For all non-overmoulded inserts, hole depth must always exceed insert height. The provision of a hole exceeding insert height by at least 2 mm is recommended, in order to prevent the bolt chosen from entering into interference with the bottom of the hole.
Obtaining a hole diameter that is both suitable and repeatable will greatly improve the performance of the insert. If the hole is too big, reduced performance will result, whilst if it is too small, there will be installation difficulties and excessive stress placed on the material - which could lead to cracking. The recommended diameters must be adapted in accordance with the filler level of the material. As a guideline, increase the bore diameter by 0.07 mm beyond a 15% filler level, and by 0.1 mm beyond 35%. Taking into account the great variety of materials and fillers, gradual adjustment of the pins used to make the holes is recommended right from the mould design stage.
The moulded walls present a surface hardening which is always favourable to the mechanical anchoring of the insert, and will be preferred over a machined finish. A maximum draft angle of 1.5° is recommended.
Wall thickness dimensioning
Mechanical anchoring performance is conditional on the volume of plastic material present around the insert. Wall thickness of 0.5 times the nominal tapping diameter of the insert is recommended: For example, for an M6 insert, a wall thickness of 3 mm is recommended. During the design of the workpiece and mould, it is important to prevent the formation of an unbonded knit line in the wall.
For press-in inserts, it is recommended that the surrounding walls, which will have to bear the insertion force, should be generously sized.
Unthreaded hole diameter in the mating workpiece to be assembled Ø T2
The assembly tension should not apply through the joint between the plastic workpiece and the insert. This is why it is important to ensure that the hole diameter is smaller than that of the upper face of the insert. Although the hole must have sufficient clearance for realignment of the workpieces to be assembled, it is also important that the diameter of the insert head be significantly greater than the diameter of the unthreaded hole.
If the corresponding workpiece to be assembled is also in plastic material, use of a compression limiter is recommended to maintain the assembly tension over time. The direct application of this tension to the plastic workpiece to be assembled will result in creeping and a relaxation of the stresses causing the assembly tension to drop. In choosing the diameter of the compression limiter, it is important to closely follow the above recommendations with regard to its inside diameter.
Wherever possible, it is always preferable to mount the insert on the opposite side to that of insertion of the bolt; in this configuration, the insert head provides excellent pull-out resistance. When using in thermoplastic materials, it is best to avoid allowing the assembly tension to be applied through the joint between the plastic workpiece and the insert.
Length of bolts used with expansion inserts
The bolts must be long enough to allow correct expansion of the insert and must exceed its length by the equivalent of at least 1 screw thread.
The application engineers at LA CLUSIENNE-CLUFIX are at your disposal to advise you in the choice of the most suitable design for your technical specification. Our testing facilities will enable you to validate the insert solution retained in its actual environmental conditions. You will be provided with a qualification test report.
This is the axial force necessary to pull the insert out of its plastic workpiece. This value represents the load which the fastener can bear.
This is the torque necessary to rotate the insert in its plastic workpiece. In real-life conditions, this value represents the stress resulting from the unscrewing of the bolt, in those configurations where the bolt is seized up inside the insert.
This value is not necessarily as high as the torque applied to the bolt during tightening, because the friction force absorbs a significant proportion of this torque.
Mechanical performance values are shown on the data sheets. Please note that, given the great variety of plastic materials, fillers and mounting configurations, these values are provided only as a guideline, and for the purposes of comparison between one and another.
The installation process is critical to the achievement of optimal performance, particularly the coaxial insertion of inserts into their housings, which must be carried out with great care. LA CLUSIENNE-CLUFIX has sufficient experience in a diverse range of projects and applications to be able to advise you. However, there is no substitute for a qualification test, which we systematically offer to carry out for you, in order to define the best parameters for your application.