Introduction: Designing Silicone Parts
Albright Technologies is pleased to provide potential and current customers with guidelines for designing silicone parts for molding. The purpose of the guidelines is to ensure that your prototype and production silicone parts are designed with manufacturability in mind. These guidelines provide a basis to work off of when we design and manufacture your silicone parts at Albright. Our guidelines are made for silicone part designs set in compression, transfer and injection molding tools. You will find that most of the guidelines here will pertain to injection molding tools, since they are far more complex than simpler compression and transfer tools. If you have questions regarding your silicone design or feel that your part is more complex in design and has tighter requirements, feel free to contact our Sales Department at 978.466.5870 or via email to determine if it is a good fit for our capabilities.
Material Considerations When Designing Silicone Parts
Silicone material grades available to our customers include:
- General Purpose
- Medical (Non-implantable)
- Short-Term (Restricted) Implantable
- Long-Term (Unrestricted) Implantable
High elongation materials may provide opportunities to achieve greater undercuts during part ejection. High elongation materials may also be more challenging to remove from the mold as they flex, instead of lending themselves to be pulled out of the mold.
Durometer is a measure of hardness and for many silicones also corresponds to many mechanical properties such as elongation, modulus, and other properties. Each material manufacturer has a slightly different material with slightly different properties, but most silicone materials are specified by name and or durometer and grade.
The durometer often also correlates to the viscosity where lower durometer materials have a higher viscosity (examples exist showing peaks not always centered at the ends of the part.) For manufacturing, mid-durometer materials such as 40-60 Shore A durometer materials are preferred because they balance improved shutoff performance of higher viscosity and better with high flow and reduced bubbles from that of the lowest viscosity materials.
The size of your part and volume often affect the manufacturability. For example, parts with extremely large volumes require longer cure times and more material, resulting in higher fabrication costs. Micro-sized parts, parts that are or have features that are in the magnitude of 0.020 inch or smaller may require specialized handling practices to demold and post process as the size starts to complicate handling. Very thin parts may be difficult to fill without surface blemishes although surface finish plays a significant role in this. Highly polished surfaces inside the tooling cavities will make surface irregularities the most visible.
When designing your parts to be injection molded, please remember that Albright is best suited for part designs up to 10 Cubic Centimeters. Our optimal shot size range is between 0.5 CCs and 8 CCs. This allows us to use our Arburg press for prototype and low volume injection molded parts. In addition, we have a larger Boy and an Engel injection press for part designs requiring shot sizes over 10 Cubic Centimeters. These two presses are best suited for medium and large sized parts.
Drafts & Undercuts
Drafts in silicone are not required, although they may make demolding (removing part from the mold during ejection period) easier. It is worth noting that most silicones are deformable enough that drafts are not required. Care should be made when choosing draft locations so that parting line locations are not limited for the mold when entering production.
Undercuts are achievable through side actions as in traditional thermoplastic part design or through flexing of a deformable part to remove it from the mold. See Figure 1 of the cross section of a bulb, where the inside ball shown in blue has to come through the top section shown in beige. The physical property of maximum elongation directly correlates to the limit of the difference between the opening and undercut size.
Figure 1: Undercut Diagram
Parts with numerous under features may lead to very complex tooling as shown in Figure 2. Features that create sharp tool edges pose two potential risks during manufacturing including damaging the part and damage to the mold tool cause sharp edges to become slightly filleted or bent over time resulting in a change in features. Adding fillets allows for easier demolding during production.
Figure 2: Undercut Cut Risk
Parting Line Location and Shut-Offs
When designing parts in silicone materials, as in thermoplastics, some parts require surfaces to be very smooth. A parting line is where the mold halves come together to create a closed cavity where the part is formed. Figure 3 shows two potential parting line configurations for the bulb above. The inside would likely be formed by a single core, so that no witness lines or flash should occur inside the part. The outside of the part would have a witness line and/or potentially flash along the line where the halves of the mold come together. Applications where the tip of the bulb makes skin contact may opt for parting line 2 to avoid making contact. Parting lines are often aligned with the parting surfaces in seals and gaskets so that fluid traveling along the parting line does not pass across the sealing surfaces.
Figure 3: Parting Line Model
Irregular surfaces may occur, especially when they are at the edge of the part. This may constitute more complex tooling as the mold plates need to align with the features of the part. Figure 5 shows two parting lines that may be required in order to achieve successful functionality.
Gates are an entrance point for silicone material to enter the mold. Gates may be on the parting line or at a single point located strategically on the part. Prototyping gates are typically 0.005 to 0.020 inches in size and are removed by trimming after ejection. This may leave a small tab or mark at the point of contact and should be expected. Effective gate locations are critical to good part filling and achieving high quality parts.
Feature Contrast and Sharp Edges
Features can be thinner when parallel to the parting line surfaces but when perpendicular as shown in Figure 4, the height direction is limited in depth. Very high contract Height/Width ratios cause increased challenges for building a mold, molding, and demolding. Height/Width ratios of 3 or less are prefered although features may have much higher ratios with careful planning. Outside edges may provide an easier tooling option as the core that creates the features stands proud and the edges do not require a small cutter to cut deep into a cavity to create a feature. Filling along parting lines reduces air entrapment as venting becomes an option.
Figure 4: Cross section shows a feature in the center of a surface
Sharp edges may be prone to tearing during demolding or handling and deflashing based on the parts design. Designs including sharp edges may require a radius in some designs to improve manufacturability. In this case a core must go through the part, in order to create the hole through the center resulting in the potential for flash at the intersection.
Figure 5: Cross-Section of strain relief device with sharp edge
Standard Rubber Manufacturing Association A3 Commercial tolerances are based on a size range and the parting line direction, since pressures in the mold and the level of compression may cause slightly greater variance in the ‘Closure Direction’ shown in Figure 6, versus the ‘Fixed Direction’. These tolerances are most appropriate for silicone part designs and provide a reasonable starting point for silicone.
Figure 6: Fixed versus closure direction in molding