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The Prototyping Blog

the prototyping blog

5 Plastic Prototype Design Variables That Affect Lead Time & Cost

Posted by Matt Sweeney on Jun 1, 2015

prototype-design-drawing.jpgYou’ve determined the best strategy for the prototype to production development project. A plan has been established with a course of action that fits your budget and timeline. Now, have you considered the design variables that may occur during this development process which accrue cost and timing?

Something to consider during the prototype phase for a high volume production project is that concepts and design standards should be mirrored to the production mold as close as possible to ensure the learnings from the prototype tool are utilized. This helps validate concepts and answer questions that always arise during a production launch. If high volumes are not of concern, the use of manual hand loads could be utilized to lower your upfront cost and shorten your lead-time.

Regardless of production volume, there are several factors that should be addressed prior to plastic prototype development. Critical factors include production volume, complexity, tolerances, structural integrity, as well as the knowledge that can be gained to support design intent and end-use applications. Here are a few other design variables that should be considered in regard to time and cost.

1. Complexity

Managing the complexity of your part can play a huge role in the overall time and cost. If your part has many variables that need to be addressed, your prototyping partner should provide insight regarding what can or cannot be eliminated. Another option is to break more complex models into two pieces that can be later assembled into the final product. Time efficiencies will come with simplified builds that can provide the valuable information needed for a more complex one. Someone with both prototyping and production experience would be able to weigh these decisions and determine the most efficient project scope. 

2. Side Actions

In the conceptual design phase (when determining how your part geometry is going to look) it is not always known if a mold will need a side action. If so, this additional variable can play a role in the completion timing and cost.

Not only does a side action cause complexity within the mold but it takes time and skill to make it work properly. If there is any way the part can be designed without requiring a side action while still maintaining its functionality, this situation would be most desirable. This is why it is important to have engineering support involved in the early prototyping phases.

3. Rounded Corners

Avoiding sharp corners in the prototype design process can be done through the use of a radius to distribute the stresses and also streamline the flow of the molten plastic. During injection molding, the hot plastic conforms to turns and corners. Rounded corners will ease plastic flow, where in contrast, sharp inside corners result in molded-in stress. This happens particularly during the cooling process when the top of the part tries to shrink and the material pulls against the corners. Working with a knowledgeable engineer to identify areas for part design improvement will lead to a stronger, more dimensionally stable part that resists post-mold warpage.

4. Sufficient Draft

Allocating sufficient draft not only makes it easier to remove a part from a mold, it also minimizes tool wear. While draft facilitates the removal of the part from the mold, it is particularly important in rapid injection molding to maintain parting lines, part quality and tool functionality.  It typically takes a degree of experience to know how much and where to add draft.

In addition to draft specifications, there are other factors that should be taken into account such as resins and polymers, molding improvements, and more.

Allow for 1.5 degrees of draft for each .001″ of texture finish depth.
Is the vertical wall in question an inside or outside wall? If it is an inside wall, the part will shrink to it during molding, and you will need more draft in order to properly eject the part.
If there is nothing on the core to hold the part in place during ejection, the part will tend to hang onto the cavity and create deformities in the part or keep the mold from running at all.  This could be a situation where you could benefit from differential draft. 

5. Wiping Shut-offs

A wiping shut-off is needed when part geometry forces you to shut off two parts of the mold against each other to prevent plastic from passing through. This is not a desirable situation and sometimes can be avoided with guidance from an experienced engineer.  However, the feature is sometimes essential to form a window. When a wiping shutoff is required, 3-5 degrees of draft has to be added to avoid grinding the steel of the mold against each other. The term for this is galling.

What challenges have you faced in regard to design variables during a prototype development or production process? Learn about 7 guidelines every design engineer needs to know for pilot prototyping. 

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