Vibration welding nylon 6 and nylon 66 - a comparative study

TWI Bulletin, July - August 1995

Ian Froment is a Project Leader in the Plastics Joining Group of the Advanced Materials and Processes Department at TWI. He started his career at TWI in 1985 as a trainee research technician, during the first five years he studied and gained an HNC in Electronic Engineering, and has recently gained a B Eng (Hons) Electronic Engineering at Anglia Polytechnic University. Ian has gained a wide knowledge of plastics joining processes and equipment and is responsible for running research projects relating to specific customer applications. Over the last three years he has been responsible for the development of equipment, including a microwave oven for welding plastics, and a resistive implant welding machine.

Nylons are becoming one of the most successful materials for replacing metals in under bonnet applications. Selection of the correct grade can often be influenced by the materials weldability characteristics. This study was undertaken by Ian Froment to compare vibration welded nylon 6 and nylon 66.

Typical applications range from inlet manifolds to throttle housings. The complex design of these items force designers to use expensive lost core processing techniques to produce the components. In order to cut the production costs, these components are increasingly being injection moulded in two parts and vibration welded together.

The harsh operating environment, i.e. high temperatures under the bonnet, means that the selection of nylon material is critical. For example, nylon 66 has higher tensile strength at room temperature (65MPa, moist) than nylon 6 (50MPa, moist) ^[1] . Nylon 66 also has a higher maximum service temperature than nylon 6 (typically 199°C for nylon 6 and 241°C for nylon 66).

This study was undertaken to compare nylon 66 and nylon 6 when vibration welded. The study investigated the effect of material displacement during welding, material condition i.e. dry or wet, material orientation, and welding pressure on the strength of the welded joints. Results were obtained for three operating temperatures; 23°C, 80°C and 120°C, where 80°C and 120°C are typical operating temperatures for under bonnet components.

Materials

Identification

This project was conducted on two 30% glass reinforced nylon materials:

• nylon 6 Du Pont Zytel 73G30
• nylon 66 Du Pont Zytel 7OG30.

Material preparation and geometry

Injection moulded plaques were welded in two directions - longitudinal and transverse to the direction of the fibres in the moulding. The geometry of the moulded plaques was 3.2mm thick x 125mm x 75mm and is shown in Fig. 1.

Welds were made in both 'dry as moulded' (DAM) and 50% relative humidity (RH) conditioned materials. Because of the delay between moulding and welding the 'dry as moulded' materials were dried in a vacuum oven for 24 hours at90°C prior to welding. For the humidity conditioned materials, the plaques were boiled in potassium acetate and water solution (ratio: 1 part water/1.25 parts potassium acetate) for 24 hours prior to welding.

Vibration Welding Trials

The Process

In vibration welding the mechanism for generating heat is by the interaction of two rubbing surfaces, the relative linear motion of one against the other whilst applying a constant load. Once molten material ('molten' implies asoftened condition of low viscosity) has been generated at the joint interface the vibration is stopped and the components aligned. The applied load is maintained whilst the weld cools and consolidates.

Welding equipment

The welding machine used for the trials was a Bielomatik variable frequency machine (Type K3215/11). The frequency and amplitude of the machine were fixed by the mass of the tooling on the top plate. For these trials the machineoperated at a frequency of 206Hz and an amplitude of 1.6mm. For the vibration welding trials the machine was operated in 'weld by depth' mode.

Welding procedure

The following parameters were kept constant for all the welding trials. Six welds were made at each welding condition:

Weld Pressure - 1.8MPa
Welding Frequency - 206Hz
Vibration Amplitude - 1.6mm peak to peak

Three parameters were varied, and their effect on weld strength investigated:

Material displacement - 1, 2, 2.5mm
Plaque orientation - longitudinal/transverse
Material Condition - DAM/conditioned

An additional study was conducted to assess the effect of weld pressure.

