Injection Molding Shrinkage and Solutions

Injection Molding Shrinkage and Solutions

◆Weld lines and solutions

◆Air bubbles (cavitation) and solutions

◆Warping (deformation) and solutions

During injection molding, if insufficient pressure is generated in certain areas of the mold cavity, the thicker areas of the plastic part shrink more slowly as the melt begins to cool, creating tensile stress. If the surface hardness of the part is insufficient and there is no melt replenishment, the surface of the part will be stretched under stress. This phenomenon is called shrinkage, as shown in Figure 3-8.

Shrinkage often occurs in areas where melt accumulates in the mold cavity and in thick-walled areas of the part, such as the junctions between reinforcing ribs, support pillars, and the part surface.

Shrinkage on the surface of injection molded parts not only affects their appearance but also reduces their strength. Shrinkage is closely related to the type of plastic used, the injection molding process, and the structure of the part and the mold.

Figure 3-8 Shrinkage phenomenon of products

Plastic raw materials:

Different plastics have different shrinkage rates. Generally, raw materials that are prone to shrinkage are mostly crystalline plastics (such as nylon, polypropylene, etc.). During the injection molding process, when crystalline plastics are heated and become fluid, the molecules are arranged randomly. When injected into a cooler mold cavity, the plastic molecules will gradually arrange themselves neatly and form crystals, resulting in a large volume shrinkage. Its size is smaller than the specified range, which is the so-called "shrinkage".

Injection Molding Process:

Injection molding, shrinkage can occur due to insufficient holding pressure, slow injection speed, low mold or material temperature, or insufficient holding time.

Therefore, when setting injection molding process parameters, it is essential to check whether the molding conditions are correct and whether the holding pressure is sufficient to prevent shrinkage. Generally, extending the holding time ensures that the product has sufficient time to cool and replenish the melt.

Plastic parts and mold structure

The root cause of shrinkage lies in the uneven wall thickness of plastic products. A typical example is that shrinkage is very likely to occur on the surfaces of reinforcing ribs and support pillars. Furthermore, the mold's runner design, gate size, and cooling effect also significantly impact the product. Because plastics have low heat transfer capacity, cooling is slower the further away from the cavity wall. Therefore, sufficient melt should fill the cavity at that point. This requires the injection molding machine screw to prevent backflow and pressure reduction during injection or holding. Additionally, if the mold's runner is too narrow, too long, or the gate is too small and cooling is too rapid, semi-solidified melt will block the runner or gate, causing a drop in cavity pressure and leading to product shrinkage.

In fact, different plastics have different shrinkage rates. Table 3-2 shows the shrinkage rates of common plastics.

Table 3-2 Shrinkage rates of common plastics：

The causes and solutions for shrinkage are shown in Table 3-3：

Bumps and solutions

After the product was demolded, localized increases in volume and expansion occurred in certain specific locations, as shown in Figure 3-9.

Bubbling in plastic parts occurs because incompletely cooled and hardened plastic releases gas under internal pressure, causing the part to expand. The following measures can be taken to improve this defect:

① Effective cooling. Methods include lowering the mold temperature, extending the mold opening time, and reducing the drying and plasticizing temperatures of the plastic.

② Reducing the mold filling speed, shortening the molding cycle, and reducing flow resistance.

③ Increasing the holding pressure and extending the holding time.

④ Improving the part structure to avoid localized areas of excessive thickness or significant variations in thickness.

③ In terms of part structure design: reduce thickness inconsistencies and ensure uniform wall thickness as much as possible; avoid sharp corners to prevent air trapping.

In terms of mold design: add venting channels at the final melt filling point; redesign the gating and runner system; ensure the vents are large enough to allow sufficient time and space for gas to escape.

Process conditions: Reduce the final injection speed; set a reasonable mold temperature and extend the mold opening time; optimize injection pressure and holding pressure; reduce screw retraction to prevent air from being drawn in during retraction and carried into the next mold cycle, and reduce material temperature.      

Shrinkage (vacuum bubble) and its solutions

Shrinkage cavities, also known as vacuum bubbles or voids, generally appear in areas of plastic parts where large amounts of melt accumulate. They are caused by insufficient melt replenishment during cooling and shrinkage. As shown in Figure 3-10, shrinkage cavities often occur in thick-walled areas of plastic parts, such as at the intersection of reinforcing ribs or support pillars with the part surface.

Figure 3-10 Shrinkage cavities appearing on the plastic part

Shrinkage cavities in plastic parts occur because when the melt solidifies, the volume shrinks more slowly in thicker areas, creating tensile stress. If the surface hardness of the part is insufficient and there is no melt to replenish it, voids will form inside. The cause of shrinkage cavities is similar to that of shrinkage (or cavitation), but the difference is that shrinkage results in surface depressions, while shrinkage cavities form internal voids. Shrinkage cavities typically occur in thick-walled areas and are mainly related to the cooling rate of the mold. Different cooling rates of the melt within the mold result in varying degrees of shrinkage at different locations. If the mold temperature is too low, the surface of the melt cools rapidly, pulling the hotter melt in the thicker walls towards the surrounding surfaces, leading to shrinkage cavities.

Shrinkage cavities in plastic parts can affect their strength and mechanical properties. If the part is transparent, shrinkage cavities can also affect its appearance. The key to improving shrinkage cavities is controlling the mold temperature.

The causes and solutions for shrinkage cavities are shown in Table 3-4：

Edge flash (burrs, trimmings) and solutions

When molten plastic is extruded from the parting surface of the mold and flakes off the edge of the finished product, this phenomenon is called flash, also known as burr, or simply flash, as shown in Figures 3-11.

Figure 3-11 shows the overflow phenomenon on the plastic part.

Flash is a serious quality problem in injection molding. If flash detaches and adheres to the parting surface of the mold and is not cleaned in time, subsequent mold locking will severely damage the parting surface, leading to new flash. Therefore, special attention must be paid to the occurrence of flash during the injection molding process.

There are many reasons for flash during injection molding, such as excessive injection pressure, excessively fast end-injection speed, insufficient clamping force, wear of ejector pin holes or slide blocks, uneven mold parting surfaces (with gaps), and plastic with too low viscosity (such as nylon).

Table 3-5 Causes and Solutions for Overflow：