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JUL/AUG 2013  

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Micromolding Case Study: Thin-Walled Funnel Tip

Micromolding Case Study: Thin-Walled Funnel Tip

By Jim Liddell and Justin McPhee, Mold Craft Inc.

 

The Challenge:

Molding a pipette-shaped part with 0.370"-long wall section that is 0.015" thick followed by a 0.080"-long section with a wall that narrows to 0.008" thick is challenging enough by most mold and molding standards. Add to that, molding these microparts from PEEK (polyetheretherketone), selected for its high strength properties needed by the customer, and the challenge is even greater. Now, how about doing this in a four-cavity mold? An eight-cavity mold? Can a Cpk of 1.33 be achieved on this wall section? These are the challenges and questions Mold Craft set out to address when a customer came to them with this medical “funnel tip” component. This project was accomplished using Mold Craft’s partners in mold fill/material flow simulation, MeltFlipper technology and a molding press designed for micromolding.

Mold Craft Inc., Willernie, Minn., designs and builds high-volume, multi-cavity molds to produce small, intricate, precision plastic and metal parts.

Mold Craft also can build injection molds with extremely tight tolerances to accommodate molding of microparts with intricate geometries that require high Cpk values. Recently Mold Craft was approached by a customer whose challenge was just that: micromedical parts shaped like a pipette called the “funnel tip” with extreme length-to-wall thickness ratio.

Mold Craft 1 

The funnel tip’s wall thickness narrows from 0.015" to 0.008".


Mold Design:

The Mold Craft Design Center took on this project that was originally designed to be a single-cavity “bridge” or prototype tool, but evolved into a production mold requiring at least four cavities, and possibly eight cavities to meet the customer’s increasing production requirements. First, Mold Craft created a preliminary design model and mold layout to be used at the heart of the collaborative efforts via WebEx with the customer.

The design package consisted of fully detailed and toleranced prints which have proven to be the key to manufacturing completely interchangeable components that do not require fitting when installing in the mold. The design department is critical to the manufacture process as they design for manufacture based on our Erowa work holding and palletization system.

A flexible manufacturing system allows Mold Craft to finish components as special operation and put on the shelf until needed. This was the key to executing this project as we increased cavitation from one to four after stringent molding trials to prove the balance and capability of the single cavity.

The mold design itself needed to be flexible to accommodate a single cavity that would evolve to four and eventually eight cavities, depending on the results of the simulation and actual testing. The flexibility was achieved by using an adequately sized mold base that would allow for the addition of cavities as the customer’s volumes increased. Interchangeable cavity inserts could be added at a later date to minimize the turnaround time for the additional cavities and insure splits that allowed for adequate venting, and the ability to modify the molding geometry if needed.

To ensure proper balance, Mold Craft enlisted Beaumont Technologies, developer of the MeltFlipper technology, early in the mold design process in order to mitigate the effects of shear-induced melt variations that develop in all runners and to assure that the cavities could be filled completely. MeltFlipper technology was developed to manage shear induced flow variations which lead to problems such as flash, short shots, long cycle times and high scrap rates. 

Mold Craft 2 

 

Optimizing the mold design helped ensure that the melt delivery system (runner and gates) and process would minimize the risks of quality issues in molding the critical features of the plastic part. The thin tip region of the part was a particular focus. A filling and rheological balance was critical in order to assure consistency between parts, eliminate core shift, eliminate hesitation effects and generally maximize the molding process window. Beaumont’s melt management technologies, including MeltFlipper, were determined to be the best means to deal with the thermal, shear and filling imbalance found in all runners. This technology was designed into the runners for each of the prototype and production molds.

As part of the runner design process, Beaumont used its proprietary molding-processing-analysis software program, VeriFlo to size the runners and gates and assure the parts could be filled with the widest process window possible.  To ensure proper mold fill and balance, Mold Craft obtained a mold filling simulation analysis, using Beaumont’s VeriFlo material flow characterization study, which is used to benchmark actual molding conditions vs. simulation predictions. With all injection molding simulation programs, a certain amount of error should be expected. After all, they are simulation, not reality.

The more demanding an application, the higher the potential for error, and because this project’s application truly challenged the limits of the injection molding process, Mold Craft wanted the best possible analysis. Beaumont’s injection molding simulation in demanding applications such as our customer’s small, thin walled, PEEK medical device part provided real world molding feedback to the simulation engineering group.


Challenges:

During the design phase Mold Craft saw that the runner and gate system would be critical to the success of the project. Mold Craft recognizes that the industry standard of a “naturally balanced” or “geometrically balanced” runner for this application could result in shear-induced melt variation. This condition could develop in the runner system that would result in different melt conditions feeding different cavities and different regions of a cavity.

This could result in cavity-to-cavity material and filling variations as well as variations within each of the double gated parts. A best-case scenario with these conditions would delay product start-up, process development and result in a narrow process window. At worst, Mold Craft wouldn’t be able to mold the product or do so at a high spoilage rate.