Weld assessment

The strength of welded joints was assessed by tensile testing. Five tensile bars (14mm wide from longitudinal welds and 15mm wide from transverse welds) were cut from each welded plaque. Three test temperatures were used:

23°C

80°C

120°C

A total of ten tensile bars were tested at each of the three temperatures for each welding condition.

Results

The results of the vibration welding trials are presented in Tables 1-4. Each value in the Table is the average of ten tensile test results, five tensile tests for each of two welds. The values are expressed in MPa.

Table 1. Longitudinal, tensile test results of vibration welds

All test results are in MPa.

Table 2. Transverse, dry as-moulded

All test results are in MPa

Table 3. Longitudinal, conditioned, 50% RH

All test results are in MPa.

Table 4. Transverse, conditioned, 50% RH

All test results are in MPa.

Effect of weld displacement

The Tables clearly demonstrate that increasing material displacement between 1 and 2.5mm during welding did not affect the strength of the joint. The results from Table 1 are plotted on Fig.2 and 3. These curves clearly show that the tensile strengths showed little variation at the three displacements investigated. For example, the results for nylon 6 tested at 23°C varied by only 1.9MPa. As the weld displacement was increased the amount of weld flash outside the joint increased. As a consequence of these results the following sections use data from the 2.0mm displacement trials for discussion purposes.

Effect of test temperature

Figures 2 and 3 also show the effect of tensile test temperature. At a test temperature of 23°C, there is no difference in the tensile values of both nylon 6 and nylon 66 where the mean value is 63MPa. Both nylon 6 and nylon 66 give decreasing weld strengths as the test temperature is increased. Nylon 6 ( Fig.2) decreases from a mean of 63MPa at 23°C to a mean of 25MPa at 120°C. The data in Fig.2 and 3 show that nylon 66 gives higher weld strengths when tested at 80°C or 120°C than nylon 6. For example, when welded longitudinal/DAM with a displacement of 2mm, nylon 66 gives a 24% higher tensile strength than nylon 6 when tested at 120°C.

In the following sections, the effect of test temperature varied as the welding parameters varied.

Effect of material conditioning

Results for welds made in nylon 6 and nylon 66 'dry as moulded' are given in Tables 1 and 2 with the results of the 50% relative humidity conditioned materials given in Tables 3 and 4. The results for 2.0mm displacement are shown graphically in Fig. 4 and 5.

The results show that there is a significant effect on tensile strength of material conditioning for both nylon materials welded in the longitudinal direction. For example nylon 6 gives weld strengths up to 70% stronger when welded DAM and tested at 230°C. At 120°C the effect of material conditioning is less evident, but again in nylon 6, weld strengths up to 30% stronger were achieved when welded DAM. A similar trend was seen when these materials were welded in the transverse direction. For example, nylon 66 gave 35% higher weld strengths when welded DAM and tested at 80°C.

The weld strengths achieved in nylon 66 were higher by up to 10MPa than those produced in nylon 6.

Effect of material direction

The effect of material orientation on tensile strength for both DAM and conditioned materials are plotted graphically in Fig.6 and 7. Higher weld strengths are achieved when welding in the DAM state, longitudinal direction and tested at 23°C. For example, at 23°C, nylon 66 gives 14% higher weld strengths when welded longitudinally. At higher test temperatures of 120°C, there is little difference between welding in the longitudinal direction to the transverse direction for either material. When welding in the conditioned state, there is little effect on weld strength of different weld orientations.

The weld strengths for nylon 6 were approximately 10MPa lower than those achieved in nylon 66. When tested at 80°C, for example, nylon 6 gave weld strengths of 33.6MPa whereas nylon 66 gave weld strengths of 42.6MPa.

Effect of weld pressure

A weld displacement of 2mm was fixed. The weld pressure was varied from 1.8MPa to 20MPa. The results, illustrated in Fig.8, show that as weld pressure was increased, the strength of the joint decreased. The maximum weld strength achieved was 63MPa when welded using a weld pressure of 1.8MPa.

