After extrusion, injection molding, surface treatment and other procedures, medical devices often need to undergo secondary molding treatment. At present, common secondary processing technologies of medical plastic products include connection, assembly, drilling, thermoforming, printing, laser engraving, surface modification, tube end molding, tube butt welding, etc.
1. Connection
Adhesive connection
Adhesive to the surface of plastic parts to complete the connection, which is a widely used bonding method. It is characterized by simple process, continuous stress distribution, light weight, or sealing, and low temperature in most processes. Adhesive joint is especially suitable for the connection of different materials, different thicknesses, ultra-thin specifications and complex components. This is a bonding technology with the fastest development and wide application in modern times.
Advantages: uniform stress distribution, maximum fatigue strength, (vibration absorption), good sealing performance.
Disadvantages: limited assembly speed, toxic gas may be generated during operation, uncertain connection strength, difficult recovery, surface treatment.
Solvent connection
The solvent dissolves the surface of the plastic part to mix the materials between the surfaces. When the solvent volatilizes, the connection is formed.
Advantages: low cost, uniform stress distribution, weight reduction;
Disadvantages: limited assembly speed, toxic gas, connection strength uncertainty.
According to the summary of production practice, in order to improve the solution and joint effect, the general principle of selecting mixed solvent is as follows:
1. The boiling points of the two selected solvents should be as close as possible, so that after mixing, the surface temperature of the components with fast volatilization will not be lowered too fast and condensate will be formed at the joint, resulting in poor adhesion or weak adhesion. When the boiling points of the two added solvents are close, this adverse factor will be overcome, and the function of each component of the mixed solvent can be fully exerted and fully bonded. The boiling point of common acetone: 56.5 ° C; the boiling point of tetrahydrofuran: 66 ° C; the boiling point of cyclohexanone: 155.65 ° C.
At present, there are many solvents for PVC bonding, but there are problems of solubility and time. The best solvents are cyclohexanone and tetrahydrofuran. Other trichloromethane, ethyl acetate and acetone can dissolve PVC materials. Most of the solvents listed above are volatile and will produce a pungent smell. Besides, tetrahydrofuran has certain toxicity and the most irritating taste, so it is not commonly used. Cyclohexanone is widely used in the medical industry. Ethyl acetate and acetone are added when necessary to increase the volatility and solubility of the solvent, so that PVC materials can dissolve and / or swell rapidly.
2. When choosing the mixed solvent for bonding different materials, the mixed solvent containing the two adherends should be selected. That is to say, any component of the selected mixed solvent should be able to dissolve both material a and material B, so as to ensure that the mixed solvent composed of these components can firmly bond different materials.
At present, the common connection methods of plastics are bonding, mechanical connection and welding, among which welding is an important one. It has the advantages of high connection strength, good surface continuity, wide application range, simple process, mechanization and high production efficiency, and has been widely used.
Hot melt welding
Use heating plate or heating wire to melt the butt surface of the welded plastic part, and then connect the butt surface through pressure to achieve the purpose of welding firmly. Hot melt welding is commonly used in the manufacture of medical plastic products with round welding surface and medical packaging bags. For example, the mouth of medical plastic infusion bottle, medical plastic or paper plastic composite packaging bag, etc.
High frequency welding
High frequency welding uses thermoplastic to move with the change of electric field due to molecular polarization between high frequency electrodes, and friction occurs between molecules, which transforms electric energy into heat energy, and the plastic itself generates heat until melting, so as to achieve the purpose of connection. High frequency welding is commonly used in the manufacture of blood bags, bow I flow bags, urine bags and other bag medical products and the sealing of some medical packaging bags.
① Advantages: uniform internal heating of welding materials; fast welding speed; beautiful welding surface. For large area welding, the advantage of special-shaped welding is obvious.
② Disadvantages: material limitation - PVC is the main material.
Superwave welding
The principle of ultrasonic welding is to cause high-speed vibration at 20kHz, so that the butt joint surface of plastic and plastic is fused due to friction and heat generation; if it is used for the welding of plastic and metal, the metal can be welded in the plastic butt joint within less than ls. Ultrasonic welding is a new secondary processing technology of plastics, which is developed with its advantages of high efficiency, high quality, beauty and energy saving. Ultrasonic welding has a wide range of application and many methods can be implemented, such as plane welding, riveting, spot welding, inlaying, etc.
Ultrasonic welding has been used in the manufacture of blood dialyzer, plasma collector and other medical products, such as the welding of the cup body and the gland of the plasma single collection centrifugal separator. This welding method is used instead of chemical bonding, which solves the melting phenomenon of the adhesive caused by the high-speed rotation of the centrifugal cup and achieves good results. In addition, this method is often used in the manufacturing process of medical protective mask, protective clothing, infusion set and balloon dilating catheter.
① Advantages: no curing time, clean air, high efficiency.
② Disadvantages: limited by compatibility, thermoplastic, noise, area and shape;
③ Main factors affecting welding effect:
A. For materials with water absorption characteristics, such as nylon, polycarbonate, polysulfone, etc., a treatment shall be carried out before welding to ensure that the products are welded firmly;
B. Place in a sealed environment;
C. Bake another piece before ultrasonic welding;
D. Ultrasonic welding immediately after injection molding.