Due to the extremely high melt flow of the PEEK material the customer needed for this part, we decided to begin with a single cavity prototype in a mold base scaled and set up to eventually accommodate eight cavities.

While expanding to four and eight cavities, the Mold Craft design engineers realized this micro-sized, dual-gated funnel tip may have serious melt balance issues both within the cavity and between cavities. The need to balance the runner and parts to ensure dimensionally-accurate parts (given the extremely small dimensions), was essential to the quality of the funnel tip.

Beaumont’s VeriFlo provided Mold Craft with the answers to all of these challenges by providing real world molding information to the analyst. VeriFlo begins with molding the subject plastic material in a specially designed flow characterization tool/mold that includes a wide variety of flow geometries. Specially designed data acquisition equipment very accurately captures the melt’s reactions when processed through a matrix of injection rates, mold and melt temperatures. Beaumont’s CAE engineers then perform injection molding simulation through each of the pre-modeled flow characterization tool/mold geometries using the exact conditions as those recorded during actual molding. The results of the simulations and actual runs are then compiled and processed. CAE engineers can then use this information while analyzing a project mold thereby improving accuracy and minimizing risks.

Using the single cavity prototype and the mold filling simulation and analysis to verify the performance, Mold Craft was able to ensure that the part design, mold and molding conditions would be optimum for producing this micro-part in a production molding environment. Simultaneously a four-cavity and eight-cavity simulation was also produced.

With the increase of the mold from a single-cavity prototype mold to a four-cavity production mold, the project took a turn to one that would require extremely tight tolerances of the part geometry to ensure high Cpk. Also, it would need an advanced runner design to deliver the PEEK material to all cavities evenly, given the extremely thin wall section— 0.008"—so that these very small parts could be molded consistently and repeatedly fill the microparts given the dimensions (0.472" long x 0.054" top diameter narrowing to 0.039" at the bottom).

The runner cutaway of a mold filling analysis predicted a melt difference of more than 70° F to be developed across the runner feeding different cavities. Beaumont Technologies determined that the differences were even more severe as all of the filling simulation programs will under-predict the actual filling imbalances. Note that not only are the different cavities receiving different melt conditions, but the melt temperature at each of the two gates feeding the inside four cavities is significantly different.

Mold Craft completed the design phase of the project using the recommendations from Beaumont Technologies, engineering the one-, four- and eight-cavity design packages. Sharing and collaborating on the design with Mold Craft management gave our customer confidence that this small, intricate part could indeed be successfully molded. The design was sent to Mold Craft’s Technology Center to produce the mold. Using precision CNC high speed mills, CNC wire and sinker EDM’s the designs came to life. Specializing in high precision multi- cavity molds, Mold Craft insured productivity using a pallet fixture system along with advanced automation to produce exacting precision intricacy while reducing costs for their client.  


Molding Machine and Material Requirements:

Because of the size and complexity of the part and the mold, Mold Craft next needed a molding machine that could produce the part. Mold Craft contacted Sodick Plustech, known for its ultraprecision injection molding machines, to discuss the project. We determined that the newly released 20 ton Plustech LP20EH2 Micro Molding series with a 12mm diameter plunger would be ideal for the application. The LP20EH2 was developed due to market demand for fine pitch connectors and other thin-wall micro-featured components. This LP series uses Sodick’s proprietary Linear Motor Driven Servo Valve that can inject at a speed of 1,300 mm/second (51.1"/sec.) translating to an acceleration of 15.3G.

Mold Craft also contacted material supplier Victrex, because of its intimate understanding of PEEK processing, material characteristics and recommendations on the specific grade as we push their material to its limits in this medical application. Victrex 90G, a high-flow PEEK developed specifically for thin-walled, intricate component applications, was selected. It also offers higher mechanical properties with improvements in modulus, toughness, and coefficient of thermal expansion.

“The Victrex 90G material was chosen due to its high strength properties and the polymer’s extremely high melt flow,” said Justin McPhee, Vice President of Engineering for Mold Craft. “There was much work done by Victrex in defining the exact material to use and supporting us to get the temperatures correct to optimize the parts. With their assistance, we were able to build an application that is working well for the customer.”

A cartridge heater assembly was chosen to precisely raise the temperature of the polymer to insure melt and mold flow to the desired range of 385°F.

The new model LP20EH2 Micro Molding series from Sodick Plustech was developed because of market demand for fine pitch connectors and other thin-wall micro-featured components. The LP20EH2 features a 12mm diameter plunger to perform the molding operation for the “funnel tip” micromold.

This LP series uses Sodick’s proprietary Linear Motor Driven Servo Valve that can inject at a speed of 1,300mm/sec (51.1"/sec) translating to an acceleration of 15.3G. Sodick Plustech’s Micro Molding series has been extensively evaluated by precision molders in the U.S., especially for those who mold medical device components. Sodick Plustech believes this new series will undoubtedly open up a variety of opportunities that challenge more complex and intricate thin wall designs which are currently not moldable or require a secondary machining operation.

 

To learn more, please visit Beaumont Technologies Inc., Mold Craft Inc.Sodick Plustech or Victrex Polymer Solutions