects them to a certain amount of deflection as they move out of position and then back again. Often, they face the same deflection over and over again. Good clips handle the stress with ease, recovering fully after each deflection and remaining none the worse for wear. Bad ones often break immediately or fail to fully recover, become weakened and eventually break. Unlike TV cops, however, good and bad clips do not work in teams. Each works alone (or fails to work, as the case may be). </p> <p>There are several factors that contribute to the effectiveness of a clip. The first is choice of material. Obviously, a spring clip requires some flexibility. If, for some reason, the resin in which a part is moulded cannot be as flexible as you&rsquo;d like, other factors must compensate for that inflexibility. </p> <p>There are three factors affecting the stress caused by flexing a clip. The first is the length of the flexing arm. A longer arm creates less stress for a given deflection of the end. </p> <p>If your design limits the length of the clip&rsquo;s flexing arm, you can increase the arm&rsquo;s effective length in several ways. One is to loop or coil the arm, allowing more functional length in less space. Another is to notch the wall to which the clip is attached, making the notched part of the wall, effectively, part of the clip arm. Similarly, you can design the wall itself to flex slightly without being notched, accomplishing the same result. </p> <p>The second way to reduce stress on the clip is to limit the size of the hook at the end of the arm. Obviously, the hook must be large enough to do its job, but the smaller it is, the less the arm has to move as the hook engages. Finally, be aware of features such as sharp corners that can concentrate stress over small areas. Pay special attention to the base of the clip where rounded corners and fillets can be used to distribute stress just as they are used to strengthen other features in a moulded part.</p> One other issue that should always be considered in designing a clip is draft. Because clips are long and narrow, it is particularly important that they be properly drafted along their lengths. In addition to easing ejection of the part from the mould, this also strengthens the clip at its base, the location of the maximum bending moment. (And don&rsquo;t forget to make the through-hole at the base of the clip generously larger than the clip-head. This allows clearance for the core in the mould which forms the underside of the clip-head. If you can visualize this core, try to give it a minimum of 3 degrees of draft and make its length no more than 8 times its thickness. <div style="margin: 5px 0;"><p style="border: 1px solid #000000;padding:4px;width:350px; margin-bottom: 4px;"><img src="images/fig1.gif" width="350" height="310"></p> <p style="padding: 0; margin: 4px 0;"><span style="color: #693; font-weight: bold;">Figure 1:</span> <strong>Good clip: longer, slender , flexible. Fillets at base for strength. Generous through hole for core. Well drafted. Clip head not too large.</strong></p> </div> <div style="margin: 25px 0;"> <p style="border: 1px solid #000000;padding:4px;width:350px; margin-bottom: 4px;"><img src="images/fig2.gif" width="350" height="250"></p> <p style="padding: 0; margin: 4px 0;"><span style="color: #693; font-weight: bold;">Figure 2:</span> <strong>Short, rigid, inflexible. Clip head large, requiring large deflection. Hole barely large enough for core.</strong></p> </div> <p>For more information on spring clips and ways to analyze stress on them before you pay for a mould: </p> <ul> <li>Some CAD packages include simple finite element analysis (FEA) programs. If you use a lot of spring clips, consider buying a separate, more sophisticated FEA package; it could save you lots of time and money.</li> <li>BASF offers a snap-fit calculator at <a href="http://www.basf.com/businesses/plasticportal/pp_techRes_tools_snapfit_en.html">http://www.basf.com/businesses/plasticportal/<br> pp_techRes_tools_snapfit_en.html</a>.</li> <li>Efunda has a page on spring clip design at <a href="http://www.efunda.com/DesignStandards/plastic_design/snaps.cfm">http://www.efunda.com/DesignStandards/<br> plastic_design/snaps.cfm</a>.</li> </ul> <p>Jordan Rotheiser&rsquo;s book, <em>Joining of Plastics</em>, published by Hanser Gardner Publications in 2004 (<a href="http://www.hansergardner.com/plastic/">http://www.hansergardner.com/plastic/</a>), has an excellent chapter on snap fits and spring clips.