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how to avoid jetting when injection molding


how to avoid jetting when injection molding

When the melt flow through a nozzle at high speed, runner, sprue or other narrow area, enter into the open areas, and there is no contact with the wall and die, it will produce jetting. Snake-like jetting makes contact with the melt equivalent to each other, give rise to small-scale sutures, as shown in Figure 1. Jet stream will reduce the strength of plastic parts, resulting in multi-surface and internal defects.
how to avoid jetting when injection molding?


Figure 1
In contrast, the normal pattern of melt filling wave front won’t generate these problems. Improve the plastic parts of the jetting flawed methods described below:
Change mold design: usually jet problem occurs in the gate design; you can relocate or change the gate design to guide the melt surface contact with the mold wall. Use overlapping gate or submersible gates, as shown in Figure 2. To the gradual expansion of the melt flow area to reduce the flow rate; using tabs or fan gate, as shown in Figure 3, you can provide melt into the mold cavity from the gate smoother transition, reducing the melt shear stress and shear strain. Increase the gate and runner size, or shorten the gate length. Review whether cold slug well designed properly or to avoid jetting when injection molding

Figure 2


Figure 3
Adjusting molding conditions: Adjust for Best screw speed curve, so that the wave front melt through the gate at low speed, until the melt glue stuck outside the gate to further increase the rate of fire to eliminate the jetting, as shown in Figure 4. Barrel temperature may be also adjusted to increase or decrease by the amount of melt temperature of each section, to eliminate the jet stream, the reason for this improvement remains uncertain, but may be there is a relationship of nozzle and melt properties (such as viscosity and surface tension etc.). For most plastic materials, reducing the temperature increases a die swell effect; however, there are some plastics (e.g. PVC) at elevated temperatures because of increasing nozzle expansion effect.

Figure 4
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