Creating a 3D Solid Block Master Model
FOURTH AND FINAL SEGMENT
When I begin developing a production design I don’t think linearly. That’s a process that would solve a single detail at a time. Instead, I try to approach the process holistically. The idea is to understand the basic objectives of the project and the purpose of the product. This perspective helps establish a set of priorities to be applied to decisions throughout the design process. At this stage of development, most concepts have a defined appearance based on a manufacturing process, component layout and segmentation of parts that form the seams throughout the form.
At this point, I review the shape and each part defined in the 3D concept model. This is a critical step in design translation. Concepts often do not account for molding or assembly considerations and often require modification that will affect appearance. I also examine the proposed assembly of the product to be certain that the final design can be easily assembled and disassembled according to manufacturing requirements. All this is done before I begin any detailed CAD development.
If the overall design looks reasonable, I import the 3D concept geometry into a CAD program like SolidWorks or PTC Creo. The imported geometry of the external plastic covers is used as a template for creating the production part designs. Some designers have used this imported geometry for creating their individual parts instead of using it as a template. I strongly discourage anyone from doing this since it will result in a sloppy CAD model and be difficult to modify as the design is detailed. Models with non-parametric imported root features will eventually crash and require a complete rebuild. I strongly advise you to simply import the geometry as a template and completely rebuild a new parametric CAD model based on your CAD software.
The first step during the conversion process from concept to production design is to create a master model. This is a solid 3D block representing the entire external plastic assembly. I’ve heard other terms used to describe this root geometry, but the purpose of creating a master model is always the same. A master model or single solid part representing the entire external shell of covers is one of the great benefits of parametric modeling.
Creating an overall form provides the root geometry for the individual parts. This will assure you that every part forming an external set of covers will align with the other. Changes made to the master model will automatically change the geometry of all associated parts. This powerful CAD modeling technique enables you to easily modify a design without concern for misaligned parts.
There are many critical considerations when creating a master model. One of the most important is understanding how parts will be molded and where the parting lines will be located relative to the fit of one part to another.
Defining split lines between covers is critical and is based on many considerations. Factors affecting split lines are based on aesthetic requirements defined by an industrial designer who may not be familiar with the associated technical parameters.
When a master model is created, individual parts are defined by inserting surfaces to split the geometry into individual pieces. The location of surfaces is dictated by the aesthetic requirements set forth by the industrial designer. In addition, the optimum locations for parting lines are based on molding, ease of assembly (design for manufacturing, DFM), reliability, structural requirements and ease of service.
This is a critical starting point in the design of plastic parts that will have a cascading effect on all subsequent features and details added to the part geometry.
Most CAD software requires you to add draft to the part geometry early in the history tree. If you attempt to add it when the part is almost completely designed you may be required to either roll back many features or completely rebuild the model. It’s good practice to consider adding draft into the root geometry that is often the master model.
Addition of draft to surfaces is critical to the entire plastic part design. I guarantee that omitting draft in your part design will result in a disaster. Drafted surfaces will affect the overall appearance, fits and function of every part. The amount of draft will depend upon texture. Therefore these decisions must be made at the early stages of plastic part design.
You cannot decide on the outside texture of plastic covers after the design is completed. Conversely, you must make that commitment before you finalize your split surfaces and account for all the draft angles. Typically one-degree draft is required for any surface oriented in the direction of the draw or the direction of how the mold is opened during part ejection.
This is based on a smooth surface. When textures are applied to a surface undercuts are formed, requiring an additional draft. Recommended additional draft is adding 1 degree of draft per .001” depth of texture added to the base 1 degree. In other words, a 2 mil (.002”) deep texture will require a minimum 3 degrees of draft.
A well-planned master model is vital. You don’t want to add too many features, yet you do want to capture the major critical features of the overall design. There are no hard and fast rules for creating an optimal master model. The best suggestion is to develop your own level of detail with experience. Personally, I try to only include product features that will influence an adjacent or mating part. Features contained within a part are omitted and added when that part is detailed as an individual component.
By the way, this same methodology can be applied to any plastic part design. It is not exclusively limited to the design of injection-molded parts. We use this technique for every process including pressure forming, rotational molding, composites etc. Master modeling is ideal for the design of any plastic product where there are many inter-related parts forming an overall surface or enclosure. We don’t apply this modeling method for internal structures or assemblages of components in machinery, mechanisms etc.
In 1983, Michael Paloian founded Integrated Design Systems. Michael and his team have built an industrial design firm with a reputation for delivering world-class designs of sophisticated, often complicated products for international markets.
Michael is the widely respected founder and president of IDS. He wears many hats – industrial designer, inventor, artist, lecturer, educator and an expert in plastic product design and plastic part fabrication. He has developed a broad range of products by working closely with local, national, and international clients. Michael holds an undergraduate degree in Plastics Engineering and a Master of Industrial Design, and teaches plastic engineering, University of Massachusetts, Lowell. In addition, his extensive experience is the basis for his branded and unique insights into the field of Industrial Design.