Shrinkage:
The shrinkage of plastics signifies the volume contraction of polymers during the cooling step of the processing of polymers. A small amount of shrinkage occurs after ejection as the part continues to cool and after that the part may continue to shrink very slightly until the temperature and moisture content stabilize.
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Plastic injection molding shrinkage is the contraction of a plastic molded part as it cools after injection. Most of the part shrinkage occurs in the mold while cooling, but a small amount of shrinkage occurs after ejection, as the part continues to cool (especially for Delrin or POM).
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What is shrinkage and warpage?
Processing and design parameters that affect part shrinkage. Warpage. Warpage is adistortion where the surfaces of the molded part do not follow the intended shape of the design. Part warpage results from molded - in residual stresses, which, in turn, is caused by differential shrinkage of material in the molded part
The shrinkage factor will depends on:
Plastics material
1. Processing condition
2. Product design
Mould design: Shrinkage value is higher for Crystalline material than Amorphous material.
Plastic Shrinkage: Material shrinkage during and after manufacturing plays an important role in why injection molded plastic parts warp. Before we dive into parts warpage, it's important to understand how and why plastic materials shrink. To do that, we need to start at the molecular level with a close look at what happens when plastics melt and cool. For the most part, the melting and cooling characteristics depend on the type of polymer and whether any filler or fiber reinforcement is present.
1. Amorphous Materials One polymer type is referred to asamorphous, which includes materials such as ABS, polystyrene, and polycarbonate, among others. They have a random and entangled molecular orientation in their natural state, much like a bowl of spaghetti. As these materials melt, the forces between molecules weaken and they move away from each other. In addition, the shear experienced during the injection phase (which is similar to friction) 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. The intermelecular forces 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 more shrinkage in the direction of flow.
2. Semi - crystalline materials: Unlike amorphous materials, semi - crystalline materials have regions of highly ordered, tightly bundled molecular structures. When they melt, the crystalline structures loosen and the molecules align to the direction of flow, much like amorphous polymers. But when the materials cool, they don 'trelax. 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, however, the effect is much greater in the direction perpendicular to flow. . 3. Fiber - reinforced materials: Fibers are often combined into a polymer material to add strength and other properties. When fibers are introduced into the plastic, they may counteract shrinkage effects due to molecular orientation described above. Fibers do not expand or contract as temperature changes, so fiber - filled materials will typically experience reduced shrinkage in the direction of their orientation.
Shrinkage is very complex in injection moulding and influenced by:
1. Hold-on pressure
2. Mould temperature
3. Hold-on time
4. Distance from the gate
5. Degree of crystallanity
6. Melt temperature
7. Injection rate
8. Wall thickness of the moulding
9. Kind and amount of fillers in polymer.
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.
How do you calculate material shrinkage?
Divide the amount of shrinkage by the original size to find the shrinkage rate. In the example, divide 2 by 8 to get 0. 25. Multiply the shrinkage rate by 100 to find the shrinkage as a percentage. In the example, multiply 0. 25 by 100 to get 25 percent.
Why Variations Happen
While it is clear that varying shrinkage rates can cause warpage, it's also important to understand why these differences occur in the first place. Here are five of the most common reasons:
1. Cooling rates: With any semi-crystalline material, a higher cooling rate results in less time for the crystalline structures to form. This effect decreases total volumetric shrinkage. The same effect applies to amorphous materials, but because there is less overall shrinkage the degree to which high cooling rates reduce shrinkage is lessened.
2. Orientation Due to Filling: Initially, the orientation of long, stringy polymer molecules is caused by shear stress during flow when the polymer is still at a high temperature and shear stress is removed, the orientation will relax. (Orientation is locked - in only when shearing and freezing occur simultaneously.) When this relaxation occurs in amorphous materials, there is generally more shrinkage parallel to flow. Because the molecules of crystalline materials are aligned in the direction of flow, most crystallization will occur perpendicular to flow, causing more shrinkage in that direction.
3. Mold Restraint: While the part is in the mold, it can't shrink within the plane of its surface but it can shrink in the direction of its thickness. This has two effects. First, there is more shrinkage in the thickness direction. Second, the polymer accumulates stresses in the plane of its surface. After ejection, these stresses may. relax as the part continues to cool, causing warpage. The higher the mold temperature, the lower the cooling rate, and the more stresses relax from the part. Mold restraint is also material dependent. Materials that resist creep (and relax more slowly) have higher linear shrinkage, while materials that relax more quickly have lower linear shrinkage.
4. Temperature Differences Through the Thickness: When the mold temperature on one side of the cross - section is | different from the other, shrinkage will not be uniform from side to side. In essence, the plane on one side of the part will shrink more, causing it to be smaller than the other side creating a bending moment that can lead to warpage.
5. Thickness variations and uneven packing: when there are varying thicknesses of the part, thick areas will take longer to cool, which can lead to higher shrinkage. A similar effect occurs with areas that are far from the gate. If a constant packing profile is used, areas closer to the gate will be denser and cool at a different rate than areas further from the gate, causing shrinkage variance.
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