The second injection can not only make the instrument surface full of soft feeling, but also can increase the product's functionality and added value.
In the past 10 years, secondary injection technology has completely changed the aesthetic standards, design ideas and functional requirements of consumer products. Medical device manufacturers also recognize the potential advantages of this technology, and continue to expand its application in the medical field.
The overmolding technique is known for creating 'soft surfaces', but it also has many other features such as: ergonomic design, dual color appearance, brand identity, and feature improvements. With this technology, the functionality of the product can be increased (eg: Noise reduction, shock absorption, waterproof, collision avoidance) and added value.
Two-shot injection and co-injection molding, double injection molding and sandwich injection molding all belong to the multi-material injection molding technology. The basic idea of multi-material injection molding is to combine two or more materials with different characteristics to increase product value. In this paper, The first type of injection material is called the substrate or the base material, and the second type of injection material is called the cover material.
1, a variety of secondary injection molding technology
In the overmolding process, the covering material is injected into the substrate above, below, around or inside and combined into a complete part. This process can be completed by multiple injection molding or insert molding. The commonly used covering material is an elastic resin.
Multiple injection molding: Multiple injection molding is a good method of medical device processing if the construction of the covering material allows. This technique requires a special injection molding machine equipped with multiple barrels to inject different resins into one injection mold. The barrels should be side by side Or placed in an L-shape where one or more injection points inject the resin into the mold. When using the same injection point, it is called co-molding. The composite part produced is a core resin material that is coated with an outer layer. Use multiple injection points When it is called secondary injection molding, a material is formed on another material to produce a multilayer structure.
However, multiple injection molding does not apply to all products. During the second injection, the slider must be moved or the core moved to another cavity. Another method is to send the core to another injection molding machine.
Insert injection molding: To produce fully covered products such as injection molded handles, insert injection molding is required. To achieve full coverage, the substrate must be removed from the original cavity and placed in another core and cavity to inject the covering material.
During this process, another mold should be run on the same or another different size injection molding machine (depending on the size of the injection molding part). Usually the base material is much larger than the cover material and may need to be warmed up. The surface temperature is close to the melting point of the cover material to obtain the best bond strength.
2, in-mold assembly
Overmolding is sometimes referred to as in-mold assembly because the two materials are fully integrated at the end, rather than merely creating a layered structure, either individually or as a component material. This applies regardless of application. It is essential that the substrate and cover material achieve the required mechanical or chemical bond strength.
3, multi-material injection molding considerations
In general, to enhance adhesion, the melting temperature of the covering material resin should be the same as that of the substrate. If the melting temperature of the covering material is too low, the surface of the substrate cannot be melted, and the adhesion between the two is not strong enough. However, if the melting temperature is too high, the substrate will be softened and deformed. In severe cases, the covering material will penetrate the substrate and cause the component to fail to be processed. Therefore, the matching materials are selected to ensure good adhesion.
In general, matching materials should have similar chemical properties or contain matching composite components. When the substrate and covering material do not match, they usually form mechanical interlocking rather than chemical bonding.
Multi-material injection molding also needs to pay attention to some issues, the most common include: Insufficient chemical or mechanical bonding strength between polymers, incomplete filling of single or multiple part materials, and flash flashing of single or multiple material parts.
Injection molding machines must maintain the consistency of injection molding. In addition, the injection molding machine barrel injection rate and injection molded parts ratio is also an important factor affecting the quality of injection molding. The ratio is critical for all injection molding operations, in the second injection molding is particularly important. The non-return device can separate the cover material like a sluice gate. When the secondary injection molding materials are all metal, the non-return device is easy to operate. If the metal substrate and the more elastic plastic, the non-return device is difficult to operate.
4, material selection
The choice of overmolding resin material has several factors, depending on the properties of the substrate on the one hand, and application performance on the other hand. Specifically, there are the following points:
Chemical resistance (meeting cleaning and other operating requirements).
Flame retardancy (in line with ecological and environmental protection requirements). Eco-friendly logo is a sign that the product meets environmental and social standards.
Abrasion resistance (so as not to sink or fall off).
Shore hardness (in compliance with softness or other requirements).
Medical Specifications (FDA, USP Class VI, ISO 10993 and Biocompatibility Requirements).
