Runnerless metal molding molds

Editor's note: The author of this report, Hakim Belhadjhamida of Mold-Masters Ltd., has a unique perspective on how hot runners can improve product quality and productivity in PIM, while providing substantial cost savings in the use of expensive feedstocks. Belhadjhamida works in R&D for Mold-Masters, which uses its own hot runners to mold PIM components for the hot runners it sells.

The proper function of a hot runner is to transfer the plastic melt from the injection molding machine nozzle to the mold cavity with no effect on its rheology. In essence, it can be considered as an extension of the machine nozzle directly to the cavity. The design of a hot runner, therefore, should consider the following factors:

The benefits of hot runner vs. conventional cold runner molding include better part quality, more control over the injection molding process, and elimination of runner removal and regrind. (Figure 1 shows a schematic of a hot runner system vs. a cold runner tool.) In PIM, the benefits extend to less tool wear and the ability to mold very thermally conductive materials with low injection speeds and pressures.

An enlarged detail of two different hot runner gate designs is shown in Figure 2. The gate seal design and material depend on the plastic solidification behavior, abrasiveness, corrosion characteristics, heat sensitivity, and the quality of gate vestige on the molded part. For example, during cavity filling with crystalline polymers, premature solidification at the gate area may occur if heat is not adequately provided. Amorphous polymers, on the other hand, have slow solidification kinetics, requiring heat to be drawn from the gate area to avoid stringing at demolding.

Applications involving semi-crystalline polymers are more complicated because a compromise between the amount of heat provided and/or withdrawn from the gate area should be achieved depending on the material's degree of crystallinity and its heat sensitivity. In essence, each gate area should be designed taking the plastic material's thermal and rheological properties into consideration.

In general, PIM feedstocks can be considered as crystalline materials due to their high thermal conductivity. Therefore, heat should be provided to the material in the gate region and large gates should be used.

Gating Methods

There are a number of different gating methods that can be used with hot runners. Figure 3 shows three different gating methods that may be used: hot tip, sprue, and valve gating. What follows outlines how each gating method pertains to PIM.

Hot tip gating. This hot runner gating method is probably the most widely used with both amorphous and crystalline materials. To function properly, the temperature at the tip, geometry of the tip, gate diameter, and cooling must be considered. This gating method leaves a small mark on the molded part surface. As illustrated in Figure 4, a relatively high degree of material shearing occurs at the gate. When applied to PIM feedstocks, this shearing results in powder/binder separation and eventually leads to gate clogging.

Sprue gating. Sprue gating leaves a long projection on the surface of the demolded part. It is commonly used with cold subrunners. Since large gates can be used with this method, good packing and less molded stress occurs compared to hot tip gating. PIM binders usually consist of different components, each with a different melting or glass transition temperature, to allow for good mixing and debinding. Typically, the molding temperature is much higher than at least one of these components.

Consequently, when using this type of gating, it is common to experience material drooling in the cavity between molding cycles. This unpacked material then solidifies and is carried inside the part during subsequent cavity filling, resulting in a defective component.

For applications that require hot sprue gating, direct gating into the cavity should be avoided and sprue pullers are required. In this scenario, the cold plug is injected into the sprue puller away from the cavity.

Valve gating. In valve gating, a pin moves to an open position during cavity filling and holding and closes during cooling and demolding. Except for the added cost, the benefits of valve gating are as follows:

Melt Flow Channels

During injection, the melt is subject to shear stresses from flow in the runners. With highly filled thermoplastics or PIM feedstock, high shear is not tolerated since material separation occurs. This causes plugging of the runners at flow transitions or in the gate area. Therefore, large diameter runners should be used in this case. High material shear can also occur when using internally heated, annular hot runner channels as compared to externally heated open pipeline runners (shown in Figure 5). In general, high material shear should be avoided whether in the runner or gate.

Another area of consideration in the design of runner channels is runner transitions. The geometry of the runner where the direction of the melt stream changes should allow for smooth melt transition to avoid dead spots that are usually more rich in powder than binder. These dead spots eventually result in plugging of the hot runner system.

Process and Regrind Control

In general, metal feedstocks have high thermal conductivity as compared to plastics. In conventional cold runner molding, high injection speeds and filling pressures are required to prevent material from freezing. Such a requirement allows little control over molding machine parameters. In addition, the high pressures involved in part filling result in gate and cavity wear. When using hot runner systems, the injection molding process window is increased as molding is controlled right at the cavity.

In closing, PIM feedstocks are much more expensive than plastics. At present, depending on the binder used, most cold runners produced can be reground and reprocessed. However, multiple regrinding results in small deficiencies in the low temperature components of the binder material, which results in problems associated with the final size of the sintered part. The use of hot runners in this case is beneficial as regrind is minimized, compared to the technology of cold runner tools.