{"id":215,"date":"2015-11-10T13:18:58","date_gmt":"2015-11-10T13:18:58","guid":{"rendered":"http:\/\/www.plastikcity.co.uk\/blog\/?p=215"},"modified":"2020-08-25T14:22:05","modified_gmt":"2020-08-25T14:22:05","slug":"specific-gravity-not-that-specific-at-all-ignore-this-blog-at-your-peril","status":"publish","type":"post","link":"https:\/\/www.plastikcity.co.uk\/blog\/specific-gravity-not-that-specific-at-all-ignore-this-blog-at-your-peril\/","title":{"rendered":"Specific Gravity &#8211; Not that Specific at all. Ignore this blog at your peril!"},"content":{"rendered":"<p><strong>The definition of \u2018Specific\u2019 is <em>clearly defined or identified<\/em> or <em>a<\/em> <em>precise detail<\/em>.\u00a0 Unfortunately, when it comes to specific gravities for polymers, this definition doesn\u2019t always hold true, so in reality you may just be &#8216;sticking your finger in the air&#8217;&#8230;&#8230;&#8230;<\/strong><\/p>\n<p><!--more--><\/p>\n<p>This is because the standard way of quoting a particular material\u2019s s.g. is in its solid state, but most of us are in the business of melting plastic. In some cases, the shift in material density when moving from the solid to the molten state can be as high as 20%.<\/p>\n<h5><em>\u00a0At the bottom of this Blog we provide you with the tools you will need to avoid making costly mistakes.<\/em><\/h5>\n<p>Using the wrong source of information for the s.g. of a particular material might not sound that much of a big deal, but I have known of instances where a moulder has quoted for a job, only to later find he can\u2019t actually handle it because his machine doesn\u2019t have sufficient shot weight.\u00a0 In one extreme case, a company placed an order for a new machine based on incorrect information.<\/p>\n<p><strong>So what\u2019s the problem?\u00a0 <\/strong>We aren\u2019t talking about the slight differences between specific grades of a material, although you should always take care when dealing with filled materials.<\/p>\n<ul>\n<li>The biggest culprit for mistakes relates to how some materials change their density so radically when going from the solid to molten state. In the same way as ice is much lighter than water, some materials become significantly less dense when you melt them.\u00a0 For example, if you look at the s.g. quoted on a material data sheet for a typical Polypropylene, the figure will usually relate to the solid state, and will come in at around <strong>90<\/strong> grammes per cubic centimetre.\u00a0 What the datasheet probably doesn\u2019t tell you, is that in the molten state (just like when you are about to inject it into your mould tool) the s.g. is more likely to be around <strong>0.73<\/strong> g\/cm3 !!!<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<ul>\n<li>The other main candidate for errors relates to how machine manufacturers specify their equipment. To keep things simple and easy to compare, manufacturers usually quote their maximum shot weights in a standard material, i.e. <strong>Polystyrene<\/strong> (P.S.).\u00a0 If you\u2019re not intending to make a particular product in P.S. then this probably won\u2019t be the maximum shot weight you can actually achieve.<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>It goes without saying that it would be fairly easy to combine 1 &amp; 2 and get in a real mess!\u00a0 I have had heated discussions with moulders in the past about shot weights, usually because their material data sheet is telling them something completely different. In one instance, it took an \u2018air shot\u2019 and set of scales to convince them.<\/p>\n<p><strong>So what should you do?<\/strong><\/p>\n<p>Firstly, when it comes to your moulding equipment, try and deal in <strong>shot volumes<\/strong>, as this is a constant for the machines specified injection unit and screw and barrel diameter.\u00a0 This information should be quoted in the relevant brochure, if not stored within the controller itself.<\/p>\n<p>Secondly, try and work with <strong>molten state specific gravities<\/strong>.\u00a0 These aren\u2019t always that easy to find, but you can create a shortcut on your desktop to a <span style=\"color: #ff0000;\"><a style=\"color: #ff0000;\" href=\"https:\/\/www.plastikcity.co.uk\/useful-stuff\/shot-weight-conversion-calculator\">calculator<\/a><\/span> embedded on the PlastikCity site that uses molten state s.g\u2019s to convert from shot volume to shot weight and vice versa.\u00a0 These figures won\u2019t be exact (due to slight variations in material grades), but should hopefully prevent you from making any major and expensive mistakes.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>The definition of \u2018Specific\u2019 is clearly defined or identified or a precise detail.\u00a0 Unfortunately, when it comes to specific gravities for polymers, this definition doesn\u2019t always hold true, so in reality you may just be &#8216;sticking your finger in the air&#8217;&#8230;&#8230;&#8230;<\/p>\n","protected":false},"author":1,"featured_media":216,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[101],"tags":[90,88,7,87,89,91],"class_list":["post-215","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uk-plastic-industry","tag-material-shot-weight","tag-plastic-shot-weight","tag-plastikcity","tag-polymer-specific-gravity","tag-s-g","tag-shot-volume-to-shot-weight","post-preview"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/posts\/215","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/comments?post=215"}],"version-history":[{"count":0,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/posts\/215\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/media\/216"}],"wp:attachment":[{"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/media?parent=215"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/categories?post=215"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.plastikcity.co.uk\/blog\/wp-json\/wp\/v2\/tags?post=215"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}