Sterilization type (steam, gamma rays, etc.).
Impact resistance (in line with structural requirements).
Melting point (in accordance with the application temperature, will not be softened or deformed).
Bonding method (a mechanical interlocking effect occurs when the two materials do not match, and chemical bonding occurs when the two materials match).
Over the past five to eight years, covering materials have evolved considerably and many elastomeric resins have been developed. For example, thermoplastic polyurethane (TPU), styrene-ethylene/butylene-styrene polymer (SEBS) , Copolyesters, Copolyamides, Thermoplastic Rubber (TPR) and Thermoplastic Vulcanizates (TPV). In practice, new polypropylene resins with good adhesion to polypropylene substrates are generally chosen.
These materials have very different Shore hardnesses. In general, the higher the hardness of the material, the stronger the abrasion resistance. The texture of the material also affects the hardness. Due to the high resistance to abrasion in the resistance test Material loss is less, for example, when the spinning wheel test is performed, the resin with high hardness has less abrasion, so the material with strong abrasion resistance is often selected for the application.
The hardness of SEBS resin is very low, not to Shore A 30, and TPU resin hardness is about Shore A 60, which is similar to that of human hand. In the past, the hardness was generally reduced by adding plasticizer or mineral oil. However, these additives are used in cleaning or use. It will be precipitated (or called frosted) and does not meet the requirements of medical applications.
Due to the development of secondary injection molding resin materials, the choice of substrates has become increasingly widespread. Currently, it has included: Acrylonitrile butadiene styrene, Polycarbonate and Nylon. The expansion of the range of materials provides more space for soft design. However, the application of new materials also brings new problems, such as: material adhesion, component design and mold operation, etc.
5, process design considerations
In the overmolding process design, the non-return device, the nozzle hole, the air outlet, and the mold surface texture are the key elements.
The non-reversible device between the substrate and the cover material is critical for the bonding effect, and it should be avoided that the ejected cover material is gradually thinned or fluffy. The cover material is too thin and will result in poor adhesion, degumming and curling. The design of the non-return device should be a clear separation of the cover material from the substrate. The recess of the substrate in Figure 1 utilizes this design.
The nozzle hole design is equally important for the success of the second injection molding. The ratio of the runner length to the wall thickness is the main factor affecting the bonding effect. According to experience, the ratio should not exceed 150:1, and the ratio should be maintained at the development of the new process design. 80:1. The figure below shows the ratio of the length of the runner to the wall thickness.
In order to shorten the process as much as possible, the nozzle hole should be set in the position with the largest wall thickness. When using TPE resin, pay attention to the nozzle aperture size. TPU and other materials need large-diameter nozzles to adapt to higher viscosity and prevent shear force. High material degradation occurs. Materials such as SEBS require a high shear rate for optimum flow rate. A better approach is to use a small diameter nozzle in the initial stage and adjust the nozzle size after the initial sampling.
As with the nozzle hole, the outlet is also an important factor affecting the bonding effect. How to control the air volume is a major problem. If the control is not good, the phenomenon of loose bonding and filling of burrs may occur. The depth of the outlet is The prevention of flash is extremely critical. The depth of the air outlet should be between 0.0005-0.001 inches depending on the viscosity of the cover material.
For some part designs, decorative surface textures can be used to facilitate product ejection. Most TPE materials tend to adhere to the mold surface because of the metal affinity or because of the material during mold opening. Vacuum is formed between the mold surface. Since many materials do not form a stable chemical bond after the product is ejected, if it adheres to the mold surface, it will greatly affect the bonding effect.
This means that the parts must be handled carefully after they have been processed. If a bond test is required, it must wait 24 hours before the material can form a stable chemical bond.
When the mold surface is parallel to the mold opening direction, if the tensile force is not enough, adhesion will occur. TPU and other materials need a 5-6–6 pull force. In addition, the mold surface plating also helps the components to eject.
In addition, the surface texture design also needs careful consideration. The surface texture will affect the softness, feel and thickness of the covering material. The appropriate wall thickness and surface texture design can complement each other to help obtain the required processing characteristics. Generally, the hardness of the material is increased. Low and more soft. Optimizing the surface texture can reduce the amount of injection molding artifacts and improve product feel, making it softer than actual hardness.