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Successful molding of component with a high flow length-to-wall-thickness to produce thin wall injection molded parts |
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Demand to create smaller, lighter parts has made thin-wall molding one of the most sought after capabilities for an injection molder. Thin-wall parts find application in portable electronics parts, automotive components, etc. Adopting a high-flow resin grade in any old machine will not ensure success molding of a component with a flow- length-to-wall-thickness ratio of 100�200:1. It is vital to have tooling of robust design, consider the strength of the material as well as venting length during processing; taking into consideration the high temperatures typically adopted for thin-wall molding.
Typically, a shot size of at least 25% of the injection unit�s capacity is recommended to minimize barrel residence time. A smaller barrel also enables the high injection speed and pressure required for thin-wall molding. So, though an off-the-floor 550 ton press might normally be equipped with a 60 oz barrel whereas with a thin-wall machine, the barrel capacity might be 9 oz. To ensure robust design of the tooling, cavity blocks and backup plates need to be thicker, with increased support pillars. Preloading support pillars is recommended in the center by 3�5 thousandths of an inch (0.076�0.127 mm) to compensate for tool deflection.
Venting is vital because of the high injection speeds involved. The land length for primary vents should be no more than 45 thousandths (about 1.14 mm), to allow gas to go through it. Mold polishing and certain specialty surface treatments also helps with flow and incorporating radii at corners makes a notable difference. If the number of corners the flow front has to round is minimised, it results in reduced pressure, making gate positioning very critical. It is preferable to have larger ejector pins for pushing out thin-wall parts, particularly in case of features such as ribs or bosses, or any kind of resistance to ejection. Ribs should also measure 100% of the nominal wall thickness at their base, as opposed to 60% for standard parts, to prevent them breaking off. Fillets should also be incorporated in ribs and other areas to eliminate sharp corners and potential sources of notch propagation, particularly because many thin-wall products are portable devices such as cellphones and notebook computers that need to comply with stringent drop-test requirements.
Though liquid crystal polymer is an ideal resin for thin-wall molding different rules need to be followed for its processing. The material flow should be from thin to thick sections so that shear is initially induced, while wall thickness also needs to be kept constant. A variation of just 2% can change the flow direction, which results in loss of pressure and shear � slight loss of hesitation results in a stop in flow. Additional design hints include impinging immediately after the gate in order to form a flow front and prevent the resin jetting down the part. This can be achieved through something as simple as positioning the gate as deep as possible inside the mold and impinging onto a core pin, or using dual gates to impinge separate material flows onto each other. Runner systems are considered part of the part when designing with LCP and should be designed so that they taper or step down at every branch in a multi-cavity tool as the resin flows towards the mold. This results in shear starting to be generated just when the flow front starts to cool as it approaches the tool. The runner system should also be devoid of sharp corners. Hot runner systems are not recommended for LCP as the large manifolds could result in loss of pressure. Venting of the tool is also important. Designing a part that allows incorporation of regrind is also advantageous for LCP.
Several software tools are available to assist processors in minimizing wall thickness and optimizing part strength. Essentially, the software acts as an interface between two or more design tools, such as flow analysis and structural analysis, and enables the user to change several design parameters such as wall thickness, injection pressure, and injection rate, and investigate the effect on �objectives� such as flexural strength and sink index. A graphic representation of the results allows the designer to identify the optimum solution.
(Extracted from an article by Matt Defosse)
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