</p> <p></p> </div> <div id="right_body" style="float:right; width: 220px; margin:0 5px 0 0; padding:6px 0 0 0; font-size:10px; line-height:15px; background-color: #f9f9f8;"> <img src="images/resources_head.gif" width="214" height="36" style="margin:0 0 13px 3px;"> <p style="margin:0 17px; font-size:11px; font-weight:bold; display: block; height: 16px; padding:3px 0 0 23px; background:url('images/checkmark.gif') 1px 0px no-repeat;">ProtoQuote®</p> <a href="/quote/request.asp?s=PMDT0706"> <img src="images/protoquote.jpg" alt="Image of the protoquote" style="padding:1px; border: 1px solid #000000; margin: 7px 17px;"><br/></a> <p style="margin:0 17px 0 17px;"> <strong>The Protoquote is new and improved! Try it out today:</strong></p> <p style="margin:5px 17px 0 17px;"> <a href="/quote/request.asp?s=PMDT0706"> Upload your 3D CAD model and we will email to you an informative ProtoQuote.</a></p> <p style="margin:15px 17px 0 17px; font-size:11px; font-weight:bold; display: block; height: 16px; padding:16px 0 0 23px; background:url('images/checkmark.gif') 1px 13px no-repeat; border-top:1px solid #ebebeb;">Design Guidelines</p> <p style="margin:0 17px 1px 17px;">As always, you can visit the <a href="/designguidelines/default.asp?s=PMDT0706">Protomold Design Guide</a> for helpful Rapid Injection moulding design information.</p> <img src="images/trivia_head.gif" width="214" height="36" style="margin:22px 0 13px 3px;"> <p style="margin:0 17px;">According to The Old Farmer s Almanac, which is true of Legos?</p> <form action="/RegisterPollResult.aspx" method="post" style="margin:15px 17px;"> <input type="hidden" value="PMDT0706" name="pollName"> <!--<INPUT TYPE="hidden" NAME="email" VALUE="www">--> <input type="hidden" value="<%IF NOT ISEMPTY [email] THEN [email]%><%ELSE%><%END IF%>" name="email"> <input type="radio" value="A" name="result" style="float:left;"> <p style="margin:0 0 8px 22px;"><strong>A.</strong> Legos were originally introduced as a puzzle rather than a building system. </p> <input type="radio" value="B" name="result" style="float:left;"> <p style="margin:0 0 8px 22px;"><strong>B.</strong> The name &ldquo;Lego&rdquo; comes from the Greek word for brick.</p> <input type="radio" value="C" name="result" style="float:left;"> <p style="margin:0 0 8px 22px;"><strong>C.</strong> Early shipments of Legos were rejected by retailers due to a tendency to melt in direct sunlight. </p> <input type="radio" value="D" name="result" style="float:left;"> <p style="margin:0 0 8px 22px;"><strong>D.</strong> Six Lego blocks can be fitted together in over 100 million different ways.</p> <input type="radio" value="E" name="result" style="float:left;"> <p style="margin:0 0 8px 22px;"><strong>E.</strong> Legos have been made from acrylonitrile butadiene styrene (ABS) since their original introduction.</p> <input type="submit" value="Submit" style="margin-top:8px;"> </form> <p style="margin:25px 17px 0 17px;"><strong>Last month's question/results<br> </strong>Which of the following is not a real plastic musical instrument?</p> <ul style="list-style-type:upper-alpha; margin-top: 10px;"> <li style="margin: 0 15px 5px 0;">Tonette</li> <li style="margin: 0 15px 5px 0;">Synthatone</li> <li style="margin: 0 15px 5px 0;">Flutophone</li> <li style="margin: 0 15px 5px 0;">Ocarina</li> <li style="margin: 0 15px 5px 0;">Humanatone nose flute</li> <li style="margin: 0 15px 5px 0;">Hum-a-zoo</li> </ul> <p style="margin:0 17px;">The correct answer is<strong> B. Synthaphone.</strong> The guesses are represented in the following chart:</p> <p style="margin:10px 0 10px 32px;"><img src="images/chart.gif" width="150" height="147"></p> <p style="margin:30px 17px 10px 17px;">Please email suggested topics for future Design Tips, questions for future Designer Surveys, and obscure bits of Plastics Trivia to <A href="mailto:marketing@protomold.co.uk">marketing@protomold.co.uk</A>.<br /> </div> <div class="clearer" style="height:1px; overflow:hidden; clear:both; margin-top:-1px;"></div> <br></div> </div> <div id="protoquote_bug_shadow" style="width:637px; margin-top:10px; height:9px; background:url('images/nav_backer_inside.gif') bottom repeat-x;"> </div> <div id="protoquote_bug" style="width:624px; padding:20px 0 17px 13px; background-color:#ffffff;"> <a href="/Default.aspx?s=PMDT0706"><img src="images/protoquote_bug.gif" width="602" height="138" border="0"></a> </div> <div id="footer1" style="width:545px; padding:10px 5px 12px 0; margin: 10px 0 0 20px; font-size:9px; color:#999999; border-bottom:1px solid #cbcec5;"> To unsubscribe, please reply to this message with "remove" in the subject line.</div> <div id="footer2" style="width:545px; padding:12px 5px 0 0; margin:0 0 0 20px; color:#999999;"> <a href="/Default.aspx?s=PMDT0706">www.protomold.co.uk</a> | Unit A Hortonpark, Hortonwood 7, Telford, TF1 7GX, UK | +44 (0) 870 723 0000</div> <div id="footer3" style="width:545px; padding:6px 5px 10px 0; margin: 0 0 60px 20px; font-size:9px; color:#999999;"> Copyright © Protomold 1999-2006</div> </div> </div> </body> </html>