Designing a plastic part for hot air staking
Original article date: January 1999
Hot air staking has the ability to provide robust fixings while de-skilling the plastic part assembly process. DENNIS EAGLES Development Manager of Phasa Developments explains some of the basics.
Plastic hot air staking is being increasingly applied in the automotive sector mirroring the growth of plastic parts in trim and fascia assemblies and building on its existing use in vehicle radiator grilles and wheel trim. The system also provides a reliable and safe method of securing PCBs and other components in electronics industry applications without the use of metallic threaded fasteners or the risk of damage being caused by ultrasonic welding.
It is a method of permanent assembly relying on hot air delivered to localised areas of a moulding distributed by a manifold. There is no precise method of dictating the number or size of stakes which should be used. They cost little to mould and each additional stake will only increase the initial capital expenditure on tooling by a fraction. Factors which come into play are:
- Ultimate strength
- The time it takes to mould initially
- The time it takes to heat for staking
- The time it takes to cool when reformed
When plotting the fixture layout and stake profile the key design features are as follows:
| Number and position :identify those places at which stakes must be positioned. These may include corners long flat sections weak areas which require more support as well as those which take more stress. Having decided on the essential fixings look at aesthetics. It may be necessary to add one or two stakes to enhance the balanced appearance of the completed product even though they may not be necessary from an engineering viewpoint. |
| Diameter:Given the material to be used assess an acceptable strength for the assembly not only for the application but for handling purposes (don’t forget that handling will take place prior to staking!). Divide the number of stakes in half (to provide a safety factor of 100%!). Then calculate the cross-section and therefore size of each stake from the specification of the material in use. Bear in mind that if a stake is too large it can create a sink. In this case it is usual to core the take to leave a tube of the correct cross-sectional area. |
| Height:as a rule of thumb the height of a stake (left for forming when assembled) should equal its diameter for an o.d. of 3mm. As the diameter of the stake increases so the height for reforming should decrease and vice versa. Thus a 10mm diameter stake could be expected to be 7.5mm high and conversely a 2mm diameter stake 3mm high. The reasoning behind this is that the larger stake will have ample material to provide a strong head and any excessive height will lengthen the processing time unnecessarily. Similarly the smaller diameter will need additional height and consequently material to produce a reasonably strong head. The only justification for keeping excessive height on a large stake is when there is too much clearance between it and the hole it fits into thus making the additional material necessary to in-fill the excessive space. |
| Style:will the stakes look acceptable for the product? If they do not then increase or decrease their diameter to an acceptable size. In the latter instance additional stakes will be needed to compensate for the loss of area. Stakes (ie parts which are to be reformed) may be round or hollow pillars ribs collars or any other shape which may be reprocessed. Don’t forget that it is quite acceptable to mix sizes and shapes of stakes if required. |
| Hole size and general clearance:the clearance around the stake should be the minimum possible which will still permit rapid assembly. As a rule of thumb the aperture should be no more than 10% larger than the stake. A lead-in on the hole and/or stake will assist alignment. |
If excessive clearance is allowed around the root of the stake it is most probable that the bulk of the material designed to produce the head will find its way into the space in the hole and an inadequate retaining form will be the consequence.
When circumstances do not permit a close tolerance on the holes more effective results can be achieved by the use of dimpled or hollow stakes and special tooling designed to achieve a greater spread of the head such a bifurcated rivet.
It is not a normal function of the process to provide lateral location by expanding the section of stake within the hole (this can create extended processing cycles). If therefore positive location against such motion is required it can be achieved in two ways. Firstly by swelling at the stake base making them a closer fit at the point where they are designed to lay in the holes. Secondly by providing ribs recesses tapered dowels or other forms of location separate from the processing points.
- Position: The siting of the stakes is not only a function of strength support and a balanced layout. Other points to consider include:
- Keep the stakes away from heat-sensitive components including thin sections of a moulding (particularly vertical walls which may be nearer to the hot-air deliver point than the stake being heated)
- Leave adequate room to enable approach and retraction of the system. Headed stakes take up more room once they have been heated!
- Ensure that there are equal sections around the area being processed. When a hole is placed at the edge of a moulding and there is little or no material on one side the thinner section will melt away resulting in a total failure of the assembly method.
- Do not mould stakes on thin sections unless there is no alternative. In such circumstances the heat can travel down the stake and cause stress relieving of the thin area adjacent to it. The material on which a stake is moulded should be at least equal to the diameter or thickness of the stake (as appropriate).
A 24-page guide to hot air assembly is available free from Phasa Developments. It is aimed at design engineers looking to achieve permanent vibration-resistant fixing of thermoplastic components and assemblies.
Advantages and limitations of hot air staking
- Because the sector being re-formed has reached moulding temperature the completed assembly will be free of stress and strain with no residual memory to promote creep and loosening of parts at a later date.
- Because 80% of the working stake has been reformed and there has been no degrading of material the formed strength will be proportionally equal to that of the base moulding.
- The tools are cool or cold so there is no sticking or material transfer.
- The stakes will shrink as they cool so the final assembly will tighten even further after processing.
- Because the hot air may exceed the melt point of the plastic because of its low thermal capacity and conduction the excess energy will not cause any degradation of the resin; however it may sometimes create slight discoloration of fillers (more visible in light colours).
- It is best to use compatible materials where the melt temperature of the stake is equal to or less than that of the material it passes through. Observe extreme caution where the material from which the stake is moulded is of a much higher melt index than the material being assembled to it or where a thin thermoplastic washer is being fitted.
Methods of permanent assembly for plastics
Welding
- Hea ting the faces to be joined and bringing them together when a melt temperature has been reached.
- Heat can be applied by several methods.
- Relies upon total compatibility of the materials.
Solvent Bonding
- Uses a solvent or bonding agent to create a mechanical and/or chemical adhesion of the surfaces. Bonds can occur between dissimilar or incompatible materials.
Staking/Swaging
- Re-form ing of pegs ribs or collars built into as moulding as retention features for additional parts.
- Can be used to assemble dissimilar or incompatible materials.
- Several methods can be used to achieve the reforming process.
- Simultaneous processing (heating re-shaping clamping and cooling) can be carried out at many locations on the part or assembly
- Phasa Developments
- Tel: 01279 657711
- Contact: Dennis Eagles
January 1999