Leader pins, also referred to as the guide pins, are an essential mold component in the injection molding process that are made with hardened alloy steel, specifically designed for use in plastic molds to offer precise alignment between the mold halves. With the use of a specifically designed shoulder and a retaining bolt, you will be a lot less prone to problems pertaining to press-fit type of guide pins or leader pins.
All mold components, including the leader pins, are all integral parts of the injection molding system. Components such as straight leader pins play the role of ensuring positive clamp plate alignment while sprue bushings provide the seal for extrusion nozzle, as well as the subsequent transfer of material. Other essential components like the shoulder style ejector pins all play their own role in kicking off finished mold at process conclusion.
Leader pins are typically used for maintaining the consistency in a handful of mechanical applications. They are mostly used in applications where consistency in repetitive motion of moving parts within a mechanism is required. As mentioned previously, leader pins are typically made of alloy steel, but can also be made of other material materials depending on the application.
Moreover, you’ll often see that leader pins or any other parts of the ejection mold system come in various dimensions and sizes based on the mechanism or the applications in which they are used. These pins can be designed to become a stationary component of a chassis, or stay as an immovable component within a mechanical application wherein pins are not required to be a permanent fixture.
In the plastic injection mold application, there are many types of leader pin structures. In such system, the leader pins must serve as the cylindrical surface of concentric circles of different diameters. Depending on the material requirements and the structural dimensions, an appropriately sized, round steel bar can be directly chosen as the material. Additionally, the leader pins in the ejection mold system must comply with the related technical requirements which are addressed below:
1. At the joint between the fixed mold plate and the leader pin, the diameter concentricity tolerance should go beyond half of the diameter tolerance in the working portion.
2. The leader pins’ cylindrical tolerance in the working portion should also comply with the technical requirements.
3. The precision, surface quality and thermal treatment of each part of the guide pin should satisfy the specifications stated in the drawing after being machined. The carburized coating on the working surface must be uniform, usually with a thickness of 0.8 to 1.2 mm, if appropriate.
The purpose of leaders pins is quite simple – to maintain the optimum clamping of the core and cavity and the correct shape of the cavity upon clamping. And the actual machining process is actually just as simple:
1. Material Preparation and Cutting:
As aforementioned, the most commonly used material of the leader pin is steel. Of course, the materials can be selected based on the attributes of your drawing. After cutting, you should allow about 3 to 5 mm of length remained for facing, and 3 to 4 mm for cylindrical turning.
2. Facing and Centering:
Next, turn one end of the pins to retain about a 1.5 to 2.5 mm turning allowance, and proceed to drilling the centered hole. Then, turn the other end to the predetermined requirements and drill the centered hole again.
3. Cylindrical Turning:
Now, turn the cylinder by a tad bit, and retain an allow of about 0.5 mm on each side for grinding. You can groove the leader pin to the required specification if necessary.
Once finished with step 3, inspect the finish sizes resulted from the previous step.
5. Thermal Treatment:
Continue with the process, and ascertain that a carburization thickness of 0.8 to 1.2 mm is produced. Upon carburization, the quenching hardness should be about 58 to 62 HRC.
After undergoing thermal treatment, have one end of the centered hole lapped, and then have the end of the other one lapped as well.
Next, grind the cylinder by applying both the centerless grinder and the cylindrical grinder. You should then have about 0.01 to 0.05 mm of allowance ready for lapping.
8. Lapping again:
Upon grinding, you need to lap the surface of the cylinder to reduce the roughness of surface, thereby achieving the required specification of surface finish.
9. Final inspection:
Last but not least, inspect the finish size of all of the steps above.
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