Often, controlling shrinkage happens after the fact, when the tool is complete and parts are running. Using a combination of personal experience, educated guesswork, and trial and error, mold engineers intuit ways to change process settings enough to reduce the issue.
This approach is proven to work, but it can be extremely time-consuming. With a more detailed understanding of shrinkage behavior, and with assistance from simulation software, mold engineers can address shrinkage earlier in the design process, save time, and improve part quality.
Five factors that influence shrinkage
Shrinkage starts at the molecular level when plastics melt and cool. For the most part, these dynamics depend on the type of material and whether any filler or fiber reinforcement is present. There are also processing and part design factors to consider.
materialsIn a state of equilibrium, ABS, polystyrene, polycarbonate, and other amorphous polymers have a random and entangled molecular orientation. A frequent analogy is the "bowl of spaghetti." As these materials melt, the forces between molecules weaken and move away from each other. In addition, the shear force experienced during the injection phase causes individual molecules to uncoil and align to the direction of flow. When flow stops, the molecules relax and return to a state of random orientation. Intermolecular forces continue to pull them closer together until the temperature drops enough to freeze them in place. These forces result in uniform shrinkage, but the relaxation effect causes significantly more contraction in the direction of flow.
materialsUnlike amorphous materials, semi-crystalline materials have regions of highly ordered, tightly bundled molecular structures. Instead of a bowl of spaghetti, these materials resemble springs connected by bungee cords. When they melt, the crystalline structures loosen and the molecules align to the direction of flow, much like amorphous polymers. But when these materials cool, they don't relax. Instead, they maintain their orientation in the direction of flow and the molecules begin to recrystallize, resulting in significantly higher shrinkage rates. In this case, the effect is much greater in the direction perpendicular to flow. More crystalline materials, such as polytetrafluoroethylene (PTFE), isotactic polypropylene, and high-density polyethylene, shrink even more than semi-crystalline materials. As these polymers cool, the molecules form crystalline regions. This structure allows the material to fit together more tightly, making them denser and able to shrink more.
3. Fiber-reinforced and filled materials
When fibers are introduced into the plastic, they may counteract shrinkage effects due to molecular orientation. Fibers do not expand or contract as temperature changes, so they tend to reduce shrinkage in the direction of their orientation and increase shrinkage transverse to their orientation. Polymers filled with long glass fibers, for example, will shrink more in the cross direction than the longitudinal direction, making them unsuitable for projects with close tolerances. Resins that are filled generally shrink less than resins that are unfilled. Resins can be filled with a variety of materials, including glass fiber, wood, and mica, in order to change a part's properties.
4. Wall thickness
Wall thickness is a factor in shrinkage because it affects the amount of crystallinity in materials, which in turn affects the total potential shrinkage. Non-uniform wall thickness causes different cooling rates throughout the part. Where walls are thinner, cooling is faster, and crystallinity and shrinkage are lower. Where walls are thicker, cooling is slower, and crystallinity and shrinkage are both higher. In amorphous materials, increasing wall thickness tends to reduce orientation effects. Maintaining uniform wall thickness helps avoid variations in shrinkage that can lead to warpage.
5. Processing conditions
Adjusting processing conditions is the most familiar way for mold engineers to address shrinkage. By changing temperatures, pressures, and packing and cooling times, it is possible to mitigate shrinkage. By applying pressure to a liquid plastic, you can compress the molecules into a smaller volume and then inject more material into the mold to compensate for shrinkage. Gating from thick areas to thin areas of the part can help in this effort, providing more efficient packing for thicker sections. (By the same token, gating from thin to think will cause thin sections to freeze off first, limiting the packing of the thicker sections.) Another approach to mitigating shrinkage is speeding up the cooling rate, allowing less time for crystals to develop and increase shrinkage. One important tradeoff to note is that speeding up the cooling rate may reduce crystallinity so much that it compromises part performance. In these cases, the material loses the crystalline properties that made it an appropriate choice for the part in the first place. Non-uniform cooling can also cause variations in shrinkage and subsequent warpage, so caution should be taken to ensure cooling occurs evenly.