How to Design Better Plastic Parts

Document created by SW Admin on Jan 13, 2010Last modified by SW Admin on Oct 13, 2011
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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
  • Assembly

 

 

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


Design intent

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.

 

 

Material selection

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.         

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Figure 1 – Part with a strengthening rib

Surface finish
The type of surface finish specified will have an effect on the amount of draft required for the part. There are a wide variety of finishes available and a number of different methods are used to apply the texture. The important point here for the design is if the texture selected requires 3 degrees of draft and less draft is modeled on the surface, or worse yet no draft, the texture make be rubbed off or worse the part stick in the tool. Surface texture is usually applied to the tool after the first samples are shot from the tool. This allows for changes to be made to adjust for design or moldability issues without re-texturing the tool. Once textured, it often is more difficult to make a change without causing a cosmetic defect.
Determine parting lines, gates
The tool designer can provide valuable input as to the possible location and types of tool related features that need to be accounted for within the design (i.e., parting lines, draft, gate location). The sooner these design related features can be discussed with your tool designer, the better. Many times this becomes a negotiation with the mechanical, industrial, and tool designers. The parting line is the location at which the two halves of the mold meet. The selection of the parting line is important for both functional and cosmetic reasons.  Parting lines can be simple or more complex (stepped) based on the part design. The tool designer typically drives the location of the parting lines with input from the design engineer. There is always a trade-off between aesthetics, design considerations, and moldability.
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Figure 2 – Parting lines on a plastic mold part
Another factor to consider when designing plastic parts is the effect of the location, size, and type of gate and runner system.  The diagram below shows a four-cavity mold with a runner system that feeds the gate of each part. The gate is the point at which the plastics will flow into the part.
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Figure 3 – Runner system for plastic parts

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
Once the tooling features have been defined, the parting line(s) will drive the direction of draft within the part. The tool designer may request additional accommodations for gates and shutoffs. Some tool designers also make the changes to their models and leave the engineering version as-is. Adding draft to the engineering model is typically better because the designer can see what changes happen as a result of draft.
20208-004.png
Figure 4 – Neutral plane draft

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.
20208-005.png
Figure 5 – Selection of the draft plane will have drastic effects on draft shapes
A parting line draft feature allows for the creation of draft along a non-planar parting line. The parting line is created using the Split Line function. The direction of pull is created from a part edge, face, or plane that defines the normal (perpendicular) direction of the draft feature. The location of this feature does not affect the tilt location for the draft feature. The feature is tilted from the parting line location. The direction of pull defines the normal direction for the draft feature.
20208-006.png
Figure 6 – Parting Line Draft
The draft analysis tool allows for a quick review of a part design to see if all the surfaces were drafted.
The tool allows the user to select a face to parallel to the pull direction and set a minimum draft angle. The draft is graphically displayed as shown.
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Figure 7 – Colors on a part showing draft angle with respect to a pull direction

Cosmetic rounds

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


Conclusion

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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Do not distribute or reproduce without the written consent of Dassault Systèmes SolidWorks Corp.

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