Silicone molding is a versatile manufacturing method but it is complex and there can be many pitfalls. Fortunately, these potential issues can be resolved with a highly competent design. Successful silicone molded products must not only perform as intended, but must also be designed from the beginning to be manufacturable. By making the right design choices OEMs can create molded devices and components that can be reliably manufactured in large volumes with minimal waste generation.

Achieving the best design the first time around is crucial as there is a lot at stake. If OEMs do not get the design right, product rejection rates will increase, productivity will decline, and a host of other issues will ensue – all negatively impacting the bottom line. Additionally, modifying a product or mold design during the production stage can be very costly – so it is worth the time to get the design phase right. Below are some key product features that must be considered upfront in order to design molded projects for success. Click here for additional design tips regarding material selection and part functionality.

Design Features

In order to consistently make high quality parts without expensive revisions, the design needs to incorporate features such as adequate draft angles, consistent wall thicknesses, and generous radii for perpendicular features such as walls and ribs. Below is more information about these and other design features.

  • Wall Thickness: simply put, uniform wall thickness is best. Inconsistent wall thickness increases the risk of mold defects such as warping, sink marks, and flow lines. If uniform wall thickness cannot be achieved, there are ways to overcome this design concern. The change in thickness should be as gradual and smooth as possible, and in the case of injection molding, the thicker features should be closest to where the molten feed passes through the gate and into the mold cavity. Use of a fillet or chamfer should also be considered to help ensure the material flows evenly and fills the entire cavity. There are also process changes that may minimize the impact of uneven wall thickness; for example, operating temperature can be increased to ensure the material does not cool too quickly in the thinner sections. In addition, the silicone injection speed can be raised to help the mold fill more uniformly.
  • Reinforcing Ribs: continuing with the topic of wall thickness, designers often want to increase the thickness in order to improve product strength. However, as previously discussed, uneven wall thickness can result in various production and product defect issues. The addition of reinforcing ribs oriented in the direction of the part’s bending forces is often a better option. These ribs improve strength and rigidity of thin sections, and avoid distortion of the part. Generally speaking, rib thickness at the rib base should be around 60% or less of the wall thickness and rib height no more than 3 times the wall thickness.
  • Corners: when it comes to corners or edges, smooth is best. Sharp corners add stress that creates weak points or cracks, impacting the part’s failure rate and even its manufacturability. To overcome this design concern, sharp transitions between wall sections should be avoided and a radius should be added to all edges or corners to more evenly distribute stresses and permit easier material flow and part ejection. Additionally, sharp corners in a mold are a great place for undesired materials to collect, such as dust, oil, air, and rust. These contaminants do not bond properly with the silicone, often creating a mold defect known as surface delamination where thin surface layers appear on the part due to a contaminant material. Incorporating smooth corners, as well as regular and proper mold maintenance, often resolves potential surface delamination issues. In general, a larger radius should be used when part design permits. As a general guideline for corners, an inside radius of at least 0.5 x the wall thickness is suggested, as well as an outside radius equal to 1.5 x the wall thickness. This guidance helps ensure a more consistent wall thickness throughout the part.
  • Draft Angle: draft is an essential element in part design and is simply the angling of walls to allow easier separation and removal of the product from the mold. A lack of draft, or zero draft, makes it nearly impossible to remove the part. Additionally, walls with zero draft may result in drag marks on the product surface due to higher friction during removal. The draft should be in the direction in which the mold moves, parallel to the mold opening and closing. The ideal draft angle depends on the depth of the part in the mold; however, a general guideline is a minimum draft angle of one degree on an untextured finish and at least three degrees for a textured finish due to the additional friction between the mold and the product. While using a draft angle has its advantages, it should be noted that the addition of draft can cause challenges especially for mating parts. In situations where zero draft must be used, try to minimize it to just a portion of the face, not the whole surface.

ProMed Molding Capabilities

ProMed has expertise in working with the full spectrum of silicones covering a wide range of properties and characteristics. We will assist in your material selection to help ensure all design requirements are met. Our manufacturing facilities and equipment are designed for a single purpose—to mold medical and implantable silicone, combination components, and bio-material grade plastics with uncompromising quality and service. We currently have four divisions that are located within two manufacturing sites. All are certified class 10,000 / ISO Class 7 cleanrooms. We can identify the right manufacturing solution for any project and have extensive experience in a wide range of molding techniques including:

  • Automated Injection Molding
  • Multi-cavity tooling
  • Micro molds and micro molding
  • Servo-controlled de-molding capabilities
  • Insert molds, overmolds, and automation integration
  • Transfer molding
  • Compression molding

Contact ProMed today at 763-331-3800 to discuss how we can help design your next molded project for success.

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Designing Molded Projects for Success
Article Name
Designing Molded Projects for Success
Description
Tips to design molded projects for success including design features such as wall thickness, draft angle, corners, and ribs.
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ProMed Molding
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