What should be paid attention to when processing conductive rubber products?

The core of processing conductive rubber products is to ensure stable conductivity, physical properties up to standard, and controllable molding accuracy. It is necessary to control the whole process from raw material pretreatment, molding process, vulcanization control, post-treatment to finished product inspection, and at the same time avoid the core problems such as uneven dispersion of conductive fillers, poor interface contact, product deformation, etc. The following are key points for attention in each link, covering core processes such as mixing, molding, vulcanization and post-treatment, and adapting to mainstream processing methods such as molding, extrusion and injection.

First, raw material pretreatment and mixing: the core ensures the uniform dispersion of conductive fillers.

The conductivity of conductive rubber is determined by the dispersion of conductive fillers (carbon black, silver powder, nickel powder, carbon fiber, etc.), which is the basis and directly affects the conductive stability of finished products.

Storage of raw materials: conductive fillers (such as silver powder and nano-carbon black) are easy to absorb moisture and oxidize, and need to be sealed and stored in a dry and ventilated environment (humidity ≤60%, temperature 20 ~ 25℃); Raw rubber (NBR, silicone rubber, fluororubber, etc.) is recycled and softened in advance to avoid the influence of hard rubber blocks on mixing and dispersion.

Cleaning of mixing equipment: Mixers (open mill and internal mixer) should be thoroughly cleaned to avoid the mixing of ordinary rubber materials and impurities, otherwise "insulation points" will be formed and the continuity of conduction will be destroyed; There is no oil stain and rust on the equipment roller and internal mixing room. It is suggested that special equipment be used to process conductive rubber.

Mixing process control

The feeding sequence is soft first, then hard, then less, and then more: first, the raw rubber is plasticated until it softens, then additives such as plasticizer and antioxidant are added, and finally conductive fillers are added in batches (to avoid agglomeration caused by one-time feeding).

Control the mixing temperature: the mixing temperature of silicone rubber is ≤60℃ and that of nitrile rubber is ≤80℃. If the temperature is too high, the raw rubber will be burnt and the conductive filler will be oxidized (for example, the silver powder will lose its conductivity when oxidized at high temperature).

Control the mixing time: take the uniform dispersion of filler as the standard to avoid the molecular chain breakage of raw rubber caused by excessive mixing and reduce the physical strength of products; The roll gap of the open mill is gradually narrowed, and the dispersion is improved by several thin passes.

Rubber compound parking: the mixed rubber compound should be parked at room temperature for 4-8 hours, so that the filler and raw rubber can be fully infiltrated, reducing internal stress and avoiding material shortage and cracks during molding; Sealed during parking to prevent moisture absorption of filler and rubber pollution.

Second, the molding process: avoid the damage of the conductive structure and ensure the accuracy of the product.

According to the product structure (sealing ring, keys, conductive strips, etc.), die pressing, extrusion and injection molding are selected. The core is to reduce the orientation and agglomeration of conductive fillers during the flow of rubber compound and ensure the accurate molding size.

Molding (suitable for special-shaped and sealed products)

Mold design: the surface of the mold cavity is smooth (Ra≤0.8μm) to reduce the flow resistance of rubber compound; Set a reasonable gate and vent (the depth of vent is 0.03~0.05mm) to avoid bubbles and material shortage in the product caused by gas storage in the cavity, and bubbles will form insulation gaps and reduce the conductivity.

Rubber weighing and molding: accurate rubber weighing, avoiding too thick flash caused by too many materials and lack of rubber caused by too few materials; When filling the mold, the compound is spread evenly to avoid the packing agglomeration caused by local extrusion. For complex cavities, the flow distance of the compound is reduced by subsection charging.

Clamping speed: the clamping speed is moderate, too fast will lead to instant extrusion of rubber compound, and the filler orientation is serious, too slow will lead to early vulcanization of rubber compound; The clamping pressure shall be based on filling the cavity without obvious flash.

Extrusion molding (suitable for strip and tubular conductive products)

Extruder screw: low shear screw is used to prevent high shear force from breaking conductive filler (such as carbon fiber) and preventing local overheating and scorching of rubber compound; The screw speed is controlled at 10~30r/min. If the screw speed is too fast, the filler will be oriented and the surface of the product will be rough.

Die design: the die size is matched with the finished product size, and the vulcanization shrinkage of the rubber compound (2-4% for silicone rubber and 1.5-3% for NBR) is considered; The surface of the die is smooth, which reduces the flow resistance of rubber compound and avoids the accumulation of filler at the die.

