Why Does Insufficient Clamping Force Cause Mold Bulging, While Excessive Clamping Force Causes Poor Venting?

In injection molding, clamping force is a parameter that "pleases no one." Set it too low, and the mold is pushed open by cavity pressure—the product flashes and dimensions go out of tolerance. Set it too high, and the vent grooves are crushed shut—air in the cavity cannot escape, and the product suffers from burning, short shots, and deeper weld lines.

The same parameter causes problems whether you go left or right. To understand this contradiction, we need to separate the two roles clamping force plays during the molding process: it is both the "supporting force" that resists cavity pressure and the "pressure source" that determines the fit state of the parting surface. These two roles demand clamping force in opposite directions.

Why Does Insufficient Clamping Force Cause Mold Bulging?

During injection, melt is injected into the cavity under high pressure, and the pressure inside the cavity tries to push the moving and fixed halves apart. The magnitude of this pushing force equals the cavity pressure multiplied by the projected area of the product on the parting surface. If the clamping force is insufficient, it means the "pressing force" provided by the injection molding machine is not enough to offset this "pushing force."

The result is that the parting surface is forced open by a gap. This gap may be only a few hundredths of a millimeter—almost invisible to the naked eye—but for low-viscosity plastic melt, it is already enough to penetrate. The melt escapes through the gap, and after cooling, it becomes flash. If the cavity pressure distribution is uneven, the opening amount in a particular local area is greater, and flash concentrates there.

Mold bulging brings more than just flash. Once the parting surface is forced open, the actual depth of the cavity changes, and the product's thickness dimension becomes oversized. If the opening amount is unstable during the molding process, product dimensions will fluctuate. For parts with strict assembly requirements, this fluctuation may directly cause assembly failure.

A more serious situation is mold bulging causing structural damage to the mold. If clamping force is severely insufficient, the parting surface is repeatedly forced open and closed, and the edges of the parting surface gradually collapse or wear. Moving components such as slides and lifters also wear faster due to repeated impact.

Why Does Excessive Clamping Force Cause Poor Venting?

When clamping force is too high, the problem goes in the other direction. The parting surface is pressed extremely tight, and the originally designed vent grooves are flattened or even completely closed.

During injection, the cavity does not contain only plastic. As the melt enters the cavity, the air inside the cavity needs to be expelled. Vent grooves are the channels left for this air. Normally, vent groove depth is between 0.02 mm and 0.05 mm—enough to expel air without letting melt escape.

But if clamping force is too high and the parting surface is pressed tight, the effective depth of the vent grooves is compressed. Air cannot escape and is compressed at the end of the cavity or in the weld area. The compressed air temperature rises sharply, potentially causing localized burning of the plastic, with black spots or burn marks appearing on the product. At the same time, the trapped air forms a back pressure that prevents the melt from filling the end of the cavity, causing short shots. For frame-type products, where multiple melt streams converge at the end of filling, poor venting makes weld lines very obvious, sometimes with visible flow marks.

One Parameter, Two Sets of Logic

Putting these two problems together, we can understand the "dilemma" nature of clamping force.

Mold bulging is about "whether clamping force is enough to offset cavity pressure." This is a force balance problem. The higher the cavity pressure and the larger the product projected area, the greater the clamping force required. If clamping force is insufficient, the parting surface is forced open, and flash and dimensional deviation follow.

Poor venting is about "whether the parting surface leaves enough gap for air to pass through." This is a clearance control problem. The greater the clamping force, the tighter the parting surface is pressed, the smaller the gap, and the harder it is for air to escape.

Insufficient clamping force is a "not pressed tight enough" problem; excessive clamping force is a "pressed too tight" problem. The two directions are exactly opposite, but both occur on the same parting surface.

How Do You Find the Clamping Force That Is "Just Right"?

Since clamping force can be neither too small nor too large, how should a reasonable clamping force be determined?

Step one: calculate how much clamping force is needed. The clamping force requirement equals cavity pressure multiplied by the projected area of the product on the parting surface, divided by a safety factor. Cavity pressure is not constant—it rises during filling and peaks during packing. For different plastics and different product structures, the empirical value of cavity pressure differs. Generally, plastics with good flowability (such as PP and PE) have lower cavity pressure, while plastics with poor flowability (such as PC and PA) have higher cavity pressure. The larger the product projected area, the greater the clamping force required. For frame-type products, the projected area is the area enclosed by the outer contour of the entire frame, not the actual area of the frame itself.

Step two: reserve venting space. Vent grooves must be designed on the parting surface, and their depth must remain effective under clamping force. If the mold lacks rigidity and clamping force causes the mold plate to deform, crushing the vent grooves shut, then additional support must be added in the mold base design, or the vent grooves must be placed where they are less likely to be compressed. For products that are difficult to vent, auxiliary means such as insert venting, ejector pin venting, or vacuum venting can be considered.

Step three: verify by trial molding. Calculation gives a theoretical value, but actual production still requires adjustment through trial molding. During trial molding, clamping force can be adjusted gradually while observing the product condition. If clamping force is on the low side, flash appears at the product edges; if clamping force is on the high side, burning or short shots may appear at the end of the product. Finding the point where "flash just barely does not appear and venting is just barely unobstructed" is the reasonable clamping force for this mold on this injection molding machine.

Step four: pay attention to the distribution of clamping force. Clamping force is not uniformly distributed across the parting surface. The injection molding machine's clamping force is transmitted to the mold through the platens. The rigidity of the platens, the arrangement of support pillars in the mold, and the actual contact area of the parting surface all affect the actual distribution of clamping force. If the center of the mold is empty (as with frame-type products), the parting surface in the central region may not receive enough pressing force. Even if the total clamping force is sufficient, local bulging may still occur. In this case, adding support pillars or adjusting the closing surface layout of the parting surface needs to be considered.

Ultimately, Clamping Force Is a "Window Parameter"

Insufficient clamping force causes mold bulging, while excessive clamping force causes poor venting. The fundamental reason is that clamping force simultaneously undertakes two contradictory tasks: it must press the parting surface tight to prevent melt from escaping, and it must leave a gap for air to be expelled.