{"id":7804,"date":"2018-03-02T08:00:16","date_gmt":"2018-03-02T14:00:16","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=7804"},"modified":"2023-02-01T10:29:11","modified_gmt":"2023-02-01T16:29:11","slug":"pe-injection-moulding-backpressure","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/7804\/pe-injection-moulding-backpressure\/","title":{"rendered":"Injection Moulding: The Role of Backpressure"},"content":{"rendered":"<p>Injection moulding has been described as &#8220;a jungle of disconnected facts and fairy tales.&#8221; There are over 200 different parameters that must be established and controlled to achieve proper injection moulding of a plastic part. But over time, there have been many developments to machines designed to make the injection moulding process more effective and predictable.<\/p>\n<p>These parameters fall within four major areas:<\/p>\n<ul>\n<li>Pressure<\/li>\n<li>Temperature (barrel, nozzle and mould temperatures)<\/li>\n<li>Time<\/li>\n<li>Distance<\/li>\n<\/ul>\n<p>All four areas are important, but pressure and temperature are the ones most varied during process control and optimisation. Based on the requirements of any particular plastic material, the pressure must be sufficient to inject the plastic material and to hold the mould closed. In addition, the temperature of the injected plastic and mould must be correctly maintained.<\/p>\n<p>Pressure is primarily of interest in the injection area, but there is also pressure found in the clamp unit of the moulding machine. Read on to learn about different areas of pressure.<\/p>\n<h3>Minimize downtime with Prospector&#8217;s essential processing data &amp; tools<\/h3>\n<p>Access an injection molding troubleshooter, compare up to 5 materials side by side, and even view typical properties to help fill in missing product data points. All this right from the shop floor since Prospector is mobile-friendly. Instantly use advanced data and tools to minimize downtime &#8211; subscribe now!<\/p>\n<p><a role=\"button\" href=\"https:\/\/www.ulprospector.com\/plastics-processors?st=31\" target=\"_blank\" rel=\"noopener\"><br \/>\nLearn more<br \/>\n<\/a><\/p>\n<h3>Injection pressure<\/h3>\n<p>This is the primary pressure for injecting 95 percent of the molten plastic into the closed mould. Normally, the highest pressure and fastest fill rate are the best condition. However, high pressure will increase moulded-in stress. So, determine the minimum amount of pressure necessary to fill the mould, and then use all of it. The hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mould.<\/p>\n<h3>Holding pressure<\/h3>\n<p>Once the majority of the plastic (95 percent) has been injected using standard injection pressure, the machine should drop to <strong>hold pressure<\/strong>. This pressure is about half of the injection pressure and is used to finish filling the mould by packing the molecules together in an orderly fashion.<\/p>\n<p>Holding pressure is required until the gate freezes off, normally in three to four seconds. Once that happens, hold pressure has no more effect on the molecules on the other side of the gate. If hold pressure is released before the gate freezes, the material in the cavity is still molten and will be sucked back out of the cavity. At the very least, there will be insufficient pressure to pack the molecules together and uneven shrinkage and cooling will occur. If valve gating of a hot runner system is used, holding pressure can be released earlier than with standard surface gating.<\/p>\n<h3>Clamp pressure<\/h3>\n<p>At the other end of the machine, <strong>clamp pressure<\/strong> keeps the mould closed against injection pressure. Therefore, the amount of clamp pressure required is based on the material being moulded. Easier-flow materials require less injection pressure; thus, they require less clamp pressure. Conversely, stiffer-flow materials will require more injection pressure, thus more clamp pressure.<\/p>\n<h3>Backpressure<\/h3>\n<p>The main focus of this article, <strong>backpressure<\/strong> is created during the return action of the screw after injecting material. It is a controllable process parameter that is often mistakenly overlooked.<\/p>\n<p>The screw turns to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A build-up of pressure is created at the front end of the screw. This pressure is used for better mixing of the plastic (especially if colours are added at the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/ulprospector.ul.com\/media\/2018\/02\/112-Molding-Parameters-1024x757.png\" alt=\"11 + 2 Molding Parameters chart - learn more about the role of backpressure in injection moulding in the Prospector Knowledge Center.\" width=\"1024\" height=\"757\" \/><\/p>\n<p>Users should keep this pressure as low as possible and in any event, should not go beyond 20 percent of the machine\u2019s maximum rated injection moulding pressure. Some backpressure may be required to simply stop the screw pushing itself (auguring) too easily out of the barrel, or cylinder; for example, 5 bar (73 psi0).<\/p>\n<p>The backpressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.