How to ensure the dimensional accuracy in the processing of rubber and plastic dust cover?

In order to ensure the machining dimensional accuracy of rubber-plastic dust cover, we need closed-loop control from five key links: material control, mold design compensation, precise control of molding process, post-processing and shaping, and whole-process detection, so as to solve the core problems of rubber-plastic material easy to shrink and deform. The specific measures are as follows:

Precise control of materials to reduce size fluctuation from the source.

Unified material grade and shrinkage parameters: The shrinkage of different grades of rubber (such as NBR and FKM) or plastics (such as PU and PA) is quite different, so it is necessary to purchase the same grade of raw materials regularly and test its molding shrinkage in advance (the shrinkage of rubber is usually 1.5%-3.5% and that of plastics is 0.5%-2.0%) as the core basis of mold design.

Strict pretreatment to eliminate internal stress of materials: rubber compound should be allowed to stand for 24 hours at * * 25 2℃ * * in advance to eliminate internal stress generated during storage and transportation; Plastic particles need to be dehumidified and dried (for example, the drying temperature of PA material is 100-120℃ for 4-6 hours), so as to avoid bubbles and shrinkage cavities after molding caused by moisture content in the material, which indirectly affects the dimensional stability.

Avoid material pollution and impurity mixing: raw materials should be stored in sealed containers, and impurity particles should be removed by sieving before feeding, so as to prevent uneven filling of mold cavity caused by impurities and local size deviation.

Optimize mold design and processing, and reserve shrinkage compensation allowance.

Die accuracy and shrinkage compensation design: the machining accuracy of die cavity and core should be controlled within * **±0.02mm** *, and the surface roughness Ra should be less than 0.8 μ m; According to the measured shrinkage of the material, the compensation amount is reserved in the die design (for example, if the shrinkage of the material is 2%, the die size needs to be enlarged by 2% compared with the drawing size of the product), and the key dimensions such as sealing lip and inner diameter are mainly compensated.

Optimization of mold cooling and exhaust system: mold cooling channels should be evenly distributed around the cavity, and the cooling water temperature should be controlled at 20-30℃ to ensure the consistent cooling rate of all parts of the product and avoid uneven shrinkage and warping deformation caused by local temperature difference; In the corner of dust cover, sealing lip and other parts that are easy to accumulate gas, open an exhaust slot with a width of 0.03-0.05mm and a depth of 0.1-0.2mm to prevent dimensional deviation caused by lack of materials and bubbles.

Accurate mold positioning and guidance: the mold should be equipped with high-precision guide posts and guide sleeves, and the fit clearance should be ≤0.01mm to prevent the cavity from being misaligned during mold closing and ensure the dimensional consistency of each molding.

Precise control of molding process parameters to reduce deformation during molding.

Control of rubber molding parameters

Vulcanization temperature: set according to the type of rubber compound (NBR is 150-160℃, FKM is 170-180℃), and the temperature deviation is controlled within 2℃. If the temperature is too high, the material will be excessively crosslinked and the shrinkage will increase; if the temperature is too low, the vulcanization will be incomplete and the dimensional stability will be poor.

Vulcanization pressure: maintain a stable pressure of 10-20MPa, and the pressure acts on the cavity evenly to avoid uneven thickness of the sealing lip; The holding time should be adjusted according to the product thickness (the holding time will be extended by 1-2min for every 1mm increase in thickness) to ensure that the material can fully fill the cavity.

Timing of demoulding: it is necessary to wait until the mold is cooled below 60℃ before demoulding, so as to prevent the high-temperature products from springback due to elastic deformation.

Parameter control of plastic injection molding

Temperature subsection control: the temperature of the barrel is gradually reduced from the rear section to the front section (for example, 220℃ in the rear section, 200℃ in the middle section and 180℃ in the front section of PU material) to avoid material degradation; The mold temperature is controlled at 40-60℃ to reduce the difference of product cooling shrinkage.

Injection and pressure maintaining parameters: the injection pressure is 80-120MPa, and the injection speed is controlled by stages of "slow-fast-slow" (initial slow filling of the gate, rapid filling of the cavity in the middle section and slow compaction at the end); The holding pressure is 50%-70% of the injection pressure, and the holding time is 3-8s, which makes up the shrinkage allowance of the cavity material and prevents the product from being sunken and small in size.

Specification for demoulding operation: use soft demoulding tools (such as silicone scraper and nylon thimble) to avoid scratching or squeezing the product; The release agent should be a special rubber-plastic release agent with uniform Bo Tu, so as to prevent excessive release agent residue from causing product surface adhesion or size deviation.

Standardize the post-treatment process, eliminate internal stress and stabilize the size

Deburring and trimming: Accurately control the processing capacity: use frozen deburring (temperature-70~-50℃) or manual fine grinding. When grinding, use fine sandpaper with 800 mesh or more, and focus on the gate and flash. The grinding capacity should be controlled within 0.01-0.03mm to avoid insufficient sealing lip thickness caused by excessive grinding.

Secondary setting treatment: after the rubber product is demoulded, it can be put into a constant temperature oven (temperature 80-100℃ for 2-4 hours) for secondary vulcanization to eliminate internal residual stress and further stabilize the size; Plastic products can be aged (kept at a constant temperature of 25℃ for 24 hours), and the size can be naturally set before testing.

Avoid post-treatment pollution and deformation: during post-treatment, products should be placed in a single layer, and stacking and extrusion are strictly prohibited to prevent warping and deformation.

Whole process inspection and environmental control to ensure the consistency of precision.

Establish a multi-level detection mechanism

First article inspection: Before each batch of production, the first article is selected and the key dimensions (inner diameter, outer diameter, sealing lip thickness and warpage) are detected with a two-dimensional image measuring instrument, and then batch production is carried out after it is confirmed that it meets the requirements of the drawings.

Process sampling: for every 50-100 pieces produced, sample 3-5 pieces, monitor the trend of dimensional fluctuation, and immediately adjust molding parameters (such as adjusting holding time and mold temperature) if there is deviation.

Full inspection of finished products: all finished products shall be screened for size, the warpage shall be ≤0.5mm/m, and the tolerance of key dimensions shall be controlled within 0.05 mm..

Control the detection and the temperature and humidity of the production environment: the detection should be carried out at a constant temperature and humidity of 23 2℃ and 50% 5% to avoid the detection error caused by thermal expansion and cold contraction of rubber and plastic materials; Keep the temperature and humidity stable in the production workshop to reduce the influence of environmental changes on product size.