If parts had completely uniform shrinkage, they wouldn’t warp, and they wouldn’t be injection molded. Non-uniform shrinkage exists in every part. The ugly ones warp, have sink or voids and the better ones meet specifications with some molded-in stress—the more uniform the shrinkage, the better the part.
Warpage is a symptom of non-uniform shrinkage, but it doesn’t tell you much about the root cause. Without analysis, you must solve these problems based on your experience and experiments at the molding machine. Wouldn’t it be better to look at the actual parameters throughout the part and the molding cycle to see how you can influence the results?
First, some definitions. Volumetric shrinkage is the change in volume of any discrete volume of material in the part from injection through ejection. Area shrinkage is the effect of volumetric shrinkage throughout the part broken down in the x, y and z directions. Mold shrinkage is a value used to scale-up the cavity so that the part meets nominal dimensional specifications.
Every molding parameter has some effect on volumetric shrinkage. Even though a single mold shrinkage value is typically used, shrinkage is never uniform, and, surprisingly, this works as well as it does. Non-uniform shrinkage is resisted by the inherent rigidity of the part based on its features and material. A very rigid part may not warp, but will still have molded-in stress.
Imagine if you squeeze the part around the outside and try to reduce the diameter. The middle will tend to dome upward. If the plastic shrinks more on the outside diameter, then that is precisely what happens. That volumetric shrinkage difference would be a typical result for a part packed with a single pressure until the part has reached the ejection temperature. The volumetric shrinkage would have larger values at the outside diameter and smaller values near the gate. The packing is not uniform.
With simulation by moldflow, the overall volume that makes up a part is broken down into smaller, tetrahedral volumes (finite element mesh) used to solve these problems. Since we can measure a myriad of molding parameters, including temperature, pressure and volumetric shrinkage throughout the cycle, it is equivalent to having thousands of sensors and probes in the tool.
With mold shrinkage, you are answering the question, “On average, how much bigger do you need to build the cavity for the molded part to meet dimensional requirements?” For volumetric shrinkage, you are answering the question, “How evenly and well packed is the part?” If you do a good job attending to volumetric shrinkage and pick a reasonable average material shrinkage value, then the part will meet dimensional requirements. One depends on a little luck, and the other depends on a bit of analysis.
The longer plastic cools until it reaches the ejection temperature, the more it shrinks. Thick areas of a part that have high values of volumetric shrinkage may result in dimensional issues and may have sink on the outside walls. With very high shrinkage, sink may effectively occur internally by forming vacuum voids. These are often mistaken for air traps that are due to melt-front advancement.
Analysis allows you to evaluate the root cause effects of all of these factors and the effect of modifying wall thicknesses, moving gates and improving cooling. This information will allow you to make informed, timely decisions whether you are designing a part, designing a mold or developing a process.
While shrinkage tends to happen often, it must be avoided when dealing with some delicate and sterile products, such as medical device injection molding. There must not be any form of distortion in the medical parts to ensure safety in healthcare procedures.
Plastic Medical Products Design
Optimize the Cooling Effects
One of the most effective ways to avoid shrinkage problems in injection molding is to optimize the cooling effects. Cooling channels or plates in the mold design should be used to ensure that the cooling process is uniform throughout the mold cavity. This will help regulate the temperature of the mold and prevent the formation of hot spots, which can cause uneven cooling and shrinkage.
Mold temperature controllers can also help with the cooling effects and maintain a consistent temperature throughout the mold. It is essential to maintain a balance as excessive cooling can cause warpage, which can lead to a defective molded part.
Reduce Wall Thickness Reasonably
Careful design and engineering of the part should be done to ensure that the wall thickness is consistent and optimized for the specific material being used. Thinner walls will result in a shorter cooling time, which will help minimize shrinkage.
However, it is important to note that reducing wall thickness can also cause the part to become more fragile and prone to breakage. Thus, it is essential to strike a balance between wall thickness and overall part strength. Furthermore, other factors such as the mold design and processing parameters can also affect the wall thickness of the molded part.
Reduce the Plasticizing Temperature
Lowering the plasticizing temperature can be done by adjusting the temperature of the plasticizing unit or by using a material with a lower melting point. A reduced plasticizing temperature will decrease the viscosity of the molten plastic, which will reduce the shrinkage caused by uneven cooling.
However, the plasticizing temperature should not be too low as this can cause other defects such as flash or incomplete filling of the mold cavity. It is essential to optimize the plasticizing temperature for the specific material being used to achieve the desired results.
Enhance Injection Pressure Speed
Increasing the injection pressure speed will force the material into the mold cavity more quickly and with greater force, resulting in a more uniform and consistent molded part. However, increasing the injection pressure speed can also cause other defects such as flow lines or burn marks on the molded part.
Therefore, it is important to optimize the injection pressure speed according to the customized plastic injection molding specifications to achieve the desired results.
Improve Back Pressure
Back pressure is the resistance of the molten plastic as it flows through the mold cavity. By increasing the back pressure, the material is forced to fill the mold cavity more completely and with greater force, resulting in a more uniform and consistent molded part.
However, excessive back pressure can cause other defects such as sink marks or voids on the molded part. It is essential to optimize the back pressure for the specific material and mold design to achieve the desired results.
Why is it Important to Maintain Uniform Wall Thickness?
Appearance
Uneven wall thickness affects the aesthetics of an injection-molded part. Defects such as warping and sink marks may arise due to the uneven thickness of walls. Superficial issues like flow lines may also be noticed.
Gating challenges
Gating is done from thick sections into thinner sections. Uneven wall thickness may result in the flow of molten plastic from thin sections to thicker sections. This results in problems as the plastic material may begin cooling in the thin section, effectively blocking the flow to the thicker section. Defects like sinking and warping arise as a result of this.
Shear stress
The shear stress of the plastic in flow can be influenced by varying wall thickness. The shear stress of the flowing plastic moving at a constant rate of filling increases as the wall thickness reduces. Uneven wall thickness hence results in different degrees of shear stress. This is a major contributor to warping in plastics.
Cooling rate
Thicker sections take longer to cool. A part with uneven wall thickness will have to remain in the mold until the whole part cools off. This leads to an increase in cycle time and the overall manufacturing process.
Cost
Thicker walls require more plastic materials to fill. They also require more time to cool. More materials and a longer time on the injection molding machine will only increase the cost of production.
Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature . During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses. These so-called residual stresses act on a part with effects similar to externally applied stresses. If the residual stresses induced during molding are high enough to overcome the structural integrity of the part, the part will warp upon ejection from the mold or crack with external service load.