<\/p>\n<p>The screw must generate &#8211; and exceed &#8211; this pressure before it can move back. This backpressure is generated by rotating the screw against the restriction of the plastics material, which is contained in the barrel, or cylinder.<\/p>\n<p>Material melting and colour dispersion can be improved with the use of high backpressures, but it also increases stress on the injection moulding machine, reduces fibre lengths in filled systems and increases the screw retraction time.<\/p>\n<p>The backpressure can also be programmed on some machines in order to compensate, for instance, for the efficient reduction in screw length which takes place during plasticisation; this type of reduction means less heat input and therefore a drop in temperature. Conversely, on several machines it is difficult to set the pressure as there is no simple way to measure the effects of the changes.<\/p>\n<p>Contrary to popular belief, the purpose of backpressure is not to raise the temperature of the resin &#8211; increasing backpressure does not significantly raise the temperature of the melt. The effect of increased rpm is much higher.<\/p>\n<p>One change on an injection moulding machine usually results in multiple effects. Increasing backpressure means that it takes longer for the screw to recover \u2014 often longer than the set or allowed cooling time. This, in turn, lengthens the cycle time, as the machine does not open the clamp until the screw is at shot size. To get the screw-recovery time back down below the set cooling time to keep the cycle consistent, we increase the screw rpm. As the data shows, a melt-temperature increase results when rpm is increased.<\/p>\n<h4>Table: Advantages and limitations of high backpressure<\/h4>\n<table>\n<tbody>\n<tr>\n<td><strong>Advantages<\/strong><\/td>\n<td><strong>Limitations<\/strong><\/td>\n<\/tr>\n<tr>\n<td>\n<ul>\n<li>\u00a0Better melt-temperature control (not increasing melt temperature)<\/li>\n<li>Better melt uniformity in viscosity and temperature<\/li>\n<li>Good colour and additive blending, if done at the press and with a correctly designed screw<\/li>\n<li>Better shot-size control<\/li>\n<li>Better filling of the flights to prevent black specks and degradation where the flights meet the root diameter of the screw<\/li>\n<li>Prevent screw augering due to pellets being wedged between flight land and feed throat<\/li>\n<\/ul>\n<\/td>\n<td>\n<ul>\n<li>Vent flooding in a vented barrel<\/li>\n<li>Fibre breakage in reinforced resins<\/li>\n<li>Degradation of resins such as PVC and acetal, if you have to use high screw speeds (rpm)<\/li>\n<li>Minimise screw-recovery time when it is the determining factor in cycle time (subject to an acceptable part)<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>&nbsp;<\/p>\n<p>Other pressures include:<\/p>\n<p><em>Cavity Pressure<\/em><\/p>\n<p>The plastics material exerts this pressure inside the cavity and tries to open and distort the mould. It may be sensed using a transducer and the produced signal that is used to actuate the switch from first stage pressure to second stage, holding pressure. This process is known as cavity pressure control (CPC).<\/p>\n<p><em>Line Pressure<\/em><\/p>\n<p>This pressure is also known as gauge pressure, as it is frequently measured by means of a gauge which is set in the hydraulic pipe or line. This pressure is not the injection pressure, but is actually the pressure present in the main supply line from the pump. Line pressure can usually be altered, and should be modified in order to suit the specific moulding run.<\/p>\n<p><em>Nozzle Pressure<\/em><\/p>\n<p>Nozzle pressure refers to the pressure measured within the nozzle. It is approximately the pressure that causes the material to flow. This pressure does not have a constant value but increases based on the growing complexity of mould filling.<\/p>\n<p>There is a direct relationship between injection pressure, line pressure, and nozzle pressure. In screw machines, the nozzle pressure is about 10 percent lower than the injection pressure (as calculated from the gauge in the hydraulic line). Pressure losses in ram machines are much greater and can reach 50 percent.<\/p>\n<h3>Additional injection moulding resources:<\/h3>\n<ul>\n<li><a href=\"https:\/\/ulprospector.ul.com\/1457\/pe-backpressure-injection-molding-machine?st=31\" target=\"_blank\" rel=\"noopener\">Backpressure Setting on an Injection Molding Machine<\/a><\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/7366\/using-cavity-sensors-optimise-injection-moulding?st=31\">Using cavity sensors to optimise injection moulding<\/a><\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/6036\/plastic-defects-avoid-injection-molding-bubbles-blisters-sinks-voids?st=31\" target=\"_blank\" rel=\"noopener\">Plastic Defects: How to Avoid Common Injection-Molding Issues<\/a><\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/3635\/pe-troubleshooting-flipchart-for-plastic-injection-molding?st=31\" target=\"_blank\" rel=\"noopener\">Troubleshooting FlipChart for Plastic Injection Molding<\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Injection moulding has been described as &#8220;a jungle of disconnected facts and fairy tales.