Improving Cost Effectiveness of Injection Molding Designs

In 1999, Yuh-Min Chen and Jang-John Liu published a paper discussing various methods to improve the cost effectiveness of injection molded designs [1]. In 2001, Corrado Poli published his version of the same general methodology for reducing the cost of injection molded goods [2]. Though these methods are similar in that the end goal is the same, there are several procedural differences spread throughout the literature.

Using the Poli method, the design engineer seeking to evaluate a product must first consider the general geometry of the product. Envelope dimensions are recorded to determine if the part is flat or box-shaped. Afterwards, the product can be categorized based on the longest dimension being above, in, or below the range from 250mm to 480mm. This defines the overall size of the product.

The next step is determining the size of the product, the engineer must consider external undercuts. These are defined as shapes which will require some modification to the die tooling to accommodate while not requiring additional core molds. An example of an external undercut could be a hole in the sidewall of a plastic box. Once the number of external undercuts has been decided, the engineer must consider the internal undercuts. These are the same as external undercuts; however, they do require additional tooling to create. This could be an internal ridge in a plastic box that requires a core mold to be installed prior to injection molding.

After determining the undercuts present, the parting surface must be considered. The planarity of the parting surface is determined along with the shape of the product within the mold. Once all of these geometric constraints have been recorded, the engineer selects the box according to the provided relative die cost table. This provides the engineer with a relative basic part complexity.

The design engineer will then consider the individual features present in the product. These features range from ribs, shutoffs, to irregularly shaped holes. These features are used to rate the overall complexity of the product. After determining the cavity detail, the number of undercuts are also factored in. The design engineer will then have a subsidiary factor for relative die cost.

Once you have considered the previous two factors, you will select a surface finish complexity factor based on the Society of the Plastics Industry rating for that finish. This is multiplied together with the previous two factors to find an overall die complexity.

Next, the design engineer will consider the ratio of length to height of the mold and use a special graph to find a correction factor to use. This correction factor feeds into a number of equations that are used to find the overall projected area of the mold base. This is used to chart out the relative die material cost which is then used, with the die complexity, to determine the overall relative mold cost.  

Once the mold cost has been calculated, the design engineer will use the volume of the mold to determine an approximate relative cost of the material used in the mold to make the part. This is then combined with a geometric analysis of the part to find an overall processing time for the product. After estimating the machine tonnage based on the total area of the part projection, the expected relative processing cost can then be determined. All of the previous calculations may then be combined in one final weighted equation to determine the relative cost of the part normalized to the volume of that part.

Using the Yuh-Min Chen method, the design engineer will consider many of the same attributes. However, Chen differs from Poli in that a computer-based system is considered to be mandatory to correctly identify cost factors. Chen primarily focuses on variations in the product cost as the result of product design. Chen considers the same focus areas of tooling, materials, and processing; however, Chen goes much further in depth on each topic.

While Poli merely makes use of a simple volume calculation to determine used material, Chen makes use of calculated sprue volume, tare volume, biscuit volume, losses due to overfill and recycling inefficiencies to form a much more in-depth calculation. Chen considers fuel usage in melting the plastic as well as labor rate particular to each machine. Trimming costs are also considered which Poli does not make use of.

After considering material costs, Chen moves on to consider tooling costs in the same way that Poli does. Similar to Poli, Chen considers tooling cost to be amortizable over the entire production volume of the product. However, Chen considers both setup costs and in-depth mold construction costs. Chen moves on to consider the processing cost associated with the product. As this process is much more complicated, it must be performed using automated computing systems.

Chen’s automated computer-aided design systems that utilize Design for Injection Molding do so in order to simplify an extremely complex set of questions that are not easily solved by a human. As there are numerous constraints to solve, an iterative approach is used to systematically redesign the product until all constraints are met and all cost inefficiencies are removed or accepted. After design has been completed, individual parts are combined and examined to ensure that global requirements are met. This ensures that tolerance stacking does not lead to excessive product rejections.

Ultimately, both the Chen-defined method and Poli-defined method of Design for Injection Molding. Both utilize similar methodology; however, Poli tends to favor a quicker solution with less detail required. Despite this, both methods allow for iterative product design to obtain the greatest cost savings for the manufacturer.

References:

[1] Chen, Y., Liu, J., (1999), “Cost-Effective Design for Injection Molding”, Robotics and Computer-Integrated Manufacturing15, pp. 1-21.

[2] Poli, Corrado. (2001). Design for Manufacturing – A Structured Approach. Elsevier. Online version available at: http://app.knovel.com/hotlink/toc/id:kpDMASA001/design-manufacturing/design-manufacturing

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