Moldflow Analysis Slashes Time To Market

Toolmaker and mold maker Minimould (Buckinghamshire, UK) achieved remarkable improvements in quality and fill time as a result of using Moldflow software early in the manufacture of a new range of laryngoscopes. These devices are now entering into production courtesy of medical device manufacturer Penlon (Abington, UK) and Minimould.

Penlon specializes in equipment related to anesthesia and has make its name and metal laryngoscopes. These are spatula-like instruments with a light source used by anesthetists during surgery to open patients' throats to allow a breathing tube to be inserted. The devices come in different shapes and sizes at accommodate patients of different ages and sizes.

Penlon migrated to a plastic material for its new laryngoscopes because surgical instruments that may carry bacteria or infection are disposed of after one-time usage. The reason is governmental concern over spreading of the new variant CDJ, where infection is extremely resistant, even to high temperature sterilization.

For Penlon, this circumstance meant redesigning the product for manufacture in plastics. Amazingly, more than 700,000 tonsillectomy-related procedures are carried out annually in the UK alone. So, the volume is sufficient for a range of disposable units to be considered for injection molding. It is expected that other European countries will now progressively move to disposables.

Not having injection molding capability at its Abingdon plant, Penlon approached Minimould, a local toolmaker and molder operated by manager director and entrepreneur Peter Clark. "One reason we used Minimould is because, although there are many knowledgeable molders, Peter Clark is very helpful and has a number of key facilities, including Moldflow software, which was used extensively during this development cycle," says Penlon technical director Alan Green.

Using the full power and potential of Moldflow and SolidWorks, Minimould designed a one-piece blade that afforded the clarity required, yet retained the full properties of the intended polymer. While this gave the most economic production capability, the design was deemed radical in what is an established market. Minimould and Penlon engineers then evolved a design that mimicked the geometry of the stainless steel version. They then used Moldflow to optimize the design and develop a version that addresses the market's need, as well as reduces production costs using the new polymer material.

Minimould engineers designed the tooling from the approved drawings. However, the design changed extensively throughout the development, as attempts were made to manage the implications on moldability of the complex variations in wall thickness and curvature demanded by the optical and medical requirements. Changing the original 'as-machined' design into a smoothly radiused product conducive to manufacture by injection molding altered the path of the guided light to the extent that the position of the lenses had to be moved, while the design was adjusted to reduce or smooth wall thickness changes where possible.

In addition, Minimould also converted the straight split-line format in the tooling to that of a complex split-line, to eliminate visual evidence of the split-line and possibly sharp edges. This was particularly helpful where contact would be made with patients' soft internal tissue, tongue, and palate. The final product exceeds its goals as a transparent blade with light guide, using a series of in-built lenses to guide the light to the end of the rod. The blade attaches to a universal metal handle and battery holder.

Selecting a material was tricky because of the limited choice of materials. Design specifications called for a material that is highly transparent with low haze. The best materials are polycarbonate (PC) and acrylic (PMMA). PMMA is not known for its strength, and some sterilization processes can attack polycarbonates. During prototype trials, PMMA gave improved results for light guidance compared to PC. Careful geometry management improved the flexibility of the PMMA design. The choice of PMMA as the intended production material required a carefully considered design approach.

Clark is an enthusiastic convert to Moldflow Plastics Advisers (MPA) software, using the Moldflow Part Adviser (MPA) module with Cosmos FEA software and the SolidWorks solid modeler. In this example, Moldflow analysis of initial prototypes implied serious problems. The main problems identified were surface defects, poor definition of fine details (particularly around the embossed lettering and logo), and long cycle times with a likelihood of voids in the thicker sections. This problem was caused by the massive and sudden wall-section ratio changes, with the wall thickness changing from chunky to thin with steep gradients between the two.

Normally, the mold technician would increase injection speed and pressure while raising mold and melt temperatures to improve theses problems. But in doing so, the molding tolerance parameters would be reduced, resulting in a higher reject incidence. .Poor quality and frequent comprehensive quality assurance checks would be required.

The snap-fit features on the initial design would also have been very poor, as the flexibility around them was very limited, resulting in difficult-to-control snap in-out forces and shearing or chipping of the snap-fit lugs. The thick section would also be high in molded-in stress, likely to result in poor service strength and optical purity.

The thicker wall sections also dictated a large gate surface area. A self de-gating, sub-type gate was not recommended in this design. The cooling of this cavity was critical. The predicted cycle time exceeded one minute and the material use expected was also excessive (part volume 19.72 cc.).

Using SolidWorks 3D modeling and real-time fine-tuning of gate positions with MPA analyses, further iterations were made to generate a design likely to give the best compromise among molding, mechanical, and medical requirements. During this process, it was determined that the best compromise between function and moldability meant that the wall section thickness should get progressively thinner as the distance from the gate point increased. Some wall sections were modified to force the melt flow to run smoothly along the blade, thereby reducing the possibility of air traps and weld lines.

Thicker sections remained unavoidable due to the optical requirements: sections thinner than six millimeters substantially reduce light collection at the source point and emission from the lens on the blade. Using prototype modeling techniques, the lens position and angle were set by calculation and subsequent trial results.

The material eventually selected was PMMA Oroglas V825 T, which offers very high optical qualities. While the mechanical properties are not the optimum, some Cosmos FEA trials predicted a satisfactory flexure of 30 millimeters at the blade tip. Impact resistance remained a concern and, coupled with the possible desire to sterilize the packaged product using gamma rays, some experiments were carried out on impact-modified grades offering improved impact resistance and gamma radiation stability. However, the light guidance and output were reduced through hazing in these materials.

Overall, design changes based upon advice from the MPA analyses enabled the cycle time to be reduced by 37.31 seconds and the shot size to just 14.86 cc. Even so, problems were still anticipated with cooling, ejection, and fill confidence in some very small and isolated points on the raised text and logo.

However, the lettering on this model was at an embossed height of 0.15 millimeter and the small points of concern are almost invisible to the naked eye.

Sink is now limited to the thicker sections, but the required strength and stiffness along the blade prevents further localized reductions in wall thickness, thus reducing the sink in this area. The majority of the sink predicted by MPA software was less than 0.03 millimeter deep, making it virtually undetectable on the finished product.

Using results from MPA cooling quality predictions, cooling circuits in the tooling were concentrated at the handle end. It was impractical to cool the side action core directly. A generous indirect cooling circuit was located immediately beneath the slide component pockets. With the thicker sections, the limiting factor was on cycle times. So, efficient cooling circuits were designed into the insert blocks, as well as into the surrounding bolster.

An interesting element of Minimould design ingenuity derived from the gate position. The gate placement offered the least risk of failure in use cased by molded-in stresses, while being in a position least likely to cause damage to the patient's soft tissue. It was noted that, if the gate was moved too close to the handle, it would introduce an additional stress concentrator in a high stress area. The option of placing the gate in the thickest sections was not considered, as the optical properties would be compromised and the gate would be in a highly visible area, which would be further exacerbated by the light 'leaking' out through any imperfections. To Penlon's advantage, the gate location situates Penlon's logo in a position to light up when the blade is in use.
Contact: By Laura Carrabine, 440-247-8653, Email Contact

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