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Piston Filler

After the piston of a cylinder filling machine has been operating for a while, a black substance can be seen forming inside the cylinder. Is this due to a problem with the cylinder itself, such as poor material or design? Therefore, is it necessary to install a dust-proof oil seal on the open end of the piston to prevent ambient dust from entering the cylinder and causing the black substance to form due to friction? Is this also necessary in a production environment that is practically a clean room? Or, perhaps, different product requirements may exist. For example, when using a piston filling machine to fill pharmaceuticals, food, or skincare products, hygienic requirements and product appearance prohibit the presence of colored matter in the product. If the piston is in reciprocating motion, will the friction between the O-ring and the cylinder cause particles to escape due to the prolonged back-and-forth movement of the piston rod and the cylinder wall? How can this problem be addressed?  

This is a very common question—the appearance of black material inside the piston rod/cylinder is a common problem on filling machines. The causes, risks, and solutions actually have a clear set of inspection and improvement strategies. Below is a list of causes, diagnostic methods, short-, medium-, and long-term solutions, as well as specific recommendations for high-hygiene applications such as pharmaceuticals, food, and skincare products. You can apply this directly or submit it to your facility's engineers or quality assurance department for reference.

1) Possible Causes (ranked by frequency and importance)

Wear and loss of O-rings, scrapers, or other elastic components (most common)—Elastomers (such as silicone, NBR, EPDM, and Viton) can produce particles due to friction, chemical degradation, or temperature, which can appear as black or gray deposits over time.

Rough or poorly finished cylinder surface—Unpolished, uneven, poorly coated surfaces, or machining residues can cause wear and even transfer black debris to the O-ring.

Material incompatibility/chemical attack—The product or cleaning agents (such as strong alkalis, acids, or certain solvents) attack the O-ring or coating, causing embrittlement or discoloration.

Foreign contaminants (dust, carbon dust, lubricant) enter the piston end. Without effective oil scraping/dust protection, ambient dust or lubricant from the machine's external source can also be introduced.

Insufficient or improper lubrication: Excessive friction can accelerate wear. Furthermore, using incompatible grease can dissolve color or contaminate the product.

Installation/design issues (misalignment, O-ring overextension, indentations) can lead to localized overwear or chipping.

2) How to Identify the Source (Quick Test Steps)

Sample Analysis: Send the black material to a laboratory for FTIR/SEM or compositional analysis (which can distinguish between rubber, grease, or stainless steel debris).

Visual Inspection: Disassemble the piston and inspect the O-ring and scraper for cracks, deformation, or wear marks. Inspect the cylinder wall for scratches or a black film.

Review Cleaning/Formula History: Has the O-ring material been changed recently, a new cleaning agent used, or a change in production formula been made?

Environmental Inspection: Are there any sources of dust, oil mist, or other contaminants near the machine?

Operation Test: Conduct a short test run in a pure medium (water or a quality assurance test fluid) to observe whether the black substance is still produced (eliminating any potential source from the product itself).

3) Solution (from most recommended to optional)

Short-term (immediately available)

Replace the dynamic seal material with the appropriate material: For food/drug applications, prioritize FDA/USP-compliant materials (if EPDM or silicone are not suitable for certain temperatures/solvents, use FKM or FFKM/PTFE hybrids instead). Select seals designed for reciprocating motion.

Install a wiper/scraper on the piston opening to prevent external dust and oil mist from entering the cylinder.

Discontinue use of cleaning agents or solvents that attack elastomers. Switch to CIP/SIP chemicals approved by the machine model or increase the flushing frequency.

Mid-Term (Design Improvement)

Improve cylinder surface finish: Use 316L stainless steel, polish well (Ra ≤ 0.2–0.4 μm is recommended), and perform electrolytic polishing and passivation to reduce microcracks, machining residue, and wear.

Use low-friction or frictionless sealing designs, such as PTFE cup seals, PTFE sliding pads, or backed O-rings/anti-extrusion rings, to reduce debris and color loss.

Use rolling diaphragms or bellows pistons: These designs have virtually no sliding elastomer contact with the product surface and are extremely friendly to pharmaceuticals and high-cleanliness environments.

