What matters should be paid attention to in the processing of rubber sealing ring oil seal products?

Rubber sealing ring (oil seal) is the sealing core component, and the processing core is centered on the sealing surface accuracy, physical properties of rubber compound, defect-free molding and dimensional stability. It is necessary to avoid the problems that affect the sealing effect, such as lack of rubber, air bubbles, flash, scratches on the sealing surface, out-of-tolerance and so on, and adapt to the processing mode of molding and injection. The following are the key points for attention in the whole process from raw materials to finished products, covering the structural characteristics of oil seal products (including lip and skeleton fit)

First, raw material pretreatment: adapt to the characteristics of oil seal compound to ensure the basic performance.

The common rubber compounds for oil seals are nitrile rubber (NBR), fluororubber (FKM) and acrylate rubber (ACM), which need to be matched according to the working conditions (oil resistance, high temperature resistance and medium resistance). The core of pretreatment is to ensure the uniformity of rubber compounds and no impurities, so as to avoid affecting the subsequent molding and sealing performance.

Storage and recycling of rubber compound: rubber compound is sealed and stored in a dry environment (humidity ≤60%) at 20~25℃ to avoid high-temperature aging and low-temperature hardening; Before use, the temperature shall be warmed up 4-8 hours in advance, and the open mill shall be softened for 3-5 times, and the roll distance shall be controlled at 0.5-1 mm, so as to ensure the uniform plasticity of the rubber compound, without hard core and agglomeration, and avoid the lack of rubber during molding.

Impurity removal: The mixed rubber material needs to be filtered by a filter screen (mesh number: 80-120 mesh for NBR and 60-80 mesh for FKM) to remove particulate impurities and burnt lumps in the rubber material, so as to prevent impurities from forming bulges at the sealing lip and cutting edge, resulting in oil leakage when the oil seal is used.

Pre-treatment of skeleton (for skeleton oil seal): the metal skeleton needs to be degreased, phosphatized and glued. Ultrasonic cleaning is used for degreasing (to avoid the influence of surface oil stain on the adhesion of rubber compound), and the thickness of phosphating layer is controlled at 5~10μm (to improve the adhesion between skeleton and rubber compound). After gluing, it is dried (at 120~150℃ for 30~60min) to avoid bubbles and falling off of the rubber coating, and to prevent the rubber compound from falling off during use.

Second, the molding process: the core protects the key parts of the seal to ensure the dimensional and structural accuracy.

Oil seal products have precise structures such as lip, sealing edge, joint surface, spring groove, etc. It is important to avoid damage, deformation and defects in these parts during molding. Molding is the main molding method of oil seal, and injection molding is suitable for large quantities of high-precision oil seals, and each process has clear parameter control requirements.

(1) Molding (suitable for all kinds of oil seals, especially special-shaped and small-batch skeleton oil seals)

Mould requirements

Accuracy of mold cavity: surface roughness Ra of sealing lip and cutting edge is ≤ 0.4 μ m, joint surface Ra is ≤ 0.8 μ m, cavity dimension tolerance is ≤±0.02mm, and there is no step at mold splicing, so as to avoid joint marks and bulges on oil seal surface;

Exhaust groove design: exhaust grooves (0.02~0.04mm in depth and 0.5~1mm in width) are set at the dead corner of the cavity and the joint of the skeleton to avoid air bubbles caused by gas accumulation during molding (air bubbles will damage the sealing surface and reduce the pressure resistance);

Demoulding structure: Ejection demoulding is adopted, and ejection rods are evenly distributed, so as to avoid the stress deformation of the lip and cutting edge of the oil seal during ejection, and the mold cavity needs anti-sticking treatment (spraying Teflon coating) to reduce demoulding resistance.

Mold loading and closing

Skeleton-free oil seal: the rubber compound is accurately weighed (error ≤ 0.5%), and the material is evenly spread according to the shape of the cavity, so as to avoid the over-thick flash caused by too much local material and the lack of rubber compound caused by too little material, especially the lip part, which needs to be fully filled with rubber compound;

Skeleton oil seal: accurately put the pretreated skeleton into the positioning groove of the mold to avoid skeleton deviation (deviation will lead to uneven packing of rubber compound and inconsistent lip thickness), and the rubber compound will spread around the skeleton during loading to avoid material entrapment between the skeleton and the cavity;

Closing speed and pressure: close the mold at a low speed (speed ≤5mm/s), and gradually increase the closing pressure (the final pressure is adjusted according to the hardness of the rubber compound, and the Shore 70 rubber compound pressure is 15~20MPa), so as to avoid lip deformation and skeleton displacement caused by instantaneous extrusion of the rubber compound.