The time taken to achieve the required material displacement was significantly less when using 20MPa welding pressure. The typical vibration time for a weld pressure of 1.8MPa was 6 sec to achieve 2mm displacement whereas welds produced with a 20MPa weld pressure had a vibration time of 2.3 sec.

At the higher welding pressures the tensile strength results for nylon 6 dropped off slightly more rapidly than for the nylon 66.

Discussion

Tensile strengths achieved in nylon 66 were higher than those achieved in nylon 6 when vibration welding injection moulded plaques.

In an ideal situation, to achieve the maximum weld strength, it is preferable to weld the components as soon after moulding as possible. In practice this scenario does not exist because welds made in the dry state will actually experience a conditioned environment. The strength of the welds was affected by the temperature of the environment, in some cases giving up to 50% reduction in strength over the test temperature range investigated.

The characteristics of the moulded part (i.e. fibre/flow direction) did affect the strengths of welded parts when tested at 23°C and welded in the DAM state, but did not have a significant effect when subjected to typical operating temperatures of 120°C. When welded conditioned, weld orientation did not affect the strength of the welds.

In order to achieve an optimum weld time, the displacement of material during the weld cycle must be kept to a minimum without affecting the quality of the weld. The results showed that increasing the weld displacement above 1mm did not improve the weld strength but only increased the weld time. In vibration welding the material displacement during the welding phase follows a distinct curve. ^[2]

At the start of the welding phase, the material is not in a molten state and therefore material displacement is small. When the material becomes molten, the rate at which it is moved to the outside of the joint, i.e. as flash, is increased. This happens until the rate of change becomes linear and no significant gain in weld strength is achieved by prolonging the weld cycle. In reality the accuracy of the moulding, specifically the surface to be welded, must be considered before determining the required weld displacement.

When welding nylon 6 and nylon 66 the maximum weld strength was achieved at the lower welding pressure. This can be explained by considering the stresses built up in the weld area during the welding process. At the higher weld pressures the final stresses in the material would be expected to be higher, resulting in a decrease in the tensile strength of the joint. At the lower pressures the reverse will be true. However, at the reduced pressure, a longer weld time is required to reach the steady state condition on the time/displacement curve. This has been recognised by equipment manufacturers, and it is now possible to specify a two pressure stage welding machine, i.e. a high pressure at the beginning of the vibration, allowing rapid generation of heat, followed by a reduced pressure at the end of the vibration, allowing the maximum strength to be reached.

Testing showed that regardless of material orientation, material condition, and displacement during welding, welds made in nylon 66 gave significantly better weld strengths than welds in nylon 6 when tested in an 80°C or 120°C environment.

Conclusions

• Welds made in nylon 66 give higher weld strengths than nylon 6 when tested at elevated temperatures regardless of material condition, orientation or displacement.
• Welding nylon 6 and nylon 66 in a 50% RH conditioned state significantly reduces the weld strength at 23°C when compared to material in the DAM state. Nylon 66 gives weld strength up to 10MPa higher than nylon 6 when welded conditioned.
• When tested at 23°C, welds made in the longitudinal direction gave weld strengths up to 20% higher than welds made in the transverse direction. At 120°C welds made in the DAM state were not affected by material vibration direction. In both directions, nylon 66 gave higher weld strengths than nylon 6.
• Increasing the testing temperature of the tensile test decreases the strength of welds made in nylon 6 and nylon 66. Nylon 66 gives up to 28% stronger welds at 120°C than nylon 6.
• Increasing material displacement above 1mm through prolonged weld times does not increase the strength of the weld but produces excessive flash.
• Increasing weld pressure gives lower failure strengths in both nylon 6 and nylon 66.

Acknowledgements

The author wishes to thank DuPont De Nemours International SA for the supply of Zytel materials to carry out this work.

References