How to ensure the dimensional stability of rubber and plastic materials during processing?

To ensure the dimensional stability of rubber and plastic materials during processing, it is necessary to systematically control the four core dimensions of material pretreatment, process parameter control, mold/equipment optimization and post-processing technology, and combine the characteristics of rubber and plastic materials (rubber, plastic) such as thermal expansion and cold contraction, creep and moisture absorption. The following are specific methods:

First, material pretreatment: eliminate the hidden danger of size fluctuation from the source.

Moisture content, internal stress and formula uniformity of rubber and plastic materials are the primary factors affecting dimensional stability, which need to be standardized before processing.

Drying treatment (for hygroscopic plastics)

Applicable materials: Nylon (PA), Polycarbonate (PC), Polyoxymethylene (POM) and other plastics with strong hygroscopicity. Excessive moisture content will lead to shrinkage fluctuation and surface bubbles after processing.

Process parameters: set the drying temperature (for example, PA66: 80-100℃, PC: 120-140℃) for 4-8 hours according to the brand of the material, and control the moisture content below 0.2% after drying; The dried material should be sealed and stored to avoid secondary moisture absorption.

Rubber material: avoid exposure or low-temperature hardening, and store at 23 2℃ for 24 hours before processing to eliminate elastic modulus fluctuation caused by uneven temperature.

Formulation and mixing control

Ensure that fillers (such as calcium carbonate and glass fiber), plasticizers and stabilizers of rubber and plastic materials are evenly mixed, so as to avoid inconsistent shrinkage caused by local component differences (for example, glass fiber reinforced plastics need to control glass fiber dispersion to prevent directional shrinkage).

The amount of vulcanizing agent added to rubber materials should be strictly controlled. Excessive amount will lead to high crosslinking density and increased brittleness, while insufficient amount will lead to incomplete vulcanization and subsequent creep deformation.

Preforming and preheating treatment

Preheat plastic particles before injection/extrusion to reduce the temperature gradient during processing; Rubber materials are pre-pressed before vulcanization to exhaust the air inside the rubber compound, so as to avoid the dimensional deviation caused by pores after vulcanization.

Second, the processing parameters: accurately control the thermal-mechanical coupling effect.

The core of the dimensional change of rubber and plastic materials comes from thermal expansion and cold contraction caused by temperature change and post-shrinkage caused by stress release, and these two effects need to be offset by precise control of processing parameters.

Hierarchical control of temperature parameters

Shooting Technique

Temperature of the barrel: temperature is controlled by sections (such as increasing temperature gradient in the feeding section → melting section → homogenizing section) to avoid material degradation caused by local overheating; Mold temperature should be stable (such as ABS mold temperature 50-80℃), so as to reduce the temperature difference between plastic parts and mold and reduce the non-uniformity of cooling shrinkage.

Cooling time: set according to the wall thickness of the material (the cooling time will be extended for 2-3 seconds for every 1mm increase in the wall thickness), so as to ensure that the plastic parts will be demoulded after completely cooling and avoid elastic recovery deformation caused by early demoulding.

Extrusion/calendering process

The screw speed is matched with the die temperature to prevent melt temperature fluctuation (fluctuation range ≤ 5℃); The temperature of the calendering roller should be uniform (temperature difference ≤3℃) to avoid uneven material thickness caused by temperature difference on the roller surface.

Rubber vulcanization process

Strictly control the curing temperature (such as natural rubber curing temperature of 140-160℃) and curing time to avoid under-sulfur (large shrinkage) or over-sulfur (increased brittleness); Adopt two-stage vulcanization process (such as silicone rubber) to eliminate the residual stress inside the product.

Stable control of pressure and velocity parameters

Injection molding process: Holding pressure and holding time are the key-the holding pressure is 60%-80% of the injection pressure, and the holding time should cover the solidification stage of plastic parts to compensate for the cooling shrinkage of melt; The injection speed is controlled by stages of "slow-fast-slow" to reduce the molecular orientation caused by melt shear stress (orientation will lead to an increase in unidirectional shrinkage).

Rubber molding process: the molding pressure should be uniform (for example, 30-50MPa), and the holding time should be adjusted according to the thickness of the product, so as to ensure that the rubber compound can fully fill the cavity and avoid dimensional deviation caused by lack of materials or flash.

Avoid stress concentration during machining.

