Medical injection molded parts often require secondary molding after extrusion, injection molding, and surface treatment. Common secondary processing technologies for medical injection molded parts include joining, assembly, drilling, thermoforming, printing, laser engraving, surface finishing, tube end forming, and tube welding.
1. Joining Medical Injection Molded Parts
Adhesive Joining
Adhesive bonding is a widely used method where adhesives are applied to the surface of the plastic part. It features simple processing, continuous stress distribution, light weight, good sealing, and low processing temperatures in most cases. Adhesive bonding is particularly suitable for joining different materials, thicknesses, thinner dimensions, and complex components. This is a rapidly developing and widely applied bonding technology in recent times.
Advantages: Uniform stress distribution, high fatigue strength (vibration absorption); good sealing.
Disadvantages: Limits assembly speed; may generate toxic gases during operation; uncertain connection strength; difficult recycling; surface treatment required.
Solvent Bonding
Solvents dissolve the surface of plastic parts, causing the materials to mix. A bond is formed when the solvent evaporates.
Advantages: Inexpensive, uniform stress distribution, weight reduction;
Disadvantages: Limited assembly speed, toxic gases, uncertain bond strength.
Based on practical production experience, mixed solvents are often used to improve bonding performance. General principles for selecting mixed solvents:
1. The boiling points of the two selected solvents should be as close as possible. This prevents the surface temperature of the faster-evaporating component from dropping too quickly, forming condensate at the joint, leading to poor or weak bonding. When the boiling points of the two added solvents are close, this adverse factor is overcome, allowing each component of the mixed solvent to fully exert its effect and ensuring sufficient bonding. Commonly used solvents: Acetone boiling point: 56.5°C; Tetrahydrofuran boiling point: 66°C; Cyclohexanone boiling point: 155.65°C.
Currently, there are many solvents for bonding PVC materials, but issues with solubility and time remain. Cyclohexanone and tetrahydrofuran are good solvents for PVC materials. Other solvents such as chloroform, ethyl acetate, and acetone can also dissolve PVC materials. Most of the solvents listed above are volatile and produce a pungent odor. Tetrahydrofuran is also highly toxic and has a strong odor, so it is not commonly used. The medical industry often uses cyclohexanone, sometimes adding ethyl acetate and acetone to increase its volatility and solubility, allowing the PVC material to dissolve and/or swell quickly.
2. When choosing a mixed solvent for bonding different materials, a mixed solvent containing both materials should be selected. That is, any component of the selected mixed solvent must be able to dissolve both material A and material B to ensure a strong bond between the different materials.
Currently, common methods for joining plastics include bonding, mechanical joining, and welding. Welding is an important method due to its advantages such as high joint strength, good surface continuity, wide application range, simple process, mechanization, and high production efficiency, leading to its widespread use.
Thermomelt Welding
Thermomelt welding uses a heating plate or heating wire to melt the mating surfaces of the plastic parts to be welded, and then pressure is applied to connect the mating surfaces to achieve a strong weld. Thermomelt welding is commonly used in the production of medical plastic products and medical packaging bags with circular weld surfaces. For example, the necks of medical plastic infusion bottles, and medical plastic or paper-plastic composite packaging bags.
High-Frequency Welding
High-frequency welding utilizes the fact that thermoplastic plastics move due to molecular polarization between high-frequency electrodes as the electric field changes. Friction between molecules converts electrical energy into heat energy, causing the plastic itself to heat up and melt, thus achieving the connection. High-frequency welding is commonly used in the production of medical bags such as blood bags, IV bags, and urine bags, and for sealing some medical packaging bags.
① Advantages: Uniform internal heating of the welding material; fast welding speed; aesthetically pleasing weld surface. Significant advantages for large-area welding and irregular-shaped welding.
② Disadvantages: Material limitations—primarily PVC.
Ultrasonic welding works by creating high-speed vibrations at a frequency of 20kHz, causing the plastic surfaces to fuse together due to frictional heat. When used for welding plastic to metal, metal can be welded to the plastic joint in less than a second. Ultrasonic welding is a novel secondary processing technology for plastics, developed for its aesthetic appeal and energy efficiency. It has a wide range of applications and can be implemented using various methods, such as planar welding, riveting, spot welding, and embedding.
Ultrasonic welding has been applied in the manufacture of medical products such as hemodialysis machines and plasma collection devices. For example, in the welding of the cup and cap of a plasma collection centrifuge, this welding method replaces chemical adhesives, solving the problem of adhesive melting caused by high-speed rotation of the centrifuge cup, achieving excellent results. Furthermore, this method is frequently used in the manufacture of medical protective masks, protective clothing, infusion sets, and balloon dilation catheters.
① Advantages: No curing time, air purification, high efficiency.
② Disadvantages: Limited by compatibility and thermoplastic properties, noise, area, and shape limitations;
③ Main factors affecting welding results:
A. For materials with water absorption properties such as nylon, polycarbonate, and polysulfone, pretreatment is necessary to ensure a strong weld;
B. Store in a sealed environment;
C. Dry the other parts before ultrasonic welding;
D. Perform ultrasonic welding immediately after injection molding.
④ Applicable scope for different materials
Ultrasonic welding is not suitable for welding every type of plastic material. Welding two materials should follow the principles outlined in the table below:
Vibration Welding
Vibration welding is a self-limiting heating welding method that generates heat through friction. Sufficient linear or angular displacement is used to generate frictional heat, melting the contact surfaces of the two parts. It can be used for most thermoplastics, especially crystalline plastics such as PE, PA, and PP, which are difficult to weld using ultrasound or fusion welding.
Vibration welding offers numerous advantages, including high welding speed, automatic temperature adjustment, no overheating in the weld, and minimal impurities in the weld area. It is widely used in medical plastic products. It is particularly suitable for long linear joints that are difficult to achieve with ultrasonic welding and joints where hot plate welding requires a long time.
2. Insert Connection for Medical Injection Molded Parts
Insertion Connection Method:
Generally refers to the connection between a plastic tube and a connector; the connection is achieved through tolerance fitting and bonding, and is mostly used for tubing products.
Advantages: Convenient connection, suitable for mass production, and unlimited length of the connector.
Disadvantages: Suitable for connecting tubular components.
Factors Affecting External Insert Connection Methods:
① Inner diameter, wall thickness, tolerance, and hardness of the plastic tube.
③ Larger and smaller diameters of the connection portion between the connector and the plastic tube.
④ No axial webbing line at the connector end.
⑤ Excessive height of the webbing line can easily cause leakage.
⑥ The formulation of the connection process, the method of inserting the connector, the quality of the clamping tape, and the tools for tightening the clamping tape.
Multi-part molding
Rotary, molding threads, tapping threads, hot air, vibration, induction, electromagnetic, laser, fasteners, inserts, hinges, hot plates, snap-fits, etc.
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