We offer fastener that are designed specifically to be used with plastics, intended for predrilled or moulded in pilot holes. They create threads in plastics by thread forming as opposed to a traditional self tapping screw that cuts into the material:
Thread forming – Where the fastener pushes the material apart into a new shape.
Self tapping – Where the fastener cuts into the material, creating chips and swarf as well as applying more stress to the material.
Radial stress needs to be avoided as it can damage or cause cracking in the material.
In the diagram to the right the force applied from the fastener is directed perpendicular to the surface of the thread.
This outward force is reduced by around half as it is proportional to the angle at which it's applied. Where that portion of the force is equal to the sine of the angle on the inclined plane (sin θ).
Sin 15- 0.259
Sin 30 - 0.50
Fasteners for plastics also have a thinner minor diameter and longer threads than standard fasteners.
Below you can see while on major diameter of both screws are the same, due to the smaller minor diameter, there is more material engaged with the fastener it is more resistant to pull out. This is particularly important as plastics tend to be softer.
Most standard self tapping fasteners for woods and metals have a wide flank angle of 60 degrees.
Fasteners for tapping into plastics require narrower flank angles, such as 30 or 45 degrees. This reduces radial stress and expansion when the fastener is driven into the material. When these are reduced it allows for boss used to also be reduced. Bringing wall thicknesses down in plastic moulding allows for reduction in cost as less material is used and cycle time can be increased.
In the diagram to the left, you can see the 30 degree thread profile allows with the same amount of displacement in the material, a deeper thread that engages with more material.
Under load or heat plastics can be susceptible to creep, a slow permanent deformation of the material. This will reduce overall clamping load.
In your design this can be compensated by:
Flat washers can spread load over a wider area in a fastening assembly.
Spring elements such as DIN127B washers can compensate for creep in your assembly.
Identifying the correct torque requirement is important as these can vary depending on the fasteners, as longer screws generally require more torque, as well as plastics having a wide variety of properties and hardnesses.
As a general recommendation we advise:
Trilobular threads on fasteners can be identified by having 3 sides. Despite appearance the width through the centre is constant at any point across the thread.
This design allows thread forming by pushing material aside rather than cutting into the material, and it allows this at a lower torque than if the thread was round. This is achieved by concentrating the force at those points.
The below diagram is exaggerated and the exact values for the screw diameter and the outer circle diameter are on the tolerance charts at the bottom of the page.
As such, as a starting point please reference to the Flexural Modulus of the plastic, we have some indicative values below but it can vary from grade to grade, even on the same base plastic.
| Recommended Screw | Material Code | Full Material Name | Flexural Modulus (PSI) |
Flexural Modulus (MPa) |
|
| Soft | PT 30 | LDPE | Low Density Polyethylene | 150,000 | 1,000 |
| PP | Polypropylene | 200,000 | 1,400 | ||
| PC | Polycarbonate | 340,000 | 2,300 | ||
| ABS | Acrylonitrile butadiene styrene | 350,000 | 2,350 | ||
| PA 66 | Nylon 6/6 | 350,000 | 2,350 | ||
| POM | Acetal / Polyoxymethylene | 400,000 | 2,700 | ||
| PS | Polystyrene | 430,000 | 2,900 | ||
| PP TV20 | Polypropylene - 20% Talc Filled | 500,000 | 3,300 | ||
| PPS | Polypheylene Sulfide | 550,000 | 3,700 | ||
| Moderate |
PT30 / PT45 |
ABS GF20 | Acrylonitrile butadiene styrene - 20% Glass Filled | 650,000 | 4,300 |
| PA 66 GF12 | Nylon 66 - 12% Glass Filled | 800,000 | 5,300 | ||
| PC GF20 | Polycarbonate - 20% Glass Filled | 850,000 | 5,700 | ||
| Stiff | PC GF30 | Polycarbonate - 30% Glass Filled | 1,100,000 | 7,300 | |
| PBT GF30 | Polybutylene terephthalate - 30% Glass Filled | 1,100,000 | 7,300 | ||
| PA66 GF30 | Nylon 6/6 - 30% Glass Filled | 1,200,000 | 8,000 | ||
Our screws for plastics are all available in three materials.
