Injection moulding: Difference between revisions

'''Injection moulding''' (U.S. spelling: '''injection molding''') is a [[manufacturing]] process for producing parts by injecting molten material into a [[Molding (process)|mould, or mold]]. Injection moulding can be performed with a host of materials mainly including [[metal]]s (for which the process is called [[die-casting]]), [[glass]]es, [[elastomer]]s, [[Confectionery|confections]], and most commonly [[thermoplastic]] and [[thermosetting]] polymers. Material for the part is fed into a heated barrel, mixed (using a helical screw), and injected into a [[mould cavity]], where it cools and hardens to the configuration of the cavity.<ref name=manPRG>{{cite book|last1=Todd|first1=Robert H.|last2= Allen|first2=Dell K.|last3=Alting|first3=Leo|title=Manufacturing Processes Reference Guide|year=1994|publisher=Industrial Press, Inc.}}</ref>{{rp|240}} After a product is designed, usually by an [[industrial design]]er or an [[engineer]], moulds are made by a mould-maker (or toolmaker) from [[3D Metal Moulding|metal]], usually either [[steel]] or [[aluminium]], and [[Machining|precision-machined]] to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in [[3D printing]] technology, using [[photopolymer]]s that do not melt during the injection moulding of some lower-temperature thermoplastics, can be used for some simple injection moulds.

Injection moulding is used to create many things such as [[Cable reel|wire spools]], [[packaging]], [[bottle cap]]s, automotive parts and components, toys, [[Comb|pocket combs]], some musical instruments (and parts of them), one-piece chairs and small tables, storage containers, mechanical parts (including gears), and most other plastic products available today. Injection moulding is the most common modern method of manufacturing plastic parts; it is ideal for producing high volumes of the same object.<ref>{{cite web|title=Application Overview: Injection Molding|url=http://www.yaskawa.com/site/Industries.nsf/applicationDoc/appinjmold.html|publisher=Yaskawa America, Inc.|access-date=2009-02-27|url-status=dead|archive-url=https://web.archive.org/web/20060412020436/http://www.yaskawa.com/site/Industries.nsf/applicationDoc/appinjmold.html|archive-date=2006-04-12}}</ref>

The German chemists [[Arthur Eichengrün]] and Theodore Becker invented the first soluble forms of cellulose acetate in 1903, which was much less flammable than [[cellulose nitrate]].<ref>{{cite web|url=https://books.google.com/books?id=kwA3AQAAMAAJ&pg=PA288|title=Chemical Age|first1=Richard Kidder|last1=Meade|first2=Harry|last2=McCormack|first3=Laurance T.|last3=Clark|first4=Alexander G.|last4=Sclater|first5=Lloyd|last5=Lamborn|date=27 April 2018|publisher=McCready Publishing Company|access-date=27 April 2018|via=Google Books}}</ref> It was eventually made available in a powder form from which it was readily injection moulded. [[Arthur Eichengrün]] developed the first injection moulding press in 1919. In 1939, Arthur Eichengrün patented the injection moulding of plasticised cellulose acetate.

The industry expanded rapidly in the 1940s because [[World War II|World War&nbsp;II]] created a huge demand for inexpensive, mass-produced products.<ref name="About Injection Molding">{{cite web|title=About Injection Molding|url=http://www.xcentricmold.com/aboutinjectmold.php|publisher=Xcentric Mold & Engineering, Inc.|access-date=30 January 2013|url-status=live|archive-url=https://web.archive.org/web/20121122105321/http://www.xcentricmold.com/aboutinjectmold.php|archive-date=22 November 2012}}</ref> In 1946, American inventor [[James Watson Hendry]] built the first screw injection machine, which allowed much more precise control over the speed of injection and the quality of articles produced.<ref name="Merril 1955">{{cite book|last=Merril|first=Arthur M.|title=Plastics Technology, Volume 1|year=1955|publisher=Rubber/Automotive Division of Hartman Communications, Incorporated, 1955|url=https://books.google.com/books?id=CC4nAAAAMAAJ}}</ref> This machine also allowed material to be mixed before injection, so that coloured or recycled plastic could be added to virgin material and mixed thoroughly before being injected. In the 1970s, Hendry went on to develop the first [[Gas-assisted injection molding|gas-assisted injection moulding]] process, which permitted the production of complex, hollow articles that cooled quickly. This greatly improved design flexibility as well as the strength and finish of manufactured parts while reducing production time, cost, weight and waste. By 1979, [[plastic production]] overtook steel production, and by 1990, aluminium moulds were widely used in injection moulding.<ref>{{cite web|url=http://www.avplastics.co.uk/a-short-history-of-injection-moulding|title=A Short History of Injection Moulding|first=James|last=Torr|date=11 April 2010|website=AV Plastics Injection Moulding - Get Stuff Made}}</ref> Today, screw injection machines account for the vast majority of all injection machines.

