Threaded inserts for plastic, metals and composite materials - A helpful ten point guide for Designers, Engineers, Specifiers and end users alike...
Design engineers who use plastics face increasing problems when trying to specify high performance
inserts that will give reliable cost-effective assemblies. Miles Harding, engineering manager of Tappex Thread Inserts Ltd (
www.tappex.co.uk) presents ten points to consider when selecting a threaded fastener for use in plastics.
1 The first essential is to understand the intended use of the insert and the mechanical performance specification that it is expected to meet. The mechanical performance of any insert in plastic will depend
upon how it interfaces with any mating parts, i.e., is it to be clamped to a mating surface or is it to be unsupported and,therefore, rely on its grip in the plastic to prevent pulling out. Most fastener manufacturers
offer a technical service so designers and moulders should not hesitate to seek their advice.
2 A working knowledge of the proposed production process and the type of plastic to be used is very important. In most instances plastic components are produced by a moulding process. There are a
number of different processes used - injection, compression, rotary or blow, resin-injected processes for GRP and composites, and vacuum forming which allow an insert to placed into the mould tool before the plastic is introduced. This process is described as mould-in and, in general, gives rise to the strongest
mechanical performance of an insert. However, the design of a component does not always allow this so, by positioning core-pins in the mould tool, moulded holes can be produced for one of several post-mould insertion processes. Alternatively, parts can be fabricated from plastic sheet; this usually involves machining holes for a post-mould insert. Use of thermoplastic grades which remelt after moulding or thermoset grades which do not melt is an important consideration when a suitable insert is being selected.
3 Where possible it is advisable to choose an insert design from a manufacturer's
standard range of parts. Usually, test data covering various materials will be available to support use in a particular application. A further benefit is that standard parts can be obtained in small quantities off-the-shelf for the support of all pre-production requirements. And they will be readily available to meet production
requirements at whatever volume, cost effectively.
4 Although the purchase cost of a fastener is very important, designers need to consider the proposed production process in order to evaluate the true 'in-place' cost of the fastener, i.e., the sum total of the
fastener piece-part cost plus the cost of installing it into the plastic.
5 Consideration of the proposed production volumes also plays a part in deciding whether to mould-in inserts or install them after moulding, either beside the moulding machine or as a separate postmould
process at a later date. While processes such as rotary moulding lend themselves to hand-loaded mould-in
inserts they can slow the injection moulding process; unless, that is, the production of very large quantities justifies the capital cost of robotics to simultaneously remove the mouldings and load the inserts automatically within the required moulding cycle time.
6 The choice of insert material is worth noting. Brass is the most popular because, although more expensive than steel, it can be machined faster, and it can be recycled economically requiring less energy to process overall. For moulding-in, brass components are less likely to damage the mould tool should they become misplaced during the mould cycle. For most applications the resistance of brass components to corrosion does not require any additional and costly plating finish which, in itself, makes recycling difficult.
7 The choices for post-mould insert design cover press-fit, either cold or with heat or ultrasonics, and self-tapping. Generally, a designer should not choose to press-fit inserts into thermoset material. They are ideally suited to press-cold into softer grades of thermoplastic materials, such as abs and polypropylene, or with
heat into harder grades, such as acrylic and nylons, particularly if they are glassfilled or mineral-filled types.
8 Larger sizes of insert can pose problems if they required to be installed with heat. As significantly more power is required to heat them to the defined temperature, after installation the heat takes more time to dissipate and to allow the boundary layer of plastic to cool sufficiently to fix the insert position in the
moulding. Therefore, care in the design of fixtures and in the handling of the moulding is important.
9 For many post-mould applications - particularly in all thermoset grades and in harder thermoplastics with glass reinforcement - a self-tapping design of insert provides a cost-effective and reliable solution.
For small production volumes it can be a flexible process that does not require expensive dedicated installation equipment. However, for larger volumes installation can be arranged to take place alongside the moulding machine, often by under-utilised labour during the mould cycle period, This will provide significant
cost savings over the lengthier heat insertion methods of installation.
10 From a quality standpoint self-tapping inserts, in both male and female threaded forms, provide a monitored production process utilising torque-controlled clutches, and, because the thread is used for the
installation, every insert in a finished moulding has to have the correct thread form to have been installed at all.
Summary
All options available for a particular application must be considered based on material grade, process capability, and finished performance to specification. A standard part should be used if at all
possible. The alternative is to seek out a specialist manufacturer and ask for a quotation for the production of a special part, placing reliance on the experience of its personnel to incorporate the essential features and tolerances relating to the interface with the plastic.
We hope you have found this guide useful. If you have any further queries or need help with identifying an insert to suit your needs, please do give us a call on 01789 206600 or visit our website...
www.tappex.co.uk
