Topic 11

Joining


11.1.1 Friction Welding

Friction welding is a completely mechanical solid-phase process in which heat generated by friction is used to create the ideal conditions for a high-integrity welded joint between similar or dissimilar metals.

11.1.2 How two metal parts are welded using friction
Friction welding is a solid state welding process which produces coalescence of materials by the heat obtained from mechanically-induced sliding motion between rubbing surfaces. The work parts are held together under pressure. This process usually involves the rotating of one part against another to generate frictional heat at the junction. When a suitable high temperature has been reached, rotational motion ceases and additional pressure is applied and coalescence occurs.
In the friction welding process one part is held stationary and the other part is rotated by a motor which maintains an essential constant rotational speed. The two parts are brought in contact under pressure for a specified period of time with a specific pressure. Rotating power is disengaged from the rotating piece and the pressure is increased. When the rotating piece stops the weld is completed. This process can be accurately controlled when speed, pressure, and time are closely regulated.

11.1.3 Plastic Welding
Hot air welding of plastics, is a plastic welding technique which is analogous to gas welding metals, though the specific techniques are different. A specially designed heat gun, called a hot air welder, produces a jet of hot air that softens both the parts to be joined and a plastic filler rod, all of which must be of the same or a very similar plastic. Welding PVC to acrylic is an exception to this rule.

Friction welding of plastics, the two parts to be assembled are rubbed together at a lower frequency (typically 100–300 Hz) and higher amplitude (typically 1 to 2 mm ) than ultrasonic welding. The friction caused by the motion combined with the clamping pressure between the two parts creates the heat which begins to melt the contact areas between the two parts. At this point, the plasticized materials begin to form layers that intertwine with one another, which therefore results in a strong weld. At the completion of the vibration motion, the parts remain held together until the weld joint cools and the melted plastic re-solidifies. The friction movement can be linear or orbital, and the joint design of the two parts has to allow this movement.

11.1.4 How two plastic parts are welded together?
Only thermoplastics that do not burn or decompose when heated to their softening temperature can be welded.

11.1.5 Permanent joining techniques
A permanent joint is one that is intended to last the life time of a product where as a temporary joint is one which is intended to hold the item together but is removable easily at any stage without causing too much damage to the main product.
Examples of permanent joining techniques are:
11.1.6 Range of permanent joining
  • Pop-rivets
  • Brazing
  • Welding
  • Adhesives
  • Soldering
  • Mortice & tenon joints
  • Finger joints
  • Dovetail
  • Nails

11.1.7 Permanent joining techniques lead to planned obsolescence and environmental issues
Permanent joins do not allow for the disassembly and easy maintenance.

11.1.8 Adhesives
adhesives_2.jpgcolle-doigts.jpg
An adhesive is a compound that bonds two items together. Adhesives may come from either natural or synthetic sources.
Adhesives offer what conventional fasteners such as pop-rivet, nuts & bolts cannot offer. Adhesives allow different materials to be bonded together
  1. Continuous connection between adherents; thus providing an aesthetic finish and allows for smooth designs.
  2. It has higher strengths in terms of static and fatigue compares to other forms of fasteners.
  3. Adhesives offer liquid and gas tight connections.
  4. Adhesives offer corrosion resistance.
  5. Adhesives offer permanent fastening to temperature sensitive components.

11.1.9 Range of adhesives suitable for joining metals, plastics, and woods.
  • PVA (polyvinyl acetate)
  • Epoxy resin
  • Contact adheisive
  • Cascamite
  • Tensol cement
  • Superglue (cyanoacrylate)

11.1.10 Advantages and disadvantages of using adhesives bonding in products
When using adhesive to join materials together, firstly you must consider preparation of surfaces, clamping, bonding time, types of materials, and health and safety as identified in the table below (reference Bostik Website) and check out the 3M Website and www.adhesives.org

Advantages
Disadvantages
Preparation of surfaces
  • No drilling of holes, etc
  • Surface must be carefully cleaned
Clamping
  • Join all shapes and thicknesses
  • Provide smooth contours
  • Jigs and fixtures may be needed
  • Heat and pressure may be required
Bonding time
  • Long curing time allows for repositioning (Disadvantage as well)
  • Long cure times may be needed
Type of material
  • Join all shapes and thicknesses
  • Dissimilar materials can be bonded
  • Certain adhesives are needed for the type of material.
Health and safety