(2) Injection molding (suitable for large quantities and high-precision frameless/frameless oil seals)

Injection parameter control: adopt low-pressure and low-speed injection (injection pressure is 80~120MPa, injection speed is 5~10mm/s), especially when filling the lip and spring groove, it is necessary to reduce the speed to avoid cavity damage and oil seal structure deformation caused by high-speed impact of rubber compound; The barrel temperature is controlled by stages (NBR: 60 ~ 80℃, FKM: 90 ~ 110℃) to avoid scorching the rubber compound in advance.

Gate design: The gate is set in the non-sealing part of the oil seal (such as the outer diameter end face and the back of the frame) to avoid the influence of gate residue on the precision of the sealing surface, and the gate size matches the flow characteristics of the rubber compound to prevent the rubber compound from being under-filled or over-flashed.

3. Vulcanization link: When accurately controlling temperature, avoid aging and deformation of sealing surface.

Vulcanization directly determines the degree of cross-linking of oil seal compound. Insufficient cross-linking will lead to poor elasticity and poor oil resistance of the compound. Overvulcanization will lead to embrittlement of the compound and hardness of the lip, which will affect the sealing effect. It is necessary to ensure the vulcanization fit between the compound and the skeleton of the skeleton oil seal.

Temperature and uniformity: the temperature deviation of the heating plate of the vulcanizer is ≤ 1℃, and the temperature of all parts of the mold cavity is consistent, so as to avoid the over-vulcanization of the oil seal caused by local high temperature (such as hard lip) and insufficient vulcanization caused by low temperature; Vulcanization temperature of different compounds: NBR 140~160℃, FKM 180~200℃, ACM 160~170℃.

Vulcanization time: according to the thickness of the thickest part of the oil seal (not the average thickness), the NBR oil seal with the conventional thickness of 2~5mm is vulcanized for 3~5min, and the FKM oil seal is vulcanized for 5~8min. For every 1mm increase in thickness, the vulcanization time increases for 1 ~ 2 min. The vulcanization time of the skeleton oil seal is appropriately prolonged by 5-10% to ensure that the rubber compound and the skeleton are fully vulcanized and bonded.

Secondary vulcanization (for FKM/ACM oil seal): The fluororubber and acrylate rubber oil seals need secondary vulcanization to remove low-molecular-weight volatiles (to avoid volatiles from polluting the sealing surface during use). Parameters: FKM 200~230℃/4~6h, ACM 180 ~ 200℃/2 ~ 4h; During secondary vulcanization, the oven is well ventilated, and the temperature rises gradually to avoid the deformation of the oil seal lip due to thermal expansion and cold contraction.

Timing of demoulding: after vulcanization, demoulding shall be carried out when the mold temperature drops below 80℃ to avoid thermal deformation of oil seal caused by high-temperature demoulding (especially in lip parts, the rubber compound is elastic at high temperature and is easy to be permanently deformed by stress).

IV. Post-treatment: focus on trimming+sealing surface protection, no damage or residue.

The core of post-treatment is to remove flash and burr, clean the sealing surface and protect the lip/cutting edge from being damaged. This is the key process of oil seal processing, which directly affects the final sealing performance. It is strictly forbidden to grind and scrape the sealing surface.

Flash processing

Sealing surface (lip, cutting edge, joint surface): using freezing trimming (liquid nitrogen temperature-196℃, time 1~2min), using rubber compound to embrittle at low temperature to remove flash, so as to avoid lip scratch, missing corner and uneven thickness caused by manual trimming; After freezing trimming, check the lip, without burr and flash residue, and keep the cutting edge sharp and without deformation;

Unsealed parts (such as outer diameter and back of skeleton): manual trimming or grinding with grinding wheel can be used, and the edge after trimming is rounded (R0.1~0.2mm) to avoid sharp edges from cutting the rubber compound.

Surface cleaning and protection

Wipe the sealing surface of the oil seal with absolute ethyl alcohol or special cleaning agent after trimming to remove the residue of release agent, dust and rubber crumbs, so as to avoid the residual substances from forming an isolation layer and affecting the joint between the oil seal and the sealing mating surface; Wipe gently with a soft non-woven fabric to avoid scratching your lips;

A small amount of special silicone oil can be applied to the lip of the oil seal (food-grade/industrial-grade is matched according to working conditions), which plays a role in lubrication and protection and prevents lip adhesion and aging during storage and transportation; It is strictly forbidden to smear grease such as butter and engine oil that can easily pollute the sealing surface.

Setting and storage

The post-treated oil seal is shaped at room temperature for 24 hours to reduce the internal stress of vulcanization and avoid uneven dimensional shrinkage; The setting environment is dry and dust-free to avoid moisture absorption and pollution of the oil seal;

When stored, a special material box is used, and the oil seal is placed in a single layer with the lip facing upward, so as to avoid the deformation of the lip caused by heavy pressure and stacking, and the skeleton oil seal prevents the metal skeleton from bumping and rusting.

