What should be paid attention to in injection molding of precision rubber and plastic components?

Key points for attention in injection molding of precision rubber and plastic components

The core requirements of injection molding of precision rubber and plastic components are stable dimensional accuracy, defect-free appearance and up-to-standard performance, which need to be strictly controlled from five dimensions: raw material control, mold design and maintenance, process parameter optimization, equipment guarantee and quality inspection. The specific matters are as follows:

First, raw material pretreatment and control

Raw material drying treatment

Rubber and plastic materials (especially hygroscopic materials such as nylon, PET and PC) must be fully dried before injection molding, otherwise it will lead to defects such as bubbles, crazing and brittle cracks. Drying parameters should be set according to material characteristics: the temperature is usually 80-120℃ for 2-4 hours, and the moisture content of raw materials after drying should be controlled within 0.02%-0.05%. The dried raw materials should be sealed and preserved to avoid secondary moisture absorption.

Mixing and recycling of raw materials

It is forbidden to mix rubber and plastic raw materials with different brands and materials, and it is necessary to store them in different zones. If additives (such as antioxidants and release agents) need to be added, they need to be evenly mixed with high-precision mixing equipment to ensure the dispersibility of additives.

The proportion of recycled scraps should be strictly controlled (usually not more than 20%-30%), and the scraps should be crushed to a uniform particle size to avoid affecting the melt fluidity and product accuracy.

Raw material performance verification

The incoming materials shall be tested for key indexes such as melt flow rate (MFR/MVR), density and tensile strength, so as to ensure matching with the production process and avoid product quality fluctuation due to batch differences of raw materials.

Second, the mold design, manufacture and maintenance

Mold is the core factor that determines the dimensional accuracy of precision rubber and plastic components, and the following points should be paid attention to:

High precision mold design

Machining accuracy of cavity and core: CNC machining center, EDM, wire cut and other high-precision equipment are required to ensure that the dimensional tolerance of cavity is controlled within ±0.005-0.01mm, and the surface roughness Ra is less than or equal to 0.4 μ m, so as to reduce the demoulding resistance and appearance defects of products.

Design of gating system: hot runner system is preferred to reduce the loss of material head and ensure the uniform filling of melt; The gate position should avoid the key size area of the product to prevent gate residue and stress concentration; The cross section of the runner needs smooth transition to avoid melt retention and degradation.

Optimization of cooling system: Adopt conformal cooling water channel design to ensure uniform temperature of mold cavity (temperature difference ≤2-3℃) and reduce warping deformation caused by uneven cooling of products; For complex structural parts, water-proof sheets can be added or insert cooling can be adopted.

Mould material and surface treatment

Wear-resistant and corrosion-resistant steel (such as S136, H13, etc.) should be selected for the mold cavity, and nitriding treatment or hard chromium plating should be carried out to improve the surface hardness and service life of the mold and reduce the adhesion of rubber and plastic melt.

Daily maintenance of mould

Before each production, it is necessary to clean up the residual materials and impurities in the cavity and runner, and check the lubrication of the guide post and guide sleeve to avoid the dimensional deviation of the product caused by die wear.

Regularly check the size of mold cavity, and repair or replace spare parts in time if the wear is out of tolerance; When the mold is idle, it should be coated with antirust oil to prevent moisture and dust.

Third, accurate control of injection molding process parameters

Precision injection molding is extremely sensitive to process parameters, so it is necessary to optimize parameters through orthogonal test to ensure stability:

Temperature parameter

Cylinder temperature: it needs to be set in stages (feeding stage → melting stage → homogenization stage), and the principle of "gradually increasing temperature" should be followed, so as to ensure the full melting of raw materials and avoid high-temperature degradation. For example, the barrel temperature of PP material is usually 180-220℃, and that of PC material is 260-300℃.

Nozzle temperature: 5-10℃ slightly lower than the temperature of the homogenization section of the barrel to prevent the melt from salivating.

Mold temperature: according to the material characteristics, rigid materials (such as PC and POM) need higher mold temperature (60-120℃) to improve the crystallinity and dimensional stability of products; The mold temperature of flexible materials (such as TPU and TPE) can be appropriately reduced (20-50℃).

Pressure and velocity parameters

Injection pressure: It can be divided into injection pressure and holding pressure. The injection pressure should be enough to push the melt to fill the cavity (usually 80-150MPa), and the holding pressure should be 40%-60% of the injection pressure. The holding time should cover the solidification stage of the product to compensate the melt shrinkage and avoid shrinkage marks and size shrinkage.

Injection speed: adopt "slow-fast-slow" subsection control, prevent melt injection at initial low speed, ensure rapid filling of cavity at middle high speed, and reduce stress concentration at end low speed; For thin-walled precision parts, it is necessary to increase the injection speed to avoid premature solidification of the melt.

Cooling time

The cooling time should be determined according to the thickness of the product and the thermal properties of the material, and the product is completely solidified and does not deform after demoulding, usually 10-60 seconds; Insufficient cooling will lead to warping of products, while too long cooling will reduce production efficiency.

IV. Performance Guarantee of Injection Molding Equipment

Equipment accuracy requirements

High-precision injection molding machine should be selected, and the repeatability of clamping force should be ≤ 1%, and the repeatability of injection quantity should be ≤ 0.5%. Equipped with servo control system to realize accurate closed-loop control of pressure and speed and reduce process fluctuation.

Daily maintenance of equipment

Check the wear of the barrel and screw regularly. If the screw is seriously worn, it needs to be replaced in time to avoid uneven plasticization of the melt.

Maintain hydraulic system and temperature control system to ensure stable pressure and accurate temperature, and prevent products from being scrapped due to equipment failure.

Five, product post-treatment and quality inspection

Post-treatment of products

For materials that are prone to internal stress (such as PC and ABS), it is necessary to anneal. Put the products in an oven, keep the temperature at 50-80℃ for 1-2 hours, and slowly cool them to room temperature to eliminate internal stress and improve dimensional stability.

When removing the gate residue, it is necessary to use precision grinding or ultrasonic cutting to avoid damaging the surface and size of the product.

Whole process quality inspection

On-line inspection: visual inspection equipment is used in the production process to monitor the appearance defects of products (such as flash, lack of materials and bubbles) in real time; Use calipers and micrometers to detect key dimensions to ensure that they meet the requirements of drawings.

Off-line inspection: the mechanical properties (tensile strength, bending strength, impact strength), heat resistance and corrosion resistance of the products are sampled to verify whether they meet the use requirements; For high-precision components, it is necessary to carry out dimensional inspection in a constant temperature and humidity environment (temperature 23 2℃, humidity 50 5%) to eliminate the influence of ambient temperature on the measurement results.

VI. Environmental control

The production workshop should be kept clean, at constant temperature and humidity to avoid dust and impurities entering the mold cavity and causing product defects; At the same time, the temperature fluctuation in the workshop should be controlled within 2℃ to prevent the mold and equipment from thermal deformation due to temperature change and affect the machining accuracy.