Design for Manufacturing

Welding and Joining Processes

This process is not as strong as fusion welding since the filler metal is less strong than base metal. In contrast, it is perfect for complex shapes. It creates watertight and airtight joints which are used for copper tubes, air conditioners, boilers, tanks, trumpets, sauce pans and jewels. Finishings are needed for aesthetic products.

Design of Joints

Welding of Plastics

Plastics can be welded too but only thermoplastics polymers (not thermoset polymers). We can use the welding manufacturing process in order to create closed/hollow shapes in a more complex way instead of blow molding or rotational molding. We can firstly manufacture parts by using injection molding (it reduces the cost of material and production) or extrusion, choosing the one or more of the following processes to fish the product: ultrasonic welding, vibration welding, hot-plate welding, (spin welding), (induction welding) and (laser welding). They all use different ways to heat up and melt the parts together.

interface between them. Pay attention when using crystalline thermoplastics such as PE, PP, PA, POM etc, since only 10-20°C are sufficient to make them change from solid to liquid state (small gap range of temperature). This doesn't happen with amorphous polymers such as PS, PVC, ABS, PC etc, since they need 50°C to change from solid to liquid state (lower shrinkage).

Ultrasonic welding Vertical vibrations at a 20.000 Hz frequency create a weld in about 1 or 2 seconds. High energy density is transferred from one part to the other by using energy directors (little protrusions that "melt" and create the joint). This process requires a really short cycle time sec= load parts (~2 sec) + welding (~1 sec) + upload p. (~2 sec) and ~5 low equipment cost. It can be used to weld mid-to-small parts which means around 200/300mm max. We can't have very strong and large joints, otherwise some parts may not melt in the correct way. The materials that are used in this

with chemical similarity (such as PP-PP and not PP-ABS), rigid (no PE, Nylon doesn't work), no additives, fillers, fibers, and regrind. In general, it is an inexpensive process but with strong limitations on materials, strength of the product, and its shape.

Vibration welding

Horizontal vibrations (180 cycles per sec), generated by using springs and electromagnets, create friction between the parts until they melt together. This process is based on sliding and rotating, not compressing. It can be used only for limited complexity shapes such as uniform section profiles and flat surfaces. Vibration welding requires a short cycle time (≈5 joints per minute) but a high equipment cost. It is ideal to weld mid-to-large parts since heat is created in the exact surface without any heat transition as in ultrasonic welding. The materials that are used in this process should be: equal (or compatible), with a similar melting point,

With chemical similarity, additives, fillers, fibers, and regrind. It is ideal for PA (Nylon). We can obtain very strong joints (fuel tanks→watertight joints).

Hot-plate welding Ultrasonic and vibration welding are similar processes because they both use mechanical energy produced from vibration. However, they have some constraints and restrictions about the geometry of the joints that must be very simple to allow the vibration but they have other limits too. This last process, called hot-plate welding, removes the limitations of these two previous processes. It has no limitations about size, shape, etc. In this process, we don't need vibration to generate heat. We simply produce heat by using a heating plate. It is basically a sequence of operations: we take the two parts and put them in a fixed position (heat-plate holder). The fixer opens so that the heating plate can go inside and then it closes. The interfaces are melted, then the fixer opens again so that the heating plate can be removed.

The two parts close again to mold the pieces through compression: the parts are finally connected. We could weld PVC and elastomers through this method. One problem about this kind of process could be the flash (abundance of material on the sides of the connection). It requires a long cycle time and you can use it to join large parts. Joints have a higher strength and they can also have a very complex shape (something that we can't produce with the previous processes since it requires fixed surfaces). The materials used can be dissimilar, flexible (non rigid, like PVC or elastomers since there is a little compression) and non-sticking. It is inexpensive as the other processes (like vibration) but it has many advantages. The price to pay (so the bad thing about this process) is that we use inexpensive tools for a long cycle time. Design of joints Vibration and hot plate welding Whenever we want to weld parts we cannot have flat edges, this is to say that we must increase the thickness of

The piece (through a way called flange joint, a visible artifact of the joint that we are not really happy to have on our product) because both in vibration and hot-plate we need 17 same space for the sliding. We could also have some flashes after the joint, some plastic material can go out creating a visible defect, an irregularity on the product (but we don't really care about it). How to remove flashes: through a flash trap we reach a better result by avoiding the external flash (removing the internal one is usually not necessary for the aesthetic).

