Molding Introduction

Introduction

Overview of Injection Molding Process

Injection molding is one of the most common processes used for the production of polymer parts. This is a cyclic process and often used with thermoplastic polymers. A polymer in the form of pellets is mixed with other additives, then heated to a melt state, and finally pressurized in a single screw extruder. This pressurized polymer melt is injected into the mold at a high flow rate to fill the mold cavities. These cavities are made in the form of the final part accounting for the shrinkage. Then the mold is cooled and the part is ejected from the mold as soon as it is stable enough for ejection. This is a cyclic process and this sequence repeats. These stages can be listed as

• Mold closes and a new cycle begins

• Filling Stage:

  • 1st Stage - velocity-controlled injection (until the mold fills approximately to 80%)

  • 2nd Stage - pressure-controlled injection (in industry called as packing, both filling and packing happen in this stage)

• Packing Stage:

  • In the industry, this stage is called holding as the pressure at the inlet is held until the gates are frozen. Once the cavity is full, the gates are still molten and pressure is further applied to account for the volume of shrinking polymer in the cavities. This packing of additional material stops when gates are frozen. In simulation, this stage is called packing.

• Cooling Stage:
  • The mold is rapidly cooled until the part reaches a safe ejection temperature. For thick parts, the center of those thick regions may still not be frozen completely.
• Mold opens and the part is ejected
• Warpage:
  • Part continues to cool outside the mold and may warp due to thermal stresses and few other reasons.

Please watch this video for a quick and good introduction to the injection molding process

Key Process Variables

There are four primary process variables that control the injection process. They are:

1. Plastic Flow Rate,

2. Polymer Melt Temperature,

3. Plastic Pressure, and

4. Cooling Rate

The words "polymer" (technically accurate) and "plastic" (commonly used) are used interchangeably in this document. These four factors are used to control the molding process. From the simulation point of view, different phases of the injection molding processes are modeled using system of equations derived from the laws of conservation of mass, momentum, and energy; together with the material model and the equation of state. This enables to numerically study the effects of these four and other process variables on the manufacturing process.

The polymer melt temperature needs to be within a range where the polymer can be made to flow reasonably easily, but is still below the specific material's degradation temperature. Each material has a "no flow" and "degradation" temperature. The "no flow" temperature is not the glass transition temperature, but rather the temperature at which the polymer ceases to flow with an applied pressure. Often, this value much higher than the glass transition temperature. The degradation temperature is tested in the lab with DSC and is the temperature at which the polymer chains break and the physical properties of the melt are expected to be materially altered.

Given the generally high viscosity of polymers, one of the difficult parameters to control is the plastic pressure throughout the cavity in the mold. The more even the pressure, the more consistent the shrink and warp will generally be, aside from other effects. Molecular orientation, and fiber direction also play a significant role in post-mold shrink and warp, but those are controlled by the cavity design, flow patterns, and fill rate. As a process variable, plastic pressure, and the minimizing of its gradient, play a key role in shot-to-shot consistency.

The cooling rate determines the cycle time. Aside from scrap, one of the most heavily reviewed and scrutinized elements of part cost is the cycle time. All manufacturers seek to reduce overhead cost by minimizing the "in-process" time, cycle time in this case. It is, again apart from scrap, the most costly element in the manufacture of any product. If one can cut 10% off of the cycle time of a part, then that's 10% of the cost of the machines and labor to make those parts and it opens up more time for other products to be made in the same machines. The degree to which the cooling rate is even between the two halved of the mold will impact the post-mold warping of those parts. It should also be noted that cold molds that "freeze-in" the stretch of the polymer from molding will creep over time and eventually release that stress. One need only look as far as extruded floor runner in most public buildings to see how this works. Also, downstream processes like painting or assembly can heat the polymer and release those stresses, causing warp to manifest after the product was inspected at the molding press and approved for sale, or subsequent processing. The cooling rate is affected by the coolant used, the design of the cooling system in the mold, the material the mold is made from, and the temperature & flow rate of the coolant as it is pumped through the mold.

Other Factors:

The long chains in polymers will stretch when made to flow and when they are aligned in the direction of flow, they will shrink more in that direction than across it. The faster the polymer flow, the smaller the shear region is and thus the more uniform the shrink will be. Fibers, based on their aspect ratio (L/D, or "length / diameter"), have the greatest effect on the warping of plastic parts. The gross size of the fibers is less important than the aspect ratio, so nano-fiber filled polymers can be expected to see similar warp results to standard fiber filler of the same basic shape and aspect ratio.

Benefits of Injection Molding Simulation

Numerical simulation of the injection molding process helps to increase the process productivity and improve the part quality. It enables an in-depth understanding of the entire process and optimizes it. Through numerical experiments, one can understand the effects of process conditions for successful injection molding. This helps to reduce the mold tryouts and efficiently bring the mold in to production. These simulation also help to identify and eliminate potential defects that may arise in the part. Overall, simulation helps to save time, money, energy, and also reduce waste.

Some Terminology

Press Parameters Vs. Plastic / Physical Parameters