Tips to simplify designs
This is a shallow dive article, so we are aiming to deliver good basic knowledge. But silicone rubber molding is a big topic, so this blog is backed up by a much more detailed white paper we will be publishing soon. Subscribe to our newsletter for updates.
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What is Injection Molded Liquid Silicone Rubber (LSR)?
Liquid silicone rubber (LSR) is a thermoset elastomer that can be used to make large volumes of durable, complex and highly detailed silicone rubber parts. This starts with the very thorough and bubble free mixing of a typical two part silicone – the polymer and a Platinum catalyst which are mixed thoroughly while kept chilled to reduce the reaction rate, otherwise they’d start cross linking as soon as they meet.
LSR molding is a little equipment intensive, but delivers great quality parts at reasonable cost. The material is really elastic, it resists compression-set and it can tolerate really extreme temperatures, for a polymer. It makes parts that have great cosmetics, it’s relatively easily colored and the parts are durable under demanding applications.
LSR is such a widely applied technology, across medical, aerospace, automotive, consumer and industrial products that you’re guaranteed to encounter some parts made this way every day – earbuds to kitchenware, phone cases to door handles. It brings a touch-friendly grip and a resolute toughness to so many applications.
How Does the Process Work?
LSR injection molding takes the newly mixed and chilled pre-cure material and passes it into the chilled barrel of a specialist molding machine, then the barrel pushes this low viscosity liquid into a tool, where it is hot-cured to make a perfect copy of the cavity it filled. The simplified steps look like this:
- The mold tool/machine: These mold tools range in complexity, depending on the molding process. We are focussing on injection molding for now, so they are built tough and heavy, like any inƒjection mold tool. They differ in two regards.
First there’s often no ejector pins, as they just don’t work so well on such soft parts. Instead, the molds are often peeled off by hand or robot and sometimes they’re blown off with compressed air.
Second, the cooling channels you’d normally find in an injection mold tool are actually heating channels, running hot water or hot oil to trigger the cure.
The tool is put up onto an LSR dedicated molding machine. No hopper for granules, they have a metering mix system and a chilled barrel but otherwise they look much like any other injection molding machine.
- Material preparation: The precursor liquids in two containers are piped into the metering/mixing system, where they are drawn in and blended in the required ratio. Dyes and fillers can be added, though some care is needed as these can disrupt the catalyst and leave a sticky mess!
In any case, the mixed and uncured polymer is passed into the chilled machine barrel and held ready for injection.
- Charging the mold: The liquid passes through a heated nozzle and is pushed into the cavity under pressure, where it forces out the air and fills the cavity, conforming to every detail.
Pressures and flows need to be well regulated, to avoid a whole load of potential defects in the cured part.
- Curing: With the mold filled and pressure retained on the liquid, the tool is heated and the cure really kicks off. It can take seconds, it can take minutes – this depends on the formulation of the LSR, the temperature of the tool and the maximum section thickness of the part.
Not all parts are fully cured in the tool, as this can really speed up processing. But partially cured parts do need careful handling.
- Cooling and demolding: With the cure sufficiently advanced that the part can be handled without damage, the clamping is released and the part is peeled or blown off the tool and passed for the next steps.
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Benefits of LSR Injection Molding
Injection molding LSR, even more than with many rigid polymers, delivers excellent detailed representation of the cavity. The material is low viscosity, suffers very little from shear issues and doesn’t see the sudden solidification that thermoplastics see on contact with the tool. This all makes for perfect reflection of the cavity features, down to fine surface finish details.
The benefits of LSR injection molded parts are significant:
- Endurance: LSR moldings offer great temperature, UV and ozone resistance, high levels of electrical insulation and chemical durability that makes them great for seals and gaskets in demanding and hot and cold conditions.
- High precision: LSR parts are precise and detailed, with great dimensional accuracy and predictable molding repeatability because the materials are really consistent and they flow really well pre-cure and a low and predictable after-cooling shrinkage rate of around 3%.
- Toughness: LSR parts are really elastic, allowing up to 700% elongation before tearing. What’s more, highly stretched parts have a strong memory of their molded shape and take little or no elongation or compression set.
- Automation and high volume: LSR and the equipment to mold it is intrinsically precise, so it really lends itself to high volume and hands free robotic production processes, when the volume justifies the investment.
- Low waste, but severe downcycling: Because LSRs are essentially from a mineral source, they are talked of as environmentally friendly.
We don’t speak to the truth of this. It is rarely if ever recycled, and takes centuries to break down in the environment because it’s so chemically stable. Though on the up-side, its breakdown doesn’t release microplastics!
Equipment for LSR Injection Molding
LSR injection molding setups look like standard injection molding equipment, but there are some extra steps that are unique to this family of materials:
Metering: A dedicated precision metering unit pumps precursor materials from the supply drums (sometimes called plungers) in the correct proportions, to supply them to the mixer. This can have an input for metering liquid additives such as colorants or diluents at the same time. Careful metering guarantees that the polymer/catalyst proportions are correct to deliver the expected material properties in the part..
Mixer: The metering unit output goes to a mixer, This can be a convoluted continuous flow mixer through which the precursors are pumped to blend them, or it can be a batch based stirrer chamber. There are advantages to both.
Molding machine: The output from the mixer is pumped into the barrel of the injection machine, which is kept cold to slow down the cure-rate to allow processing to not be rushed.
Nozzle: A heated nozzle connects the barrel to the mold tool, so the post-mix is heated as it enters the tool to start the cure process.
