LSR injection molding is not merely a more sophisticated form of compression molding – it is a process of high control, with automation and high-precision mold design, material handling, and tolerance planning to attain consistent mass production.
Liquid silicone rubber (LSR) injection molding involves a two-part, platinum-cured liquid system which is metered, combined and injected at pressure into a heated mold and rapidly cross-linked to a stable elastomer. This type of production is usually selected when a large-volume production of high precision silicone components is required, particularly when the production requires tight tolerances, clean surfaces and repeatability (medical devices, automotive seals, baby products, and consumer electronics parts).
Most of the buyers tend to think that LSR injection molding is always better as compared to compression molding. Actually, it can only be applied more appropriately when the part geometry, tolerance demand and the volume required to manufacture the part warrants the extra cost of tools.
LSR injection molding is superior in terms of dimensional stability and efficiency in automation only when part design, mold engineering, and material behavior are well-coordinated.
How LSR Injection Molding Works (Process Overview)
Over the decades of operation of LSR production lines, the true advantage of this process is its closed-loop automation and short and repeatable cycles, however, each step requires to be carefully controlled to prevent variability.
The two-part liquid silicone rubber (Part A with catalyst and Part B with cross-linker) is kept in two drums. Precision metering pumps provide definite proportions (usually 1:1), and then, after casting metering, there is homogenous blending without introducing air.
High pressure injection of the mixed material at a cold-runner system is then done into a heated mold (typically 150200 C). It has low viscosity, it flows easily into intricate geometries, and the platinum-catalyzed curing occurs fast – commonly in 1060 seconds, depending on the thickness of the part and the temperature of the mould.
After the curing, automated ejector pins or robotic demolding are used to remove the parts with cold-runner separation reducing wastes.
| Process Stage | What Happens | Risk Factors | QC Focus |
| Metering | A/B ratio control | Ratio deviation | Pump calibration |
| Mixing | Static mixer homogenizes components | Air entrapment | Visual bubble check |
| Injection | High-pressure fill into heated mold | Jetting, short shots | Gate design, pressure profile |
| Curing | Heat crosslinking in mold | Under-cure, over-cure | Mold temp stability |
| Demolding | Automated ejection | Tear risk | Draft design, ejection speed |
| Post-processing | Minimal trimming, inspection | Flash, defects | Mold precision, visual/AOI |
This automation guarantees cycle repeatability much better than in the case of manual processes so that LSR molding process is best in stable and high-output production.
Advantages of LSR Injection Molding
Consistency and decreased secondary operations In practice, the largest wins in injection molding with liquid silicone rubber occur due to consistency and decreased secondary operations – particularly where the parts must have tight regulatory or functional requirements.
| Advantage | Manufacturing Impact |
| High precision | Better dimensional stability, tight tolerances |
| Automation | Reduced labor cost, consistent quality |
| Short cycle time | Higher output, faster throughput |
| Clean production | Ideal for medical/food-grade, low contamination |
| Low flash | Reduced trimming, better aesthetics |
| Complex geometries | Excellent flow for thin walls, undercuts |
The LSR molding excels with high precision silicone molding which is required to be biocompatible, reproducible and low flash when medical components are needed.
Limitations and Cost Considerations
The other side is obvious by experience: LSR injection molding cost increases dramatically in the very first place, and it cannot always be profitable in all the projects.
Tooling includes complicated cold-runner mechanisms, accurate venting, and high grade steel to manage heat and pressure – typically 35 times the cost of more straightforward compression molds. The catalyst of platinum and purity make the material costly.
When the quantity is small, e.g. batches, tooling amortization becomes prohibitively expensive on a per-part basis. The very thick sections (>1015 mm) may result in a longer cure time or uneven crosslinking.
| Cost Factor | Impact | Explanation |
| Tooling cost | High | Complex cold runner system |
| Mold design complexity | High | Precise venting, gating required |
| Material cost | Higher | Liquid silicone premium |
| Small batch production | Less efficient | Tooling amortization spreads poorly |
Investment in LSR tooling is only applied at volumes that exceed some threshold, usually thousands of tens of thousands, or situations where tolerances and cleanliness warrant it.
Typical Tolerances in LSR Injection Molded Parts
Real-world tolerances Real-world tolerances in LSR injection molding are more repeatable and tighter than compression tolerances and still require high sensitivity to part size, geometry, mold quality, and shrinkage (typically 23%).
We regularly hold under good mold compensation and process stability:
| Part Size | Typical Tolerance | Notes |
| <50 mm | ±0.05–0.15 mm | High stability possible |
| 50–150 mm | ±0.15–0.3 mm | Dependent on geometry, shrinkage direction |
| >150 mm | Case-dependent | Mold design critical, may need tighter control |
The other advantage for silicone molding is that LSR has a consistent shrinkage, coupled with low sink, which is much more useful with tight-tolerance silicone parts than compression.
LSR vs Silicone Compression Molding: Key Differences
The decision that LSR injection molding or the [silicone compression molding process](insert URL here) should be made usually depends on volume, precision requirements and budget constraint.
| Factor | LSR Injection | Compression Molding |
| Tooling cost | Higher | Lower |
| Automation | High | Low |
| Tolerance | Better (±0.05–0.3 mm) | Moderate (±0.2–0.5+ mm) |
| Labor | Low | Higher |
| Ideal volume | High | Medium/low |
| Cycle time | Short | Longer |
| Flash control | Excellent | More trimming needed |
LSR is superior in places where repeatability and cleanliness is of paramount importance.
DFM Considerations for LSR Injection Molding
Most of the risks are eliminated in the early stages of DFM, neglected and you will run after defects in one cycle after another.
Pay attention to equal flow, adequate ventilation to avoid air traps, equal wall thickness to avoid hesitation or overpacking and sufficient draft (= 0.5-1 -) to facilitate demolding without tearing.
| DFM Factor | Recommendation | Risk if Ignored |
| Gate size/location | Balanced flow paths | Jetting, short shots |
| Wall thickness | Uniform, 0.5–10 mm ideal | Flow hesitation, sink |
| Venting | Adequate micro-vents | Air trap, burn marks |
| Draft angle | ≥0.5° | Tear risk, demold damage |
| Undercuts | Minimize or use collapsible cores | Complex tooling, cost up |
Good DFM transforms possible headache into quality production.
Common Defects in LSR Injection Molding
Despite experience, some defects are visible when the parameters drift out of control – here are some of the parameters we monitor on an everyday basis:
- Flash: Material that spills in at parting lines — due to overpressure or improper parting fit, or too little clamping.
- Short shot: Incomplete fill No flow due to low pressure, cold mold, or entrapment of air.
- Jetting: Snake-like flowing marks – gate too small or injection speed too high.
- Air trap: Bubbles or voids – weak venting or quick fill trapping gas.
- Overcure/Under-cure: Tacky or brittle components – balance or ratio of temperatures.
The majority of them are related to the design of molds, process settings, or material consistency – root-cause solutions typically involve adjustment of temperature, pressure profiles, or venting.
Conclusion — LSR Injection Molding Is Precise, But Not Universal
The features of LSR injection molding include high precision, the benefits of automation, and the capability to manufacture clean and repeatable high precision silicone parts in volumes. However the increased tooling requirement and cost of materials imply that it does not work well on all silicone projects as the default.
It provides the most successful outcomes in high-volume, high-tolerance, or high-cleanliness production including high-volume, complex geometries or high-cleanliness applications especially in medical, automotive and consumer processes.Early DFM collaboration, realistic tolerance expectations and process discipline are the keys to success. In cases of these alignments, LSR has given stable mass production, which can be matched by few other methods.