Causes and countermeasures for deformation of injection molded parts - 4

Time:2024-03-04 19:00:44 / Popularity: / Source:

Serial No. 4 (Friends who are interested can follow Gud Mold and check out previous series.)

4. Factors affecting deformation

1. Plastic raw materials

Characteristics of plastic raw materials have a huge impact on deformation of molded products. Different raw materials have different molecular structures and intermolecular forces, which manifest themselves in different fluidity, orientation characteristics, shrinkage characteristics, mechanical and physical properties. Therefore, shrinkage rates produced cannot be same. Different molecular structures of plastic materials determine type of plastic material; and different internal additives determine different properties of same plastic, including fluidity, anti-degradation performance, flexibility, flame retardancy, and UV resistance. Although most of time, as mold factories and injection molding factories, we cannot decide which plastic materials to use, but understanding differences of plastic materials and their impact on product deformation is very helpful for us to analyze and solve problems. With this knowledge, we can predict deformation to a greater extent in the early stage. On the one hand, we can give customers reasonable suggestions (such as well-founded advice to customers to relax unrealistic shape and size tolerance requirements; suggest reasonable design structures to compensate for deformation). On the other hand we can design effective preventive measures on mold (such as targeted design of gating system and cooling system).
There are countless types of plastics, and more importantly, degree of freedom in plastic formulations is very high. Production of plastic materials can be customized or modified according to user or application needs, which allows types of plastic materials to be expanded to almost unlimited. In general, there are two main categories of plastics used in injection molding production: thermosetting plastics and thermoplastic plastics. Thermosetting plastics are not within scope of this article, so they will not be described in detail here.
Thermoplastics are divided into crystalline plastics and amorphous plastics according to whether they have a tendency to crystallize. In actual production, it is almost impossible to provide sufficient conditions for crystalline plastics to completely crystallize. Since in most cases, degree of crystallization of crystalline plastics is incomplete, so most of time we call it semi-crystalline plastics.
So-called crystallization refers to orderly and neat folded arrangement structure formed by plastic molecular chains during cooling and solidification process. Characteristics of crystallization make shrinkage rate of crystalline plastics naturally higher than that of amorphous plastics. Because molecular chains are arranged neatly and occupy a relatively small space, shrinkage rate is larger. Just like a diligent person tidying up a house makes it appear that there is a lot of space, same principle applies. But not all plastic materials have a tendency to crystallize. Molecular chains of some plastic materials have no tendency to crystallize during cooling and solidification process, but tend to be randomly and freely entangled. Molecular chains are randomly entangled with each other tightly, become more and more tightly entangled until they are completely solidified. One of direct manifestations of crystallization and free winding behavior in properties of plastic materials is that shrinkage rate of crystalline materials is larger, usually greater than 1%; shrinkage rate of amorphous materials is smaller, usually less than 1%. Through this, we can roughly distinguish crystalline materials and amorphous materials based on their shrinkage values.
Why do molecular chains of some plastic materials tend to crystallize during cooling and solidification process, while others do not? This is determined by molecular structure. Relatively speaking, molecular chain structure of crystalline plastic is relatively simple, with no or few complex branches. This molecular structure makes it conducive to folding itself and forming crystals. Molecular chain structure of amorphous plastic is relatively complex, with many branches. This complex structure itself is not conducive to its orderly folding and inhibits its crystallization tendency. See diagram below for a 3D model diagram of a molecular chain.
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Figure 4.1 The overall 3D model of PP molecular chain. PP is a crystalline plastic with a relatively simple molecular chain branch structure and few branches, so it can be folded and arranged in an orderly manner during cooling and solidification process, that is, crystallization.
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Figure 4.2 Overall 3D model of PC molecular chain. PC is an amorphous material. As you can see from picture, its molecular chain structure is very complex and has many branches. This structure is naturally not conducive to crystallization.
Generally speaking, the more complex molecular monomer structure of a plastic is, the worse its fluidity will be when it forms long chains during polymerization reaction. This is because the longer molecular chains are, the more entangled they are with each other, and plastic flow behavior is essentially movement of molecular chains to each other. The longer molecular chains, the more difficult it is for material to flow. Regardless of whether it is crystalline plastic or amorphous plastic, its molecular shape in molten state is same. Molecules are in a naturally curled state, like earthworms mentioned above. It can be imagined that the more complex molecular chain structure and the longer molecular chain, the more difficult it is for molecules to move between each other when driven by external forces, which manifests as an increase in viscosity and difficulty in flow of plastic.