Ultrasonic welding

In this case we cannot melt the whole thickness of the part. Simple joints with an energy director are used to concentrate the heat in a small section. To avoid flash we can use step joints. If we produce a joint through this method we reach less strong joints. Through the use of a tongue and groove joint we can avoid both internal and external flashes. There is another type of ultrasonic joint called shear joint and it

is mainly used for crystalline plastics that melt in a very short temperature interval. The problem here is that we have two parts that can't go together like a puzzle, so the small area in contact melts and the upper part goes perfectly on the other part. The advantage of the shear joint is that we don't need to melt the whole part of the piece. The last type of joint is called staking which is not limited to one type of welding, the advantage is that it melts different materials. The lower part has some bosses and the upper part has some holes in correspondence. The tool transfers energy from the lower part to the upper one. That is why the lower part creates a mushroom shape. It is an alternative to melt dissimilar materials.

Examples:

Ultrasonic welding: the first example is a piece for the dishwasher which is usually made of PP (polypropylene). We have two shapes and we want to weld them together. The second is about the fact that Apple was the first company to use ultrasonic

welding to produce plastic parts of headphones. The third example is a swatch, this process is used in several points like the crystal that is welded to the ABS part. The last one is a disposable lighter made of POM. Vibration welding: the first example is about two parts for the car welded together. The seconds are two housings for speakers. The cover is made of glass fiber and plastics at the same time. The last example is the light of a car, if it's a very small vibration is fine, if they are really big ones then we need to use a hot-plate process. Adhesive bonding: It is basically the same as gluing (hobby world), we glue some parts together on a large scale. The glue is called the structural adhesive. We apply the glue on the selected surfaces and we wait for the curing, the process that allows the glue to become hard. When the adhesive is hardened something different happens, on a microlevel we see that the adhesion fills completely the little gaps that we could have between.

The parts (no air gap anymore). Heat is not required, it can be used to make the process faster but it is not necessary since we already use a chemical substance like glue.

The main techniques used for apply adhesive bonding are:

Spray application: a spray gun compresses the adhesive and spray it through little drops on wide surfaces. The limitation is that we need large areas otherwise we would waste a lot of it in the air. What we need to do before is to prepare the surface and then, after the process, clamp and curing the adhesive. The clamping phase requires a long cycle time.

Dispensing: it is mostly used for small areas. It is not as common as the other method. In the picture we can see an automatic dispenser with some pedals (on/off). In the example the adhesive is applied on the head of a doll.

Adhesive bonding is considered a simple and fast technique but it is not that simple as we are using a chemical substance that may bring to some problems with volatile emissions and the safety.

of workers. Another reason is that we need to prepare the surfaces (something that we don't need in the other processes for making joints) because we want to remove contaminants. Even after the process we need to proceed with clamping and curing of adhesive. This process has a long cycle time, so much longer than the welding of plastics. It is also the most expensive technique since the time required is a lot. The main applications of adhesive bonding are: non-weldable or dissimilar materials. Joints: they may have a complex shape, airtight, no holes or additional parts. In addition, the joints may have some insulating properties. Conditions for the joint strength: we have to check the operating temperature (<250°C) since the adhesive could burn and degrade by being a chemical substance. We also need a large surface area and when we apply external forces to the parts the stress must be uniform on the joint. What does it mean? In the picture we see that we apply a force.

(tension) to a part and we have an uniform force applied, that's good. We can reach the same uniformity through shear since it brings uniform stress. In contrast, a bending force can't be used because we would reach a variable stress which doesn't contribute to the strength of the joint. It may bring the joint to cleavage (the break of the adhesive). Another negative thing that could happen is peeling. We kind of have to predict which will be the external forces.

Adhesive bonding is usually used in the automotive sector like for Jaguar which was the first car with an aluminum body instead of steel (so much lighter). The problem with the aluminum body is that we could not use spot welding because it has little strength (strengthening of welds or fastenings). In the automotive sector there is a continuous search for alternative processes to spot welding. One possible substitution is adhesive bonding which is implemented with self-piercing rivets in order to strengthen.