Causes and Countermeasures of Deformation of Injection Molded Parts - 3

Time:2024-02-29 21:14:51 / Popularity: / Source:

Serial 3 (Readers who are interested can follow Gud Mould to view previous serials 1 and 2)
We understand characteristics of earthworms, then we use earthworm theory to describe molding process. This will help us understand what happens to earthworms, so we can understand why plastics deform.
Countless small earthworm particles (each particle contains countless frozen and inactive earthworms) are put into hopper of injection molding machine and enter material tube under its own weight. Rotation of screw pushes particles towards front end of material tube. Due to shearing effect and heating effect of material tube heating tube, earthworms gradually regain their activity under action of high temperature and slowly stretch out. These stretched earthworms are transported to the front end of material tube by screw. These earthworms are now able to move freely. Motile earthworms are continuously sent to the front end of screw until a sufficient number can fill mold cavity and flow channel. At this time, screw stops rotating and waits for injection (above plasticizing stage). Everything is ready, screw starts to push forward, earthworms are squeezed out of nozzle and into mold. Nozzle is a very narrow passage, earthworms must be stretched out and arranged in an orderly manner to pass through smoothly. Therefore, earthworms automatically pass through nozzle in an orderly manner and enter mold. Since front channel has never been very spacious and very narrow (injection molded parts are generally shell parts, and wall thickness generally does not exceed 3 mm), earthworms must continue to maintain an orderly queue in order to move forward smoothly. Because mold wall is very cold, earthworms that first came into contact with mold wall were immediately frozen and lost their activity. Because time was too short, they did not even have time to curl up before being frozen. These frozen earthworms form a good insulation layer, protecting subsequent earthworms from moving forward smoothly until the entire mold cavity is filled (filling stage). Earthworms finally filled mold cavity, and rapid movement of earthworms in mold cavity stopped. Insulation layer formed by sacrifice of forerunner earthworms protected earthworms in the middle from being frozen quickly, so earthworms inside can entangle each other according to their nature, becoming more and more tightly entangled (amorphous plastic) or stacked in an orderly manner (semi-crystalline plastic) until they are frozen and inactive. Due to winding shrinkage or crystallization shrinkage, some spaces are released. Screw has not stopped moving at this time, and gate has not yet frozen. Earthworms continue to replenish these spaces later (pressure-holding and feeding stage). If gate freezes before full shrinkage, there will not be enough new earthworms to make up for shrinkage release space, and plastic product will shrink too much; if pressure holding ends before gate freezes, it means that injection molding machine has released pressure. At this time, pressure in mold cavity will be higher than pressure in runner, gate is not fully cured, uncured earthworms in mold cavity will flow back from gate back to runner, resulting in excessive local shrinkage near gate, or even excessive shrinkage of the entire product. After gate freezes, holding pressure becomes meaningless because melted glue cannot enter mold cavity. This is when earthworms in mold cavity begin to wrap around freely or fold in an orderly manner. Degree of this depends on how much time heat conduction function of mold can provide for shrinkage (cooling stage).
Deformation of Injection Molded Parts 
Figure 3.9 Schematic diagram of typical injection molding machine structure
Due to complexity of mold cavity structure, temperature in each area is uneven, and amount of earthworms added to each area is different, so final degree of tight entanglement or orderly folding arrangement (degree of crystallization) of earthworms in each place is different. Some have enough time to wrap very tightly (center of melt, thicker wall, the area where heat accumulates on mold), some lose their activity halfway through wrapping, and some places cannot fill space left by subsequent earthworms after wrapping, etc. In short, degree of tightness of earthworm entanglement is different in all areas inside mold cavity. Even in same area, cross-sectional direction is different between forerunner earthworms and inner earthworms.