Since moulds have been expensive to manufacture, they were usually only used in mass production where thousands of parts were being produced. Typical moulds are constructed from [[hardened steel]], pre-hardened steel, aluminium, and/or [[Beryllium copper|beryllium-copper]] alloy.<ref name="Concise Encyclopedia of Plastics">{{cite book|first1=Donald V.|last1=Rosato|first2=Marlene G.|last2=Rosato|title=Concise Encyclopedia of Plastics|publisher=Springer|year=2000}}</ref>{{rp|176}} The choice of material for the mold is not only based on cost considerations, but also has a lot to do with the [[Product life-cycle management (marketing)|product life cycle]]. Generally speaking, those who have matured, the need for mass production of the product selection of materials will be better, and hope that the mold circle time the larger the better so that the total cost will be reduced. For those who have just developed, not very mature, just want to produce a small-scale [[Market testing|market test]] products, the choice of material is often some lower cost of aluminum alloy and so on. These mould called [[rapid tooling]]. inIn general, steel moulds cost more to construct, but their longer lifespan offsets the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel moulds are less wear-resistant and are used for lower volume requirements or larger components; their typical steel hardness is 38–45 on the [[Rockwell scale|Rockwell-C scale]]. Hardened steel moulds are heat treated after machining; these are by far superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium moulds can cost substantially less, and when designed and machined with modern computerised equipment can be economical for moulding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mould that require fast heat removal or areas that see the most shear heat generated.<ref name="Concise Encyclopedia of Plastics"/>{{rp|176}} The moulds can be manufactured either by CNC machining or by using [[electrical discharge machining]] processes.

Some moulds allow previously moulded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmoulding. This system can allow for production of one-piece tires and wheels.[[File:Micro mold small feature.jpg|thumb|Micro injection molding can help create extremely precise part with micro features<ref>{{cite web| url = https://www.medicalmoulds.com|title = Micro Systems | website = Micro Systems | access-date = 3 Nov 2023}}</ref>]]

Moulds for highly precise and extremely small parts from [[micro injection molding]] requires extra care in the design stage, as material resins react differently compared to their full-sized counterparts where they must quickly fill these incredibly small spaces, which puts them under intense shear strains.<ref>{{Cite web|url=https://www.medicalmoulds.com/micro-moulding-vs-conventional-moulding/|website=Micro Systems |title=Micro Moulding vs Conventional Moulding|date=16 May 2023 |language=en|access-date=22 May 2023}}</ref>

ToolingTool steel is oneoften of the main pillars of plastic injection mouldingused. Because it directly impacts the end product, getting it right can make or break a plastic injection moulding production.<ref>{{cite web |last1=Preety |first1=Verma |title=Injection Moulding Tooling Process |url=https://originmoulding.com/tooling.php |website=Origin Moulding |access-date=26 October 2023}}</ref> Mild steel, aluminium, nickel or [[epoxy]] are suitable only for prototype or very short production runs.<ref name=manPRG /> Modern hard aluminium (7075 and 2024 alloys) with proper mould design, can easily make moulds capable of 100,000 or more part life with proper mould maintenance.<ref>{{cite web|last=Goldsberry|first=Clare|title=Aluminum vs. steel tooling: Which material is right, and how to design and maintain?|url=http://www.plasticstoday.com/articles/aluminum-vs-steel-tooling-which-material-right-how-design-how-maintain-082920122|work=Plastics Today|date=29 August 2012 |publisher=UBM Canon|url-status=live|archive-url=https://web.archive.org/web/20120902213403/http://www.plasticstoday.com/articles/aluminum-vs-steel-tooling-which-material-right-how-design-how-maintain-082920122|archive-date=2012-09-02}}</ref>

Moulds are built through two main methods: standard [[machining]] and [[electrical discharge machining|EDM]]. Standard machining, in its conventional form, has historically been the method of building injection moulds. With technological developments, [[CNC]] machining became the predominant means of making more complex moulds with more accurate mould details in less time than traditional methods.

The sequence of the events during the injection mould of a plastic part is called the injection moulding cycle. The cycle begins when the mould closes, followed by the injection of the polymer into the mould cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected.<ref name=IMhand2>{{cite book|title=Injection Molding Handbook|edition=2nd}}</ref>{{rp|13}}

*[[GasMulti-assistedmaterial injection molding|Multi-material '''injection''' molding]]

[[Automation]] means that the smaller size of parts permits a mobile inspection system to examine multiple parts more quickly. In addition to mounting inspection systems on automatic devices, multiple-axis robots can remove parts from the mould and position them for further processes.<ref name="callister">{{cite book|last=Callister|first=William D.|title=Materials Science and Engineering: An Introduction|date=2003 |url=https://archive.org/details/materialsscience00call_0|url-access=registration|publisher=John Wiley and Sons|isbn=9780471135760 }}</ref>

Specific instances include removing of parts from the mould immediately after the parts are created, as well as applying machine vision systems. A robot grips the part after the ejector pins have been extended to free the part from the mould. It then moves them into either a holding location or directly onto an inspection system. The choice depends upon the type of product, as well as the general layout of the manufacturing equipment. Vision systems mounted on robots[[robot]]s have greatly enhanced quality control for insert moulded parts. A mobile robot can more precisely determine the placement accuracy of the metal component, and inspect faster than a human can.<ref name="callister"/>

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