TIPS WITH TONY: LSR Offers Design Flexibility

There are many reasons why you should be looking at liquid silicone rubber (LSR) — I’ll highlight a few big ones in order to get you thinking about this versatile thermoset.

For a deeper dive into LSR and how it’s used in the lighting industry, please attend my free tech talk webinar hosted by Tech Briefs.

LSR Molding
LSR parts are formed in process similar to that of conventional plastic injection molding with one main difference. LSR is a thermoset material that compounds two liquids together, which is then heat cured in the mold to produce a part. The material delivery system is cooled and only the mold is heated. This is unlike thermoplastic molding, which begins with the melting of plastic pellets that are injected into a heated mold.

Optical LSR is highly flexible and can replace glass in many lighting applications.

LSR Advantages
LSR parts are strong, elastic, chemical resistant, serializable and biocompatible, and have a range of operating temperatures. The benefits of LSR lend itself well to the automotive, medical and lighting industries where gaskets, seals and lighting lenses are frequently used.

LSR parts have a very good temperature resistance, ranging between -49°F to 392°F; they are non-yellowing, UV stable and optical LSR has up to 94 percent light transmission; they offer good vibration control and offer up to 400 percent flexibility along with excellent part memory.

The chart pretty much speaks for itself when comparing LSR to PC, PMMA and glass when looking at replacing the traditional materials with LSR.

Design Flexibility
Traditional thinking of part design needs to be considered, but many can be broken:

  • Part thicknesses greater than 1 in. and less than 0.020 in. are achievable with little to no concern of any unsightly sinks or internal voids.
  • No ejector pins are used to remove parts from the mold as they are all hand-removed.
  • Gates are nearly invisible, barely thicker than flash. LSR flows like water, so the gate needs to be very shallow, but wide.
  • Negative draft angles or increased undercuts are a possibility with up to 400 percent part flexibility and part memory.
  • Ability to fill fine details or voids.
  • Ability to combine components reducing number of parts to assemble, e.g., combining a lens and seal for lighting applications.

For more information on LSR, please download our white paper or listen in to my tech talk presentation mentioned at the top of the tip. You can also visit our website at or contact one of our customer service engineers at or 877.479.3680 with additional questions on any of our services.


Proto Labs has a knowledgeable support staff able to answer nearly any manufacturing question you toss their way. If you haven’t already spoken to them, you should get to know our customer service engineers (CSEs) who can help guide you on your next additive manufacturing, CNC machining and injection molding project.

This is Tony from the popular blog series, Tips with Tony. He was a CSE for years and is now Proto Labs’ go-to technical specialist.

It’s challenging to assemble the top five most frequently questions asked — it should be more like a top 100 questions asked on a daily basis. But, I whittled it down to the top questions our CSEs are most frequently asked.

5. How many parts can you produce and how fast can I get them?

Additive Manufacturing
SL, SLS and DMLS:  1 to 50+ parts in 1 to 7 days

CNC Machining
Milling and Turning:  1 to 200+ parts in 1 to 3 days

Injection Molding
Plastic: 25 to 500 sample parts in 1 to 15 days with low-volume production of 10,000+ parts available

Liquid Silicone Rubber (LSR): 25 to 500 sample parts in 1 to 15 days with low-volume production of 5,000+ parts available

Metal: 25 to 100 sample parts in 10 to 15 days with low-volume production of 5,000+ parts available

Magnesium: 25 to 100 sample parts in 15 days with low-volume production of 5,000+ parts available

Please note that lead times are dependent on part size and current workload. As such, not all parts are eligible for a one-day turn and expedite fees may be applied turnaround times faster than standard delivery.

4. What manufacturing method(s) should I use?
First, let me ask you these questions to help you decide:

  • How many parts do you need?
  • Are these functional, cosmetic or just something to hold in your hand?
  • Do you know the material you want?
  • What type of finish do you need?

If you have a part total in mind, you can begin narrowing down the manufacturing method to either injection molding (for higher volumes) or additive manufacturing or machining (for lower volumes). Identifying the material you need will then allow you to narrow down your decision even further. Beginning with any one of these questions will start you down the correct path towards the proper manufacturing method.