Move dynamic seals out of the product contact area (using magnetic coupling or isolation mechanisms)—if feasible, locate the shaft seal outside the product area.

Long-Term (High-Grade/High-Risk Products)

Use fully hygienic pistons (compliant with standards such as 3-A/EHEDG) made of extremely low-extractable/low-wear materials such as PTFE or FFKM, and conduct extractables and leachables testing.

Design with dual seals and monitoring: The outer layer has two seals and a leak detection channel, which will trigger exhaust/alarm failure if the first seal fails.

Select seals that are CIP/SIP-compatible and operate within the approved temperature/pressure/chemical compatibility ranges provided by the material supplier.

4) Is it necessary to use dust-proof oil seals in "almost cleanroom" production environments?

Even in environments close to cleanrooms, it is still recommended to use appropriate oil scrapers/dust-proof parts and seals made of suitable materials. Reason:

Cleanrooms reduce large particles in the air, but they cannot completely eliminate micro-wear and particulates generated by the seal itself.

A clean environment reduces extraneous dust, but frictional wear (wear of elastomers against each other or the cylinder wall) can still occur, especially with long-term reciprocating motion. Therefore, in clean processes (food/pharmaceutical/cosmetic), design solutions that minimize contact between sliding elastomers and the product (such as PTFE cups, bellows, or rolling diaphragms) should be employed, along with external oil scrapers.

5) Regarding "Will long-term reciprocating friction introduce particles into the product?"

It's possible: Any sliding seal will wear due to friction, especially if the material or surface treatment is inadequate, or if chemical or thermal conditions accelerate aging.

Risk Mitigation: Select low-wear materials (PTFE, FFKM), optimize cylinder surfaces, add dust-proof scrapers, adopt rolling/isolated designs, and regularly replace seals. These measures can reduce visible particles, stains, or detectable impurities to below regulatory standards.

6) Specific Recommendations for Different Products (Pharmaceuticals/Food/Cosmetics)

Pharmaceuticals (most stringent): Use materials that comply with pharmacopoeias and GMP (316L, PTFE, FFKM, etc.); perform extractables/leachables testing, particulate testing (USP <788>), and endotoxin/sterilization process testing; prioritize designs with non-sliding elastomer contact (rolling diaphragms or bellows).

Food: Use food-grade (FDA) materials and avoid greases that may leach colorants or odors; polish cylinder walls and ensure easy CIP cleaning.

Cosmetics/Skin Care Products: Select seal materials based on the ingredients (oily/alcoholic/acidic/alkaline); pay attention to appearance requirements (color, suspended solids), and perform batch sampling.

7) Maintenance and Management (Reducing Recurrence)

Establish a replacement cycle (based on operating hours, number of strokes, or inspection results).

Perform regular disassembly and inspection, taking photos and documenting seal wear patterns (which can be used to identify root causes).

Use appropriate lubricants (if necessary) and record usage quantities and specifications (certain additives are prohibited in food and drug products).

Flushing SOPs (CIP) and chemical compatibility checks are performed before and after operation.

Quality release testing: Particle testing, color/appearance inspection, and, if necessary, FTIR testing for source identification.

Conclusion: Black matter does not necessarily indicate "bad cylinder material"—the most common causes are O-ring or scraper wear, poor cylinder surface preparation, or chemical compatibility issues. In clean/hygienic environments, design efforts should focus on reducing product contact between sliding elastomers and improving surface treatment/adding scrapers. If necessary, utilize non-sliding solutions such as rolling diaphragms or bellows, and perform extractables/particle testing to verify safety.

Conclusion: 

Black matter does not necessarily indicate "bad cylinder material"—the most common causes are O-ring or scraper wear, poor cylinder surface preparation, or chemical compatibility issues. In clean/hygienic environments, design efforts should focus on reducing product contact between sliding elastomers and improving surface treatment/adding scrapers. If necessary, utilize non-sliding solutions such as rolling diaphragms or bellows, and perform extractables/particle testing to verify safety.

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