Therefore, in this process, earthworms formed three states (for sake of vivid description, let's call them three forces): first, some earthworms were frozen (solidified layer) before they could wrap themselves around; second, some earthworms were frozen during entangling process and was not fully entangled. Although it was frozen, its tendency to entangle still existed. Tendency of entanglement or crystallization did not disappear, but was just frozen in product; third, some earthworms are fully entangled and has the highest degree of density. In addition, after product is ejected when it reaches ejection strength (product is not ejected after it is 100% solidified), it does not mean that earthworms inside have completely lost their activity. Earthworms inside can still make small movements and move until they are completely solidified. This is why product shrinks after it appears. Because these three forces are distributed within earthworm production products, and distribution situation is very complicated, it seems that the three warring forces are intertwined, there are contradictions, conflicts, (stress) between them. When product breaks away from mold and loses restrictions of mold, these three forces interact with each other, just like external forces are exerted on product. If product itself has strong structural rigidity, stress generated by interaction of the three forces is not enough to overcome rigidity of product, product will not deform or deform very little; if product itself has poor structural rigidity and is not enough to resist internal stress, deformation will occur. Even if product as a whole does not deform or deforms very little, internal forces will appear in other forms, such as stress marks, cracking when subjected to external forces, etc.
(Three different types of uneven shrinkage naturally exist inside any injection molded part: uneven shrinkage in wall thickness section; uneven shrinkage between regions; uneven shrinkage in melt flow direction and vertical flow direction. These three types of unevenness exist objectively and are unavoidable. All countermeasures to reduce deformation are to take all measures to reduce degree of these three types of uneven shrinkage. Uneven shrinkage in wall thickness direction basically has little impact on deformation of product. For unfilled plastics, amorphous plastics basically show isotropic shrinkage, and crystalline plastics show a certain degree of anisotropic shrinkage, but not too much. Therefore, the biggest factor affecting deformation of unfilled plastic products is uneven shrinkage between regions. However, for fiber-filled plastics, shrinkage difference between melt flow direction and vertical flow direction is very large, so it has a great impact on product deformation. This is also one of reasons why deformation of fiber-added plastic products is more difficult to control.)
(So-called uneven shrinkage between regions is caused by complexity of product and mold structure. For product structure, due to need to meet various functions, product must be designed with different features: main body, side walls, bump holes, ribs bits, pillars, flanges, even uneven body wall thickness, etc. These different features must be designed with different thicknesses, and changes in their geometry make heat conduction process they experience in mold never same, which determines absolute existence of uneven shrinkage between product areas. From mold perspective, one is impact of cooling design. Usually front mold has a lot of freedom in designing water path, and front mold is usually steel wrapped in plastic for product. Heat of melt is relatively easy to conduct away; rear mold is usually made of plastic-wrapped steel, which is relatively small in size and is generally less capable of absorbing heat than front mold. Moreover, rear mold usually has restrictions on ejection structure, and degree of freedom in arranging water channels is far less than that of front mold. This causes heat transfer capacity of rear mold to be generally inferior to that of front mold. The other is influence of gating system. Area close to gate undoubtedly has a better chance of obtaining better pressure-holding and shrinkage effect, while area far away from gate is naturally less able to obtain pressure-holding and feeding than area near gate. This difference will be greater if melt flow is very long. )
(Complexity of product and mold structure leads to uneven shrinkage between regions. An excellent mold designer can skillfully use mold design to make up for shortcomings of product design, use unevenness of mold to make up for unevenness of product, that is, by appropriately optimizing product structure, rationally designing pouring system and cooling system to minimize this shrinkage unevenness. However, a poor mold design will amplify it, that is, product unevenness + mold unevenness will turn into greater shrinkage unevenness.)
Above three situations exist in any injection molded product, so it can be said that product cannot achieve an ideal and absolutely uniform shrinkage state. Deformation is absolute, non-deformation is relative.
Conclusion: Uneven shrinkage of product produces internal stress. Role of internal stress is essence of deformation, and cause of internal stress is uneven shrinkage, which is fundamental cause of deformation.

4. Factors affecting deformation

Through description of front mold, we know root cause of deformation. Next, we will discuss factors that affect deformation. We will discuss it from following five aspects.
1. Plastic raw materials
2. Product structure
3. Mold design
4. Mold processing
5. Molding process.
(To be continued: Series 4. A paragraph will be published every day. If you are interested, you can follow Gud Mould. If you continue to follow, there will be surprises.)

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