Why Does Increasing Mold Temperature Sometimes Improve Weld Lines but Sometimes Worsen Sink Marks?

In injection molding, mold temperature is a parameter that people both love and hate. Raise the mold temperature, and weld lines become shallower and surface gloss improves—but sink marks may deepen instead. Lower the mold temperature, and sink marks are controlled—but weld lines become deeper and more obvious, sometimes accompanied by flow marks and rough surfaces.

This "press one gourd down and another pops up" phenomenon confuses many machine operators. But behind it is no mystery—it is simply that mold temperature acts on the two defects through completely different mechanisms. The improvement of weld lines relies on "healing at the molecular level," while the worsening of sink marks stems from "the accumulation of thermal shrinkage." The two mechanisms occur in different time windows and respond to mold temperature in opposite directions.

Why Can High Mold Temperature "Cure" Weld Lines?

The essence of a weld line is that when two melt streams meet in the cavity, the already-cooled "skin" at their fronts has not truly fused together. It is not just a visual line—it is a mechanically weak plane. Polymer molecules in the weld region do not have enough time to diffuse and entangle with each other, and what remains after cooling is an interface with reduced strength.

The effect of high mold temperature on weld lines strikes exactly at this pain point. It slows the cooling rate of the melt front, allowing the two melt streams to maintain higher temperature and lower viscosity when they meet. The molecular chains have greater mobility at the interface, diffusion is more complete, and the degree of healing is higher. Studies have measured the width and length of weld lines and found that both decrease significantly as mold temperature increases. In micro injection molding, when mold temperature is raised above 100°C, the total length of weld lines can be reduced by as much as 40%, while tensile modulus doubles.

From the perspective of molecular motion, the logic is clear: the strength of a weld line essentially depends on the molecular mobility of the polymer at the weld interface. And molecular mobility is directly determined by the temperature at that location. The higher the mold temperature, the better the "welding" quality at the weld, and the less noticeable the weld line. So for weld lines, higher mold temperature is beneficial, and the direction is clear.

Then Why Do Sink Marks Worsen?

The logic of sink marks is a completely different matter. Sink marks originate from the fact that during cooling, the interior of the part undergoes more volumetric shrinkage than the surface layer. By the time the surface has solidified, it can no longer compensate inward, so the surface is "pulled down" by the interior, forming a depression.

High mold temperature plays an unhelpful role here. It brings two effects, both of which worsen sink marks.

The first effect: increased total thermal shrinkage. When plastic cools from the molten state to room temperature, volumetric shrinkage is unavoidable. The higher the mold temperature, the higher the temperature of the part when it is demolded, and the greater the temperature difference between demolding temperature and room temperature—meaning the greater the amount of thermal shrinkage that must be released. In other words, high mold temperature lets the part leave the mold "carrying more heat," and this heat continues to release after demolding, manifesting as greater post-shrinkage. If the part wall thickness is uneven, the thick-wall areas store more heat and shrink more, making sink marks more pronounced.

The second effect: the battle over the compensation window. There is an easily overlooked detail here: before the gate freezes, higher mold temperature actually extends the time the gate remains open, meaning injection pressure and packing pressure have more time to push melt into the cavity to compensate for shrinkage. From this angle, high mold temperature is favorable for sink marks. But the problem is that this favorable window is time-limited. Once the gate freezes, the compensation channel closes, and subsequent thermal shrinkage can no longer be compensated. At that point, the additional thermal shrinkage brought by high mold temperature is converted into sink marks without reservation. For molds with small gates or early gate freeze-off, the "compensation dividend" of high mold temperature has not been fully collected before the "shrinkage bill" arrives.

One Process Parameter, Two Sets of Time Logic

Putting these two mechanisms together, we can understand why mold temperature acts in opposite directions on weld lines and sink marks.

The improvement of weld lines occurs at the moment of filling and welding. The role of mold temperature is to keep the melt in an "active" state when the streams meet, allowing time for molecular diffusion. This window is very brief—just at the moment the two melt streams converge.

The worsening of sink marks occurs during cooling and demolding. The role of mold temperature is to change the "thermal state" of the part when it leaves the mold—the higher the mold temperature, the more sensible heat the part carries away, and the greater the volume that needs to shrink after demolding. This window lasts until the part has completely cooled to room temperature.

Weld lines care about "how hot it is when they meet"; sink marks care about "how hot it is when it leaves." High mold temperature raises both temperatures simultaneously, so weld lines improve while sink marks worsen.

Is There a Way to Have Both?

In conventional injection molding, mold temperature is a "fixed value" that remains essentially constant throughout the molding cycle. This means you must make a trade-off between weld lines and sink marks: higher mold temperature gives better appearance but heavier sink marks; lower mold temperature gives more stable dimensions but more obvious weld lines.

But there is a technology that unties this knot: Rapid Heat Cycle Molding (RHCM), also known as high-gloss mark-free injection molding.

Its idea is simple but ingenious: turn mold temperature into a variable within the cycle, rather than a constant.

Before and during injection, the mold is rapidly heated to a level approaching or even exceeding the resin's heat deflection temperature (for example, 150°C). At this point, the melt flows under a high-temperature mold wall, the front remains in a viscous flow state, and when the two melt streams meet, the molecules are still active—so weld lines are naturally eliminated.

After injection and packing are completed, cooling water is rapidly introduced, and the mold temperature drops sharply to below 40°C within seconds. The part solidifies quickly under the low-temperature mold wall, the gate freezes rapidly, and thermal shrinkage is "locked in" at a lower level, controlling sink marks and warpage.

The key lies in misalignment in time: when weld lines are forming, the mold temperature is high; when sink marks are forming, the mold temperature has already dropped. One parameter takes different values at two points in time, thereby satisfying both requirements simultaneously.

Of course, the price of this technology is that the mold requires special design (steel resistant to thermal fatigue, matching temperature control channels), and rapid cooling/heating temperature control equipment is needed. Both mold cost and equipment investment will rise. But for products with high appearance requirements where sink marks are also unacceptable, this is currently the most effective systematic solution.

Ultimately, Mold Temperature Is Not "The Higher the Better" or "The Lower the Better"

The reason weld lines and sink marks respond to mold temperature in opposite directions is fundamentally that the "temperature" each cares about is not the temperature at the same point in time. Weld lines care about the interface temperature at the moment the melt streams meet; sink marks care about the overall thermal state of the part at demolding. Conventional injection molding uses the same mold temperature to influence both moments simultaneously, so contradiction is inevitable.