product development | CAE software
stress distributions, uniform shrinkage and ultimately, moulded parts that are much less likely to warp out of shape compared to parts that have significantly varying wall thickness.” Avoiding abrupt transitions from thick to thin is also
essential. “It is inevitable that your parts will require some variations in wall thickness due to the incorpora- tion of structural features such as ribs, bosses and gussets. However, the transition from thin to thick should be as gradual as possible in order to avoid mould-filling phenomena such as flow hesitation or race-tracking,” he says.
Right: JK Mold Design has
implemented SolidWorks software and
has reduced its mould design cycle by 50%
Go with the flow “Given a choice, molten plastic flowing inside of an injection mould cavity will always take the path of least resistance, typically towards the thicker wall sections. Flow hesitation occurs when the melted plastic flows into a thicker section while the flow in the thinner section stalls and sometimes freezes off completely, causing major problems. Race-tracking occurs when the molten plastic ‘races’ around the edges of a part due to thicker wall sections around the perimeter of the part compared to the interior wall sections. Maintaining gradual transitions from thin to thick can help reduce these phenomena or eliminate them altogether, resulting in higher quality moulded parts with fewer manufac- turing defects,” he explains. Ruckinski adds that ribs are a frequent
cause of problems. Commonly used in plastic parts to provide structural integrity, prevent part warpage and to aid in the integration of internal components, if ribs are not designed properly relative to the surfaces they are attached to problems such as sink marks, warpage and part failure can occur. “The following rib design guidelines work well for most plastics materials,” he says. “Rib thickness at the base should be between 50-70% of the nominal wall thick- ness, while rib height should be 2.5-3.0 times the nominal wall thickness. In addition, ribs should have 0.5-1.5 degrees of draft for ejection, and rib base radii should be 0.25-0.40 times the nominal wall thickness. The distance between two ribs should also be 2-3 times the nominal wall thickness.” Finally, Ruckinski emphasises that sharp corners
46 INJECTION WORLD | January/February 2016
should be avoided. “Sharp corners in plastics parts act as stress concentrators that can lead to crazing, cracking, increased susceptibility to chemical attack and ultimately, part failure. So it is a really good idea to avoid them at all costs. However, it is usually pretty easy to add fillets or chamfers to avoid sharp corners altogether.” Simple rules, of course, but not always easy to
implement in today’s increasingly complex component and mould designs. The SolidWorks 3D design applica- tion aims to ensure that all part designs, however complex, are problem-free. The latest release is SolidWorks 2016, which the company says is powered by Dassault Systèmes’ 3DExperience platform and has been developed to cover all aspects of product develop- ment. New enhancements include the ability to flatten any surface, visualise and help validate design perfor- mance, more efficiently communicate with manufactur- ing, quickly create marketing-quality images, and more easily access commands. The company says the improvements, together with a new user interface, mean that designers and engineers can solve complex design problems, streamline parallel design process- es, and fast-track
development through manufacturing. JK Mold Design of
Sacramento, in California in
the US, has implemented SolidsWorks software and claims to have reduced its mould design cycle by 50%. Owner John Kreutzberger says he evaluated several solid modelling packages
before settling on SolidWorks, which he says was selected for its ability to
handle diverse CAD data and for its access to a
robust set of add-on mould design and production applications. It also includes inherent mould design interrogation capabilities, as well as producing engineering drawings and detail views automatically. “The tools that Dassault Systèmes SolidWorks has
added over the past few years are very helpful for resolving mouldability issues,” says Kreutzberger. “I like to use the cavity function in SolidWorks software to do my core cavity splits, using the shrinkage capability to scale parts. Once I have a model of the mould, I can either send a SolidWorks software model or a Solid- Works eDrawings file to the customer to show how I have resolved a mouldability issue, such as sufficient draft.” Plazology is a UK-based consultancy that offers a range of services across the development and produc-
www.injectionworld.com
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