Traction speed: the traction speed is synchronized with the extrusion speed, so as to avoid stretching or compression of the product. Stretching will lead to the fracture of conductive filler, while compression will lead to the aggregation of filler, which will destroy the continuity of conductivity.

Injection molding (suitable for high-precision and large-scale conductive products)

Injection pressure and speed: low pressure and low speed injection, high pressure and high speed will lead to violent flow of rubber, serious orientation of conductive filler, and even separation of filler and rubber; The injection pressure is adjusted according to the thickness of the product, the pressure of thin-walled products is appropriately increased, and the pressure of thick-walled products is gradually increased.

Drum temperature: the drum temperature is controlled by stages, and gradually increases from the feed inlet to the nozzle, and the overall temperature is 5~10℃ lower than the mixing temperature, so as to prevent the rubber from vulcanizing in the drum in advance.

3. Vulcanization process: control temperature and time to ensure full and uniform vulcanization.

Vulcanization is the key to conductive rubber molding. Insufficient vulcanization will lead to poor physical properties and unstable conductive properties of products. Overvulcanization will lead to aging of raw rubber and oxidation of fillers, which will affect the quality of finished products.

Vulcanization temperature control

Temperature uniformity: the temperature deviation of the heating plate of the vulcanizing machine (flat vulcanizing and vulcanizing tank) is ≤ 2℃, and the temperature deviation in the vulcanizing tank is ≤ 3℃. Local high temperature will lead to over-vulcanization, while low temperature will lead to under-vulcanization.

Calibration of actual temperature: calibrate the temperature of vulcanizing equipment regularly to avoid the discrepancy between the temperature displayed by the instrument and the actual temperature; For thick-walled products (thickness > ＞10mm), step heating vulcanization is adopted, first preheating at low temperature, and then gradually rising to the set temperature, so as to avoid internal rubber vulcanization lagging behind the surface.

Vulcanization temperature of different rubber compounds: silicone rubber vulcanization temperature is 160~180℃, nitrile rubber vulcanization temperature is 140~160℃, fluororubber vulcanization temperature is 180~200℃, and when the conductive filler is silver powder, the vulcanization temperature should not exceed 180℃ to prevent silver powder from oxidation.

Vulcanization time control

Determine the time according to the thickness of the product: follow the principle of "extending the time for thick-walled products and shortening the time for thin-walled products", and vulcanize the products with conventional thickness (2~5mm) for silicone rubber for 3~5min and NBR for 5 ~ 8 min; For every 1mm increase in thickness, the curing time increases by 1~2min.

Avoid early demoulding: premature demoulding will lead to insufficient vulcanization of products, which is difficult to make up for after secondary vulcanization, and will also lead to deformation and sudden drop of electrical conductivity of products.

Secondary vulcanization (suitable for silicone rubber and fluororubber conductive products)

Silicone rubber products need secondary vulcanization (200~220℃ for 2~4h) to remove low molecular volatiles and improve the heat resistance, physical properties and conductive stability of products. Ensure that the oven is well ventilated during secondary vulcanization, and avoid the surface pollution of products caused by volatile accumulation.

The secondary vulcanization temperature is gradually increased to avoid product deformation and filler oxidation caused by instantaneous high temperature.

Fourth, post-treatment: reduce the damage of products and ensure the clean conductive interface.

The core of post-treatment is to remove flash and burr, clean the surface of the product, avoid mechanical damage and destroy the conductive structure, and control the deformation of the product at the same time.

Flash processing

Freezing trimming (liquid nitrogen temperature-196℃) is adopted, which is suitable for special-shaped and high-precision conductive products, and avoids corners and scratches caused by manual trimming and grinding wheel trimming, as well as the fracture of internal conductive filler caused by mechanical force; Freeze trimming time should be controlled within 1~3min to avoid embrittlement of rubber compound caused by too long time.

Manual trimming is only suitable for simple products. When trimming, use a sharp blade to gently scrape off the flash along the edge of the product to avoid scratching the surface of the product with excessive force, which will lead to poor conductive contact (such as conductive keys).

Surface cleaning

There is no oil stain, dust and release agent residue on the surface of the product, otherwise an insulating layer will be formed, which will affect the conductive contact performance; Wipe the surface gently with anhydrous ethanol and isopropanol, and dry it naturally after wiping (to avoid oxidation of filler caused by high temperature drying).