&#8221; There are over 200 different parameters that must be established and controlled to achieve proper injection moulding of a plastic part. But over time, there have &hellip; <a href=\"https:\/\/ulprospector.ul.com\/7804\/pe-injection-moulding-backpressure\/\">Continued<\/a><\/p>\n","protected":false},"author":22,"featured_media":7805,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"episode_type":"","audio_file":"","podmotor_file_id":"","podmotor_episode_id":"","cover_image":"","cover_image_id":"","duration":"","filesize":"","filesize_raw":"","date_recorded":"","explicit":"","block":"","itunes_episode_number":"","itunes_title":"","itunes_season_number":"","itunes_episode_type":"","footnotes":""},"categories":[29,21],"tags":[222],"ppma_author":[1238],"class_list":{"0":"post-7804","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-processing","8":"category-plastics-2","9":"tag-equipment-services","10":"entry"},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Injection moulding: the role of backpressure | Prospector<\/title>\n<meta name=\"description\" content=\"Over 200 different parameters are involved to achieve proper injection moulding of a plastic part. 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Having originally qualified as a metallurgist at Cambridge University, Andy spent a period as a consultant, where he specialised in advanced composites, asbestos substitutes and the methodology of materials selection, subjects on which he has published several books and technical papers. Since the early 1980s, he has edited many of the leading manufacturing and engineering titles in the UK, firstly cutting his teeth as a technical journalist on Design Engineering. Known as \\\"The Materials Man\\\", he covered many of the early innovations in engineering plastics. He was promoted to editor in 1985 and subsequently moved on to edit Engineering magazine (1992), and Industrial Technology (1994). In 1999, with former colleagues, he launched Pro-Talk, which founded the first online publications for engineers in Europe - the then thriving business was sold to Centaur Publications in 2006. 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Having originally qualified as a metallurgist at Cambridge University, Andy spent a period as a consultant, where he specialised in advanced composites, asbestos substitutes and the methodology of materials selection, subjects on which he has published several books and technical papers. Since the early 1980s, he has edited many of the leading manufacturing and engineering titles in the UK, firstly cutting his teeth as a technical journalist on Design Engineering. Known as \"The Materials Man\", he covered many of the early innovations in engineering plastics. He was promoted to editor in 1985 and subsequently moved on to edit Engineering magazine (1992), and Industrial Technology (1994). In 1999, with former colleagues, he launched Pro-Talk, which founded the first online publications for engineers in Europe - the then thriving business was sold to Centaur Publications in 2006. Since then, Andy has continued to publish online, including his own title New Materials International (www.newmaterials.com). He is also a regular contributor to many specialist engineering titles in the UK and Europe, including Controls, Drives &amp; Automation (CDA), Engineering &amp; Technology (E&amp;T), and Environmental Technology. As technology companies strive to manage their own websites, they are recognising the need to develop their writing and editing resources. Andy now works directly for companies in the manufacturing sector, delivering technical content through this medium for their current and prospective customers. Working with end users, PR agencies and website designers, this business is growing rapidly and Andy is aactively building a team of expert writers to fufil the demand for feature articles, news items and white papers.","sameAs":["http:\/\/www.ulprospector.com"],"url":"https:\/\/ulprospector.ul.com\/author\/andy-pye\/"}]}},"authors":[{"term_id":1238,"user_id":22,"is_guest":0,"slug":"andy-pye","display_name":"Andy Pye","avatar_url":"https:\/\/ulprospector.ul.com\/media\/2017\/10\/Andy-Pye_avatar_1508792576-96x96.jpg","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/posts\/7804","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/users\/22"}],"replies":[{"embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/comments?post=7804"}],"version-history":[{"count":0,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/posts\/7804\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/media\/7805"}],"wp:attachment":[{"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/media?parent=7804"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/categories?post=7804"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/tags?post=7804"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/ulprospector.ul.com\/wp-json\/wp\/v2\/ppma_author?post=7804"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}