For semi-crystalline plastics, degree of crystallization affects shrinkage rate of product. Crystallization requires corresponding conditions. Time is a necessary and sufficient condition, and another condition is content of nucleating agent. High mold temperatures provide more time for molecular crystallization, so degree of crystallization is higher. A high degree of crystallization results in increased product shrinkage, i.e. smaller size. Increased crystallinity increases density of product weight and therefore product's sturdiness, but also increases brittleness. But from another aspect, the greater shrinkage, the more difficult it is to control deformation and size of product. Therefore, when size and deformation of a product need to be focused, it is unwise to choose crystalline materials, such as PP. (However, for some mechanical transmission products such as gears and crankshafts, PA or POM materials need to be used. Design of these products determines that product is very rigid, so its shrinkage will not cause serious deformation of product. Natural self-lubricating and good mechanical properties of PA and POM materials meet application of these products. Although size requirements are very high, they are still selected. Both POM and PA are semi-crystalline materials.) The higher nucleating agent content, the faster plastic can crystallize, so degree of crystallization can reach a very high level within a limited time. Generally speaking, semi-crystalline materials, due to their crystallization characteristics, determine that their molecular structures are not too complex. Therefore, generally speaking, crystalline materials have better fluidity, such as PP, PA, POM, PE, PBT, etc.
Amorphous plastics generally have more complex molecular structures than semi-crystalline plastics. Due to complexity of molecular structure, it is naturally not conducive to crystallization. In glassy state, molecular chains of amorphous materials are tightly and randomly entangled. Tightness of entanglement depends on conditions during its production process. Like crystallization phenomenon of semi-crystalline materials, random entanglement of molecular chains of amorphous materials also requires certain conditions, time is a necessary and sufficient condition. Therefore, the higher temperature of mold, the longer winding time is provided, therefore the higher degree of winding, the greater shrinkage. The tighter winding, the better mechanical properties product usually exhibits, such as strength, impact resistance, etc. Due to complex molecular chain structure of amorphous materials, their fluidity is relatively poor and requires greater injection pressure and holding pressure than semi-crystalline materials. The higher melting temperature of material, the greater temperature difference between it and glass transition temperature. When material drops below glass transition temperature, it will release more heat, so it will take longer, which also provides longer time for plastic molecules to wrap around, so shrinkage is greater. The shorter molecular chain and simpler structure of amorphous materials, the better their fluidity. Therefore, it is easier to transmit pressure during injection molding process, pressure-holding and shrinkage effect is better, shrinkage and deformation are easier to control.
It should be noted that although above-mentioned increase in mold temperature and melt temperature can promote shrinkage of plastic materials, shrinkage of product will increase. But on the other hand, higher mold temperature and melt temperature also provide more favorable conditions for pressure-holding and shrinkage. Crystallization process of semi-crystalline materials and random winding process of amorphous materials is a process of material shrinkage. Shrinkage is accompanied by release of excess space, released space is compensated by incoming melt glue maintained and compressed. At this time, the higher mold temperature and the higher melt temperature, which means the lower melt viscosity, and more melt can enter mold cavity for feeding. In this case, if size of runner gate is large enough and holding pressure lasts long enough, product will not necessarily shrink more under high mold temperature and high material temperature; in mold cavity of same volume, more material is packed into product, and degree of molecular crystallization/winding is higher, so product has a denser microstructure. Generally speaking, this is beneficial to product quality. In this case, it is possible to make shrinkage of product smaller. Molecular microstructure of thermoplastics in glassy state is shown in Figure 4.5.
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Figure 4.5 Schematic diagram of microstructure of amorphous plastics and semi-crystalline plastics in glassy state. Picture on the left shows amorphous plastic, whose molecular chains are randomly entangled with each other. Picture on the right shows a semi-crystalline plastic. Neatly arranged part in the middle is crystalline area, and outside is surrounded by amorphous area.
As shown in Figure 4.5, molecular chains of amorphous plastic show a random winding pattern. When absorbing heat, gaps between molecules expand, and molecules slowly unwind. As temperature rises, gaps between molecules gradually become larger. To a certain extent, molecules can move between each other, material can flow. Therefore, for amorphous plastics, they only have a relatively clear glass transition temperature (Tg), but do not have a clear melting point. Melting process is like a piece of butter, gradually melting as temperature rises. Semi-crystalline plastics, however, exhibit different melting patterns. When temperature rises above glass transition temperature (Tg), molecular chains in amorphous region wrapped around crystalline region begin to loosen and expand. Their melting behavior is same as that of amorphous plastics. When temperature rises to a certain temperature (Tm), crystallized area absorbs heat. As heat is absorbed, crystal lattice is gradually melted. Only when crystal lattice is completely melted can plastic flow. During lattice melting process, although material absorbs heat, its temperature does not rise significantly. This temperature point is melting temperature (Tm) of material. Like ice melting into water. Therefore, semi-crystalline plastics usually have a relatively obvious melting point. Since semi-crystalline plastics have crystalline blocks, their lattice stability is very high, so stability of product is also very high. In semi-crystalline materials, only crystallized area is area that is truly solidified, while non-crystalline area, even after injection molding, may recrystallize attached to crystallized area under certain suitable conditions. Result of this phenomenon is post-shrinkage of product. Amorphous materials, strictly speaking, have never truly solidified their internal structure. Even after injection molding, when conditions are right, molecules will continue to tangle, which is why amorphous plastics are relatively less stable.