It is forbidden to use release agent containing silicone oil, which will seriously affect the conductivity. Talcum powder and water-based release agent can be used (it needs to be verified in advance that it has no effect on the conductivity).

Setting and storage

The post-treated products are set at room temperature for 24 hours to reduce internal stress and avoid deformation; The setting environment is dry and free of impurities, so as to avoid moisture absorption and pollution of products.

Avoid heavy pressure when products are stacked, and prevent the easily deformable parts such as conductive strips and keys from being crushed and destroying the internal conductive structure.

5. Process assistance: auxiliary materials, equipment and environmental requirements

Selection of auxiliary materials: all additives (plasticizer, antioxidant and vulcanizing agent) should be compatible with conductive filler and raw rubber to avoid chemical reaction leading to filler failure and poor cross-linking of rubber; Give priority to the selection of low-volatility, non-insulating additives to reduce the impact on electrical conductivity.

Equipment maintenance: the processing equipment is maintained regularly, and the rollers, molds and screws are not worn or deformed. The worn parts will lead to uneven local stress of the rubber compound and poor dispersion of the filler; The equipment is well grounded, so as to avoid the conductive filler from adsorbing impurities due to static accumulation.

Production environment: the production environment is clean, dry and dust-free (ten thousand/one hundred thousand cleanliness is recommended, suitable for high-precision conductive products), and dust and fiber impurities will be mixed into the rubber compound to form insulation points; The ambient temperature is controlled at 20~30℃ and the humidity is ≤60%, so as to avoid moisture absorption of the rubber compound and oxidation of the filler.

6. Quality inspection: control the whole process to avoid the outflow of unqualified products.

Conductive rubber testing needs to pay attention to both conductive properties and physical properties, and the core testing items and requirements:

Conductivity: detect volume resistivity and surface resistivity (according to GB/T 1672-2008), and the resistivity deviation of different parts of the same product is ≤ 10% to ensure uniform conductivity; For conductive products (such as keys), check the contact resistance to ensure stable contact without sudden jump.

Physical properties: Test Shore hardness, tensile strength, elongation at break and compression set (according to GB/T 528-2009 and GB/T 7759-2015). If the physical properties are not up to standard, the products will be deformed and cracked in use, which will indirectly affect the electrical conductivity.

Appearance and size: The product has no bubbles, lack of materials, cracks and flash, and the dimensional deviation meets the requirements of the drawings (the dimensional tolerance of high-precision products is ≤±0.05mm). The dimensional deviation will lead to poor installation and false contact.

Batch sampling: every batch of rubber compound and products should be sampled. The rubber compound should be tested for filler dispersity (which can be observed by slicing microscope), and the products should be tested for electrical conductivity and physical properties. If problems are found, the process should be adjusted in time.

Seven, special types of conductive rubber processing additional considerations

Silver-based conductive rubber: Silver powder is expensive, easy to be oxidized and easy to agglomerate. It needs to be added in batches, and the vulcanization temperature is ≤180℃. After-treatment, the surface is prevented from scratching (silver powder is easy to be oxidized when exposed), and it is sealed and protected from light when stored.

Carbon fiber conductive rubber: carbon fiber is easy to be cut and broken. Low-shear equipment is used in mixing and extrusion to avoid high pressure and high speed, reduce the flow distance of rubber compound during molding, and prevent the fracture of carbon fiber from affecting the conductivity.

Compound conductive rubber: When compounding multiple fillers (such as carbon black+silver powder, carbon fiber+nickel powder), it is necessary to premix different fillers evenly before adding raw rubber to avoid delamination and agglomeration of different fillers.

Eight, common problems and solutions

Solutions to the core causes of common problems

The mixing process is optimized for uneven conductivity, high resistivity, poor dispersion of fillers, bubbles and lack of materials, and the number of thin passes is increased; The mold is provided with an exhaust slot, and the mold is evenly loaded.

There are impurities, mold release agent residues and scratches on the contact resistance surface, so clean the surface and replace the mold release agent; Using freezing trimming to avoid scratches

The product is deformed, the size is out of tolerance, the internal stress of vulcanization is large, and the post-treatment pressure prolongs the setting time, so step vulcanization is adopted; Stack without heavy pressure, and then package after setting.

Poor physical properties, easy cracking, insufficient vulcanization, excessive mixing and calibration of vulcanization temperature and time to avoid scorching of rubber compound; Control the mixing time without breaking the bad molecular chain.