What Is the Manufacturing Process of Rubber Molded Parts?

Every day, clients ask me about rubber molding processes. Understanding these processes is crucial - choosing the wrong one can lead to quality issues and unnecessary costs.

Rubber molded parts follow a repeatable flow: compounding, preforming, molding (compression/transfer/injection), post-curing, finishing (deflashing, trimming), and quality verification with documents. Each step locks in performance and consistency.

I run this workflow daily at Julong Rubber. I reduce risk up front, then I hold the line with data. Buyers see reliable parts and on-time shipments.

How do I turn raw rubber into a stable compound?

Problems start with poor compound control. I prevent them during mixing.

I blend base polymer, curatives, fillers, plasticizers, and additives on an internal mixer or open mill, then I record batch data. I aim for uniform dispersion, stable viscosity, and clean, traceable batches.

I set a formula to match the application. EPDM¹ handles weather and steam. NBR² handles oil. HNBR³ lifts heat and oil resistance. FKM/Viton⁴ covers fuels and solvents. Silicone⁵ serves wide temperature and FDA contact when needed. I select hardness by Shore A⁶ and compressive behavior by compression-set targets.

🛠️ What I control in compounding

• ✅ Material lot traceability: I log polymer lot, batch number, and mix time
• ✅ Temperature profile: I keep the mix below scorch limits
• ✅ Dispersion quality: I check carbon black and silica streaks with cut tests
• ✅ Mooney viscosity⁷: I stay within a narrow window for molding repeatability
• ✅ Preforming: I calender or extrude blanks to weight tolerance for stable fill

Typical compounding targets

I also plan for shrinkage. I record historic shrink rates by compound. I use them in cavity design and later verify with first articles.

Which molding process should I choose—compression, transfer, or injection?

The wrong process wastes money. I choose the one that fits geometry, volume, and tolerance.

Compression is simple and cost-effective. Transfer gives better fill on thin ribs. Injection scales, balances flow, and shortens cycle time for high volumes. I match process to part and forecast.

I prepare preforms for compression to control flash. I design pots and sprues for transfer to move compound cleanly. I balance runners and vents for injection to avoid knit lines and trapped air.

⚖️ Process comparison for buyers

Key setup choices I make

• Gates and runners: I minimize shear, avoid jetting, and place weld lines away from sealing lips
• Vents: I machine micro-vents at last-fill zones; I keep them shallow to prevent flash
• Flash land: I tune land width and depth to hold flash thin without starving cavities
• Cure window: I run DOE on temperature, pressure, and time to stabilize dimensions

What happens after molding—post-curing, deflashing, and inspection?

The press is not the finish line. Stability and appearance come after.

I post-cure when chemistry or compliance needs it, deflash to clean edges, and inspect to spec. This is where odor, compression set, and dimensional stability fall into place.

I post-cure silicone and some peroxide-cured compounds to remove volatiles and complete crosslinks. I choose oven profiles by compound and part thickness. I deflash by cryogenic blasting, tumbling, or manual trim depending on geometry and edge quality targets.

🔧 Post-molding controls

I verify hardness (ASTM D2240), dimensions, and surface condition. I sample by AQL or per PPAP¹² plan. I photograph CTQ features for traceability. I record press parameters to link quality to process.

How do I control tolerances, hardness, and consistency?

Data beats guesswork. I lock tolerances and hardness with standards and repeatable methods.

I mark the drawing with DIN ISO 3302-1 tolerance class (M1–M4) for molded rubber, state hardness as “70 ShA ±5, ASTM D2240, 3 s at 23 °C,” and verify with a clear gauge plan.

I choose class by geometry and function. Sealing lips often need M2. Large simple profiles may use M3. I keep “steel safe” on high-risk dimensions so I can polish down to size after first articles. I track shrinkage per batch and adjust inserts if needed.

🧭 Tolerance choice guide (quick view)

H3: My verification routine

I calibrate the durometer and use a bench stand to remove human error. I measure hardness on flat, 6 mm-equivalent thickness or stacked coupons. I fix dwell at 3 s and room temperature at 23 °C. I measure dimensions on a CMM or calibrated gauges. I log 3–5 points per CTQ and compute average and range. I compare to the drawing and tolerance class. If a dimension trends near a limit, I adjust cure time or polish the insert. I repeat after any compound lot change.

How do I qualify the process—DFM, DOE, and first articles?

A smooth launch begins before steel is cut. I use DFM and small experiments to learn fast.

I review draft angles, parting line, flash land, and undercuts. I set a shrink target. Then I run a DOE on temperature, pressure, and time to find a safe cure window before first-article inspection.

I make a simple sample mold when the risk is high or geometry is new. It saves cost later. First-article lots come with full measurements, hardness data, and material certificates. I include leak or compression-set tests when the application needs them.

📋 Launch checklist I follow

I still remember a project where a thin lip leaked in assembly. My DOE showed a narrow cure window. I changed vent locations and added 0.1 mm land. The leak vanished, and the buyer avoided a field recall.

What documents prove quality—PPAP, EN 10204 3.1, and test reports?

Buyers need proof. I provide it in a compact, auditable pack.

I deliver a PPAP-lite or full PPAP on request, plus EN 10204 3.1 material certificates. I attach hardness, tensile, tear, and compression-set results, and I include process parameters for traceability.

I tailor the scope to the project. HVAC seals often need dimensional reports and hardness only. Automotive programs may require control plans, PFMEA, MSA (gage R&R), and capability (Cp/Cpk). I can add aging tests, fluid immersion, and low-temperature brittleness per standard.

📄 Common documents I send

How do I choose materials and design for the application?

Right material saves cost later. I match polymer, hardness, and design to the job.

I pick EPDM for weather and hot water, NBR for oils, HNBR for heat-oil, FKM for fuels/chemicals, and silicone for wide temperature or food contact. I set Shore A⁶ by load and sealing line pressure.

Material and performance matrix

I design seals with generous radii at corners, proper draft for demold, and stable cross-sections. I avoid sharp internal corners that trap air. I place knit lines away from sealing edges. I size lips for compression-set limits and choose hardness that seals without over-compression.

What makes the difference in delivery time and cost?

Clear drawings and early decisions remove friction. Small choices add up.

I confirm tolerance class, hardness spec, inspection plan, and shipping terms before tooling. I choose the right process for the forecast. I plan bridge runs if volumes grow.

⏱️ Cost and lead-time levers

• 🧩 Process fit: compression for low volume; injection for scale
• 🔁 Cavities: start small, add later with proven geometry
• 🧪 Sample mold: de-risk new designs before multi-cavity tools
• 📐 Standards on drawing: DIN ISO 3302-1 class and hardness method save emails
• 📦 Shipping plan: decide EXW/FOB/DAP/DDP early to protect dates

Work with me

• ✅ I run B2B only, factory-direct.
• 📩 Email: info@rubberandseal.com
• 🌐 Website: www.rubberandseal.com
• 🔧 Products: rubber seals, custom rubber parts, rubber wheels, hoses, gaskets

Conclusion

A good rubber part comes from a good process: compounding, molding, post-cure, finishing, and proof. I follow this flow every day to ship stable parts on time.