A common silver colour coating on case hardened steel.
Mechanically identical to bright zinc plated, but offering a different aethetic finish.
The most corrosion resistant material available.
This drive, also known as kaxalobular, offers the best torque transfer.
This is our most common head, suitable for a wide range of implementations.
This head style allows for the surface of the screw head to be flush against the surface of the panel.
This is the widest head we offer, the wider head is better as spreading stress over a wider area, particularly of relevance when working with softer plastics.
This is the most commonly and readily available drive.
Every fastener we can provide is available with these three head styles.
Those three head styles above are all available with both Pozi or Torx drives
Due the different thread forms and different properties of various plastics, the recommended hole sizes are below.
We also recommend the general considerations with all shapes and styles of fastener;
| d | Nominal Screw Diameter |
| dA | Boss Diameter |
| dH | Hole Size |
| ts | 0.4 x d |
| h | Minimum - LE + (1 x d) |
| LE | Length of Thread Engagement |
| S | No specific value |
Material Code |
Full Material Name |
Hole Size (dH) |
Boss Diameter (dA) |
Thread Engagement (LE) |
| ABS | Acrylonitrile butadiene styrene | 0.80 x d | 2.00 x d | 2.00 x d |
| ABS/PC Blend | Acrylonitrile butadiene styrene + Polycarbonate | 0.80 x d | 2.00 x d | 2.00 x d |
| ASA | Acrylonitrile styrene acrylate | 0.78 x d | 2.00 x d | 2.00 x d |
| PA 46 | Nylon 4/6 | 0.73 x d | 1.85 x d | 1.80 x d |
| PA 46 GF30 | Nylon 4/6 - 30% Glass Filled | 0.80 x d | 1.85 x d | 1.80 x d |
| PA 6 | Nylon 6 | 0.75 x d | 1.85 x d | 1.70 x d |
| PA 6 GF30 | Nylon 6 - 30% Glass Filled | 0.80 x d | 2.00 x d | 1.90 x d |
| PA 66 | Nylon 6/6 | 0.75 x d | 1.85 x d | 1.70 x d |
| PA 66 GF30 | Nylon 6/6 - 30% Glass Filled | 0.82 x d | 2.00 x d | 1.80 x d |
| PBT | Polybutylene terephthalate | 0.75 x d | 1.85 x d | 1.70 x d |
| PBT GF 30 | Polybutylene terephthalate - 30% Glass Filled | 0.75 x d | 1.85 x d | 1.70 x d |
| PC | Polycarbonate | 0.85 x d | 2.50 x d | 2.20 x d |
| PC GF30 | Polycarbonate - 30% Glass Filled | 0.85 x d | 2.20 x d | 2.00 x d |
| LDPE | Low density Polyethylene | 0.70 x d | 2.00 x d | 2.00 x d |
| HDPE | High density Polyethylene | 0.75 x d | 1.80 x d | 1.80 x d |
| PET | Polyethylene Terephthalate | 0.75 x d | 1.85 x d | 1.70 x d |
