When it comes to joining plastic parts, there are a number of methods that can be employed, depending on the task at hand. Perhaps the most straightforward approach is to design a fastening element (such as hinge or latch, for example) into the parts. You need to ensure you are dealing with stronger plastics, in this instance, as the joint must be able to endure the rigours of assembly, service load and possible repeated use. So really you are talking about lightly loaded, non-rigid assemblies where precision is not so essential.
Two other common means used for plastics joining are solvent bonding and UV bonding. In solvent bonding, the plastic is softened first with a coating of solvent. The two plastics components are then clamped together. The molecules mix together and the components bond as the solvent dries and evaporates. This process works solely with thermoplastics. “The level of pressure applied is critical,” according to Craftech Industries, “as too much may cause the components to distort. In addition, 24 to 48 hours at room temperature or several hours at a higher temperature may be necessary to facilitate the bonding.”
UV (Ultraviolet) bonding, on the other hand, utilises a curing process with high-intensity ultraviolet lighting that instantly dries or cures inks, adhesives or coatings. Moreover, UV bonding works very quickly to bond plastic components, even when the plastic material blocks UV light. In addition, plastic adhesives bond together a wide array of substrates, making this method an efficient way to bond plastic pieces to other materials, such as ceramic, glass and metal. “This process provides many advantages, such as reduced rejection rates, increased production speed, improved solvent and scratch resistance, as well as superior bonding,” it adds.
ACTION STATIONS
It’s worth taking a close-up look at some examples of plastics joining in action. The Relyon Plasma Piezobrush PZ2 from Intertronics, for example, has nozzle attachment options for surface activation across a wide range of applications for the improved adhesion on surfaces that are otherwise difficult to bond, print on, coat or laminate.
Treatment of metals, and smaller and more precise applications, is now possible by choice of a suitable nozzle, while the use of a variety of special gases also enhance the possibilities of PDD (Piezoelectric Direct Discharge) on challenging substrates. Based on the direct, electric discharge at an openly operated piezoelectric transformer, low input voltage is transformed, resulting in high electric field strengths. The Piezobrush PZ2’s ionised energy output of cold active plasma gives successful pre-treatment and surface activation, enhancing wetting and adhesion with Standard, Nearfield, and Multigas-and-Needle nozzle options.
“When used on conductive surfaces, the Piezobrush PZ2 Nearfield Nozzle helps to resolve issues where direct plasma discharges can damage some substrates or even the plasma device itself,” advises Intertronics. “Inside the PZ2 Nearfield Nozzle, a glass inlet forms a dielectric barrier and changes the type of discharge, distributing the power from the direct discharge uniformly over the treatment area, therefore eliminating the possibility of damage to the surface. This allows metals and other conductive substrates to be treated with confidence.”
ELASTOMERIC MATERIALS
What about the bonding of elastomeric materials, which elastically return to their original shape after deformation by compression or tension? Materials fitting this category are numerous and include rubbers, plastics and polymers; all differ in their polymer structure, polarity and surface properties, and that is why it is so difficult to stick these materials together.
“The growing number of design applications for elastomeric substrates has greatly increased the need for information on assembly techniques using these materials,” says Jane Powell, marketing executive, Techsil. “Adhesives offer several benefits over mechanical fasteners for joining elastomeric materials. Unlike solvent bonds or ultrasonic welds, adhesives perform well with thermoset rubbers. In addition, adhesives distribute stress over a joint's entire bond area rather than in a single location. Unfortunately, the wide selection of elastomers and adhesives can make it difficult to identify the optimal combination for a given design.”
What is that makes elastomeric materials so difficult to bond? “Elastomeric materials are used in a myriad of industrial applications such as gaskets, hoses, seals, automotive vibration damping and medical devices; and are formulated specifically for their resistance to harsh chemical and environmental conditions,” she points out. “As a result, these substrates also tend to be difficult to chemically bond. Often characterised by low surface energies, low porosity and non-polar surfaces, elastomeric materials feature no surface roughness onto which an adhesive can secure itself. In addition to this, they are stretchy!”
Although polyurethane, epoxy and hotmelt adhesives can be used, states Powell, the most effective chemistries of adhesives for bonding elastomeric materials are:
Cyanoacrylates: one-part, solvent free, rapid room temperature curing adhesives. “They adhere to most substrates, are easy to dispense in automated systems and come in a range of viscosities. They exhibit strong shear and tensile strength. Whilst cyanoacrylates offer an easy bonding solution, they do have decisive drawbacks: they embrittle quickly and exhibit a very low impact and peel strength. They show poor durability on glass and poor solvent resistance. They are not good gap fillers, have low temperature resistance, the bond skins quickly and may stress crack some plastics.”
RTV Silicone Adhesives: solvent free, one-part systems which range in viscosity from self-levelling liquids to non-slumping pastes for gap filling. “They cure to soft, flexible thermoset elastomers with excellent property retention over a wide temperature range. They adhere to a wide variety of substrates and there are UV cure formulations available to initiate cure. However, some can be slow to cure, so they are not always practical for small item assembly on a fast production line. They can have poor cohesive strength and a limited depth of cure.”
Light Curing Acrylics: cure within seconds to form a tough thermoset polymer with excellent adhesion to a wide variety of substrates. “The ability to cure ‘on demand’ offers significant processing benefits,” she comments. “In addition, the wide range of viscosities and properties make light curing acrylics ideally suited for fast speed automated bonding processes. Other advantages include:good environmental resistance; solvent-free; good gap filling; 1-part; invisible bond lines; rapid fixture and complete cure; the bond formed is elastic, so there is no stress cracking and they can be sterilised without losing integrity.” Drawbacks are minimal, she contends.
“Selecting the best adhesive for a given application involves more than selecting the adhesive that provides the highest bond strength,” Powell concludes. “Other factors such as speed of cure, flexibility, environmental resistance, thermal resistance, suitability for automation and price will play a large role in determining the optimum adhesive system.”
In addition to some of the companies mentioned in this article, there are many more experts in the field of plastics joining to be found exhibiting at The FAST Exhibition on 21 September. Included amongst them are suppliers of mechanical fixings, adhesives and plastics welding.
Craftech
