Complete FAQ Compilation for Rubber & Plastic Industry
Q: What working conditions are NBR/EPDM/VMQ/FKM/CR and natural rubber suitable for respectively?
A:
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NBR (Nitrile Butadiene Rubber): Resistant to engine oil and hydraulic oil; operating temperature range -40℃ to 120℃; widely used for hydraulic parts and oil seals with high cost performance.
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EPDM (Ethylene Propylene Diene Monomer): Resistant to water, steam, ozone and outdoor exposure; incompatible with mineral oil; applied to water pipes, shock absorbers and door/window seals.
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VMQ (Silicone Rubber): Wide temperature range of -60℃ to 230℃; food and medical grade, electrically insulating; not resistant to fuel oil.
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FKM (Fluororubber): Tolerant to extreme high and low temperatures, strong acids, strong alkalis and fuel oil; used for automotive engine components and high-temperature chemical equipment.
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CR (Chloroprene Rubber): Weather-resistant, flame retardant and wear-resistant; for transmission belts and protective sleeves.
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NR (Natural Rubber): High elasticity and excellent shock absorption; used in tires and shock pads; poor resistance to oil and aging.
Q: How is rubber hardness marked? What is its range and tolerance?
A: Shore A hardness is the universal standard with a range of 15A to 90A; standard tolerance is ±5A. Soft rubber: 10A–30A; general sealing rubber: 60A–70A; wear-resistant hard rubber: 80A–90A.
Q: Why is there a huge price gap between seals of the same size made from different materials?
A: Raw material costs vary drastically; FKM costs 8 to 15 times more than NBR, followed by silicone rubber and EPDM. Formulation fillers and food-grade/automotive-grade additives also push up costs.
Q: What materials and certifications are required for food-grade/medical-grade rubber?
A: VMQ silicone rubber is the preferred material, which must pass FDA and LFGB certifications. Recycled materials and toxic plasticizers are prohibited; the rubber shall have no precipitation and no peculiar odor.
Q: What are the differences between PU (Polyurethane) and ordinary rubber?
A: PU features outstanding wear resistance and tear resistance, with a broad hardness range (10A–95D) and good oil resistance. Its drawbacks include poor hydrolysis resistance and easy softening under high temperatures. It is mainly used for rubber rollers, shock pads and wear-resistant parts.
Q: What application scenarios are suitable for PP/ABS/PC/PA66/POM/PVC?
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PP (Polypropylene): Acid and alkali resistant, food-safe; for home appliance housings and containers.
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ABS (Acrylonitrile Butadiene Styrene): High toughness; for equipment casings and toys.
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PC (Polycarbonate): Highly transparent and impact-resistant; for lamp covers and protective shields.
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PA66 (Nylon 66): Wear-resistant and self-lubricating; for gears and bushings.
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POM (Polyoxymethylene / Acetal): Low friction and stable dimensional accuracy; for precision transmission parts.
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PVC (Polyvinyl Chloride): Low cost and flame retardant; for pipes and sleeves, prone to brittleness at low temperatures.
Q: What distinguishes modified plastics from virgin raw plastics?
A: Additives such as glass fiber, tougheners, flame retardants and UV stabilizers are blended in modified plastics. Glass fiber reinforcement improves tensile strength; tougheners reduce low-temperature cracking; UV stabilizers are used for outdoor products.
Q: How to classify plastics by high-temperature resistance grade?
A: Long-term service temperature: PP 80℃, ABS 70℃, PC 120℃, PA66 100–130℃; special engineering plastics including PPS and PEEK withstand temperatures above 200℃.
Q: What causes blooming (white powdery deposit) on rubber surfaces and does it affect service performance?
A: Blooming is the precipitation of additives (vulcanizing agents, anti-aging agents), resulting from under-vulcanization, excessive additives in formulas or drastic temperature fluctuations during storage. Slight blooming does not impair sealing performance, while severe blooming reduces adhesion and accelerates aging.
Q: Why do products exude oil, turn sticky and feel tacky?
A: Inadequate vulcanization (insufficient temperature or time) leads to precipitation of low-molecular-weight oil. In the short term, it contaminates matching components; in the long run, mechanical strength and aging resistance drop sharply, making such products defective.
Q: Why do seals harden, crack and leak liquid/gas within a short service period?
A: 1. Improper material selection (EPDM used in oil service environments); 2. Long-term thermal aging under excessive temperature; 3. Corrosion by contacting media; 4. Outdoor aging caused by ozone and ultraviolet rays; 5. Shaft surface scratches during installation.
Q: Why do oil seals leak shortly after installation?
A: Reversed lip orientation, scratched shafts or unqualified surface roughness, dry friction without lubrication, excessive cavity tolerance, working pressure exceeding the rated sealing limit, and unchamfered metal edges cutting the rubber lip.
Q: Why do rubber components peel off from bonded metal parts?
A: Residual oil contamination on metal surfaces, mismatched primer adhesive, incompletely dried adhesive coating, insufficient vulcanization pressure, and smooth metal surfaces without textured finish.
Q: What causes sink marks, depressions, air bubbles and short shots on plastic molded parts?
A: Sink marks stem from insufficient packing pressure or uneven wall thickness; air bubbles are caused by undried raw materials or excessive injection speed; short shots result from low injection pressure/temperature and undersized runners.
Q: What leads to whitening, brittle fracture and stress cracking of plastic parts?
A: Moisture in raw materials, excessive mold release agent, excessive ejection force, rapid cooling, and concentrated stress induced by glass fiber orientation.
Q: What are the rubber molding processes, along with their advantages and disadvantages?
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Compression Molding Vulcanization: Suitable for small batches and diverse specifications with low mold cost, ideal for non-standard seals; disadvantage: relatively low production efficiency.
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Injection Vulcanization: Applied to mass-produced precision parts with stable dimensions and fast production speed; disadvantage: high upfront mold investment.
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Extrusion: For rubber strips, pipes and sealing strips with continuous production capacity, unable to fabricate complex special-shaped structures.
Q: What are the standard vulcanization parameters (temperature & time) for rubber?
A: For conventional NBR/EPDM, vulcanization is carried out at 150–170℃ for 5–15 minutes. Extend vulcanization time for thick-walled parts and shorten it for thin small components; excessively high temperature will scorch and embrittle rubber.
Q: What are the differences between plastic injection molding and extrusion processes?
A: Injection molding forms complex special-shaped shells and gears inside closed molds; extrusion continuously produces long profiles, pipes and sheets for mass production.
Q: Why do products from the same batch have deviations in hardness and dimensions?
A: For rubber: uneven mixing, fluctuating vulcanization temperature and unstable rubber compound weight. For plastic: variations in melt temperature, mold temperature, packing pressure parameters and raw material batch differences.
Q: Can leftover rubber/plastic trimmings be recycled for reproduction?
A: Recycled scraps can be mixed at a ratio of 10%–30% for ordinary industrial components. Recycled materials are prohibited for food-grade, medical and automotive safety parts, as they cause additive precipitation and reduced mechanical strength.
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