  • Fumes can be toxic
  • Human tissue can be bonded
Adhesives present several distinct advantages over conventional methods of bonding, however, there are also disadvantages which may make adhesive bonding impractical:
Advantages

Disadvantages
  • Provide large stress-bearing area
  • Provide excellent fatigue strength
  • Dampen vibration and absorb shock
  • Minimize or prevent galvanic corrosion between dissimilar metals
  • Join all shapes and thicknesses
  • Provide smooth contours
  • Seal joints
  • Join any combination of similar or dissimilar materials
  • Often less expensive and faster than mechanical fastening (can be automated)
  • Heat, if required, is too low to affect metals parts
  • Provide attractive strength-to-weight ratio.

  • Surface must be carefully cleaned
  • Long cure times may be needed
  • Limitation on upper continuous operating temperature
  • Heat and pressure may be required
  • Jigs and fixtures may be needed
  • Rigid process control usually necessary
  • Inspection of finished joint difficult - Non distinctive testing not always possible
  • Useful life depends on environment
  • Special training is often required.
Read about testing and failure of adhesives at: http://en.wikipedia.org/wiki/Adhesive


Task - Adhesives

Download the following sheet and complet using the resources available on this page.


Moulding

11.2.1 Terminology used in moulding
  • Sprue
  • Flash
  • Parison
  • Die
  • Draft angle
  • Injection moulding

11.2.2 How is an injection-moulded product made?
You need to be able to recognize and sketch the following diagram and label the hopper, hydraulics, heaters, screw, sprue and mould.

11.2.3 The advantages of injection moulding
Consider initial capital investment, tooling accuracy, quality control and quantity of product.

11.2.4 Discus how standardising bottle caps have constrained bottle design, but have cut costs for the manufacturers
  • Bottle caps can be classed as standardized part.
  • Bottle tops are injection moulded, while bottles are normally made by blow moulding.
  • It is financially beneficial for a blow moulding company to use off-the-shelf bottle tops instead of purchasing an injection moulding machine and new tooling.
  • The constrained bottle design - allows bottles from different manufacturers to be sold in vending machines.

11.2.5 How is a blow-moulding products made?
You need to be able to recognize and sketch the following diagram and label the extruder, parison, the mould, and air inlet.

11.2.6 How is a rotational-moulded product made?
You need to be able to recognize and sketch the following diagram and label the mould, filling the mould, heater chamber, rotation and cooling chamber.

11.2.7 Explain how a compression-moulded product made?
You need to be able to recognize and sketch the following diagram and label the mould, pre-form, hydraulic press, finished part and flash material.

11.2.8 Discuss why some products have to be made using compression-moulding
Consider the heat the product must withstand, quality and type of product to be made. (thermosets).

11.2.9 How is a vacuum-formed product made?
You need to be able to recognize and sketch the following diagram and label the vacuum chamber, former, platen, heater, air in and out.

11.2.10 Identify manufacturing methods suitable for thermoplastics and thermosets
Plastic

Manufacturing method

Material
Thermoplastic

Vacuum forming, blow moulding, injection moulding, rotational moulding

HDP
Thermoset

Compression moulding


Casting

11.3.1 Describe Lost Waxing
11.3.2 How are lost wax cast products made
Consider preparation of the master pattern; injection of wax to create a copy, creation of a wax tree to make a wide range of small parts from the same metal; covering wax with ceramic or plaster of Paris; removal of wax; and the addition of the final chosen material.

11.3.3 Explain how a range of products are made using lost wax casting.
Jewellery, dental implants, hip replacements and wind instrument keys.

11.3.4 Describe high-pressure die casting
Die casting is mainly used for low melting alloys. Molten metal is forced into a mould under high pressure.

11.3.5 How are high-pressure die cast products are made
You need to be able to recognize and sketch the following diagram and label the holding furnace, injector, gooseneck and die.

11.3.6 Explain how a range of products are made using high-pressure die casting
For example: Hip replacements, disk drive chassis and carburettors.

11.3.7 Outline the advantages and the disadvantages of high-pressure die casting
Advantages

Disadvantages
High accuracy

High plant costs
Good surface finish

High tooling cost
Thin walls

Cannot be used for a wide range of alloys
High rate of production

Limitations on maximum size that can be cast

Forming

To be added

Resources