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TIPS WITH TONY: Wall Thickness by Resin Type

Knowing the material your parts are going to be manufactured in early on in your development process can save you time, money and a lot of frustration. You should work closely with your manufacturer during design, so they can help you identify potential material issues before any parts are actually molded. Because some part geometries inherently work better with certain resins, your manufacturer can help guide you toward the appropriate material options.

Issues that can result from selecting an incorrect material:

  • Warp
  • Sink
  • High fill pressure
  • Poor cosmetic finish
  • Shorting or burning
  • Brittleness

Uniform Wall Thickness
With injection molding, we talk a lot about how uniform wall thickness helps improve mold fill versus thin features that can restrict the material and create a number of the aforementioned issues. Having connected walls that are too thick and too thin can affect how a part cools, thus creating sink and warp. Furthermore, the same issues can arise if your entire part is too thick or thin.


Watch rib-to-wall thickness ratios. To prevent sink, rib thickness should be about half of wall thickness.

This is why the appropriate rib-to-wall thickness ratio must be followed. The appropriate thickness for a rib that is extruded from another surface is approximately half the thickness of the adjacent surface. This is the optimal part design to provide strength while at the same time reducing your chances for significant warp or sink. Learn more on uniform wall thickness in plastic parts on our website.

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TIPS WITH TONY: Ribs and Radii

Designing ribs and radii into your injection-molded part is not only important to increase strength and stability — it improves material flow, eliminates thick areas that create issues like sink, and ultimately, enhances the cosmetic appearance of your part.

A thick part will have several issues with sink or voids creating cosmetic, functional or molding concerns. The addition of ribs reduces the amount of volume in part thickness while still providing the part with the same overall height. Some believe that eliminating the volume/part thickness can decrease the strength of a part. This is not true — adding ribs can actually improve stability, strength and cosmetics depending on the material selection and part geometry.

Watch rib-to-wall thickness ratios. To prevent sink, the thickness of the rib should be about half of the thickness of the wall.

A major design consideration that is often overlooked is rib-to-wall thickness ratios. If you have a rib feature that is too thick on a wall, you’ll create thick areas that can result in unsightly sink or shadowing on the opposing surface. Avoid this by following a guide of 40 to 60 percent wall thickness for any rib.



Coring Out
Coring out is a technique where you remove material from a plastic part, leaving distinct walls and ribs that provide enough strength and mating surfaces for other parts in the assembly. It is necessary to make the part moldable and also saves cost and weight. Leave ribs in the right location and size to maintain strength, particularly in bending, and retain surfaces and features that interface with other parts in the assembly.

Ramps and Gussets
Continuing the discussion on strength and resin flow improvement, ramps and gussets are important features that you can build into your design. Sharp corners create high stress points whereas gussets and ramps are stress relievers, working to improve overall part quality.

Adding radii — edges or vertexes that have been rounded — to your part will improve how resin fills the mold as plastic can flow poorly around sharp corners. When you have sharp corners, resin can create stresses that cause the part to warp or bend, and may provide a location that can break since this is a weaker transition point. Resolve this by adding a generous angle to both the outside and inside corners.

Ideally, you should model radii so the inside and outside radii use the same center resulting in larger radii on the outside curves. This will help you retain a consistent wall thickness throughout your part.

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5 Design Considerations for Multi-Cavity Molds

Moving from a single cavity mold to one that produces two, four or eight parts at once seems like an easy way to increase production volume and reduce part costs. This can be true in many cases, but only if the right steps are taken and the requisite homework done first.

The 3D CAD model for a multi-cavity mold.

Designing a part for multi-cavity molding is not as simple as copying the CAD file for a single-cavity mold multiple times. It’s important to recognize that parts that behave perfectly in single-cavity mold might not play well with others, at least not without first making some tweaks to the part, the process or even the material.

In July’s tip, we look at important design considerations for multi-cavity molds that include gating, side-actions and pick-outs, material flow and how family molds are used differently than multi-cavity tooling.

Read the full design tip here.