Features

Medical manufacturers of single-use products have a dual challenge: achieving high levels of quality while maintaining competitive cost. From an injection molding perspective, mold design and equipment selection ultimately determine whether or not those goals are achieved. Cost targets, projected volumes, the raw material required, and unique product features have impact on mold cavitation strategy, design of the gates and runner system, amount of automation within the mold, optimum tonnage of the molding machine, and the degree of automation in the de-molding process.

The key question becomes: how can a product development team and sourcing personnel who are not expert in injection molding, best understand their options and make the correct choices in mold design? Three key areas to evaluate are potential suppliers’ degree of vertical integration, the robustness of their design processes, and their design for manufacturability expertise.

Integrated Mold Design and Injection Molding Capability

Fig. 1 – High cavity molds, such as the 16-cavity mold shown, require an engineering team focused on both mold design and manufacturing challenges.
Selecting an injection molder or contract manufacturer with both mold design and injection molding capability has several benefits. First, the mold design and fabrication team looks at the project from both a mold design and manufacturability standpoint, since they will ultimately be responsible for achieving whatever production and cost targets set during the design process.

Second, an integrated team helps shorten the overall product development process, particularly if the supplier uses a gated design process to enable tooling development to begin as early in the design process as possible. Taking a vertically integrated approach to tooling fabrication and use of in-house resources often cuts another two to three months off of product development time.

Third, integrated capabilities typically mean that tooling maintenance and repair can be performed in-house. This can substantially reduce maintenance cost and downtime, since molds do not have to be shipped to a third-party for repair and the convenience of in-house resources helps ensure regular preventative maintenance, which can increase the useable life of the tool.

Finally, in situations where a third-party mold design and fabrication house is the best choice for tooling development, a molder or contract manufacturer with mold design expertise can more effectively manage that supplier and ensure that all critical information is communicated. This also prevents situations where the molder and tool fabricator each blame the other for any tooling issues. (See Figure 1)

A Robust Design Process

Mold design for medical single-use products requires consideration of far more elements than commonly found in consumer products. Approved product materials may be challenging to mold or de-mold. Tolerances may be tighter. Quality and cost goals dictate minimized secondary finishing operations. Given that there may be higher cost for specialized raw materials and limits on the use of regrind material, scrap must be minimized. Not surprisingly, the design process must look at a wide range of tradeoffs in developing the ideal solution.

Fig. 2 – Software modeling tools help test design assumptions and reduce mold design iterations, reducing both time and cost in product development.
A good design team starts by assessing customer requirements and developing an internal design plan plus a customer specification. A standardized approach aligned with regulatory and validation testing requirements makes it easier for the device manufacturer’s product development team and the molding supplier’s team to understand what resource gaps need to be addressed at the beginning of the process.

The next stage in the process normally uses 3D CAD models to test initial assumptions. The design review process should include functional analysis and risk evaluation. Mold-flow analysis software should be used to optimize the mold design. Once the design is approved, prototyping and validation begin. In supplier selection, documented processes, experience with products of similar complexity, software modeling tools, and in-house prototyping capability should all be carefully evaluated. These elements can be critical in reducing overall design process time and eliminating design iterations. (See Figure 2)

Design for Manufacturability (DFM) Expertise

One of the challenges that must be overcome when mold design and molding are done in separate silos, is the fact the teams that exclusively design molds are often not expert in manufacturing. Achieving optimum manufacturability often requires tight tolerances and precise timing of raw material entry, along with molded product exit. When the teams are integrated, these issues are addressed earlier in the process. If manufacturability issues are not addressed until after the tool is fabricated, making necessary adjustments adds time to the design cycle and significant cost.

Example: High Cavity Mold Design for a Urinary Catheter Product

The following example illustrates the value that a strong design process coupled with in-house mold design expertise can bring to complex product development projects.

