Tooling Corner: Reducing mold/part cost through skillful engineering

Controlling production cost is a critical element of a profitable molding operation. The most successful molding companies understand this and can quickly assess the cost effectiveness of producing a given component.

However, to understand the real cost of producing a component, one must begin with a look at the initial concept. The designer?s vision of the component begins to lock in shapes and features, sometimes without much consideration as to how these can be produced. These shapes and features have a great influence throughout the design phase. It is during this phase of the process that these features and shapes become tooling and molding dreams or nightmares.

It is also during the design phase that a number of significant opportunities exist to create a product that meets performance requirements at the lowest possible production cost. The designer always needs to balance cost vs. performance.

Some of the key design and engineering factors that can influence final component cost are:

When a product is originally conceptualized, the designer can make critical decisions about how a product functions and economically feasible ways to manufacture the product. For example, an enclosure application as seen in Figure 1 could be designed as two separate molded components snapped together or as a single component using a two-shot molding process, forming a living hinge.

Understanding the cost benefits and penalties of decisions made at the concept stage will have a significant impact on the part aesthetics, function, and manufacturing steps required to produce the housing.

Production Costs

The cost of producing a molded component includes:

The largest of these factors is often the raw material cost, making it a significant percentage for most injection-molded components. It is very important to evaluate the performance criteria of the molded component since it influences the type and amount of material required in the application. Selecting a material that meets critical service requirements allows the designer to achieve the optimum performance at minimum cost. It is vital that the appropriate engineering properties be compared when selecting the right resin for the job. For example, if creep or stress relaxation (with or without temperature influence) are considerations in the application, the use of a more durable resin is necessary.

The design also plays a differentiating role in material selection and thus the final cost of a product. For example, a computer monitor could be designed using a PC/ABS blend because the load on the monitor is such that PS cannot meet the creep requirements. The monitor was originally designed with a uniform 4-mm wall thickness, but design optimizing software determined that only a small load-bearing region of ribs and walls required a 4-mm thickness to meet the structural needs.

Incorporating the 4-mm sections only where needed reduced the wall stock for the non-load bearing areas of the part to an average of 3-mm thickness. This substantially lowered the part?s volume and therefore the overall raw material cost, and consequently the molding cycle time, allowing the producer to mold the part in an engineering thermoplastic with a total component cost savings of approximately 25%.

This illustrates the fact that it is not advisable to simply look at price/lb as the differentiating factor in reducing the cost of a component. An innovative design that maximizes the material?s performance is the key to minimizing cost and maximizing performance.

From a cost perspective the engineer needs to take advantage of all the design-related opportunities available. For example, Figure 2 shows a side-wall window design that avoids the need for side-action slides, reducing tooling cost and potential process problems.

Making use of joining techniques that do not require secondary operations can take significant cost out of a component/assembly. It is critical to design these features into the product before cutting mold steel.

A snap-fit assembly could replace a more expensive metal insert/screw design. This could save money, although the part would consequently require an engineering resin rather than a commodity material. A cost comparison between using a cheaper material vs. the advantage of the reduced assembly steps (and the associated manufacturing costs) needs to be made.

Components with critical cosmetic requirements can create significant added cost from rejects and tooling changes. Often these issues are directly related to design details. For example, a poorly designed boss that causes wall thickness read-through results in part rejects, redesign, retooling, and part ejection issues; all of which could have been avoided by proper design.

Use of Simulations

It is difficult to know when you have an optimal design, but it?s vital to ensure that the component meets the requirements and is as cost effective as possible.

Generally, two considerations dictate the design?structural integrity and moldability. The goal then is to design the part such that it handles the necessary loads, and can allow defect-free filling in the mold.

Due to the filling patterns of plastic materials, finite element design techniques need to be applied to both optimize the structural and moldability issues.

In the past, components requiring impact, for example, were typically designed for quasistatic loadings and then went through an iterative prototyping phase to optimize the impact related issues.

Advances in simulation capabilities now allow for accurate prediction of high-speed impact events, which in turn can significantly reduce the product development timeline and eliminate the ?over-design? which results in increased cost.

For example, a cell phone housing analysis gives the engineer the ability to determine failure without cutting a prototype tool?he can place material where it is most needed, minimizing overall use of material.

Paramount in such an analysis is the use of the correct ?high-strain rate? material properties. Material behavior input for impact requires special nonstandard testing techniques such as we have used for PC/ABS.

Another advanced CAE tool that has been shown to help the engineer select the best material and minimize the amount of material needed is ?shape/topology/wall thickness? optimizing software.

Figure 3 illustrates an example of how a computer monitor support is designed for a given set of loading conditions. Here a PC/ABS is found to consume the least amount of material with the computer generated optimal rib pattern.

Molding and Assembly

Moldability at the lowest cost is critical. Usually the design of the injection mold is the key to successfully molding a part at minimum cycle time with minimized scrap. CAE mold-filling tools are a necessity to avoid the trial and error process in optimizing a tool.

An automotive headlamp bezel that exceeds pressure limits with a single gate can be fixed by strategically using multiple gates. Weld lines are located in acceptable areas, avoiding the need to use an expensive hot-runner system and reducing the tooling cost. The use of up-front runner balance optimization (hot-drop to cold runner) can minimize scrap and allow for a single four-cavity mold to be used. Significant efficiencies in cycle time and tooling cost are achieved.

One area where significant cost can be avoided is by assembling components efficiently. Here the ability to predict the warpage of a component can save excessive tool tweaking which can cause cost overruns.

A printer chassis, for example, can be fine-tuned for tolerance using shrinkage/warpage and cooling software. Here the cycle time is minimized by optimal cooling channel placement, and warpage is controlled within spec?all before the first tooling is cut.

Specialized Processes

Technologies such as multicomponent molding, gas-assist molding, film-insert molding, injection-compression, and especially thin-wall molding, have proven to be cost-saving options when the part is designed for the technology and the proper engineering material is selected.

When parts are not designed to fit the special technology, however, significant tooling rework and delay can result; so be sure to have a fully thought-out design before applying some of these techniques.

Thin-wall molding has been successful in minimizing material usage and lowering overall cost. Experience has shown that thin-wall parts need high-flow, durable materials that can handle high injection rates.

Designing ribs, wall thickness, part shape, and gate location are critical to a thin-walled part. Special high injection-pressure molding equipment and hot-runner systems that use valve-gating techniques should all be considered.

Editor?s note: This article was adapted by permission from The Society of Plastics Engineers, from an ANTEC 2006 article. Terry G. Davis of Bayer MaterialScience assisted the author in the preparation of the original article.