When delving into the design of a component for silicone molding, numerous challenges await. Crafting a suitable mold is the linchpin to attaining the precision and quality required for the final product to meet your production benchmarks.
Liquid Silicone Rubber (LSR) injection – molded parts are preferably manufactured with tight tolerances due to the issue of flashing. This escalates the demand for even stricter tolerances compared to traditional thermoplastic molding processes. The initial phase in the design journey is the research and development stage, which is succeeded by the creation of a prototype. This prototype serves as a safeguard against unanticipated hurdles during the production phase.
This silicone injection molding design guide will comprehensively explore tolerances, accuracy, shrinkage, gates, part specifications, undercuts, drafts, finishes, and a plethora of other related topics. Its aim is to demystify the complexities associated with LSR design and illustrate how adhering to these molding guidelines can guarantee the manufacturability of your desired part.
These LSR molding design guidelines will not only highlight the significance of designing silicone parts but also detail the diverse tools necessary for the task. Silicone rubber molding is an indispensable manufacturing process, finding applications in a wide array of projects, from crafting props for Hollywood blockbusters to producing the intricate components we use daily. One of the most prevalent applications of silicone rubber is in the healthcare industry, attributed to its compatibility with human tissue. Silicone is an outstanding material that presents unique advantages over thermoplastics. However, prior to working with this material, numerous design considerations must be taken into account.
Initial Design Considerations
Silicone is an elastic material. When it is extracted from the mold cavity, flashing might occur. Silicone molds typically consist of two or three plates. When dealing with LSR, ejector pins are not utilized in the same manner as they are in harder, more conventional thermoplastic molds. Consistency is of utmost importance. Designers need to incorporate undercuts and integrate them with other design elements for LSR molds.
Moreover, due to silicone’s elasticity, and the fact that ejection necessitates the application of force, the material is prone to hot tearing. This makes it challenging to automate the production process for certain designs if the parts are not positioned in a single location within the mold. It is also crucial to consider that silicone shrinks. The shrinkage rate ranges from 5% to 7%, which is relatively high compared to traditional thermoplastic materials.
Shrinkage rates can fluctuate depending on the type and grade of the silicone. This makes it arduous to determine the exact shrinkage rate, which is yet another reason why creating a prototype is beneficial for the design process. Prototypes can help resolve minor details long before the costly production phase commences.
Material Specifications, Grades, and Durometer
Silicone possesses unique properties that make it an ideal choice for manufacturing various types of components. One of the significant advantages of silicone is its biocompatibility, rendering it suitable for the development of medical technologies intended for use within the body. Medical – grade silicone differs from other similar materials in that it is employed for healthcare – related components. Thus, it requires sterilization and must adhere to medical standards.
Thermoplastic molding necessitates temperature control, with mold temperatures generally not exceeding 170 degrees Fahrenheit. Conversely, Liquid Silicone Rubber must be heated between 320 and 450 degrees Fahrenheit to facilitate the curing process.
The highly elastic nature of silicone enables it to achieve elongation of up to 1,000% in some of the highest – grade materials. While high elongation can pose challenges during the demolding process, it can also aid in achieving greater undercuts during part ejection. Similar to almost all other rubbers, silicone is non – conductive, offering distinct electrical and thermal properties compared to other materials.
Application is a crucial factor to consider when selecting a grade of silicone rubber. For healthcare components, implantability, sterilization, and other biological factors should be among the top priorities. However, for most products, including medical components, durometer is one of the most critical mechanical properties to consider when choosing a material.
Durometer refers to the hardness of the silicone, which correlates with its other properties such as modulus and elongation. Different manufacturers offer materials with varying properties, usually specified by durometer and grade.
Think of durometer as the material’s viscosity, which can range from 0 to 80 on a scale. For example, lower – durometer silicone may have a higher viscosity, while the highest – durometer silicone will have a reduced bubble flow. Silicone grades are available in a wide range of variations, with numerous applications for general use, medical purposes, food – related products, implantation, and specialty items.
Part Size
Part manufacturability is significantly influenced by the determination of part size and overall production volume. Compared to low – volume or small – scale productions, large – volume productions may require an extended curing time for the silicone, not to mention the higher material costs.
Component size and dimensions can have a substantial impact on the molding and ejection processes. For instance, parts with very thin geometries may be prone to blemishes or surface irregularities during manufacturing. However, the application of finishes can minimize the likelihood of such issues.
Wall Thickness and Tolerances
In LSR injection molding, tighter tolerances need to be considered due to flashing and the liquid nature of the material. Walls as thin as up to 0.040 inches (0 – 10 mm) can be achieved. However, this is contingent upon the wall’s size and its position within the component. One advantage of LSR injection molding is that liquid silicone rubber can often fill thin walls without issues. For a well – designed part, smaller linear tolerances can be achieved using other materials like resin. In some cases, specialized molds may be beneficial for LSR injection molding.
Tolerances are determined based on both the size range and the parting – line direction (where the mold halves meet). Pressures within the mold and compression can affect these tolerances, and they need to provide a starting point for the silicone. Consulting a qualified mold maker during the design process can be highly beneficial and may assist in achieving the necessary tolerance levels for component manufacturability.
