The Manufacture Of Plastic Pens

Injection Molding

Introduction

Have you ever wondered about how every day plastic items, like your pen, was made? We are surrounded by a world of plastic. Chances are that everyone here uses a pen every day. Whether or not you wonder about it, I think you would benefit from knowing how plastic items are manufactured. Plus, this will give you something to talk about during Thanksgiving dinner.

I had an internship this past summer in Minneapolis at an injection molding company called Advanced Molding Technologies who has been active since 1991 and left with a lot of knowledge about the industry. (Advanced Molding Technologies).

Injection molding is a complex yet interesting process that starts with many beads of resin and yields the majority of everyday objects. Today we will explore what injection molding is, how it is done, then look into how pens, specifically, are made.

What is injection molding? David Johnson, an engineering manager at Advanced Molding Technologies with a background of 27 years in the industry says that injection molding is a manufacturing process used in mass production. Each mold makes a specific part which creates repeatability, making every part consistent and reliable. Because every mold makes a specific part it can take months to years to get a part into production. Erik Sherburne, a quality engineer of 10 year also at Advanced Molding Technologies, told me about the various materials used by injection molding.

Metals, elastomers, glasses, and thermoplastics are the most common. We will focus on the latter. Thermoplastics is a plastic material that melts when heated up and then solidifies when cooled. Now that we know what injection molding is let’s find out what the process of injection molding is.

What is the process of injection molding? (Rosato 21). Thermoplastic (resin) is heated up in the barrel. The resin drops from hopper into a long barrel that has a screw in it and a heater around it. The screw pushes resin down barrel heating up the resin as it makes it to the nozzle. Melted resin is pushed through the nozzle into the tool. This is defined as a shot. The screw keeps pushing until the cavity is filled with molten resin. The screw then holds the pressure while resin fills the cavity. The new formed part will then be cooled and ejected. Cool water is ran through tool using hoses to cool part. Small pins eject part from tool into a drop box or on a conveyor belt. Now that we know what the process is let’s look deeper into how parts are made, specifically pen.

All parts have varying geometry and features. Pen’s geometry can hard to manufacture because of its thinness and special features to obtain that. V. Rosato, who has written and edited multiple versions of Injection Molding Handbook tells us that there are two sets of components when it comes to molding tools. The cavities and cores of the tool. The base in which the cavities and cores are mounted. A pen’s molding tool has a core and multiple cavities. Pens, like many other parts, are easiest to manufacture by injection molding because it is the most efficient way to mass produce them in the shortest amount of time.

Conclusion

All the plastic parts around us are made by a complicated process that takes a long time called injection molding. Injection molding is a manufacturing process that is used to mass produce plastic parts. It starts by melting resin, shooting it into a cavity, then cooling and ejecting it. Pens are made using the same process with some twists thrown into it. Next time you use a plastic pen, cup, tupperware and even the chairs you sit on, you will have a general understanding of how it was manufactured.