Polyfix Screws: Self Tapping Screws For Plastic
What Are Polyfix Screws?
Polyfix Screws are a type of fastener specifically designed to be fastened into plastics. Intended for use within pre-drilled pilot holes, Polyfix Screws are self tapping, meaning they create their own threads when driven into an assembly.
These screws feature a specialised trilobular shaped thread forming thread and have a sharper thread profile in comparison to standard fasteners.
This sharper thread profile creates less pressure on the walls of a plastic boss, making them suitable for injection moulded parts.
Similar to Thread Rolling Screws, Polyfix Screws are fasteners with a trilobular thread that will form the plastic around them when they are driven into a pilot hole.
The Trilobular Thread
The Trilobular shape can be identified by its three equal sides and constant width, specifically intended to allow the fastener to form threads with less driving torque.
Although it may not seem that way, the triangular profile is the same width across the centre of the shape.
Trilobular threads have three lobes designed for cutting and three recessed relief areas. This shape allows the plastic assembly to fill, or ‘cold flow’ into the free spaces during installation, ensuring a strong hold.
This also makes the installation of the screw easier as there is a reduced cutting area, and the driving torque is more concentrated at each point.
Types of Polyfix Screws
Polyfix Screws are available with 2 different thread angles, which make them an ideal self tapping screw for applications involving various plastics, including thermoplastics.
Accu offers Polyfix Screws in both 30 degree and 45 degree thread angles as standard, with a wide range of drive types and head types offering flexibility when choosing the ideal fastener for your application.
A Polyfix Screw with a 30 degree thread angle, also known as Polyfix 30, is ideal for use within low density plastics, being better suited for softer materials due to the sharper angle of the thread.
By comparison a Polyfix 45, a Polyfix Screw with a 45 degree angle, is ideal for medium density plastics.
Why Choose Polyfix Screws Over Self Tapping Screws?
With sharper threads than standard Self Tapping Screws, Polyfix Screws can cut deeper into a housing. This means that the fastener has a better resistance to pull out or stripping, and can offer improved resistance to vibration with reduced friction during the thread forming process.
With standard Self Tappers featuring a thread angle of 60 degrees, the lower thread angle offered by Polyfix Screws reduces radial stress and expansion, maximising the fasteners performance.
This special thread profile also allows the use of smaller bosses, which can reduce the overall cost of materials used within an assembly.
How To Install Polyfix Screws Into Plastics
Polyfix Screws perform best when they are used in a boss, with the thinnest part - if the boss is tapered - being at least 2 times, ideally three times, the diameter of the thread.
To reduce the risk of bursting and cracking, the pilot hole should be chamfered, ideally to the thread’s major diameter.
These fasteners should not be loaded in shear; they are designed to clamp two parts together with no space in between.
The pilot hole for this fastener should be deep enough for at least twice the diameters worth of threads to be engaged.
For guidance when using Polyfix Screws, please consult the tables below.
These tables demonstrate the recommended pilot hole sizes alongside the corresponding thread engagement number for all sizes of Polyfix Screw threads in different common plastic uses.
Please note that these figures should be used as guidance only and several factors must be considered when installing Polyfix Screws into plastics.