Liquid Silicone Rubber Injection Molding: LIM Process Parameters, Flash-Free Tooling, and Defect Troubleshooting
You just brought up a new LSR tool, and the first 50 shots are flashing at the parting line, the cavity is under-cured on one side, and a sticky patch on the ejector pin keeps tearing the part. This is the most common story we hear from process engineers qualifying a new liquid silicone rubber injection molding cell. The article below gives you the parameter windows, the flash-free tooling requirements, and a defect-troubleshooting matrix you can actually use on the floor. It is written for process engineers, tooling buyers, and product managers running or planning LIM production of seals, valves, medical and baby-care parts, and automotive connector seals.
The LIM Process at a Glance: Metering, Mixing, Injection, Cure
- Feed and meter. Component A (vinyl-functional silicone plus Pt catalyst) and component B (Si-H crosslinker plus inhibitor) are pumped from drums through static mixers into the metering unit. The unit must hold a 1:1 ratio by volume within about ±1–2% — any drift and the part will not cure fully.
- Static mix and inject. The streams converge in a static mixer and are shot through a cold-runner nozzle kept below roughly 40–60 °C into a heated mold. Injection pressure at the nozzle is typically 50–150 bar, tuned by part size and gate design.
- Fill and cure in seconds. The mold is closed and heated to 160–200 °C (commonly 170–180 °C). Cure time scales roughly with wall thickness squared: a 2 mm wall cures in 5–15 s, a 4 mm wall in 20–40 s. Demold hot.
- Post-cure. Many parts go to a post-cure oven for 1–4 h at 150–200 °C to drive off volatiles and stabilize compression set before dimensional inspection.
- Inspect and automate. First-offs are checked for flash and cure; production runs use robot demold, vision inspection, and batch CoA verification of viscosity, Shore A, tensile, and tear per ASTM D412, D624, and D2240.
Core Process Parameters and Their Windows
| Parameter | Typical window | What goes wrong outside it |
|---|---|---|
| Mold temperature | 160–200 °C (commonly 170–180 °C), cavity-to-cavity within ±2–3 °C | Too cold: short shots, tacky parts, long cure. Too hot: scorching, sticking, longer cycle. |
| Injection pressure (at nozzle) | 50–150 bar, depending on part size and gate | Too low: short shots and air entrapment. Too high: flash at the parting line and over-packed vents. |
| Metering ratio (A:B) | 1:1 by volume, ±1–2% | Drift causes under-cure (B-rich) or inhibition/hardness shift (A-rich). Verify with a daily scale shot test. |
| Injection speed | Slow-to-medium fill, tuned to gate area | Too fast: jetting, bubbles, weld-line voids. Too slow: cold flow front and short shots. |
| Cure time vs. wall thickness | ~5–15 s at 2 mm, ~20–40 s at 4 mm (scales with t²) | Under-cure sticks and tears at demold; over-cure wastes cycle time and may scorch thin sections. |
| Post-cure | 1–4 h at 150–200 °C | Skipping it leaves volatiles in the part and compression set drifts in service. |
For comparison, HTV silicone is a high-viscosity gum compounded on a mill and pressed at 150–180 °C for minutes per cycle with flash to trim — see our HTV silicone rubber compounding process guide for that workflow. LSR is liquid, metered, flash-free, and cures in seconds, which is why it dominates high-volume precision parts. Tooling cost is higher (precision cold-runner molds), but unit economics typically win above roughly 50k–100k parts/year. Compound chemistry sets the viscosity window and cure kinetics, so read our primer on liquid silicone rubber compounds before locking the tool steel.
Flash-Free Molding and Tooling Requirements
| Requirement | Why it matters | Typical value |
|---|---|---|
| Cold-runner system with valve gate | Keeps material below ~40–60 °C in the runner and gates into the hot cavity; no runner scrap because LSR is a thermoset | Single-drop or multi-drop valve-gate cold decks |
| Pre-loaded / spring-loaded mold plates | Holds the parting line shut against injection pressure to prevent flash | Hydraulic or spring pre-load, tuned to expected tonnage |
| Vacuum venting | Pulls air out before LSR fills; vent gaps are the path of least resistance so flash forms there if vents are dirty or too open | Vent gap ~0.01–0.03 mm; vacuum pulled before injection |
| Parting-line precision | Any mismatch opens a flash path that LSR will find within one or two bars of extra pressure | Low-micron parting-line flatness, polished shut-offs |
| Uniform mold temperature | Cavity-to-cavity ±2–3 °C or cure time drifts and parts stick or scorch unevenly | Heated manifolds with closed-loop control per cavity |
| Clamp force sizing | Too little clamp force is the single biggest cause of flash at startup | ~30–60 kN per 10 cm² of projected cavity area for LSR |
Cycle times of 30–90 s including demold are realistic for small-to-medium parts on purpose-built LIM machines (Engel, Arburg, Sodick LSR variants) that integrate the metering unit, the injection unit, and the robot demold. With valve-gate cold decks, the gate vestige is small enough that downstream deflashing is normally not required.