| PET GF 30 | Polyethylene Terephthalate - 30% Glass Filled | 0.80 x d | 1.80 x d | 1.70 x d |
| PMMA | Methyl Methacrylate | 0.85 x d | 2.00 x d | 2.00 x d |
| POM | Acetal / Polyoxymethylene | 0.75 x d | 1.85 x d | 2.00 x d |
| POM GF30 | Acetal / Polyoxymethylene - 30% Glass Filled | 0.80 x d | 1.85 x d | 2.00 x d |
| PP | Polypropylene | 0.70 x d | 2.00 x d | 2.00 x d |
| PP GF30 | Polypropylene - 30% Glass Filled | 0.72 x d | 2.00 x d | 2.00 x d |
| PP TV20 | Polypropylene - 20% Talc Filled | 0.72 x d | 2.00 x d | 2.00 x d |
| PPO | p-phenylene oxide | 0.85 x d | 2.50 x d | 2.20 x d |
| PS | Polystyrene | 0.80 x d | 2.00 x d | 2.00 x d |
| PVC (Rigid) | Polyvinyl Chloride | 0.80 x d | 2.00 x d | 2.00 x d |
| PEEK | Polyether Ether Ketone | 0.85 x d | 2.00 x d | 2.00 x d |
| Screw Diameter | Soft Materials | Hard Materials |
| 1.8 | 1.19 | 1.45 |
| 2.2 | 1.47 | 1.79 |
| 2.5 | 1.80 | 2.00 |
| 3.0 | 2.26 | 2.50 |
| 3.5 | 2.73 | 2.95 |
| 4.0 | 3.18 | 3.41 |
| 5.0 | 3.62 | 4.10 |
| 6.0 | 4.55 | 5.05 |
As a general rule we recommend as a beginning point;
Typical Soft plastics include LDPE, PP, ABS and Nylons.
Typical Hard plastics include Polycarbonate, glass or otherwise reinforced plastics.
| Size | Pitch | d (Min) | d (Max) | d1 |
| 1.8 | 0.8 | 1.80 | 1.90 | 1.04 |
| 2.2 | 0.98 | 2.20 | 2.34 | 1.25 |
| 2.5 | 1.12 | 2.50 | 2.64 | 1.40 |
| 3.0 | 1.34 | 3.00 | 3.15 | 1.66 |
| 3.5 | 1.57 | 3.50 | 3.68 | 1.91 |
| 4.0 | 1.79 | 4.00 | 4.18 | 2.17 |
| 5.0 | 2.24 | 5.00 | 5.20 | 2.68 |
| 6.0 | 2.69 | 6.00 | 6.20 | 3.19 |
| Nominal Length | Tolerance (mm) |
| 4 - 6 | ±0.60 |
| 7 - 10 | ±0.75 |
| 12 - 18 | ±0.90 |
| 20 - 30 | ±1.05 |
| 35 - 50 | ±1.25 |
| 60 | ±1.50 |
| | C Hole Diameter |
D Screw Diameter |
|||
| Size | Pitch | Min | Max | Min | Max |
| 1.8 | 0.8 | 1.75 | 1.85 | 1.65 | 1.75 |
| 2.2 | 1.0 | 2.15 | 2.25 | 2.05 | 2.15 |
| 2.5 | 1.4 | 2.45 | 2.55 | 2.40 | 2.50 |
| 3.0 | 1.5 | 2.95 | 3.05 | 2.90 | 3.00 |
| 3.5 | 1.65 | 3.45 | 3.55 | 3.37 | 3.47 |
| 4.0 | 1.75 | 3.91 | 4.06 | 3.81 | 3.96 |
| 5.0 | 2.3 | 4.91 | 5.06 | 4.81 | 4.96 |
| 6.0 | 2.5 | 5.91 | 6.06 | 5.80 | 5.85 |
| Size | Length | Tolerance |
| 1.8 - 2.2 | Any | ±0.8mm |
| 2.5 - 5.0 | < 20 mm | ±0.8mm |
| > 20 mm | ±1.3mm | |
| 6 + | Any | ±1.3mm |
| Size | Soft Plastics | Hard Plastics |
| 1.8 | 1.19 | 1.45 |
| 2.2 | 1.47 | 1.79 |
| 2.5 | 1.80 | 2.00 |
| 3.0 | 2.26 | 2.50 |
| 3.5 | 2.73 | 2.95 |
| 4.0 | 3.18 | 3.41 |
| 5.0 | 3.62 | 4.10 |
| 6.0 | 4.55 | 5.05 |
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