INJECTION MOLD

INJECTION MOLD

 

Once a plastic has reached its proper melt viscosity, it must then pass through the injection nozzle and into the mold. The function of the mold is to shape and then cool the plastic melt. Molds are generally made of tool steels. The cavities are machine made out of smaller blocks (inserts) of tool steel, hardened and inserted into a standard or custom mold base. A feed system consisting of a sprue, cold slug well, runners and gates feed melted plastic from the nozzle to the mold cavities.

The sprue is a tapered channel that delivers the melt from the nozzle to the cold slug well. The cold slug well receives the leading edge of plastic from the nozzles. The leading edge is always cooler with a thicker viscosity than the melt behind it and must be deposited in the cold slug well or it will block the runners. Once the pressure rises in the sprue channel the plastic melt breaks into the runners. The melt moves down the runner until it reaches a small restriction at the end of the runner, which marks the entry into the cavity. This restriction is called a gate. It holds the melt until the entire runner system is filled. Once the runner system reaches maximum pressure, the melt breaks through the small restriction of the gate and fills the mold cavity. The rapid filling of the cavities causes air compression and creates a small combustion on the surface of the plastics. This produces a burn mark, which is relived by venting the mold where combustion occurs.

The mold provides channels for cooling, heating, and venting in the mold cavity. Ejector pins and other release devices are incorporated into the mold as required. The choice of a mold base depends on the cost, configuration and production requirements of the part. For high volume production, the initial expense of quality tooling is a sound investment. The choice of tool steel depends on the quantity and quality of parts to be produced and will be primarily determined by the type of polymer being used for the application.

Important Properties for an Injection Mold:

  Balance of the runners: This means that the plastic flow to each part in the mold should be exactly the same. This helps to hold consistency in parts made on the same shot.

  Uniform water-cooling of the mold:  The core and cavity of the mold should be cooled uniformly to obtain sound parts.

  No deflections: Make sure that mold does not bend or deflect when plastic is injected into the mold.

  Vent the mold: The mold should be vented so as air pressure does not build up during injection causing air traps in the parts.

TYPES OF INJECTION MOLD

Types of mold normally used in industries are

1.   Cold runner single-day light mold:  It is the simplest mold design. It consists of two assembly units of plates with cavity and core inserts mounted in each unit as shown in figure. These units are fastened on the molding machine platens.  One unit reams on the injection side, while the other unit containing plastic part, feed system, cooling system and ejector pins, opens with the clamp. Simple parts with large gate are usually molded in single-day light mold. A disadvantage of this type of mold is that plastic parts remain attached to the feed systems. The molded part must be separated from its feed system in a post molding operation. These feed system are then ground into small pieces or pellets and mixed with the virgin plastic for reuse.  Cold runner injection molds are more economical to build than hot runner molds, however they can be less economical to run if the amount of plastic in the sprue and runner exceeds 35% of the plastic that is injected into the part cavities.

 Cold Runner Single Day Light Mold

2.  Cold runner multi-day light mold: It consists of more than two assembly units of platens such as:

a.       Stationary or Runner plate assembly unit, which is attached to the stationary platen and usually, contains the sprue and runner.

b.      Floating cavity plate assembly unit that contains the molded part and ejector system.

c.       Movable assembly unit that contains the core and ejector system.

In multi-day light mold, the molded part separates from its feed system automatically, when the mold opens as shown in figure. One part drops out on one side of the plate, and the feed systems, separated at the gates from the part; drop out on the opposite side of the plate. The advantage of a multi-day light or three-plate mold is that it allows precise pinpoint or center gating of the plastic parts. This style mold is generally less expensive than a similar hot runner injection mold. 

Figure: Cold Runner multi Day Light Mold

3.   Hot runner mold: In hot runner mold, the runners are kept hot in order to keep the plastic material in a fluid state at all the times. In general, a round runner is cut in a manifold, which is heated with the help of heaters provided to it as shown in figure. The advantage of a hot runner mold is that the sprue and runner remain molten, producing no byproducts to be separated or reground.  In addition the cycle time is often shortened because the amount of plastic to be melted and then cooled is often drastically reduced.  While hot runner molds are typically more expensive than a conventional cold runner tool. They are particularly advantageous for small injection molded parts where the sprue and runner can consist of more than 50% of the total plastic shot per cycle.

 

Figure:  Hot Runner Single Day Light Mold