Defect Troubleshooting Matrix
| Defect | Likely root causes | First corrective actions |
|---|---|---|
| Flash at parting line | Insufficient clamp force, worn/dirty vacuum vents, parting-line damage, injection pressure too high for the tool | Re-check clamp tonnage, clean vents, inspect shut-off faces, reduce injection pressure and re-qualify the shot |
| Short shot / uncured flow front | Mold too cold, injection speed too low, shot size or venting insufficient, A/B ratio drift | Raise mold temperature within ±2–3 °C per cavity, increase speed/shot, run a scale shot test on the meter |
| Bubbles / voids | Moisture on fillers or mold, poor degassing, trapped air at weld lines, injection too fast | Dry fillers and pre-heat the mold, pull vacuum before injection, slow the fill near weld lines |
| Sticky / tacky spots or uncured patches | Pt catalyst poisoning (sulfur, nitrogen/phosphorus compounds, tin, amines), contaminated gloves or release agents, metered ratio drift | Audit the cell for sulfur-cured rubber, amine-cured plastics, tin stabilizers, switch to clean gloves, verify 1:1 ratio with a scale shot |
| Tears at demold | Under-cure, draft angle too small (<~1–2°), mold temperature too low at ejection, demold too early | Add cure time, increase draft to ≥1–2°, raise ejector-zone temperature, delay demold by a cycle |
| Dimensional drift | Inconsistent post-cure, uneven mold temperature, batch-to-batch Shore A variation (±3–5 typical) | Lock post-cure recipe, tighten cavity temperature control, demand batch CoA and adjust mold compensation (~2–3% to ~2–2.5% with post-cure) |
Floor routine that prevents 90% of these issues: tactile/springback test on first-offs, daily scale shot on the A/B unit, per-cavity mold temperature logging, visual flash check each cavity, and a CoA from the compound supplier for every batch. If you are qualifying an optical or low-volatility part, the same logic applies to our LSR injection molding process for optical parts guide.
Frequently Asked Questions
What mold temperature should I set for liquid silicone rubber injection molding?
Run 160–200 °C, commonly 170–180 °C, and hold every cavity within ±2–3 °C. Below that you will see short shots and tacky parts; above it you will scorch thin sections and lengthen the cycle.
How do I get flash-free LSR parts from a new tool?
Use a valve-gate cold runner, pre-load or spring-load the mold plates, pull vacuum before injection, and keep vent gaps around 0.01–0.03 mm with low-micron parting-line precision. Size the clamp at roughly 30–60 kN per 10 cm² of projected area.
Why are some LSR parts sticky or uncured in patches?
Most often it is platinum catalyst poisoning — sulfur, nitrogen or phosphorus compounds, tin, or amines from gloves, release agents, or nearby materials — or A:B ratio drift outside ±1–2%. Audit the cell for contaminants and run a scale shot test on the meter to verify the 1:1 ratio.
Talk to Our Engineering Team
Whether an LSR part molds cleanly at target cycle time is decided jointly by the compound (viscosity window, cure kinetics, inhibition resistance) and the process setup — so process questions belong in supplier qualification, not just price negotiation. Send your part drawing, target cycle time, and cavity layout to our technical team and we will recommend a compound, a runner concept, and a parameter window you can put straight into your sampling plan.
Related Reading
- Optically Clear LSR: Equivalent Grades, Light Transmission and LIM Molding Guide
- Low-Volatility, Oil-Bleeding-Free LSR for Automotive and Medical Parts
- HTV Silicone Rubber Compounding and Mixing Process: Two-Roll Mill Procedure and Batch QC
- Silicone Rubber Gaskets: Material Selection and Design Guide
- Silicone Rubber Compounds for Industrial Applications
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