Causes and Countermeasures of Deformation of Injection Molded Parts - 2
Time：2024-02-26 19:18:29 / Popularity： / Source：
We only need to figure out what happens to these earthworms when they are injected into mold cavity and cooled after being injected into mold cavity, and we will understand true cause of deformation. We first need to understand what characteristics these earthworms have:
When earthworms are in the active state (plastic molten state) and do not flow, they tend to curl naturally instead of being straight. Moreover, each earthworm is like a rubber band and is stretchable. When you straighten a rubber band, then release your hand and lay it flat on the left side, rubber band will naturally curl. Both crystalline and amorphous plastic molecules exhibit this property in molten state.
When earthworms are in an active state (plastic melt state) and flow through a narrow area driven by pressure (shear flow effect), they will tend to flow in an orderly manner, that is, flow in an orderly manner along flow direction (flow alignment), just like many pedestrians must queue up to pass through a narrow passage. The narrower road, the higher degree of orderly arrangement of earthworms (the higher degree of flow orientation); the faster they move in mold cavity, the higher degree of orientation along flow direction.
When the entire mold cavity is filled, earthworms' rapid movement speed in mold cavity quickly decreases to almost zero. At this time, earthworms (plastic molecules in the center of melt), which are protected by frozen earthworms (solidified layer), a type of earthworms (amorphous plastic) begin to wind and shrink freely, gradually winds tighter and tighter. If time is enough, they can wind around each other to the tightest state that can be achieved in theory. Another type of earthworm (semi-crystalline plastic) has different properties than previous one, but tends to shrink in an orderly manner (crystallization), and given enough time, the largest number of earthworms can line up. This process of winding or ordering can last for a long time, from hours to weeks, or even longer.
(After filling, under heat insulation of solidified layer, molecules in the center of melt begin to slowly shrink. Molecules of amorphous plastic are naturally entangled, entanglement becomes tighter and tighter; molecules of semi-crystalline material tend to be ordered and crystallized. If cooling rate is slow enough, that is, when mold temperature is quite high, required cooling time is very long, and plastic molecules will be entangled or crystallized to the greatest extent. At this time, if there is not enough pressure holding effect to supplement new melt glue to make up for space released by entanglement or crystallization of plastic molecules, plastic product will show a large shrinkage rate. Process of molecular entanglement or crystallization does not stop after product is ejected from mold. It can only stop when temperature of the entire product drops below glass transition temperature. Moreover, even under glass transition temperature, molecules of plastic are completely inactive. When temperature rises or is acted upon by external forces, there will still be a small degree of continuous entanglement or crystallization between internal molecules, causing product to further shrink. This phenomenon is post-shrinkage. )
The longer this process takes, the tighter winding or the higher degree of orderly bending and arrangement, until it completely loses activity (complete solidification). But some varieties of earthworms are different. This kind of 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 and therefore exhibits a diametrically opposite shrinkage characteristic.
(For glass fiber-added materials, glass fiber itself does not shrink or shrinks much less than shrinkage of its base material. Due to effect of flow orientation, glass fibers are relatively neatly arranged along flow direction. Fibers are coupled to parent material molecules, constraining free winding or crystallization behavior of molecules, resulting in very low shrinkage in flow direction. In vertical flow direction, shrinkage is larger due to gap between molecules and fibers, volume shrinkage rate of plastic materials is relatively fixed. Since shrinkage in flow direction is suppressed, shrinkage in vertical flow direction will inevitably increase to compensate to achieve the overall volume shrinkage. For glass fiber-added materials, shrinkage in vertical flow direction sometimes even exceeds shrinkage of base material.)
When earthworms flow under pressure, they tend to preferentially flow in the direction of small flow resistance, just like driving a car on highway rather than on an ordinary highway.
(Stagnation effect, plastic tends to flow preferentially towards path with low flow resistance when flowing)
As soon as earthworms come into contact with cold mold wall, they will solidify and freeze immediately, have no time to shrink. Frozen earthworms form an insulating layer that protects inner earthworms from freezing and allowing them to continue moving forward. The first earthworms that come into contact with mold wall are all a group of great earthworms, making huge sacrifices because they cannot entangle or crystallize according to their nature.
Figure 3.7 Schematic diagram of melt flow cross-section. When melt moves in mold cavity, front end becomes a fountain, and a solidified layer is formed immediately when it contacts mold wall. Due to heat preservation effect of plastic, melt in the middle can flow well.
(On flowing cross section, shear between melt and solidified layer is the most severe. Due to shear heating effect, temperature here is also the highest. Heating causes temperature of melt to rise, which reduces viscosity of melt. On the other hand, shear promotes orientation of molecules during flow, making flow easier. This is shear thinning effect. High temperature generated by shearing may far exceed melting temperature of plastic, but probability of material degradation is very small. This is because filling time is very short, generally within 3 seconds for molds. Filling time of large automobile exterior parts needs to be longer, and it takes time for material to degrade. It takes a certain amount of time for anything to happen. Time is a necessary and sufficient condition. If this condition is insufficient and other conditions are sufficient, things will not happen. )
Figure 3.8 Flow section shear distribution curve. This figure does not consider solidified layer, so maximum shear is at contact surface between melt and mold wall. This phenomenon occurs in hot runner plate. In fact, during mold cavity filling process, since mold temperature is much lower than glass temperature of material, a solidified layer will definitely form.
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