Become a plastics pro: Six injection moulding arenas you need to master

SPONSORED CONTENT: US-based injection moulding solutions provider RJG, Inc. describes the six steps injection moulding specialists can take to become a 'plastics pro'.

There are those who are good at what they do, and then there are those who are the absolute best. The pros. The late NASCAR legend, Dale Earnhardt is a great example of one such pro. He was so good, in fact, that it’s thought that he could see the air moving around the car to better understand what move he should make next.

If you’ve had the honour of meeting a plastics pro, you’d know how brilliant they are. How are they so good? What do you need to know to be on that higher level?

The list of need-to-knows in order to become a plastics pro is seemingly endless, and getting started may feel overwhelming. That's why we recently released a white paper digging into six often overlooked technical arenas that are vital to become a plastics pro: mechanical, electrical, fluid dynamics, thermodynamics, metallurgy, and statistics. Here's a brief overview of each.

1. Mechanical

There are many areas that require a deep understanding of mechanical in the industry, including the machine, the intensification ratio, and the mould.

The Machine: Did you know that 80% of the energy required to melt the material comes from mechanical friction? The first step is understanding the Length to Diameter Ratio (L/D) (a comparison of its flighted length to diameter) in conjunction with the compression ratio (the reduction in depth of the space between the root diameter and outside diameter of the screw).

The Intensification Ratio: The next type of mechanical advantage that must be understood on the moulding machine is the intensification ratio. This ratio determines the amount of work, or power, the injection unit can deliver during the filling and packing phase (figure 1).

The Mold: It’s important to understand minimum and maximum angles for either to ensure the steel can be disengaged from the moulded part. If the angle is too steep, there could be galling between the metal components. If it’s too shallow, the distance of travel required to release the part could negatively impact the mould size or place excessive stress on the lifter rod or cam pin.

2. Electrical

To a certain extent, a plastics pro needs to have an understanding of how electricity is used in the machine, as well as in the mould.

The machine’s heaters are used to preheat the screw and barrel assembly before screw rotation can occur. If the heater bands are not properly sized, it can take a long time before production can begin. The heater bands also provide a small amount of energy to melt the plastic material. Understanding how to size the heater and how to verify the proper function can go a long way in making sure the machine is performing appropriately, increasing the opportunity for quality parts each cycle.

3. Fluid Dynamics

Understanding the characteristics of Newtonian and non-Newtonian fluids in the injection moulding industry is vital to create a successful process. In order to dig into fluid dynamics, you must first understand these 3 terms.

Fluid dynamics: The properties of fluids in motion. There are two different types of fluids: Newtonian and Non-Newtonian (Shear Thickening or Shear Thinning) along with understanding viscosity as shown in Graph 1.

Viscosity: The resistance to flow. The more the fluid resists flow, the higher the viscosity.

Shear rate: The change in velocity of parallel planes in a flowing fluid. Picture a river and how the water flows differently near shore in comparison to in the centre of the river. At the edges, the water flows slower. That’s why the fish hang out there, because they don’t have to work as hard to stay in one location. Out in the centre of the river, water flows faster and has higher shear rates.

4. Thermodynamics

Thermal conductivity, specific heat, and shrink rate are three aspects of thermodynamics that have a huge impact in overall part quality and process efficiency.

Thermodynamics: Physics that deals with heat and other forms of energy.

Thermal Conductivity (Tλ): The rate in which heat can be conducted through a material.

Thermal Expansion: How much a material will expand with a given temperature.

Specific Heat (Cp): The heat required to raise the temperature of the unit mass of a given substance by a given amount.

Shrink Rate: The amount a thermoplastic will shrink when cooled. The machine is responsible for putting energy into the material through screw rotation to force the pellets from a solid state to a liquid state, then reversing the process in the mould.

5. Metallurgy

There is a surprisingly significant amount of metal that is used in the manufacturing of plastic moulded parts. A few key areas include screw and barrel assembly, tie bars, and platens.

Understanding the material that is going to be processed also impacts the cavity and core steel selection. When processing materials that are abrasive, choosing the appropriate mould steel is critical.

6. Statistics

One of the least favourite topics for many people is mathmatics and statistics. But a plastics pro needs to have an understanding of it because they are likely responsible, at least in part, for the qualification of a new part.

Conclusion

Don’t be fooled—plastics pros possess no superpowers. They may not even be masters of all of these areas. But they do have a firm understanding of each area and its influence over the process as a whole. Gurus are able to see plastics from many different perspectives, which allows them to better understand the situation and how to apply the appropriate solution. You, too, can become a pro with enough training and practice.