When processing plastics, reduce shear heating caused by too fast screw speed and too high back pressure; When rubber is mixed, the roller distance and mixing time are controlled to avoid excessive shearing leading to molecular chain fracture of rubber compound and subsequent deformation.

3. Mould and equipment: Optimize the structural design and improve the processing consistency.

Mold accuracy and equipment stability directly determine the dimensional repeatability of rubber and plastic products, which need to be optimized from two aspects: design and debugging.

Key points of mold design

Shrinkage compensation: according to the theoretical shrinkage of materials (such as PP shrinkage of 1.5%-2.5%, natural rubber shrinkage of 2%-4%), shrinkage allowance is reserved in mold design; For complex products, the shrinkage compensation value (actual shrinkage = (die size-product size)/die size ×100%) needs to be corrected through die trial.

Die structure optimization: adopt uniform wall thickness design (wall thickness difference ≤1:3) to avoid uneven cooling speed caused by sudden change of wall thickness; The surface of the mold cavity is polished to Ra≤0.8μm, which reduces the demoulding resistance and prevents the plastic deformation caused by forced demoulding.

Temperature control system: cooling water channels are built into the mold to ensure the uniform spacing of water channels (the spacing is less than or equal to twice the wall thickness of the mold) and realize the uniform distribution of cavity temperature; Rubber vulcanization mold should be equipped with heating pipes to ensure that the mold temperature deviation is ≤ 2℃.

Equipment precision and stability control

Regularly calibrate the injection quantity and holding pressure accuracy of the injection molding machine, and the error should be controlled within 2%; The clearance between the screw and barrel of extruder should be detected regularly, and replaced in time when the wear exceeds 0.5mm to avoid unstable melt conveying.

Processing equipment should be installed on a solid foundation and equipped with shock-proof pads to avoid mold displacement or wall thickness fluctuation of products caused by vibration.

Fourth, post-treatment process: eliminate residual stress and stabilize the final size.

The internal stress and post-shrinkage of rubber and plastic products after processing are important reasons for the dimensional change, which need to be eliminated by targeted post-treatment technology.

Annealing treatment (for plastic parts)

Put the plastic parts into a constant temperature oven, set the temperature to 60%-80% of the thermal deformation temperature of the material (such as PC: 100-120℃), keep the temperature for 2-4 hours, and then slowly cool to room temperature (cooling rate ≤5℃/h), so as to eliminate the molecular orientation stress generated in the injection molding process and reduce the shrinkage rate (the dimensional change rate after annealing can be controlled within 0.1%).

Post-vulcanization treatment (for rubber parts)

For silicone rubber, fluororubber and other special rubber products, two-stage vulcanization (oven temperature 200-250℃, heat preservation for 2-4 hours) is adopted to remove low-molecular-weight volatiles inside the products, improve the stability of crosslinking density and avoid creep deformation during use.

Rubber products need to be trimmed after vulcanization, and frozen trimming (liquid nitrogen temperature-196℃) is used instead of manual trimming to ensure consistent dimensional accuracy.

Environmental adaptive treatment

The processed rubber and plastic products should be placed in standard environment (temperature 23 2℃, humidity 50 5%) for 24-48 hours, and then tested and assembled after the size is completely stable to avoid temporary size fluctuation caused by environmental temperature and humidity changes.

V. Process Inspection and Quality Control

On-line inspection: the production line is equipped with dimension inspection equipment (such as laser caliper and secondary imager) to monitor the key dimensions (such as thickness and aperture) of the product in real time, and automatically adjust the process parameters (such as mold temperature and holding time) when it exceeds the tolerance range.

Sampling inspection: Sampling by batches, measuring the dimensional change rate of products at normal temperature, high temperature (such as 80℃) and low temperature (such as-40℃) to ensure that the dimensional stability requirements of the use environment are met.

Data traceability: record the batch number, process parameters and size detection data of each batch, establish a database, optimize the process parameters through data analysis, and form a closed-loop control.

summary

The dimensional stability control of rubber and plastic material processing is the whole chain project of "material-process-mold-post-treatment", and the core logic is: eliminating internal unevenness of materials → controlling temperature/pressure fluctuation in the processing process → compensating shrinkage deformation → eliminating residual stress. According to the characteristics of different rubber and plastic materials (such as crystalline plastics, amorphous plastics and vulcanized rubber), the dimensional tolerance can be controlled at a high precision level of 0.1%-0.3%.