Amorphous plastics exhibit isotropic shrinkage due to random winding and shrinkage pattern of their molecular chains during cooling and solidification process. So-called isotropic shrinkage means that shrinkage rate of plastic in flow direction and vertical flow direction is basically same. Isotropic shrinkage is relatively uniform, so it is easy to control, and its deformation is also relatively easy to control.
Due to influence of crystallization, shrinkage characteristics of semi-crystalline plastics show slight anisotropy, that is, shrinkage rates in flow direction and vertical flow direction are inconsistent. Generally speaking, shrinkage in flow direction will be slightly larger than shrinkage in cross-flow direction, and the higher degree of crystallization, the greater difference. This is due to influence of flow orientation. During flow, molecular chains are roughly arranged neatly along direction of flow. When crystallization occurs during cooling, extension direction is folded and arranged in an orderly manner, and size shrinks larger relative to vertical direction, resulting in shrinkage anisotropy.
In short, shrinkage of semi-crystalline plastics is more difficult to control than amorphous plastics, physical changes in their molecular structure during cooling and solidification are more complex. Therefore, when designing molds for injection molding semi-crystalline plastics, it is even more necessary to carefully consider rheology and shrinkage characteristics of material.
Additives to plastic materials include plasticizers, flexibility agents, flame retardants, etc. These additives that improve processability and usability of plastics will change flow characteristics and shrinkage characteristics of plastics to a certain extent. Generally, additives such as plasticizers that improve material fluidity are beneficial to product deformation control, because improved fluidity means that pressure required for injection molding is reduced, and internal stress of product is reduced, thus improving deformation.
Fillers of material, including high aspect ratio fillers such as fiber and low aspect ratio fillers such as talc, also have a great impact on shrinkage characteristics of plastics. Additives with low aspect ratios such as talc have relatively little impact on shrinkage characteristics of plastics. They only reduce shrinkage rate of material without increasing anisotropy of material's shrinkage. Fillers such as talc and glass beads are isotropic in shape and therefore do not cause anisotropic shrinkage of material. Because it does not shrink or shrinks much less than plastic base material, it only reduces the overall shrinkage rate of material. Since these additives are harder than plastic base material, their addition increases strength and elastic modulus of material.
However, high aspect ratio additives such as fibers have a huge impact on shrinkage characteristics of plastics. It can be said that it completely changes shrinkage characteristics of plastic, increases anisotropy of material shrinkage, and makes it exhibit an opposite shrinkage characteristic compared to unfilled plastic. Reason is that fibers and polymers are coupled together, constraining shrinkage of polymers. Since fiber itself does not shrink and does not tend to wind in a free state, explanation of earthworm theory mentioned above is: this earthworm is tied with a slender stick to increase its strength (added with fiberglass material). Due to constraints of stick, its free winding nature is restricted, so it shows a completely opposite shrinkage characteristic. Therefore, shrinkage along flow direction is very small, while shrinkage in vertical flow direction is large due to molecular gap. In addition, volume shrinkage rate of material is certain. When shrinkage in one direction is suppressed, there will inevitably be greater shrinkage in other direction to compensate. Due to entangled characteristics of molecular chains, unfilled materials shrink slightly more along flow direction than perpendicular to flow direction.
Length of molecular chain of material and complexity of molecular chain structure have a great impact on molding process and therefore affect impact of process on deformation. Molecular chain length and molecular chain structure of material simultaneously determine mechanical properties of material, which will affect structural stability of product and impact of product structure on deformation.
Structure of molecular chain of plastic determines type of plastic, but addition of various additives and fillers changes characteristics of similar materials, including physical properties, rheological properties, and resistance to various degradations.
Complexity of plastic's molecular chain determines to a certain extent whether plastic has a tendency to crystallize. Since crystallization is a phenomenon in which plastic molecules are folded and arranged in an orderly manner during cooling and solidification process, an overly complex molecular chain structure is not conducive to crystallization. Therefore, semi-crystalline plastics usually have relatively simple molecular chain structures, while amorphous plastics have relatively complex molecular structures.
The higher degree of crystallization of semi-crystalline plastics, the greater shrinkage. Crystallization requires conditions, and time is the most important condition. The longer product takes to cool, the more crystallized it will be.
During cooling and solidification process of amorphous plastics, molecular chains wind and shrink randomly, so its shrinkage rate is smaller than that of semi-crystalline plastics. Random winding and shrinkage also takes time. The longer time, the tighter winding and the greater shrinkage.
Semi-crystalline plastics shrink slightly anisotropically, and the greater crystallinity, the higher degree of anisotropy. Shrinkage characteristics of amorphous plastics are basically isotropic. Material exhibits anisotropic shrinkage, and its shrinkage behavior is more difficult to control, which means that deformation of product is more difficult to control.
Addition of various fillers will change rheological and shrinkage properties of material. Addition of low aspect ratio fillers such as talc powder will basically not change anisotropy of material shrinkage, but addition of high aspect ratio fillers such as fibers will lead to severe anisotropy of material shrinkage. Therefore, shrinkage and deformation of fiber-added plastics are more difficult to control. It is necessary to pay attention to flow and shrinkage behavior of fiberglass materials to develop high-quality molds.
(To be continued: Series 5. A paragraph will be published every week. If you are interested, you can follow Gud Mould.)

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