Causes and Countermeasures for Deformation of Injection Molded Parts - 6
Time：2024-03-08 18:46:39 / Popularity： / Source：
3. Mold design
In terms of mold design, factors that have a greater impact on product deformation mainly include rheology function design of mold and heat conduction function design of mold. Mechanical function design of mold also has a certain impact on product deformation, but it is not as significant as the previous two. Following is a detailed description of effects of these three aspects of mold design on product deformation and analysis of their causes.1) Rheological functional design of mold
So-called rheological functional design of mold is, in layman's terms, design of mold's gating system. Design of pouring system must follow rheological principles of plastic materials to achieve good results. Rheology refers to physical mechanics of studying material deformation and/or flow from aspects such as stress, strain, temperature and time. Rheological properties of plastics refer to flow characteristics of plastics, including effects of temperature, pressure, speed and other factors on flow behavior of plastics (principles of these aspects have been explained previously using earthworm theory). Rheological characteristics of plastic materials directly determine flow pattern, flow distance, welding method, etc. of plastic melt. Flow pattern directly affects orientation of molecules/fibers, thereby affecting shrinkage; flow distance directly affects transmission of pressure, which determines distribution of pressure in mold cavity and has a great impact on pressure holding effect, thus affecting shrinkage; welding method directly affects length of bonding line, determines escape direction of air in mold cavity and also affects shrinkage.Long-term experience has summarized some basic gating system design principles that follow principles of plastic rheology. Designers need to have a deep understanding of following principles regarding gate location selection:
Feed glue from thick part of glue position to facilitate pressure transmission. The thicker wall thickness, internal melt glue remains molten for a longer time, which is beneficial to downstream pressure-maintaining and feeding. Moreover, gate naturally withstands greater and longer holding pressure, thicker walls can obtain a longer shrinkage time, so it is beneficial to obtain the overall shrinkage balance of product. Injecting glue into thick parts is mainly beneficial to maintaining pressure, but does not have a great impact on filling. Therefore, for gas-assisted molding processes, you can choose to inject glue into thin parts of heavy products, because shrinkage of thick parts can be formed by gas injection to form an inner hollow shape to compensate for shrinkage.
Balanced glue feeding means that melt flowing into each gate reaches end almost at the same time. In concept of balanced glue feeding, it is very important that distance from vertical runner to each gate is equal. Try to ensure that this is the prerequisite for uniform product shrinkage. Balanced glue feeding can effectively avoid molding defects such as underflow and over-packing. There is a common misunderstanding here. Balance mentioned here does not refer to equal amount of glue injected at each gate (many people focus on balance of glue injection), but balance of melt flow distance of each gate.
Fully consider flow length of melt. Sometimes number of gates will be reduced as much as possible due to number of bonding lines and mold cost considerations (especially hot glue channel molds). Reason is based on material flow length ratio data provided by material supplier, or based on results of mold flow analysis. In fact, this consideration is detrimental to reducing deformation. Flow length ratio data provided by material supplier is obtained under experimental conditions with special molds, and it has great limitations. If flow length ratio given by material supplier is 200, then it is a good idea to take 100 when designing mold. Reason here is obvious. The longer flow length, the greater pressure loss during filling and pressure holding, and the higher degree of uneven shrinkage. In addition, the longer flow length, the greater temperature drop at flow front, which is harmful to quality of bonding line.
Consider minimizing pressure losses in gating system.
Make full use of frictional heating effect of flow channel to help flow.
Shape and size of gate determine initial mode of melt entering mold cavity, which largely determines orientation of molecules or fibers when melt flows in mold cavity. Degree of orientation of molecules and fibers determines impact of flow factors on shrinkage unevenness. Gate size affects degree of injection pressure loss during filling, affects gate shear rate, determines gate freezing time during pressure holding, thus determines pressure holding effect. Pressure-holding effect has a crucial impact on uneven shrinkage of product.
