Scientific management of pressure loss in injection molding process is essential to ensure rationality of process window and improve production yield. There is more or less pressure loss in every link of plastic from barrel to cavity, and these pressure losses are directly related to product design, mold design, material selection, injection molding machine and process. Reasonably reducing pressure loss can significantly increase injection molding process window, improve injection molding yield, and reduce defective products.

Whether plastic product design is reasonable determines success of injection molding project. Good product design can reduce a lot of risks for subsequent injection molding production, so what problems should product design pay attention to in reducing pressure loss?

Part thickness plays an important role in difficulty of molding process. Typically, product wall thickness of an injection molded part should be between 1 mm and 6 mm (0.040 to 0.250 inches). The thinner wall thickness, the greater pressure loss of plastic during filling. Of course, if wall thickness is thicker, production cycle of product will be elongated, and problems such as voids may also occur. Therefore, wall thickness should be reasonably selected according to functional requirements of the later stage of product.
For wall thickness of product, another important principle is to maintain consistency of wall thickness to avoid large differences in wall thickness of product at different locations.

 

Another factor that can significantly affect molding success is size of part. For ease of understanding, we will use OAL (Overall Length) of part as a consideration. OAL of injection molded parts can also vary widely, as does thickness. In medical industry, OAL of some parts may be as small as 2-3 mm. In automotive or home appliance industry, OAL may be as long as 1.5 meters. The longer OAL, the greater pressure loss.

The first step in material selection is to evaluate viscosity characteristics. Viscosity is resistance of molten plastic to flow. The higher viscosity of material, the more pressure injection molding machine needs to provide to overcome resistance encountered during flow of plastic, and the greater pressure loss. If you want to reduce pressure loss and expand process window, you should choose a lower viscosity material.

MFI is generally used to represent ease of flow of molten resin, and the better flow, the lower pressure loss of material. Same material also has different MFI grades. Under same conditions, you should try to choose a material with better fluidity. This may cost more materials, but considering late defect rate and cost of solving problem, it may be more cost-effective overall.
Material batch fluctuations
Quality fluctuations between material batches are inevitable. Material viscosity changes caused by material batch fluctuations can reach +/-20%. It is not surprising that viscosity of material fluctuates by +/- 40% if regrind is also added. Therefore, rational management of material batches is also very important to reduce and stabilize pressure loss.

So-called aspect ratio refers to ratio of flow length of plastic in mold to wall thickness, which is generally expressed as xxx:1 (such as 80:1). Flow length is calculated from gate to end of cavity. The greater aspect ratio design, the greater pressure loss. Therefore, it is necessary to choose a reasonable aspect ratio to minimize pressure loss. Reasonable design of gate location, runner arrangement and runner length are all important considerations for reducing aspect ratio.

Size of gate plays a crucial role in how well material flows. It also affects ability to "pack" parts. If gate is too small, a higher injection pressure will be required during filling stage, and pressure loss at gate will also be greater.

Performance requirements of an injection molding machine are critical to success of any process. Here are three factors to evaluate when choosing a machine.

We must ensure that injection device has correct volume to avoid unmelted particles or degradation.

We must understand volume flow of injection molding machine, not linear speed. Thin-walled parts generally require higher volumetric flow rates to ensure complete filling of cavity before flow front freezes.

When judging whether injection molding machine is suitable, an important part is to judge whether injection molding machine can provide pressure for plastic to reach required volume flow, that is, whether there is a pressure limitation in filling stage of injection molding process. If pressure is limited, injection molding machine cannot meet requirements.

As market leader in cavity pressure, RJG believes that for most plastics, cavity end pressures above 3,000 PSI are required to provide a relatively wide window for process. Of course, this is not to say that injection molding is impossible with pressures below 3,000 PSI at the end of cavity, it is possible. However, cavity end pressure and amount of risk associated with mold failure exhibit an exponentially negative relationship, ie, the lower pressure, the higher risk.
In standard Decoupled Molding II process, under premise that other process conditions remain unchanged, if viscosity of material increases, pressure at the end of cavity will decrease, because increase in viscosity will bring about pressure loss increase. When pressure loss is greater than verification value, some defective products related to insufficient pressure are easy to appear, such as undersize, warpage, depression and short shot, etc.

 

Graph below shows trend data curve of material viscosity and cavity end pressure for a 15 hour continuous production run. Mold is equipped with a cavity pressure sensor at the end of cavity, machine is equipped with a hydraulic sensor to monitor viscosity changes, and data is displayed on an eDART® or CoPilot® summary graph. Inverse relationship between plastic viscosity (red) and peak pressure at the end of cavity (blue) can be easily seen from curve.

 

When we are trying to manage pressure loss during injection molding, we first have to consider thickness and size of part. Second, MFI of material is evaluated based on geometry through which material must flow. Not only MFI should be considered at the beginning of project, but also material changes over time (batch-to-batch variation). Third, mold design needs to be considered to ensure correct aspect ratio. In addition, performance standards of injection molding machines responsible for production need to be determined. Finally, it is time to determine whether correct molding strategy has been selected. If any one of these key areas is ignored, it will be difficult to consistently produce high-quality injection molded products.
In short, if part has thin walls and long OALs, uses a high viscosity, low MFI resin and high aspect ratio design, chances of reaching 3000 PSI pressure at the end of cavity are extremely low. All steps need to be thoroughly evaluated and better process control methods applied.