What should be paid attention to in the processing of precision rubber and plastic components in the field of sports equipment?

Precise rubber and plastic components in the field of sports equipment (such as shock absorbers, seals, grip sleeves, pulley bushings, etc.) need to meet the requirements of high wear resistance, fatigue resistance, dimensional stability and mechanical properties in sports scenes, so it is necessary to control the material characteristics, process parameters and quality during processing. The following are specific considerations:

I. Precautions for Material Pretreatment and Selection

Material compatibility matching

Choose rubber and plastic materials according to the usage scene of sports equipment;

Damping parts (such as treadmills' damping pads): Choose high elastic EVA and polyurethane (PU), and pay attention to Shore hardness (40-60HA) to avoid losing damping effect if it is too hard or being too soft and easy to deform.

Wear-resistant parts (such as pulley bushing and bicycle pedal sleeve): nylon (PA), polyoxymethylene (POM) or nitrile rubber (NBR) shall be selected, and the wear-resistant coefficient of the materials shall be ≤0.3 (dry friction state).

Outdoor equipment parts (such as trekking pole handles): Weather-resistant materials (such as TPU and silicone rubber) shall be selected, and they shall pass the UV aging test (tensile strength loss ≤10% after 1000 hours of irradiation).

Avoid mixing different brands of materials, such as ordinary rubber and food-grade silica gel, which will lead to performance stratification and affect the safety of use.

Material drying pretreatment

Rubber and plastic materials with strong hygroscopicity (such as PA, PC, ABS) should be dried in a drying oven at 80-120℃ for 2-4 hours before processing, and the moisture content should be controlled below 0.2%, otherwise bubbles and crazing will occur during injection/extrusion, which will lead to the decline of the mechanical properties of the components.

Rubber materials need to be mixed in advance (initial mixing temperature is 50-70℃) to ensure that vulcanizing agent and reinforcing agent are evenly dispersed and avoid local performance differences.

Second, the key control points of molding process

(1) Injection molding (suitable for complex structural parts, such as pulleys and connectors)

Precise control of temperature parameters

Cylinder temperature: adjust according to the material, such as setting the temperature of PU material cylinder to 160-180-190℃ in stages to avoid bubbles generated by high temperature decomposition; Rubber injection molding needs to control the vulcanization temperature (150-170℃). If the temperature is too high, the material will be burnt, and if it is too low, the vulcanization will be incomplete.

Mold temperature: precision components need constant temperature mold (temperature difference ≤ 2℃). For example, if the mold temperature of PU damping pad is controlled at 40-60℃, the dimensional deviation caused by uneven cooling shrinkage can be reduced (shrinkage rate is controlled at 0.5%-1.5%).

Pressure and velocity matching

Injection pressure: high pressure and low speed strategy should be adopted for precision parts, with a pressure of 80-120MPa and a speed of 30-50mm/s, so as to prevent the shear stress generated by high-speed injection from causing molecular orientation of materials and warping and deformation of components.

Holding pressure and time: The holding pressure is 60%-80% of the injection pressure, and the holding time is adjusted according to the wall thickness (the holding time is extended by 2-3 seconds for every 1mm increase in the wall thickness) to ensure that the cavity is full and reduce the shrinkage marks.

Mold and demoulding design

The mold accuracy should be higher than the tolerance level of components (such as component tolerance of ±0.05mm and mold tolerance of ≤±0.02mm), and the cavity surface should be polished to Ra≤0.4μm to prevent mold sticking.

The demoulding inclination is more than or equal to 1 degree, so as to avoid scratching the surface of the component by forced demoulding; For thin-walled parts (wall thickness ≤1mm), it is necessary to install ejector plate buffer device to prevent ejection deformation.

(2) Extrusion molding (suitable for strip-shaped components, such as grip sleeves and sealing strips)

The screw speed is synchronized with the traction speed.

The screw speed is controlled at 20-50r/min, and the traction speed is matched with the extrusion speed (the speed ratio is 1:1.05), so as to avoid the tensile deformation of the assembly caused by too fast traction, and the surface ripple will appear if it is too slow.

