Author Question: Explain the purpose of a preplasticizing unit. What will be an ideal ... (Read 18 times)

PhilipSeeMore · **on:** Aug 19, 2018

Explain the purpose of a preplasticizing unit.

What will be an ideal response?

Question 2

How does high crystallinity in a resin affect the way the resin is injection molded, including any post-molding operations that might be done?

What will be an ideal response?

brittrenee · **Reply #1 on:** Aug 19, 2018

Answer to Question 1

In some operations, chiefly where the melting of the resin is the limiting factor in decreasing the cycle, a second barrel or chamber is added to the injection molding machine. This second chamber is the collection point for the melted resin as it exits from the screw or ram melting unit. The second chamber has a ram injector that can push the melt of resin into the mold even while material is being introduced from the melting unit. The combination of a continuous melting operation and an on-demand injection operation can speed molding significantly in some cases.

Answer to Question 2

Crystallization normally gives materials a sharper melting point. That is favorable in injection molding because it makes the material easier to heat to a uniform viscosity. Furthermore, the crystallinity usually means that the material will cool and solidify more sharply, thus decreasing the time that the material must be cooled. After the part is removed, residual crystallization is common. This continuing crystallization can distort the part. Therefore, cooling fixtures are often needed to ensure that the part will crystallize and complete cooling without part distortion.