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How to Improve Part Design for Manufacturability

Working together, part designers and tool designers can make recommendations to enable better manufacturability
Photo courtesy of Plastikos

In a collaborative approach, part designers tap the expertise of their mold-making and molding partners

Rob Cooney and Ryan Katen

Just because a part looks good on paper, doesn't mean that it will turn out well in production. Beyond merely creating an aesthetic and functional design, experienced engineers understand the importance of optimizing a part for manufacturability. The design must be moldable, the mold must be durable and well planned, and the part must be manufactured with precision.

These considerations are especially important when creating parts for medical markets. Poor designs can create product flaws that can present significant safety concerns for doctors and patients. Any weak link in the design and fabrication processes can compromise the integrity of the final part--and lead to expensive redesign down the line.

To ensure that a part is well designed for manufacturability, designers must rely on the expertise of their mold making and molding partners. The best parts are produced through a joint effort of a part designer, tool designer, and manufacturer--all in constant communication from the earliest development stages. Through cooperation, a team of designers can avoid many common molding pitfalls and produce better, more efficient, more cost-effective parts.

Let's take a look at several areas where this collaborative approach can deliver positive results.

Part Design and Material Selection

Part design is the starting point for producing a quality finished piece. It drives the entire process and sets the course for both mold design and production.

One of the most important part design considerations is maintaining consistent wall thickness. Even thickness makes the part much less likely to contain imperfections.

Sink marks, which are slight dips in the surface of a part, are common part imperfections that are caused by insufficient packing, resulting from inconsistent wall thickness. Generally, a part should be gated thick to thin, so that the last area to pack out is at the gate end of the part. That way, the gate end freezes off last and ensures adequate packing throughout the part.

Similar to sink marks, warp is another type of distortion to a part's surface. Whereas a sink is more localized, warp generally occurs over an entire part. It is also usually the result of inconsistent wall sections, which cause temperature variations during cooling.

Voids are internal imperfections in a part that occur most often in thick-walled areas. They occur when there is a significant difference in cooling rate between skin and core material. These imperfections aren't as physically apparent as sinks or warp, but they can be identified through X-ray.

In addition to wall thickness, designers must also consider the durability of the part upon ejection from the mold. They should avoid incorporating delicate, fragile features that have a tendency to break during the ejection process.

Although gating is more a concern of the mold designer, it is also important for part designers to be aware of gate design. The mold designer must determine if the part is able to have gate vestige, and if so, the best placement of the gate. If possible, gates should be located at the thickest area of the part, so that plastic flows from there to the thinner sections. Otherwise, it's difficult to pack out the thick areas.

It's also important to choose the best material for a given application. Certain materials do a better job than others in filling out certain wall thicknesses, and many materials have a more uniform shrinkage than others. This also influences the quality of the final part because a more uniform shrinkage equals lower warp, which is caused by differential shrinkage.

For example, if a customer wants to use polyvinyl chloride (PVC) to make a part that is traditionally constructed using liquid crystal polymer (LCP), they'll have trouble filling out the wall. Because the viscosity of PVC is higher than that of LCP, the resistance of flow will inhibit the material from filling in the thin areas.

Mold Design and Fabrication

A good mold maker will take responsibility for working closely with the part designer and driving the mold design. Based on the part design, the mold maker will provide insight and recommendations to the customer on how to create the best mold to produce that part.

Using computer-aided design (CAD) software, mold designers and engineers can create an initial blueprint for the mold. With that as a guide, they can easily react to any changes that may occur on the part design, material selection, or cavitation, and adjust the mold accordingly.

To design a good mold, a mold maker must first ensure that the part design is physically moldable. Occasionally, it's possible to design a part that can't be molded. Additionally, a mold maker must be sure the steel can be fabricated according to the mold design. During fabrication, the steel can behave differently from what's assumed during design. Designers must ensure the steel is thick enough to withstand the injection process.

It's also important to make sure that the steel walls have adequate support in relation to the injection pressure. If the walls are too thin, a cavity can deflect under the pressure, causing flash or defects. Designers must ensure that walls are supported in high pressure areas to prevent this problem.

Alignment is another key to a well-designed tool. When molds close, all components must align properly. Any misalignment will lead to premature wear.

The Scientific Molding Process

After the part and mold have been designed, a molder should run samples of a part to be sure that everything is functional and molding correctly. Expert molders use what is known as the scientific molding process to evaluate the molding process and make any necessary adjustments. This provides a consistent, repeatable production of the part.

Through this process, process engineers determine both the optimal molding conditions and the molding window, or the best speed at which plastic should be injected. Using real-time production monitoring systems and advanced quality inspection equipment, the process also examines how easily the part can be manufactured and how consistently the mold runs, based on several criteria.

After determining the optimal molding conditions and molding window, mold makers conduct a first article inspection. They examine all the critical features of a part, and then all the non-critical ones. Often, mold makers will also do a capabilities study, in which they examine critical dimensions over an extended production run. This enables them to detect any variations over a longer period of time and adjust the tool or production print accordingly.

Another common monitoring component is a cavity pressure sensor to the mold, especially for higher-volume applications. The sensor is usually installed opposite the gate end of the mold and close to the last section to fill. Most sensors have pre-determined levels. Otherwise, the molder can set a threshold on the sensor, generally a low limit for short shot and a high limit for flash.

If the pin and the cavity sensor on the mold don't register the required pressure, the press will automatically divert the part with a chute or conveyor. This lets the molder know whether a part is good or bad before the mold even opens.

The Benefits of Working Together

Plastikos and Micro Mold, sister molding and mold making companies based in Erie, Pa., specialize in working together to improve part and tool design in ways that reduce cost, improve efficiency, and make products easier to manufacture. Earlier this year, the designer of a medical valve component came to them for assistance. The customer was producing the valve using an existing mold that was causing internal voids and sinks in the part. Additionally, one of the assembly requirements involved pushing the part onto a metal piece that fit inside it. The force required for this process was causing the product to fail at an unacceptably low force.

Designers at Micro Mold and Plastikos, working closely with each other and the customer, saw opportunities to modify the walls of the part for greater consistency. They cored out the thick-wall sections in a way that did not compromise the overall strength or integrity of the part. Through modified part and mold designs, they eliminated all the part sinks and voids upon the first shot out of the mold. And the tolerance for the force of pushing the part onto the metal component more than doubled.

Rob Cooney is the manufacturing manager at Plastikos, and oversees design and engineering at Micro Mold. Ryan Katen is the general manager at Micro Mold and engineering manager at Plastikos.

Plastikos is a custom injection molder; its sister company, Micro Mold, focuses on medical mold building for small, tight-tolerance components. More information on the companies is available at www.plastikoserie.com.