One key area of focus in the mold design process is determining the number of cavities needed to achieve production goals. High cavity molds enable medical manufacturers to tap economies of scale when fabricating high volume single-use products by reducing the number of machines needed to achieve required production volumes. That said, a higher number of cavities drives higher tooling costs and, in some cases, larger machine tonnage, which adds additional cost. With medical products, typically high cavity molds are only used for high-volume products, such as syringes. However, in this case, the contract manufacturer had an existing customer with a small part used in a specialty catheter application that was forecasted to do 12 million pieces in its first year of production and then grow to 24 million in the second year. The team at a contract manufacturer determined that a 64-cavity mold would provide the most cost-effective option for meeting the anticipated second year volumes.

There were several areas of challenge in this mold design effort including:

  • Optimizing the mold to fit an existing 220-ton injection molding machine
  • Designing the correct assembly layout of the mold
  • Designing the right de-molding process for the part release from the mold as this product is not rigid
  • Hitting the 22-second cycle time necessary to stay within cost targets.

The contract manufacturer’s engineering team used its standardized process to assess customer requirements and develop a Design Development Plan (DDP). The tooling design process included a DFM phase, followed by development of the mold specification. The DFM document outlined requirements for core cavity, sliders, gating position/size and location, and the results of computer-simulated mold flow analysis. The goal was to ensure efficient molding with minimal scrap and elimination of secondary finishing processes with as few design iterations as possible. The DFM recommendations also proposed use of a robot arm for the de-molding process.

An advanced CAD/CAM/CAE software package was used for tool, hot runner, and cooling system designs. Moldflow software was used for mold-flow analysis and to support Design of Experiments (DoEs) to optimize the design and molding parameters that were not part of the contractor’s existing library of injection parameters. Mold flow software was also utilized for molding process simulations to test assumptions prior to tool fabrication.

As originally designed, the mold was too large for a 220-ton injection molding machine. Utilizing a higher tonnage machine would increase cost since the machine consumes more space and energy, so the team needed to change the mold assembly layout to reduce the size of the mold. A key challenge was that this part was small and formed by the both the core, cavity, and sliders. The less complicated parts typically found in high cavity molds are usually formed with just core and cavity.

The team needed to develop a design that had sufficient space between cavitation, runners, and a comfortable ejection system. They designed a mold without thin steel, positioned the layout accordingly, so that a single hot runner tip would inject the plastic to two parts, and designed a hot runner system that met those requirements. This reduced the space required enough to utilize the lower tonnage (220-ton) machine.

A specially formulated low density polyethylene (PE) material selected for this part also presented a challenge. This specialty PE resin is flexible, which doesn’t work with a standard ejection system for de-molding. A robot arm assisted by a balloon-type pneumatic system was added to the de-molding process to eliminate this issue.

The final area of focus involved optimizing the process to ensure a 22-second cycle time was achievable. One key constraint in achieving that cycle time was cooling requirements. The parts needed sufficient time to cool during the mold opening and closing process. Providing sufficient cooling time required optimization and synchronization of the mold opening, cavity release, and part ejection process. The design team estimated cycle times during mold flow analysis and then worked with production time to fine tune process to the targeted 22-second cycle time.

The mold completed the validation process and the production process is achieving desired quality levels. The customer’s goals for cost and scalability were achieved.

A key element of any supplier selection process is assessing the degree to which that supplier can modify existing processes to accommodate challenges driven by material selection, unique product features, anticipated production volumes, and cost targets. As this example illustrates, there were a number of a challenges to address in developing the optimum mold design. Design assumptions needed to be modified to address cost targets and constraints related to raw material choices.

The final process needed to be fine-tuned to achieve the cycle time target while achieving the required degree of cooling time. The ability to test assumptions via modeling software helped reduce overall time and design iterations.

Conclusion

Having an integrated mold design and production team helped ensure all issues were identified early in the tooling design process instead of after production commenced. When evaluating new contract manufacturers, a well-designed supplier survey tool should include detailed questions that look at the processes and solutions that those suppliers under consideration have used to address challenging product development projects.

This article was written by Daniel Benze, Senior Vice President Technical, Forefront Medical Tech nology, Farmington, CT. For more information, Click Here.