Gate Position

The position or location of the gates is closely tied with the previous two topics. the number of gates, the flow pattern, and position cannot be separated. All are interrelated. There are several considerations for determining a part's gate location, including:

• Place gates to achieve balanced filling—primary importance as discussed above

• Place gates in thicker areas to better pack out the part

• Place gates far from thin features to prevent hesitation

• Place gates against a wall to prevent jetting

• Place gates to prevent weld lines from forming in weak regions of the part or where they will be visible

• Place additional gates to prevent overpacking

• The type of tool being used—is it a two- or three-plate mold?

• Hot or cold runners, or a combination?

• The type of gate desired: edge, tunnel, etc.

• Restrictions on gate location because of part function

• Restrictions on gate location because of tool function 7.3.3.1 Balanced Gate Locations

Throughout this book there has been a discussion of balanced flow within a part. Balanced flow, however, is not necessarily easy to define. Balanced fill is achieved when the extremities of the part fill at the same time and pressure. Below are four examples of gate placement to achieve a balanced flow in a simple rectangle. All have advantages and disadvantages that are discussed.

End-gated Part: The end-gated part, shown in Figure 7.18, is considered balanced because the flow is unidirectional, and material continues to flow through every area of the part once it is filled (with the possible exception of the extreme left corners). Placing a gate on the end of the part will produce an orientation that is aligned down the axis of the part. This type of gate location generally reduces warpage, in particular with amorphous and fiber-filled materials.

The disadvantage is that the flow length is quite long, so fill pressures will be relatively high, and packing may be a problem. With constant packing pressures, the variation in volumetric shrinkage can be high but this problem can be overcome with a decaying packing profile.

Design Rules 117

Figure 7.18 End gate with balanced fill

Center-gated Part: The center-gated part in Figure 7.19 is reasonably well-balanced but not as good as the end-gated part. The problem is that the flow front starts out radial, but then straightens out and becomes linear. There is some degree of underflow because the flow length to the middle of the long side is very short compared to the flow length to the long end of the part. This can result in warpage, depending on the material and structure of the part. A center-gate location is usually better for round or square parts.

Figure 7.19 Center gate, mostly balanced

Two Gates, Uniform Flow Length: If a second gate is needed, then the positioning of the two gates is critical. In Figure 7.20, the gates are placed so the flow length between the gate and end of the part is the same as the flow length to the weld line. The spacing was calculated by breaking up the length into twice as many sub-moldings as the number of gates. The gates are placed at the boundary between every other sub-molding. This gives the best possible balance within the part with multiple gates. Whenever gates are added to the part, each gate should fill about the same flow length and volume. This is difficult-sometimes impossible-with nonsymmetrical parts, but this should be the initial goal.

One potential problem with this gate location is the weld line. The temperature of the flow front and the pressure on the weld line when it forms determines the quality of the weld line.

With this gate location, the weld line forms at the end of fill, therefore the pressure drop between the gate and weld line will be higher than if the gates were closer, and potentially at a lower temperature when the weld line forms.

When considering warpage, this gate configuration will not overpack the center of the part, possibly reducing the warpage of the part.

118 Gate Design

Figure 7.20 Two gates with uniform flow length, mostly balanced

Two Gates, Closer to the Center of the Part: This gate configuration is very similar to the previous one. The gate spacing is calculated by splitting the length into a number of sub-moldings equal to the number of gates plus one. This places the gates closer together and the flow length to the ends of the part is longer, as shown in Figure 7.21.

As a result, there is overpacking between the gates, possibly leading to warpage. The potential for warpage makes this gate configuration less desirable than the one in Figure 7.20; however, the weld line may be of higher quality than the previous case. This is because the weld line is formed closer to the gate at higher temperatures, and will see higher pressures. If quality of the weld line is of primary importance, this gate location may be better than the previous example.

Figure 7.21 Two gates closer to center, not balanced

7.3.3.2 Gate in Thicker Areas

In Figure Figure 7.22 the part has a 5 mm thick section and a 2 mm thick section. An edge gate was placed in the thick section on the part in the center, and in the thin section on the part to the right. Each part has the same gate, runner size, and processing conditions. The

results indicate that the part with a gate placed in the thick section has much lower and uniform volumetric shrinkage compared to the part with a gate placed in the thin section.

Depending on the objectives of the part, it may be beneficial to place a gate in a thicker area, even if the balance of the part may not be quite as good. This situation will most likely be the case if the material is semicrystalline and/or if sink marks and voids are critical defects to be

avoided.

L

L/4 L/4

L

L/3 L/3

Design Rules 119

Figure 7.22 Gating in thick and thin sections, comparing volumetric shrinkage

7.3.3.3 Gate Far from Thin Features

When there is a wide variation in wall thickness, place the gate as far as possible from thin features to avoid hesitation. In Figure 7.23 the part has nominal wall is 2 mm and the rib is 1 mm thick. Both examples have the same processing conditions.

In the top part, the gate is close to the thin rib. When the flow front reaches the rib, the flow splits. Polymer flow takes the path of least resistance. Since the pressure required to go into the thick nominal wall is much less than the thin rib, most of the material goes in the nominal wall. The material going into the rib is hesitating so there is little shear heat, and the material gets quite cold and eventually freezes off, creating a short shot.

In the bottom part, the gate is at the far end of the part. In this case, when the material gets to the thin rib, there is not much of the part left to fill. The material hesitates going up the rib, but there is not enough time for the rib to freeze off. The last place to fill is still in the rib, but it does fill. This problem is more likely to occur with semicrystalline materials because they tend to freeze faster.

thickness Wall

Gated in thick wall

Gated in thin wall

120 Gate Design

Figure 7.23 Gates near and far from thin feature

7.3.3.4 Add Gates as Necessary to Reduce Pressure

You often need to place gates so that the fill pressure is within the capacity of the injection-molding machine. As a general rule, the maximum fill pressure of the part without a feed system should be about half the machine limit. For a typical machine, the pressure to fill a part should not exceed 70 MPa (10,000 psi).

When a single gate has a pressure that exceeds a pressure limit or guideline, the pressure must be lowered. Changing the gate location to reduce the maximum flow length in the part is a good way to lower the pressure to fill. Once you have achieved the shortest possible or practical flow length and the pressure is still too high, add a second gate.

As you add additional gates, place them so all gates have about the same volume to fill, the same flow length, and a balanced fill. This will reduce the pressure (Figure 7.24).

Gate far from thin rib

Rib fills

Gate near rib Hesitation in rib

short shot occurs

Design Rules 121

Figure 7.24 Adding a second gate reduces the pressure by reducing the flow length

7.3.3.5 Prevent Overpacking by Adding Gates

Depending on the geometry of the part, adding gates can sometimes improve the packing of the part by improving its uniformity. In Figure Figure 7.25 the single-gated part has a nearly balanced filling pattern. However, due to the center rib close to the gate, the volumetric shrinkage in the rib is very low because it is overpacked. This may cause a problem with warpage, but it also may cause a problem with ejecting the part. There may be other overriding factors in the decision to place a gate, but overpacking may be important. The double-gated part fills the center rib toward the end of fill. The volumetric shrinkage in that center rib is much better than the single gate location.

Figure 7.25 Single gate, overpacked rib Gate

Gates

Single gate Center rib fills early and overpacks

122 Gate Design