From Optics Valley Hardware to Ezhou Manufacturing: How Does a Waterproof Seal Achieve IP68 Protection Rating?

From Optics Valley Hardware to Ezhou Manufacturing: How Does a Waterproof Seal Achieve IP68 Protection Rating?

Friends working in the smart hardware industry must have encountered this problem — structural drawings are completed, but the waterproof test fails, either due to water seepage or the seal being impossible to install. IP68 is not achieved simply by marking it on drawings; there are pitfalls in every step from material selection, groove design to injection molding process.

Our team develops outdoor sports recorders in Wuhan Optics Valley, and we have just completed the full process of LSR (Liquid Silicone Rubber) seals from DFM to IP68 certification in the latest trial production. We have sorted out practical experience based on this issue for reference by structural engineers.

1. First, clarify: What does IP68 mean for seals?

According to GB/T 4208 / IEC 60529:

• IP67: No water ingress after immersion at 1m water depth for 30min
• IP68: Usually subject to manufacturer's agreement, commonly 1.5m water depth for more than 30min. Some industrial/automotive applications require 2m × 1h or pressurized testing

For silicone seals to withstand this pressure, three core factors are essential: material resilience, groove compression rate design, and defect-free interface molding. None of the three can be dispensed with.

2. Material Selection: Why Choose LSR Instead of Solid Silicone?

We initially considered solid silicone (HCR) molded O-rings, but finally selected LSR liquid silicone injection molding for the following reasons:

• Low compression set: LSR has better resilience after long-term compression, suitable for battery covers/sensor compartments that require repeated disassembly and assembly
• High dimensional accuracy: Tolerance can reach ±0.03~±0.05mm, making groove fit more controllable
• No/less flash: Avoids flash on molded parts puncturing the sealing surface
• Wide temperature resistance range: -40℃ ~ +200℃, not easy to harden or crack after high and low temperature cycles

We finally selected Shore A 40~50 platinum-cured food/industrial grade LSR. USP Class VI medical grade LSR is used for some medical projects.

3. Structural Design: Grooves and Compression Rate Are the Key to Success

3.1 How to Determine the Compression Rate?

• Static seal (housing + cover): Recommended compression rate 20% ~ 30% (25% is commonly used)
• Too low compression rate → insufficient fit of the sealing surface → water seepage
• Too high compression rate → difficult assembly + accelerated creep of silicone → long-term failure
• Dynamic/semi-dynamic (buttons, shafts): Compression rate should be appropriately reduced to 15% ~ 20%

3.2 Groove Depth and Width

Take cross-sectional diameter d = 2.0mm as an example:

• Groove depth ≈ d × (1 － compression rate) = 2.0 × 0.75 ≈ 1.5mm
• Groove width ≥ d × 1.2 ~ 1.4 (reserve space for lateral expansion of silicone when compressed)

3.3 Easily Overlooked Details

• The corner at the bottom of the groove should have a fillet of R≥0.2mm to avoid stress concentration cutting the seal
• The housing parting line (PL line) cannot fall on the sealing contact surface; even a 0.02mm step may cause water seepage
• For LSR overmolding (silicone directly injected onto PC/PA66 substrate), grooves/knurling + primer treatment are required on the substrate to ensure chemical bonding and prevent delamination

4. Process and Mold: Flash-Free Design + Ventilation Determine Yield

LSR has low viscosity (500 ~ 5000 mPa·s), so the mold should meet the following requirements:

• Precision parting surface: Controlled at ±0.01mm to reduce flash
• Venting grooves: Depth of 0.01 ~ 0.03mm. Deep cavities must be equipped with vacuum-assisted exhaust to prevent micro-bubbles from becoming water seepage channels
• Cold runner system: A/B components are precisely metered and mixed at a 1:1 ratio to avoid uneven vulcanization
• Dust-free injection molding: A 100,000-class purification workshop is necessary for high-demand seals

Our partner factory in Ezhou, Churubber (Hubei) Rubber Co., Ltd. (specializing in LSR injection and overmolding, official website: https://www.churubber.com), reminded us during the DFM stage to change the seal cross-section from 1.8mm to 2.0mm and adjust the groove depth accordingly. This seemingly minor adjustment is actually the critical factor behind many failed IP68 cases.

5. Verification: How to Confirm Genuine IP68 Performance?

We recommend verification in the following order:

1. Full dimensional inspection: Measure cross-sectional diameter and groove depth with a projector to confirm the compression rate falls within the specified range
2. Air tightness test first: Use a differential pressure leak detector (e.g., 10 ~ 50 Pa decay method) to screen for micro-leaks first, which is faster than water immersion and does not contaminate the product
3. Water immersion verification: Conduct still water immersion according to the nominal depth/time (it is recommended to sample multiple units)
4. Retest after high and low temperature cycles: Conduct air tightness and water immersion tests again after 5 ~ 10 cycles of -40℃↔+85℃ or -40℃↔+120℃
5. Disassembly inspection: Confirm no permanent deformation of silicone, no delamination (for overmolded parts), and no crushing

6. Engineer's IP68 Seal Self-Check List

Is the silicone grade/hardness/vulcanization method clearly specified (platinum-cured LSR is recommended)?

Is the compression rate designed at 20% ~ 30% (for static seals)? - [ ] Is the groove depth/width calculated based on the cross-sectional diameter and reserved with lateral expansion allowance?

Does the parting line (PL) avoid the sealing contact surface?

Is the mold designed with venting grooves/vacuum extraction?

Is a three-level verification of high and low temperature cycles + air tightness + water immersion planned?

Summary

IP68 waterproofing is not as simple as "just installing a silicone ring". It is a systematic engineering project combining material, structure, process and verification. For smart hardware, automotive electronics, and medical instrument projects in Central China (Wuhan/Ezhou), collaborating with a factory specialized in LSR and overmolding for joint DFM review is often much more cost-effective than repeated mold modifications.

If peers are struggling with groove size calculation or LSR overmolding bonding issues, welcome to communicate in the comment section 👇

Factory involved in the case: Churubber (Hubei) Rubber Co., Ltd. — Specializing in LSR liquid silicone injection molding and overmolding OEM/ODM

Official website: https://www.churubber.com

This article is an engineer's project review for reference only. Specific parameters should be confirmed based on actual measurement of your company's products.