Generally speaking, large gates are beneficial for filling and pressure holding, but because they are not highly restrictive to flow, in multi-cavity molds, overly large gates are detrimental to achieving a filling balance for each cavity. Moreover, large gate removal residues have a fatal impact on appearance of product, and usually require additional labor to remove. Large gate can remain open for a long time, so it can obtain effective pressure holding for a long time, which is beneficial to overcome product deformation. Small gates such as pinpoint gates are generally favored by mold factories and customers because their gate residue has little impact on appearance of product and generally does not require secondary processing to remove. Small gates are beneficial to improving filling to a certain extent, because small gates have great restrictions on plastic filling. Shearing effect of plastic flowing through gate has a great effect on reducing viscosity of melt, thus improving filling to a certain extent, but it also leads to excessive shear stress. This situation is beneficial for filling balance of multi-cavity molds. However, due to natural limitations of small gate size, its development time is relatively short, which is detrimental to pressure holding effect. Fan-shaped gate is of great benefit to dispersion of melted glue into mold cavity. It can maintain uniform orientation of plastic molecules or fibers to the greatest extent, which is beneficial to promoting uniform shrinkage. Moreover, its size is naturally large, and it has a very positive pressure-holding effect on pressure-holding effect. Gate is large and initial flow rate of melt into mold cavity is easy to control. Therefore, injection molding can control appearance defects caused by filling in area near gate, such as air lines, flow marks, etc.
Gates of various shapes and sizes have their own pros and cons. Selection of gates requires full consideration of product quality, characteristics of plastic materials, and production costs. Therefore, it is difficult to formulate a very detailed standard to specify what type of gate must be used for which product structure. But as a mold designer, you must have necessary knowledge to comprehensively analyze these factors to achieve a good compromise.
2) Design of heat conduction function of mold
So-called heat conduction design of mold refers to design of cooling system of mold. In essence, thermoplastic injection mold also plays the role of a heat conductor during injection molding process, that is, heat released by plastic melt injected into mold is quickly conducted out. Thermal properties of plastic determine shrinkage characteristics of plastic during cooling and solidification process, thus determining direction of cooling design of mold.Mold is a heat exchanger, its heat exchange efficiency and heat conduction uniformity have a great impact on plastic shrinkage. For temperature of mold, we must use a dynamic and changing concept to understand it, instead of statically and rigidly thinking that mold temperature must be uniform. It should be pointed out that uniformity of mold temperature is not purpose, but a means to ensure uniform heat transfer and achieve uniform product shrinkage. Uniform product shrinkage is our goal. This concept must be clear, otherwise it will fall into a mechanical and dull solution to problem.
Regarding understanding of mold temperature, there is a common misunderstanding in industry. Many people think that mold temperature is needed for heating, which is wrong. For a certain plastic material, a certain mold temperature is required by plastic properties, that is, plastic needs to be cooled and solidified within such a mold temperature range to obtain qualified quality, rather than requiring heating. Because no matter what kind of thermoplastic plastic material, required mold temperature range is far lower than its glass transition temperature and even further lower than its melting temperature. Therefore, no matter what mold temperature requirement is, for injection molding, it is cooling, not heating. This point must be clearly understood.
As mentioned before, the higher uniform shrinkage of melt glue in each area of mold cavity, the smaller chance of product deformation. Countless practical experiences have verified this. Mold temperature uniformity is critical for uniform product shrinkage. It should be noted that uniformity of mold temperature is a dynamic uniformity, not a static uniformity. So-called dynamic uniformity means that temperature of each area on mold must maintain a certain uniformity during injection, pressure holding and cooling processes, rather than relying on a mold temperature controller or mold temperature control system to maintain uniform mold temperature when mold stops production. Uniform temperature is not goal, uniform heat conduction is the goal. In order to achieve uniform heat conduction, it is necessary to set different temperatures in each area of mold. Because mold absorbs heat during injection molding process, which is a dynamic process. Process of melting glue into mold cavity and releasing heat during pressure-holding and cooling process is a dynamic process, resulting change in mold temperature is also a dynamic process. In short, we must do our best to ensure that cooling rate of melt in each area of mold cavity is consistent, so as to minimize uneven shrinkage caused by cooling.
Due to complexity of product structure, mold structure is also complex. This complexity is reflected in the fact that melt heat absorbed by each area of mold steel is different, and heat capacity of steel in each area is also different. Some areas are wrapped in plastic, and some areas are covered in steel wrapped in plastic. Result is that heat capacity of each area is different, and degree of temperature rise is also different after absorbing same heat. Since temperature rise caused by heat absorption in each area of mold cavity is different, we must consider this situation when designing water path, purposefully adjust cooling design of each area, connect cooling media of different temperatures to achieve uniform heat transfer and uniform shrinkage.
(To be continued: Series 7. A paragraph will be published every week. If you are interested, you can follow Gud Mould.)
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