Mold clamping: The mold clamping looks like every other injection machine setup. Clamping force holds the toolshut against injection pressure, then opens up after the cure is complete to allow mold removal.
Applications of LSR Injection Molding
Liquid silicone rubber injection molding has slowly moved to center stage in the manufacture of elastomer parts in the last few years, as the equipment and expertise have become more commonplace. It’s now a key manufacturing process across most sectors:
Medical/healthcare
LSR parts bring a lot to the healthcare sector. The material makes soft and comfortable parts, or finishes to parts that must be in patient contact. It’s biocompatible, hypoallergenic, hard wearing, and tolerant of sterilization processes, making it ideal for so many patient contact and treatment associated applications. LSR is particularly important in drug delivery systems like drips, peristaltic pumps and fittings for transfusion/dialysis and for clean-room equipment.
Automotive
Injection-molded LSR is widely used in automotive components which benefit from its toughness, stability and extreme temperature tolerance in the engine bay.
The sector uses LSR components for flange and duct seals, airbox seals and restraint straps. In particular, windscreen wiper rubbers benefit from LSRs resistance to abrasion and UV and the softness that avoids scratching the window glass.
Industrial
In industrial equipment applications, LSR injection moldings serve as seals and gaskets, strain relief and vibration damping elements and cable grommets.
These applications benefit from the toughness, softness and UV/chemical stability of the materials, giving long service life in harsh environments like mining and oilfield systems.
Electronics
LSR is very widely used in molding tactile keypads, often in two-step molding processes where a color overlay is molded onto a clear component – so the color can be laser etched off and the clear material can serve as a lightpipe. It also serves in connectors, seals, water-tight gaskets, insulators, shock mounts and switch pads.
Household/consumer products
LSR injection moldings serve in kitchenware, clothing branding elements, phone cases, toys, baby equipment, wearables, and personal hygiene – personal care products. LSR is soft touch and tactile, tough and capable of taking rough handling that accommodates real-world usage.
Design and Manufacturing Considerations for LSR Injection Molding
Designing parts for LSR injection molding follows its own set of rules. These originate with the nature of the material, both in the raw and molded state, because these create opportunities and constraints that should be worked with, for best possible outcome
Part Design and Tooling Rules
In many regards, design of silicone parts actually reduces the design rules that guide and restrain design of injection molded rigid plastic parts. Using the opportunities that the material enables can facilitate exciting design opportunities that should be understood and exploited.
A basic fact of the fluid mechanics of the uncured LSR pumped into a tool drives design. LSRs are shear-thinning materials, showing non-Newtonian behavior. They typically have a shear resistance (viscosity) which decreases with increasing shear rate – the thinner the section, the lower the viscosity as the material flows. This makes these materials perfect at forming fine details.
Add to this the very high elasticity, and the parts will peel from relatively deep undercuts very reliably. This means tools for complex and wrap-around parts don’t need slide or collapsing cores, they can be built as basic cavities without complexity. Undercuts are not to be avoided, but celebrated and exploited!
Another tooling detail that simplifies both part design and tooling is that ejector pins and stripper plates don’t work, because the material is just too soft. So no allowance need be made for these in part design. Inseta, tool designers will opt for hand/robotic peeling to remove the part – and this can be assisted with air pressure to help lift the part.
This low and flow related viscosity makes a tooling burden in that part-line leakage is more possible, so try to design for short part lines and encourage the toolmaker to make them as clean/close as possible. Then get the molder to experiment with clamping pressure to minimize the flash.
For most parts, minimal draft is required. LSRs are not sticky once cured, so they come off the tool easily. And low drafts on the cavity side can help to pull the part off, making demolding easier.
Venting is important – and relying on the part line for venting increases the risk of flash, so adding some small vent channels to waste chambers that have lower quality blanking (part line sealing) means easy venting and easy trimming of the vent areas. Allowing for these small spurs in part design is a good idea.
Finally, there’s a risk that the gate will be more evident than for most moldings, so its best to place it where it won’t be seen – hidden on the underside, covered by another component etc.
Different strokes
In a rigid or elastomeric thermoplastic, the polymer is heated to make it liquid, ready to inject into a ‘cold’ tool that must be cooled to accelerate hardening. LSRs are different.
The blended pre-cursors are liquid and low viscosity at room temperature and quickly react to form the elastic solid. Machine barrels are often chilled, material is typically injected through a heated nozzle into a heated tool, and as the material heats up it rapidly cures.
If you’re overmolding with LSR, the same rules apply but you need to be extra careful about bonding compatibility with the substrate. Choose a suitable material and apply the overmolded LSR to the freshly molded and still warm substrate.
As with all molding processes, shrinkage is an issue and you will need professional advice about your part, to get the tooling ‘oversize’ right. Remember that the part will shrink more after molding, so the allowance must be carefully estimated.
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Product Design
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Choosing between LSR Injection Molding and Compression Molding
You can reasonably conclude that LSR injection, compared with compression molding the same materials is lower cost and good enough. You’d be right, as long as processing time isn’t an issue, you’re not trying to get the finest detail and thinnest sections and you don’t mind some post-processing to handle a lot more flash.
LSR injection is fast, costs more to set up but can handle large production volumes that are deeply unsuited to compression molding. The biggest issue is cure times, which are quite a lot longer in the much simpler compression molding setup.
Conclusion
LRS injection molding is a powerhouse process, when applied according to these guiding principles. It delivers great components quickly, it involves very low material waste and it can handle finnesses that is not attainable by the alternative approaches.