The size of the die should consider the shrinkage rate of the material. For example, the die size of the silicone extrusion should be 10%-15% larger than the finished product, and the cooling shrinkage should be compensated.

Cooling setting control

The vacuum setting box is adopted, the cooling water temperature is controlled at 20-30℃, and the water flow is uniform, so as to avoid the internal stress concentration of the module caused by local cooling too fast and cracking in the later stage.

For hollow profiles (such as pipes), it is necessary to control the internal pressure to be stable to prevent uneven wall thickness.

(III) Vulcanization process (core process of rubber components)

Precise control of three elements of vulcanization

Temperature, time and pressure should strictly follow the requirements of material formula, such as vulcanization temperature of butyl rubber is 160℃, time is 15-20min and pressure is 10-15MPa. Insufficient vulcanization will lead to low hardness and poor elasticity. Overvulcanization will make the material brittle and easy to crack.

Adopt two-stage vulcanization process (such as silicone rubber): after one-stage vulcanization (170℃×10min), then two-stage vulcanization at 200℃×4 hours to remove low molecular volatiles and improve the aging resistance of components.

Prevent sulfide pollution

The mold should be clean and free of oil stains, and sulfur-containing release agent should be avoided to prevent frosting on the rubber surface, which will affect the appearance and adhesion.

III. Matters needing attention in post-treatment process

Deburring and finishing

The burr of precision components should be treated with a freezing deburring machine (temperature-70~-120℃) to avoid size deviation caused by manual grinding; Key dimensions (such as the diameter of matching holes) need to be processed by CNC precision carving machine, and the accuracy is controlled within 0.02 mm.

When rubber components are deburred, the cutting tool should be sharp and the edge fillet should be ≤0.1mm to prevent scratching the rubber surface.

Surface treatment and modification

Components to be bonded (such as grip sleeve and metal rod) need to be plasma treated or sanded (roughness Ra1.6-3.2μm) to improve the bonding strength (peeling strength ≥5N/mm).

Parts with high wear resistance requirements can be sprayed with wear-resistant coating (such as PTFE coating), and the coating thickness should be controlled at 5-10μm to ensure that the assembly size will not be affected.

Cleaning and drying

The processed components need to be cleaned with pure water to remove oil stains and debris, and then dried in a hot air drying oven at 60-80℃ to avoid mold or bonding failure caused by residual moisture.

Four, quality inspection and control points

Dimensional accuracy detection

Key dimensions are detected by two-dimensional image measuring instrument and coordinate measuring machine, such as the roundness of the inner diameter of pulley bushing ≤0.01mm and the coaxiality ≤ 0.02 mm.

The sampling ratio follows the GB/T 2828 standard, and the AQL value of precision parts should be ≤0.65 to ensure that the batch qualification rate is ≥99.5%.

Verification of mechanical properties

According to the sports equipment industry standard test:

Wear resistance: Tested by Taber wear tester, the wear amount is ≤ 50mg/1000r;

Fatigue resistance: after 1 million cycles of compression/tension, there is no crack or deformation in the assembly;

Hardness: Tested by Shore Hardness Tester, the error is ≤±2HA.

Environmental reliability test

High and low temperature cycling test (-40℃~80℃, cycling for 50 times) shows that there is no crack or deformation in the component appearance;

Salt spray test (neutral salt spray, 48 hours), the metal insert rubber and plastic components should be free from corrosion and degumming at the bonding place.

V. Precautions for Safety and Environmental Protection

Material compliance

Rubber and plastic components of sports equipment should meet the environmental protection standards of REACH and RoHS, and harmful substances such as phthalates (plasticizers) and heavy metals are prohibited, especially children's sports equipment should pass the EN 71 safety certification.

Processing environment control

The waste gas produced by rubber vulcanization needs to be treated by activated carbon adsorption and catalytic combustion, and then discharged after reaching the standard; Rubber and plastic crumbs need to be classified and recycled to avoid environmental pollution.

summary

The processing core of precision rubber-plastic components of sports equipment is * * "material adaptation+process precision+quality strict control" * *, and the process parameters need to be adjusted according to the functional requirements of the components, and at the same time, the size and performance testing of the whole process should be strengthened to ensure that the products meet the durability and safety requirements in sports scenes.