Collaboration Leads to Success

Turning resin into a well-designed medical part requires careful planning throughout each step of the process.

If you talk to anyone who has been in the medical plastics industry long enough, he will have a story about a part that looked great on paper, but created significant problems in production.

Experienced engineers know that it is not enough just to design an aesthetic and functional product. It is equally, if not more important, to ensure part designs are optimized for manufacturability, which often requires the support of a variety of collaborators.

For best results in production, the design must be moldable, the mold must be durable and well planned, and the parts manufactured with precision. Poor designs can create product flaws that 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 the best design of a part for manufacturability, designers must rely on the expertise of their moldmaking and molding partners. Production of the best is 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 us look at several areas where this collaborative approach can deliver positive results.
 

Choosing the Best
The part design process is the starting point for creating any 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, slight dips in the surface of a part, are a common part imperfection caused by insufficient packing due to inconsistent wall thickness. Generally, gating of a part should be 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 typically in one location, 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 are not as apparent, physically, as sinks or warp, but are identifiable 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.

Early and constant communication among the part designers, tool designers, and molders is critical to the success of a project.

Although gating is more a concern of the mold designer, it is also important for part designers to be aware of gate design. He 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 is difficult to pack out the thick areas.

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

For example, if a customer wants to use Polyvinyl Chloride (PVC), to make a part that is traditionally made of Liquid Crystal Polymer (LCP), he will have trouble filling out the wall. Because the viscosity of PVC is higher than LCP, the resistance of flow will inhibit the material from filling in the thin areas. It is important for designers to rely on recommendations of an expert with knowledge of the properties of various materials for advice on which materials are appropriate for a given application.
 

Design, Fabrication
A good moldmaker will take responsibility for working closely with the part designer and driving the mold design. Based on the part design, he will provide 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 moldmaker must first ensure that the part design is physically moldable. Occasionally, it is possible to design a part that is not moldable.

Additionally, a moldmaker must be sure that fabrication of steel is possible, according to the mold design. The steel can behave during fabrication differently than assumed during design. Designers must ensure the steel is thick enough to withstand the injection process.

It is 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 there is support to walls 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.

Over time, all molds wear out and need sections replaced. Tool designers must evaluate which parts of the mold will wear out most quickly due to abrasion of the materials. By identifying these sections, they can build replacement inserts. That way, the sections can be replaced, quickly and when necessary, significantly reducing downtime or production interruptions.

Keep Variables in Check
After design of the part and mold, a molder should run samples of a part to be sure everything is functional and molding correctly. Expert molders use scientific molding to evaluate the molding process and make any necessary adjustments. This effort provides for a consistent, repeatable production of the part.

In scientific molding, process engineers determine both the optimal molding conditions and the molding window, or the best speed at which to inject plastic. Using real-time production monitoring systems and advanced quality inspection equipment, they also examine how easily part manufacturing will be and how consistently the mold runs based on several criteria.

Engineers must closely evaluate the manufacturability of a part, considering the process conditions with respect to the limits of the injection-molding machine.

After determining the optimal molding conditions and molding window, moldmakers conduct a first article inspection. They examine all the critical features of a part, and then all the non-critical ones. Often, they will also do a capabilities study, in which they examine critical dimensions over an extended production run. This study enables them to detect any variations over a longer period 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. Installation of the sensor is usually 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 do not 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.
 

Cooperation, Collaboration
Plastikos and Micro Mold, sister molding and moldmaking companies based in Erie, PA, specialize in working together to improve part and tool design – reducing cost, improving efficiency, and making products easier to manufacture.

Recently, 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 the part pushing 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. The tolerance for the force of pushing the part onto the metal component more than doubled.

This case study is a reminder that it is important for designers to find knowledgeable partners and trust their expertise. Through cooperative effort, designers, moldmakers, and molders can produce better parts more cost-effectively and efficiently.

About the Authors
Ryan Katen serves as both the engineering manager for Plastikos and the general manager for Micro Mold. He holds both a Bachelor and a Master degree in Industrial Engineering from Purdue University. He has significant tooling and injection molding experience, with a focus on manufacturing process improvement and optimization.

Rob Cooney serves as the manufacturing manager for Plastikos and holds a B.S. in Plastics Engineering Technology from Penn State, Erie, PA. With more than 13 years in the industry, Cooney has considerable expertise in continuous improvement, quality systems, and the scientific molding process to achieve the highest levels of quality and efficiency for customers. Cooney also serves on the SPE (Society of Plastics Engineers) board of directors and is president of the Northwest Pennsylvania chapter of SPE.

Plastikos Inc.
Erie, PA
plastikoserie.com