BGE design analysis

Design Analysis Designing plastic parts is a complex task involving many factors that address a list of requirements of the application “How is the part to be used?” “How does it fit to other parts in the assembly?” “What loads will it experience in use?” In addition to functional and structural issues, processing issues play a large role in the design of an injection molded plastic part How the molten plastic enters, fills, and cools within the cavity to form the part largely drives what form the features in that part must take Adhering to some basic rules of injection molded part design will result in a part that, in addition to being easier to manufacture and assemble, will typically be much stronger in service Dividing a part into basic groups will help you to build your part in a logical manner while minimizing molding problems As a part is developed, always keep in mind how the part is molded and what you can to minimize stress Stress The main enemy of any injection molded plastic part is stress When a plastic resin (which contains long strains of molecules) is melted in preparation for molding, the molecular bonds are temporarily broken due to the heat and shear force of the extruder, allowing the molecules to flow into the mold Using pressure, the resin is forced into the mold filling in every feature, crack and crevice of the mold As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part Turning hard or sharp corners exerts more stress on the molecule than taking gentle turns with generous radii Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part Part stresses can cause warpage, sink marks, cracking, premature failure and other problems While some stresses in an injection molded part are to be expected, you should design your parts with as much consideration for stress reduction as possible Some ways to this are by adding smooth transitions between features and using rounds and fillets in possible high stress areas Wall Thickness Determining the best wall thickness is an important decision that is driven by several factors Functional Requirements of the Part: If you need a lot of strength and stiffness, you may choose to make your nominal wall thicker You can also make your walls stiffer by adding ribs Uniform Wall: Maintaining a nominal wall in your base feature will assure you of good resin flow through the mold Typically, your nominal wall shouldn’t vary more than +/- 10% Sometimes a complex part may require a varying thickness to meet the functional requirements of the part Try to avoid this, because this variation can cause major problems in mold filling and part warpage Since the hot plastic will always follow the path of least resistance (flowing into more open areas as opposed to smaller, tighter areas), the thinner walls of your part may not fill properly The molder can increase the injection pressure to force the thinner areas to fill, however, this can build even more stress into the part causing warpage This is particularly the case with plastics that have a higher mold shrinkage factor Alternatives: If your part is so complex that you need variations on your wall thickness, look for an alternative You may want to use design features such as coring or using ribs At the very least, try not to make the transitions between thicker and thinner sections too abrupt Try using a gradual transition or chamfered corners to minimize the dramatic change in pressures inside the mold Molding Considerations The thinner the nominal wall, the shorter the distance hot melt can flow before it solidifies or “freezes off” The flow length is dependent on material since different resins have different melt, flow, and freezing characteristics If your analysis shows that you need a particularly thick wall to satisfy your structural requirements, perhaps upgrading your material to a stiffer variety will help you to reduce your nominal wall Also, additives such as glass fibers can increase stiffness of a part with the same nominal wall But bear in mind, these additives can also affect the flow and shrinkage characteristics of the material and thus make your part design more difficult as opposed to easier Draft Most plastic parts include features such as outside walls and internal ribs that are formed by opposing surfaces of tool metal inside a closed mold To properly release the part when the mold opens, the side walls of the mold are tapered in the direction that the mold opens This tapering is referred to as “draft in the line of draw” This draft allows the part to break free of the mold as soon as the mold opens The amount of draft required can depend on the surface finish of the mold A smooth, polished tool surface will allow the part to eject with less draft than a standard tool surface Sink Marks When the hot melt flows into the mold, the thick sections don’t cool as fast as the rest of the part because the thicker material becomes insulated by the outside surface of faster cooling plastic As the inner core cools, it shrinks at a different rate than the already cooled outer skin This difference on cooling rates causes the thick section to draw inward and create a sink mark on the outside surface of the part In addition to being unattractive, the mark also represents added stress that is built into the part Parting Lines A “parting line” is the line of separation on the part where the two halves of the mold meet The line actually indicates the parting “plane” that passes through the part While on simple parts this plane can be a simple, flat surface, it is often a complex form that traces the perimeter of the part around the various features that make up the part’s outer “silhouette” Part lines can also occur where any two pieces of a mold meet This can include side action pins, tool inserts and shutoffs Parting lines cannot be avoided; every part has them Keep in mind when designing your part, that the melt will always flow towards the parting line because it is the easiest place for the displaced air to escape or “vent” Keep these factors in mind when designing your injection molded part, and remember that it is easier to avoid problems in the beginning than change your design down the line  ... when designing your part, that the melt will always flow towards the parting line because it is the easiest place for the displaced air to escape or “vent” Keep these factors in mind when designing... material since different resins have different melt, flow, and freezing characteristics If your analysis shows that you need a particularly thick wall to satisfy your structural requirements,... can also affect the flow and shrinkage characteristics of the material and thus make your part design more difficult as opposed to easier Draft Most plastic parts include features such as outside