Plastic part design offers many challenges to the design engineer. The entire process from start to finish is complex - from the modeling challenges to the analysis and tool design.
The following disciplines are common to plastic part design:
- Mechanical design
- FEA (Finite Element Analysis)
- Mold flow and thermal analysis
- Tool and mold Design
Even if the engineer has a great design, the part must be able to meet the lifecycle requirements of the product and be manufacturable. The ability for the design engineer to understand and communicate to the other disciplines can mean the difference between a success and failure with a new product design.
General steps in plastic part design
First you need to determine the purpose of the design. The time invested understanding what is known and required from the design prior to just modeling the part, is time well spent.
Robust modeling techniques
Keep it simple when modeling parts that are robust. Many very difficult designs can be broken down in fairly simple parts. Using techniques like naming important design features, keeping functionally grouped feature together, and tying the geometry to a simple design skeleton.
The type of material used for the design should be determined early because different materials will behave differently when molded. The design rules (i.e., minimum wall thickness, radius) will be derived from the type of material selected. The material properties, molding characteristics, and cost for plastics vary greatly. The aesthetics of the product and its' environment will drive the material selection. Most resin manufacturers have design guides for their resins that call out minimum and maximum wall thickness, radii, and other molding properties.
FEA and assembly analysis
Finite element analysis (FEA) and assembly analysis save time and money by alerting you to potential drawbacks of your design. If the analysis is done late in the process, the tooling may need to change. This can cause the tool to be late and/or extra charges to be added. FEA analysis, in the example shown, indicates the need for a strengthening rib. The rib should be placed in the appropriate location within the FeatureManager® design tree. Typically, it should be placed before draft or cosmetic radii.
The functional concern is that the plastic flows into the part easily and at a desirable location based on how the part will fill. The cosmetic concern is if the gate will leave a mark on the surface. It is, therefore, typically placed on an inside surface.
Many factors determine the correct size and location for the gate and runner system. Heat, pressure, and cooling cycles will have an effect on the final part geometry. This is where communication with your tool designer is critical. Many tool designers will also use analysis tools to predict the plastic flow characteristics and cycle times.
Draft can be added in features as they are created. The disadvantage is the neutral place in the sketch plane. There is no flexibility in selecting a different neutral plane, and multiple features cannot be selected.
The selection of the neutral draft plane determines the way the draft faces is created. The example below shows the effect of the location of the neutral draft plane. All examples assume the draft feature is being created to allow tool to be pulled from the top of the part. The same draft angle (15 ) has been used for the split; subtractive, and additive draft examples. The part started out at 1.500 on the top, middle, and bottom and was draft using the plane named "Draft Plane" as the neutral draft plane.
All three examples affect the size of the box differently. The user needs to determine which neutral draft plane will produce the desired results. The middle method (split) has the smallest effect on the geometry. The reason for this is a portion of the face is made smaller and a portion is made larger. The other examples either completely add or subtract from the size of the part.
The last function of the process should be to add cosmetic fillets. These fillets are necessary for cosmetic, functional, and molding purposes. A sharp corner can cause a stress condition and lead to a part failure. Care should always be taken when adding large fillets to the inside supporting structures as to not exceed the nominal wall thickness. For example, consider the part shown above, where a large fillet is on the inside of a support boss. This may result in a sink mark when the part is molded. This occurs because the plastic is thicker there and therefore cools slower than the nominal wall section surrounding the fillet. A cosmetic defect may appear on the outside of the part. The degree of the defect varies depending on material and surface finish.
General rules for cosmetic rounds:
- Create larger fillets first. Larger fillets can fail of trying to bend around a tight corner.
- The order the fillets are inserted will determine the way the fillets are blended.
- Group fillets into the same feature. Don't make each fillet its own separate feature. Different radii values can also be combined into the same feature.
- Take advantage of tangent propagation. The number of fillets required could be reduced by defining the fillets that allow the later fillet to continue along tangent edges if the Propagate along tangent edge option has been checked.
- Use loops and faces when creating fillets, where applicable. This makes the feature more robust when the geometry changes
SolidWorks can be a powerful tool for plastic part design. The key is to understand the elements that are touched by designing plastic parts and communicate within these cross-functional areas to ensure the part and the design as a whole is a success.
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