Shot Size
When it comes to injection molding, designing parts with shot size in mind is crucial. In some cases, parts with a shot size below 10 cubic centimeters may pose problems. At SHsilicagel, our primary focus is on LSR, LSR 2 – Shot, and LSR Multi – Shot injection molding of custom parts and components.
LSR overmolding, also known as twin – shot or multi – shot injection molding, is a process where LSR is directly molded onto a substrate made of thermoplastic. This occurs immediately after the substrate has been molded, all within the same mold and machine. By using the LSR Multi – Shot method, several parts or materials can be integrated into a single component, which are permanently bonded. Two – part silicone rubber molding allows for the combination of the best properties of both materials in the final product, reducing the need for post – molding assembly processes.
Dimensional Accuracy
Silicone rubber parts demand a high degree of dimensional accuracy, which should be reflected in the finished product. Prototypes can reduce production times and costs by refining and optimizing a component’s design before mass production.
For example, shrinkage values can be obtained through prototyping and testing the material intended for use in the final product. This provides a rough guideline for the required values, which can then be incorporated into the design phase.
Shrinkage
The overall quality of the product is affected by the shrinkage rate of the silicone used. Regrettably, shrinkage is difficult to predict and requires significant adjustment prior to production. In addition to the shrinkage that occurs during curing, several other factors can influence the shrinkage rate, including the temperature of the tools during molding and ejection. The pressure within the mold cavity, the compression of the silicone, and the direction of material flow from the injection point can all impact the overall shrinkage rate.
Since shrinkage rates as high as 2 – 3% can occur, it is essential to account for shrinkage during the design phase. Molding a part in an existing mold of similar shape and size can offer insights into how the material will behave. Dimensional accuracy is always of utmost importance, especially for silicone parts. Therefore, testing the material and allowing it to cure properly can help minimize design flaws before production. Given that your project has customized requirements and geometries, consulting a qualified mold maker during the design phase, along with creating a prototype, can significantly reduce the potential for unexpected expenses.
Drafts and Undercuts
Demolding, or removing the component from the mold during ejection, is a process often enhanced by the use of drafts, although their necessity depends on the component. Most silicone rubbers offer a range of mechanical properties and deform well enough that drafts are often not required. However, when designing a component, it is important to pay attention to the placement of draft locations to avoid limiting the mold’s parting line, which can improve manufacturability.
One of the significant advantages of LSR molding is the ability to produce parts with undercuts, which is not feasible in other molding methods. Most undercuts can be removed without mechanical assistance. Undercuts are achievable in liquid silicone molding, similar to traditional thermoplastic part designs. However, one drawback of working with silicone rubber is that elongation restricts the size of the undercut. Additionally, parts that require more complex tooling with sharp – edged tools may face an increased risk of damage to the finished component or the mold tool.
Parting Lines/Split Lines
Designing parts with silicone, like traditional thermoplastics, often requires smooth surfaces. Parting lines, where the mold pieces meet, create a closed mold cavity for part formation. When the mold is assembled, it forms a single core in the component’s geometry, preventing lines or flashing from occurring on the inside where the part is molded.
Parting lines should align with the parting surfaces in seals and gaskets to prevent fluid from leaking along the line or across the seal. In some cases, irregularities may occur at the part’s edges. Thus, it is important to consider the need for more than two parting lines when designing your component.
Gates
Unlike traditional materials, liquid silicone rubber parts typically require smaller gates due to the material’s properties. A gate feeds into the thickest section of the component. Since gates can leave marks on the finished component, placing them on a surface that is not critical in terms of dimensions or aesthetics during the design process can result in a better – quality product. If this is not possible, creating a recess specifically for the gate when designing the mold can help mitigate this issue.
Vents or overflow areas may also be useful when designing your component, depending on your requirements. These can enhance the quality of the finished product by allowing air to escape during the molding process.
Aesthetics
Reviewing all the design principles mentioned above for LSR molding can help create a higher – quality product and enhance the aesthetics of your component. By considering the type of material and how it behaves during the molding process, you can strategically place undercuts and gates to reduce common blemishes or surface irregularities caused by molding.
In addition to employing better design techniques when creating silicone molding prototypes, the use of mold finishes and tooling can also improve the product’s aesthetics. Silicone rubber is versatile and available in a wide range of colors and grades. In some cases, a machined finish within the mold cavity can result in smoother surfaces and reduce the need for mold release agents. Special textures can also be applied in the mold cavities to enhance the surface features of the finished product.
How SHsilicagel Can Help
SHsilicagel is one of Asia’s leading injection molding companies, specializing exclusively in LSR and solid silicone compression molding. We have a proven track record of providing our customers with excellent quality control and unparalleled manufacturing capabilities.
Contact our team of skilled injection molding professionals for a free consultation. We provide exceptional customer service and manufacturing expertise to deliver high-quality, custom-made LSR, solid silicone, and LSR multi-shot components tailored to your exact requirements and specifications.