the shrinkage of plastic parts, due to the nature of plastic ( to expand when hot and to shrink when cold), the dimension of plastic parts after molding and cooling is smaller than that of the cavity.
the shrinkage of plastic parts, due to the nature of plastic ( to expand when hot and to shrink when cold), the dimension of plastic parts after molding and cooling is smaller than that of the cavity.
The shrinkage rate is strongly depending on the polymer composition & material properties (PVT, thermal properties…), itself but also on the processing conditions (temperature, pressure, flow rate etc.) applied and part design & geometry (Wall thickness, gate location, mold constraints).
Polymer Composition
Semi-crystalline polymers (e.g. Polybutylene terephthalate or Polypropylene) always show a higher shrinkage than amorphous polymers (e.g. PS, Polycarbonate , PolyVinyl Chloride, Acrylonitrile Butadiene Styrene, Polymethyl Methacrylate). This is because semi-crystalline polymers when cooled down, they see part of their macromolecular chains re-arranged to form crystallite that is a well-organized structure, leading to less space needed for the same number of atoms.
However, a slow rate of crystallization or a low degree of total crystallization has the effect of reducing shrinkage and thereby reducing warpage in semi-crystalline polymers.
It is also important to note that the presence of side chains in molecular structure inhibits the ability of molecule to fit into a developing crystal structure. Hence, these high degrees of chain entanglements in highly branched polymers also inhibit rapid crystallization.
Nucleated resin grades show higher amounts of shrinkage. This is same of copolymers and homopolymers as well.
Molecular Weight
Degree of shrinkage is also influenced by molecular weight. HMW resins show a higher viscosity on filling and a higher pressure drop in the tool cavity during filling. Higher packaging pressure must be used to compensate for the cavity pressure drop or else the lower pressure melt will result in higher shrinkage in the final part.
Fillers and Fibers
Fillers and fibers are generally added in plastics to modify properties such as stiffness, creep resistance etc. Filler systems and fiber reinforcements in composites result in different shrinkage from the virgin polymer. Most fillers and fibers have relatively low coefficient of thermal expansion, hence when part is cooled down during processing, these additives tend to shrink significantly. The reduction on shrink is approx. proportional to their concentration.
Fiber-filled materials such as polymers filled with long glass fibers shrink less along the direction in which fibers align (typically the flow direction) compared to the shrinkage in the transverse direction.
Recycled fiber-reinforced polymers exhibit different mold-shrinkage characteristics then those of virgin resin.
Pigments
Pigments, in general, lead to increase in shrinkage. They promote shrinkage by acting as a nucleating agent. The use of pigments tends to increase the cross-flow shrinkage in semi-crystalline polymers.
Presence of pigments in polymers can affect the crystallization and hence the mold shrinkage.
Organic pigments provide crystalline nuclei from which crystals grow. Earlier initiation of crystallization and more rapid crystallization result in high amount of crystallinity in pigments resin as compared to neat polymer.
Inorganic pigments cause the same type of shrinkage change to a less degree.
Time and Stress – Dimensional Stability
In plastics, the rate of dimensional change is determined by the stress level and the temperature at which the part is held under stress. At increasing times, the part under load will deform in response to the applied load.
Loss of fluids such as plasticizers loss due to migration or boil-off with time also causes shrinkage and increases brittleness.
Further excessive shrinkage beyond the acceptable level can be caused by:
Low injection pressure
Short pack-hold time or cooling time
High melt temperature
High mold temperature
Low holding pressure
Polymer Grades with Low Shrinkage
View a wide range of polymers with low shrinkage, analyze technical data of each product, get technical assistance or request samples.
The shrinkage of plastics signifies the volume contraction of polymers during the cooling step of the processing of polymers. This contraction is partly due to the difference of density of polymers from the melt state and the cooled, rigid state.
Most of the plastic molded part shrinkage occurs in the mold while cooling. 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. In higher shrink materials such as acetal and nylon, the post-mold shrinkage can be significant.
Uncompensated volumetric contraction leads to either sink marks or voids in the molding interior. Controlling part shrinkage is important in part, mold, and process designs, particularly in applications requiring tight tolerances. Shrinkage that leads to sink marks or voids can be reduced or eliminated by packing the cavity after filling. Also, the mold design should take shrinkage into account in order to conform to the part dimension.
When the thermoplastic is processed by the injection molding method, the mold size changes during the cooling process. These dimensional changes are referred to as “shrinkage.” Shrinkage is based on the compressibility and thermal expansion of the plastic. When the thermoplastic shrinks, the volume changes. In order to maintain the desired size of the plastic part, the mold cavity increases as the amount of plastic shrinkage increases. Therefore, tool and mold manufacturers must predict the mold cavity and shape between the dimensional differences associated with shrinkage. This is not easy in many cases, because shrinkage is determined by a large number of influencing factors. If the designer incorrectly estimates the contraction, it will cause the component to deform. In addition to process control variables (temperature, pressure) and material properties (p-, v-, d-behavior, filler, amorphous, semi-crystalline), the stiffness and wall thickness of the formation also affect shrinkage. Although shrinkage is based on thermal expansion, the effective reduction in size after removal from the mold is less than expected for pure thermal expansion. Other mechanisms play a role in shrinkage, resulting in reduced heat shrinkage relative to pure thermal expansion.
Plastic shrinkage refers to the volume shrinkage of the polymer during the cooling phase of the process. This shrinkage is due to the fact that the density of the polymer is different from the melt state, the cooled state,rigid state
As all mold engineers know, shrinkage is an unfortunate fact of life. All plastics shrink as they cool from viscous liquids to solids, and every type of plastic shrinks in a slightly different way. And while eliminating shrinkage is impossible, minimizing it is essential for molding parts accurately.
When the plastic melts and cools, shrinkage begins at the molecular level. To a large extent, these forces depend on the type of material and whether there is filler or fiber reinforcement. There is also a need to consider processing and part design factors.
Wall thickness is a factor of shrinkage because it affects the crystallinity of the material, which in turn affects the total potential shrinkage. Uneven wall thickness results in different cooling rates for the entire part. The thinner the wall, the faster the cooling, the lower the crystallinity and shrinkage. The larger the wall thickness, the slower the cooling rate, and the higher the crystallinity and shrinkage. In amorphous materials, increasing the wall thickness reduces the orientation effect. Maintaining a uniform wall thickness helps to avoid shrinkage changes that can cause warpage.
When fibers are introduced into plastic, they may counteract the shrinkage effect due to molecular orientation. The fibers do not expand or contract with changes in temperature, so they tend to reduce shrinkage in the direction of orientation and increase shrinkage in the transverse direction of orientation. For example, polymers filled with long glass fibers shrink more in the transverse direction than in the longitudinal direction, which makes them unsuitable for projects with narrow tolerances. The filled resin typically shrinks less than the unfilled resin. The resin can be filled with a variety of materials, including fiberglass, wood, and mica to alter the performance of the part.
Managing contraction is a complex task, considering the number of factors involved and how each factor affects other factors. By allowing engineers to solve this problem early in the product design cycle, simulation software can make this job easier.
Using simulation tools such as Autodesk Moldflow, you can set up and run an analysis to visualize the expected amount of shrinkage given the current part material, design, and expected processing conditions. For ease of explanation, the results can be scaled. The engineer can then change the machining conditions or part design and run the simulation again to see how much the shrinkage is reduced. Simulation tools can also consider a wider range of potential solutions faster, easier, such as changing the size of a material or mold, all of which is more convenient than dealing with shrinkage that has already occurred.