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How to Mold Liquid Silicone Rubber Successfully

Communication and cooperation among partners is critical to optimizing LSR molding results.

In a medical device molding project involving liquid silicone rubber (LSR), the molder, the mold builder, and the molding press manufacturer can dramatically affect the success of the project by communicating with one another clearly and cooperating to meet customer requirements. Often involving close tolerances and precise control of critical characteristics, molding LSR-based medical devices can be especially demanding.

Example of a component made from optically clear liquid silicone rubber.

All three partners need to share not only detailed specifications with one another but also their experiences in molding LSR. Even when all three parties are familiar with LSR molding techniques, each has unique and intimate knowledge of a given specialty––molding, mold building and press performance. In addition, each acts as a knowledgeable consultant to the others, thus resulting in a stronger partnership than the traditional arrangement in which a mold builder and a press manufacturer simply serve as vendors to the molder.

The communication phase requires continuous discussions that should begin before the molder responds to a request for quotation (RFQ). While these discussions may initially involve primarily the molder and mold builder so that they can design a new mold, the participation of the press manufacturer is also essential for sharing expertise about the functional interaction between the mold and press. While a new molding press is not required for each new project, the press manufacturer’s input is vital for advising which of the molder’s existing presses will produce the highest-quality result most efficiently.

This interaction ensures that the partners will have agreed on the pricing and will have synchronized production and delivery schedules before the project begins. Preliminary information gathering and planning is an essential investment in the success of the project. Always costly and time-consuming, unforeseen obstacles during production inevitably reduce the finished product’s time to market. By sharing their own unique expertise, the partners can forge a tight team that ultimately works to the benefit of the OEM and the customer who uses the medical device.

LSR Molding Partnership

The Molder’s Perspective. In most cases, it is the molder that receives the RFQ from the customer, responds to the RFQ, initiates the project, and produces the part when the quote is accepted. Molders that are experienced with the intricacies and challenges of LSR molding should immediately reach out to their mold making partner and press manufacturer, both of whom should be equally experienced to consult about the project.

Next, the molder should provide the mold builder with those project specifications that will affect the mold’s performance: the annual production volumes of the finished piece; the type of molding machine to be used—whether vertical or horizontal; the required mold surface finish; sealing surfaces, if any; secondary operations that can affect the process; and whether a fully automatic cold runner mold will be needed.

Two halves of an assembly composed of a two-shot thermoplastic and a tactile silicone pushbutton switch.

The more detailed the specifications, the better able the mold builder will be to assess the required mold features. To provide this level of detail, a mold specification checklist such as that shown in Figure 1 can be employed for each mold RFQ. This checklist can include such specifics as the mold type; the type of mold base steel; details about sprues, runners and gating; ejection requirements; cavity qualities; cores; special features; and tool actions.

After digesting these details, the mold maker may well respond with alternative suggestions based on experience in order to achieve a more efficient molding process or a higher-quality result. For example, the mold maker may recommend the use of two 16-cavity molds instead of one 32-cavity mold or suggest the use of a different mold base steel.

At the same time, the molder should consult with the molding machine manufacturer about which of the molder’s existing presses are most effective for the project at hand. This discussion should also address whether a specific press must be modified or have such features as heating zones, vacuum, or a cold runner to suit the current project. The experience of the press manufacturer is invaluable for assessing whether such modifications are necessary.

In its ongoing relationship with the molder, the press manufacturer will have already recommended presses that provide both additional capacity and additional flexibility to the molder’s production capacity. Drawing on knowledge of the press itself and experience with other customers that have used it, the press manufacturer knows what a particular piece of equipment can do.

This is where the value of a partnership comes into play: The molder, mold maker, and machine manufacturer engage in a three-way conversation, exchanging ideas and experiences that result in the best options for the project. At this point, the molder can submit a proposal to the customer with confidence that it is based on solid experience.

Petri dish with a 0.004-in. wall of transparent silicone bonded to a green wall of polycarbonate.

The Mold Maker’s Perspective. Molding LSR is completely different from molding plastics. Mold makers familiar with creating molds for LSR applications point out that molders with experience in plastic molding but new to LSR molding enter unfamiliar and risky waters, often without realizing it. Molders of plastic materials tend to assume that years of experience with plastics translate directly to LSR. However, the opposite is true. Such molders should forget everything they know.

LSR molds must be of very high quality and must be machined to extremely precise specifications. In addition, they make demands on the molding press that plastic molds do not. As a result, an LSR mold can be significantly more expensive than a similar-sized plastic mold, shocking molders new to LSR applications.

The Press Manufacturer’s Perspective. The other critical tool in the molding process, the molding press, must work with the final version of the mold and meet such project demands as the daily production volume, the total number of parts to be molded, and the product life cycle. In addition, an LSR press must contain more heating zones than a plastic press and must include a vacuum system. The discussion between the molder and the press manufacturer often begins by evaluating whether an existing press will work or whether a new one will be required.

When designing the mold, other factors determine the best press for the project, including the shot size and number of mold cavities, the material specifications, and the part design. Because the materials used to mold LSR-based medical device components and the designs themselves are at the high end of the technology curve, the press and the mold must work together precisely and efficiently.

Molds are built for a single molding project, although the project may continue over several years. Molding presses, on the other hand, involve significant investments that must be amortized over many years and many projects. Thus, while a press must be able to work efficiently with a mold for the duration of the project at hand, it must also be able to work with other molds and other projects. To achieve the optimal interaction between the molds and presses over the long term, the press maker should seek input from the molder about the products it may manufacture in the future and from the mold maker about the most suitable presses for such applications.

A Coordinated Effort

Flexible, chemically stable, durable, and biocompatible, liquid silicone rubber offers unique properties to medical device manufacturers. Because of these valuable properties, the material will continue to grow in popularity, especially as medical devices shrink and become more complex. Moreover, this trend will proceed apace as manufacturers produce more and more products for the home healthcare market. As a result, more molders, mold makers, and press manufacturers will become involved in LSR molding. Thus, it is essential that they openly exchange information with vendors who have learned from experience that LSR is a key to efficient, cost-effective, and successful molding.


Jim Ritzema is director of operations and technology of Northbrook, IL–based Rogan Corp. Reach him at [email protected].

Geri Anderson is marketing director of Brea, CA–based M.R. Mold and Engineering Inc. Reach her at [email protected].

Kohei Shinohara is general manager of Schaumburg, IL–based Sodick Plustech. Reach him at [email protected].

Len Hampton is national sales manager of Schaumburg, IL–based Sodick Plustech. Reach him at [email protected].

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