Tappex is a UK-based original manufacturer of Threaded inserts for plastic, metals, and composite materials. Below is 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. Tappex would like to offer ten points to consider when selecting a threaded fastener for use in plastics.

1. Required mechanical performance
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 molders should not hesitate to seek their advice.

2. Production process
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 be placed into the mold tool before the plastic is introduced. This process is described as mold-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 mold tool, molded holes can be produced for one of several post-mold insertion processes. Alternatively, parts can be fabricated from plastic sheets; 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. Insert design
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. Insert parts and installation cost
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. Production volumes
Consideration of the proposed production volumes also plays a part in deciding whether to mold in inserts or install them after molding, either beside the molding machine or as a separate post-mold process at a later date. While processes such as rotary molding lend themselves to hand-loaded mold-in inserts, they can slow the injection molding process unless that is, the production of very large quantities justifies the capital cost of robotics to simultaneously remove the moldings and load the inserts automatically within the required molding cycle time.

6. Material
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 molding-in, brass components are less likely to damage the mold tool should they become misplaced during the mold 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. Installation
The choices for post-mold insert design cover press-fit, either cold or with heat or ultrasonic, 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 glass-filled or mineral-filled types.

8. Size
Larger sizes of insert can pose problems if they are 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 molding. Therefore, care in the design of fixtures and in the handling of molding is important.

9. The efficiency of self-tapping inserts
For many post-mold 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. Quality
From a quality standpoint, self-tapping inserts, in both male and female threaded forms, provide a monitored production process utilizing torque-controlled clutches, and because the thread is used for the installation, every insert in a finished molding 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 browse our products. You can also follow us on Twitter…@tappexltd