Feature Article

Understanding Silicone, From Start to Finished Device

Posted by Camilla Andersson on May 23, 2011

Moulding silicone parts and successfully integrating them into a medical device is technically challenging, but the material’s many desirable properties make the effort worthwhile.

Medical-grade formulations of silicone are biocompatibe, vapour permeable and easy to sterilise.

Silicone solutions
The use of silicone parts in medical devices creates many challenges for manufacturers that run the gamut from material selection and engineering issues to the choice of an appropriate production process. The degree of complexity increases yet again when it comes time to integrate the silicone part into a device and ensure its compatibility with other components that may be made from plastic, metal or glass. The purpose of this article is to address these issues and suggest some feasible solutions.

The desirable physical properties of silicone, which exceed those of other elastomers, include high elongation at break, compression set, heat stability, consistent mechanical properties across a range of temperatures, flexibility at temperatures down to -50°C, transparency, colourability using US FDA–approved pigments, UV and ageing resistance and environmental compatibility.

In addition, medical-grade formulations of silicone are pure and contain no organic plasticisers. The materials are biocompatible according to ISO 10993-1 and USP Class VI (selected tests), vapour permeable and easy to sterilise. Low-friction grades have been developed to facilitate the automated assembly of medical devices while self-bonding grades allow easy bonding to substrates such as plastic, metal or glass in overmoulding or bi-injection processes.

Silicone is a thermoset elastomer, but to take full advantage of the material, the manufacturer will need the tooling and process accuracy that is typically called for when working with thermoplastic compounds. This enables fully automated production, but it also demands a high level of technical expertise.

These features make silicone an optimal elastomer for use in medical devices, but they also make it challenging to conjugate material selection, part design and appropriate manufacturing methods into a seamless process. It is also worth noting that silicone parts for medical applications typically are produced in small volumes, which should be taken into consideration when calculating process setup and other costs. Process start-up is time consuming and results in some materials waste.

To ensure the successful outcome of a project, it’s important to begin with a detailed feasibility analysis and the support of a skilled engineering team. The variables to consider initially are as follows:

  • Part design. To optimise critical mechanical issues, consider not only the silicone part but also the other parts of the device with which it will come in contact.
  • Compound selection. Among the available medical grades, choose the one that most closely meets the desired physical and chemical properties.
  • Annual production volumes.
  • Target price. A compromise will need to be reached between the complexity of the tooling and desired productivity.
  • Establish specifications. What class cleanroom will be needed? What level of automation? How about packaging?

Liquid silicone rubber
When choosing a compound, one should bear in mind that there are two different types of silicones: liquid silicone rubber (LSR) and high-consistency (silicone) rubber (HCR), also known as high-temperature vulcanising (HTV) silicone rubber, although the distinction is misleading since both materials cure at high temperatures. LSR increasingly is replacing both HTV and organic rubber in many applications, as people become more familiar with the material’s benefits in terms of ease of transformation and compatibility with automated production processes. HTV rubber is associated with conventional technologies and tooling but it requires manual finishing operations. It is suitable only for small-quantity production volumes. It is possible to transform HTV using almost the same technology as with LSR, but the cost savings in tooling is lost and cycle times are longer, making it feasible only for very specific performance requirements. LSR and the clean, automated production processes that it enables are almost always the best option for medical applications.

Several LSR grades are available for medical applications including standard grades, grades engineered to bond with plastic or metal substrates, low-friction grades for automated assembly, UV-resistant grades and so forth. Application requirements will determine the most appropriate grade.

When designing a part that will be moulded in LSR and integrated into a medical device, it is important to keep in mind a number of critical issues; for example, sharp corners must be avoided when designing the part, and the part should not come into contact with abrasive components that are part of the finished device. An experienced supplier of medical-grade LSR moulding services can be a tremendous resource in terms of optimising part design to achieve the right balance of performance and manufacturabilty.

Silicone moulding tools
Developing the appropriate tooling for a silicone moulding project is half the battle. To wage a successful campaign, manufacturers will need to arm themselves with the appropriate materials, engineering and manufacturing expertise, and in-depth knowledge of mould heating and vacuum systems, ejection devices and injection systems.

LSR injection moulding systems can be based on hot-runner or cold-runner technology. For medical applications, cold-runner systems generally are the only viable option because of cleanliness requirements. Hot-runner systems require manual intervention, or die cutting, thus rendering the process unstable and subject to contamination. Hot-runner technology may be suitable for making a prototype mould or for limited-quantity production runs. Be mindful, however, that the same injection technology should be used at the prototype and mass production stages.

The next decision is choosing between a cold-runner system with open-gate or shut-off nozzles. The choice largely depends on part dimensions, the injection point and its functionality and the number of cavities in the mould. The main advantages of shut-off nozzle systems are the almost perfect mechanical balance achieved between cavities, which guarantees absolutely identical parts even with multicavity moulds, and the absence of wastage or gate marks. Reject rates are minimal—typically less than 1%—and the process is extremely clean, stable and easy to execute. Because of growing demand for very small parts and high cavity counts—more than 32—mould makers are switching to open-gate nozzles. These systems are slightly less expensive than shut-off nozzles and have the advantage of allowing a narrower interaxis between the cavities and, thus, enabling the design of smaller moulds. The disadvantage is that these systems are mainly suitable for large production quantities in shops that run three shifts. The facility also must be staffed by expert operators to obtain a stable process. Moreover, the technology leaves a 0.15- to 0.25-mm protrusion at the injection point. Shut-off nozzle technology has made great strides, however, and it is now possible to use shut off nozzles with multicavity moulds that have 32, 64 or even 128 cavities. While open-gate systems are common in automotive applications, the use of shut-off nozzles is advisable for medical moulding.

Validations and quality controls

Following validation of the silicone part’s integration with the finished device at the prototype stage, two more milestones await. The first involves verifying the installation, operational and performance qualifications of the production process; the second milestone is the establishment of quality control (QC) processes, including part and material traceability and process consistency.

Typical QC processes for LSR parts used in medical applications include measuring part dimensions and verifying cosmetic aspects such as the absence of flash. It is inherently difficult to measure LSR medical parts because of the dimensional tolerances that are required and the material’s hardness, which is typically between 10 and 70 shore A. When tolerances are on the order of 0.01 mm, the use of conventional measurement techniques will distort the results. State-of-the-art 3-D optical control equipment that measures parts to an accuracy of 4 microns is a better option.

For medical applications, the parts must be postcured to eliminate volatile materials. According to the European Pharmacopeia, postcure cycles should equal four hours at 200°C. For QC purposes, the parts are weighed before and after postcuring to verify that the decrease in weight falls within the parameters of a fixed percentage depending on part thickness and grade of compound.

There is growing demand within the medtech sector for LSR and related bimaterial parts. These functional parts generate complex issues for manufacturers in terms of engineering and their successful integration into a finished device. Performing a feasibility analysis, tapping into appropriate engineering expertise and establishing robust validation and quality control processes will ensure that the material is achieving its full potential in the development of next-generation medical devices. 1

Andrea Tomayer
is Business Development Manager and
Dominique Dupard
is Managing Director
at Top Clean Packaging Group, Z.I.
Les Torrents, 63920, Peschadoires, France
tel. +33 4 7380 9267
e-mail: info@tcgroupe.com

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