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The Art of Silicone Transfer Molding


Posted in Molding Services by mddiadmin on August 1, 2006

Transfer molding is a convenient and cost-effective way to produce silicone medical device components.


COVER FEATURE: OVERMOLDING

Transfer molding can accommodate short runs, which makes it well suited for medical devices.

Transfer molding is widely used in overmolding silicone products, especially the handles of medical devices and surgical instruments. The process creates durable components and has a reputation for short cycle and mold changeover times and for keeping production runs flexible. In addition, OEMs may find the process desirable because it can accommodate customized mold features.

Three important elements in manufacturing medical devices using transfer molding are raw materials selection, mold and cavity design, and processing equipment capabilities. This article discusses each of these elements so that OEMs better understand the process and can identify suitable suppliers.

The medical device market is characterized as a competitive, rapidly growing, technology-driven field. These factors contribute to short product life cycles. Competitive pressure to implement technological advances results in rapid product obsolescence. Therefore a company's research and development strategy is key to survival.

Research and development budgets in the medical device arena are based on factors such as potential sales, government grants, and special financing. Developing the next generation of products for the healthcare industry has shifted from the big-budget medical device powerhouses to smaller ventures that also include start-ups. Large corporations can typically finance capital-intensive R&D efforts through existing cash flows, but smaller companies and start-ups are not typically cash rich. Recent trends have placed the burden of product R&D onto small companies or start-ups, many of which are acquired in the late stage of product development. In small companies R&D budgets are constrained and often time-sensitive. Therefore, it is critical that such firms focus on product design and the use of cost-effective processes such as transfer molding.

The Transfer Molding Process

Transfer molding involves a piston-and-cylinder-like device built into a press so that raw silicone material is pushed into a mold cavity through small holes, or runners. The process transfers the material into the mold and heats it under pressure to produce a cured part.

Figure 1. (click to enlarge) The process of transfer molding consists of four relatively simple steps: loading, compressing, retracting, and
releasing. Deflashing after the component is released from the mold helps remove excess silicone.

This process is composed of four steps, which ensure a properly molded and cured product (see Figure 1). The following overview describes the four steps:

• Cut and weighed raw silicone material is loaded into the transfer pot of the molding press. The silicone is forcibly injected from the transfer pot through a series of runners.

• From the runners, the material is forced into the mold cavity. While in the mold cavity, the silicone is held under a specific temperature and pressure for a specific time to undergo the curing process. The curing process can range from 2 to 12 minutes, requiring temperatures ranging from 260° to 300°F and pressures ranging from 500 to 1000 psi.

• The transfer plunger is retracted, and any excess silicone is removed from the transfer pad.

• After the silicone is fully cured, the pressure is released and the mold is opened. Once the product is removed from the mold cavity, the silicone undergoes a secondary process of deflashing to remove any excess silicone (or flash) left on the product from the mold parting lines.

The transfer molding process works well for molding silicone components and overmolding onto inserts. To optimize bonding of the silicone, it is critical that the molded device or instrument is clean and free of any excess materials or contaminants. When using inserts during the molding process, an added cleaning and preparation step is often taken to prepare the insert.

Molding specialists use a process that eliminates the need for extensive surface preparation, enabling OEMs to choose from a variety of handle cores. This process does not require extensive grit blasting or surface modification in order to obtain a secure, strong bond that will withstand repeated steam autoclave cycles.

When less-than-ideal bond strength or delamination is observed, surface treatment or adjustments to the transfer molding process may be necessary. Silicone primers can substantially improve bonding, particularly for low-surface-energy materials like plastics. Corona or plasma treatments effectively oxidize most surfaces and may also improve bond strength. Slight modifications to the handle cores, such as longitudinal grooves, mechanical locking features, or undercuts, can also be employed to increase bond strength.

Molding equipment is another consideration that can affect the molding process. For transfer molding, it is appropriate to use 50-tn automated transfer molding machines, especially for custom-designed silicone handles and devices. A machine's 50-tn clamping pressure ensures that the mold stays closed securely during the curing process, which helps produce consistently molded parts.

Table I. (click to enlarge) Transfer molding considerations for a typical instrument handle. Each mold requires 29.47 tn of force and so needs at least 50 tn of clamping pressure.

Silicone rubber expands in the mold, which increases cavity pressure up to 400 bar. The force needed to clamp the mold can be calculated by multiplying the projected surface area of the cavity by the cavity pressure. Table I provides an example of an instrument handle.

An advantage of using 50-tn machines is that the size is appropriate for typical device handle applications. The size of the machine relates to its ability to process multiple handles at a time. In addition, a 50-tn machine is relatively small and so does not take many hours to heat and stabilize. It takes up minimal floor space and can accommodate lean manufacturing.

Setup for the machines can be standardized so the presses can handle 18 × 18-in. mold bases. These molds can be shuttled between presses with minimal setup.

The Benefits of Transfer Molding

Device manufacturers often need to reduce R&D costs without sacrificing R&D quality. Silicone transfer molding offers an easy transition to full-scale production that can help OEMs achieve this goal.

The greatest benefit of transfer molding is that it can accommodate both short-run lots and high production runs. Molding cycle and changeover times can be shorter than those of alternative silicone molding techniques because molders can use multiple mold cavities on a single machine. The molds are easy to change, and switching colors requires minimal cleanup.