V. Precautions for exclusive processing of skeleton oil seal

Skeleton oil seal is a composite product of rubber compound and metal skeleton. The core needs to ensure the adhesive strength between rubber compound and skeleton to avoid degumming during use. Pay extra attention to the following three points:

Accurate skeleton positioning: the clearance between the skeleton positioning groove and the skeleton in the mold is ≤0.01mm, and the skeleton is completely embedded in the positioning groove when the mold is installed, without inclination or deviation, so as to prevent uneven thickness when the rubber material wraps the skeleton;

Full filling of rubber compound: the gap between the skeleton and the cavity needs to be fully filled with rubber compound, without glue and gaps, otherwise it will lead to insufficient bonding area and increase the risk of degumming;

Avoid skeleton damage: avoid collision and deformation of skeleton during processing, and the phosphating layer and gluing layer are free from scratch and falling off, otherwise the adhesion between rubber and skeleton will be directly reduced.

Six, quality inspection: all the inspection, focusing on the key indicators of control and sealing.

The oil seal inspection needs to cover the size, appearance, physical performance and sealing performance, and each batch is sampled, and the unqualified products are strictly prohibited from flowing out. The core inspection items and requirements are as follows:

Dimension detection: use the second dimension and micrometer to detect the inner diameter, outer diameter, lip thickness and spring groove size of the oil seal. The tolerance meets the drawing requirements (high-precision oil seal ≤±0.02mm), and the lip thickness is even (deviation ≤0.01mm), so as to avoid poor installation and excessive sealing gap caused by out-of-tolerance dimensions;

Appearance inspection: there are no bubbles, lack of glue, scratches and burrs on the sealing surface, no degumming and gaps between the glue and the skeleton, no deformation and corner missing of the lip/cutting edge, and no obvious flash and cracks on the unsealed parts;

Physical property testing: the Shore hardness (NBR 60 ~ 80, FKM 70 ~ 90), tensile strength, elongation at break and oil resistance (according to GB/T 1690) were sampled. After oil resistance, there was no obvious swelling and hardening (swelling rate ≤ 5%). The adhesive strength (peeling strength ≥5kN/m) of the skeleton oil seal is tested, and there is no degumming;

Sealing performance test: the product oil seal is subjected to static pressure sealing test (1.2~1.5 times the service pressure), and there is no leakage after holding the pressure for 30min; The resilience of the lip is detected, and it quickly recovers to its original state without permanent deformation after pressing.

Seven, common problems and targeted solutions

Common problems in oil seal processing are directly related to sealing performance, so it is necessary to quickly trace the reasons and adjust the process. The core problems and solutions are as follows:

Solutions to the core causes of common problems

Lip deformation, uneven thickness, too fast clamping speed, skeleton deviation, and premature demoulding reduce the clamping speed, calibrate the skeleton positioning groove, and demould after cooling.

Air bubbles on the sealing surface, poor exhaust of the rubber-lacking mold, insufficient weighing of the rubber, and uneven loading optimize the exhaust slot, accurately weigh the rubber, and evenly spread the material according to the cavity.

Skeleton and rubber degumming skeleton are not clean in oil removal, the rubber coating falls off, and the vulcanization is insufficient. Strengthen the ultrasonic degreasing of skeleton, check the quality of rubber coating, and extend the vulcanization time.

Oil leakage (detection/use) of sealing surface, lip scratch, residual flash and poor elasticity of rubber compound. Freeze trimming is adopted, sealing surface is cleaned, and vulcanization parameters are calibrated to ensure full crosslinking of rubber compound.

Out-of-tolerance, uneven shrinkage, mismatched curing shrinkage of rubber compound and insufficient setting time. The mold design considers the shrinkage of rubber compound and extends the setting time to more than 24 hours.

VIII. Process Aids: Requirements for Equipment, Environment and Accessories

Specialization of equipment: the mixer, mold and vulcanizer for processing oil seals are recommended to be dedicated to avoid cross-contamination with other rubber products (such as silicone oil and impurities), and the equipment should be calibrated regularly for accuracy (such as vulcanizer temperature and clamping pressure);

Production environment: It is recommended that the class 100,000 clean workshop should have an ambient temperature of 20~30℃ and a humidity of ≤60% to avoid dust and fibers adhering to the sealing surface of the oil seal, which will affect the bonding effect;

Matching of accessories: water-based release agent or Teflon release agent is used as the release agent, and oily release agent is strictly prohibited (it is easy to remain on the sealing surface, leading to oil leakage); Cleaning agent and silicone oil should be compatible with oil seal compound to avoid chemical reaction leading to aging of compound.