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It is usually slow and inefficient to mold thermoplastics using the compression molding techniques described above. In particular, it is necessary to cool a thermoplastic part before removing it from the mold, and this requires that the mass of metal making up the mold also be cooled and then reheated for each part. دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
is a method of overcoming this inefficiency. Injection molding resembles transfer molding in that the liquefying of the resin and the regulating of its flow is carried out in a part of the apparatus that remains hot, while the shaping and cooling are carried out in a part that remains cool. In a reciprocating screw injection molding machine, material flows under gravity from the hopper onto a turning screw. The mechanical energy supplied by the screw, together with auxiliary heaters, converts the resin into a molten state. At the same time, the screw retracts toward the hopper end. When a sufficient amount of resin is melted, the screw moves forward, acting like a ram and forcing the polymer to melt through a gate into the cooled mold. Once the plastic has solidified in the mold, the mold is unclamped and opened, and the part is pushed from the mold by automatic ejector pins. The mold is then closed and clamped, and the screw turns and retracts again to repeat the cycle of liquefying a new increment of resin. For small parts, cycles can be as rapid as several injections per minute.
One type of network-forming thermoset, polyurethane, is molded into parts such as automobile bumpers and inside panels through a process known as reaction دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
, or RIM. The two liquid precursors of polyurethane are a multifunctional isocyanate and a prepolymer, a low-molecular-weight polyether or polyester bearing a multiplicity of reactive end-groups such as hydroxyl, amine, or amide. In the presence of a catalyst such as a tin soap, the two reactants rapidly form a network joined mainly by urethane groups. The reaction takes place so rapidly that the two precursors have to be combined in a special mixing head and immediately introduced into the mold. However, once in the mold, the product requires very little pressure to fill and conform to the mold—especially since a small amount of gas is evolved in the injection process, expanding the polymer volume and reducing resistance to flow. The low molding pressures allow relatively lightweight and inexpensive molds to be used, even when large items such as bumper assemblies or refrigerator doors are formed.
The importance of دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
on the productivity of a tool is often overlooked in the design of a mold. Several areas in the mold design exist where the molder must work with the mold builder in order to optimize the productivity of the mold. A good standard for mold productivity is saleable parts out of the press per hour. Cycle time and part quality are the critical aspects of saleable parts per hour. The areas of design found to be most important for increased productivity are the sprue bushing, runners and gates, hot manifold, venting, cooling, and ejection. While each of these items is specific to the mold being built, good design for each can contribute to improved part quality and optimum cycle time.
Too often the mold maker is left to decide the sizes of the sprue, runners, and gates and only when running the first samples does the molder learn that the sizes are not optimal. Much of this can be resolved beforehand by following the principles of runner and gate design found in the Injection Molding Handbook, as well as other reference materials. Again, runners sized too small affect the heat and pressure of the دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
and runners too large may slow the cycle for cooling time and cause unnecessary regrind.
Computer Numerical Control (CNC) machining is a manufacturing process in which pre-programmed computer software dictates the movement of factory tools and machinery. The process can be used to control a range of complex machinery, from grinders and lathes to mills and CNC routers. With دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
, three-dimensional cutting tasks can be accomplished in a single set of prompts.The CNC process runs in contrast to — and thereby supersedes — the limitations of manual control, where live operators are needed to prompt and guide the commands of machining tools via levers, buttons and wheels. To the onlooker, a CNC system might resemble a regular set of computer components, but the software programs and consoles employed in CNC دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
distinguish it from all other forms of computation.When a CNC system is activated, the desired cuts are programmed into the software and dictated to corresponding tools and machinery, which carry out the dimensional tasks as specified, much like a robot. In CNC programming, the code generator within the numerical system will often assume mechanisms are flawless, despite the possibility of errors, which is greater whenever a CNC machine is directed to cut in more than one direction simultaneously. The placement of a tool in a numerical control system is outlined by a series of inputs known as the part program.With a numerical control machine, programs are inputted via punch cards. By contrast, the programs for CNC دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
s are fed to computers through small keyboards. CNC programming is retained in a computer’s memory. The code itself is written and edited by programmers. Therefore, CNC systems offer far more expansive computational capacity. Best of all, CNC systems are by no means static since newer prompts can be added to pre-existing programs through revised code.
Rubber materials that are harder are more resistant to compression set, the permanent deformation of a material after prolonged compressive stresses at a given temperature and deflection. If a rubber reaches a compression set, the seal loses its ability to return to its original thickness when the compressive stress is released. Leakage may occur, and seal failure can result. Chemical resistance can be critical – and complicated. That’s why it’s important to identify all the chemical agents to which your rubber product will be exposed. For example, if you’re in the mobile equipment industry, you may need engine bay insulation that can resist both fuel oil and cleaning chemicals. The دیدن لینک ها برای شما امکان پذیر نیست. لطفا ثبت نام کنید یا وارد حساب خود شوید تا بتوانید لینک ها را ببینید.
s on fuel tanks may need to resist both diesel fuel and biodiesel blends.