A 50-tn molding machine is usually suitable for the transfer molding process, especially for manufacturing surgical handles.

Molding times vary depending on the type of process. And different molding processes have varying levels of downtime devoted to cleaning and mold changeover. In some cases, this process can take up to 6 hours. In a transfer molding operation, changing to different silicone colors can be done in less than 30 minutes. The process to switch molds can be done in an hour and a half. This flexibility and short changeover time increases manufacturing throughput and leads to shorter lead times, even for customized parts.

The transfer molding process is also suitable for molding parts that have intricate details and design features. Precision recesses or pockets, unique textures, brand logos or identification, and shutoff points that control what is seen or not on the instrument can be created with the molding process. OEMs often want such intricacies because they help differentiate their products on the market and aid in brand recognition.

Transfer molding also lends itself to overmolding onto inserts. Inserts made of stainless steel, aluminum, or high-performance plastic are placed into the mold cavity before silicone transfer, and during the molding process the silicone bonds to the insert.

Although transfer molding is a good choice for handheld instruments and devices, the process has limitations. Short run lots and high production runs of surgical instruments are easy to do, but transfer molding does not easily accommodate runs that are greater than 10,000–20,000 units. The number of cavities per mold and the number of times the molding press can be cycled in a day are limited.

In addition, the thick gum-stock silicone used in transfer molding requires separate premixing or milling of the base silicone rubber, cure catalyst, and colorant. These elements are mixed together on a two-roll mill. The down-side to mixing these materials ahead of time is that the compounded material only has a shelf life of approximately 3 months. Keeping the product cool can lengthen this shelf life, but typically smaller batches are compounded to keep product fresh.

Transfer molding can also use liquid silicone rubbers and even pourable silicone systems. This is particularly beneficial in the design phase as it gives the engineers a range of material options. Some systems can be mixed by hand, deaired, and placed directly in the transfer pot. The system pressure and temperature may need to be adjusted to avoid flow lines from premature curing.

Curing

The curing process for molded silicone is an important and delicate step. Curing is the chemical process that changes the properties of silicone through the action of heat, pressure, and catalysts. Curing transforms the raw gum-stock silicone to its usable stable form. The parameters of heat, pressure, time, part thickness, and mold design all influence the cure of the product.

In silicone transfer molding, both peroxide and platinum catalysts are typically used for curing, and similar procedures are followed for each of them.

Either a peroxide or a platinum catalyst is mixed into raw silicone base. During the molding process, the catalyst is triggered by heat and pressure, cross-linking the silicone polymer and curing the product.

Peroxide curing is more cost-effective than platinum curing. However, peroxide can leave by-products in the silicone. That residue could cause the silicone to bloom and discolor on the surface. To prevent the negative effects of the leftover residue, it is necessary to perform a postcure step for silicone cured with peroxide.

Platinum curing is a true catalytic cure. The catalyst is triggered when heated to a certain temperature that causes the silicone to vulcanize. This process is expensive, but it has benefits. Platinum creates a faster cure and decreases the cycle time of the molding process. During the curing process, the silicone is heated to 265°–300°F. No postcure is necessary.

The Benefits of Silicone

Silicone is commonly used for medical devices because it complies with FDA regulations for medical products and instruments. The material does not support bacteria growth, and it adapts well to tissue and body fluids.

The silicone used in medical devices contains silicone polymers, reinforcing fillers, colorants, and catalysts. Silicone is one of the most stable materials available on the market and has an excellent service life.

Silicone material can resist acids, bases, chemicals, oils, and water, and withstands multiple autoclave cycles and high temperatures. Silicone does not stain or corrode other materials. It offers long-term resistance to environmental extremes and withstands a wide temperature range, remaining workable from –75° to 500°F. Because of its thermal stability and relatively high melting and boiling points, silicone is often used when other materials are not applicable.

Silicone is used frequently in handheld medical devices because it can create a soft and conformed grip for the user.

Silicone also accommodates custom colors, durometers, and textures so that manufacturers can design instruments in surgical kits that have a unique look and an ergonomic feel. It is important to keep in mind, however, that silicone does have some limitations to consider.

Through consistent use, silicone collects dirt and dust from the environment. It therefore needs to be kept clean for medical use. Because silicone is susceptible to collecting dirt, the milling process must take place in a cleanroom environment to minimize contamination of the material. And because premilled silicone material has a shorter shelf life, it must be carefully stored in a clean, cool environment to ensure optimal use.

Conclusion

Transfer molding is a cost-effective way to mold silicone for handheld medical devices because it offers flexible production runs, short cycle and changeover times, and multiple insert molding configurations per mold. When evaluating silicone transfer molding as a potential molding process for medical instrument and device components, OEMs should look for suppliers that offer transfer molding expertise and the ability to accommodate custom designs for unique products. If the instrument or device has intricate design features or is going to be overmolded onto a stainless-steel, aluminum, or plastic insert, transfer molding may be an attractive option.

Michael Gauthier is president of Gauthier Biomedical. He can be reached at michael@gauthierbiomedical.com.

Copyright ©2006 Medical Device & Diagnostic Industry

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