The fourth article on the processing and use of Premix’s electrically conductive plastics focuses on the injection molding process. The electrically conductive PRE-ELEC® grades can be successfully molded with normal processing equipment. However, the processing conditions require particular attention due to their significant effect on the resistivity of the molded product.
Processing makes a difference
As you will learn later in this article, the resistance of a product made out of the same carbon black compound can vary from 1E+4Ω to 1E+9Ω. How the molten material enters, fills, and cools within the mold cavity affects the properties or the distribution of the carbon black filler within the compound. The final resistivity of the molded article depends on the effects of the shear on the carbon black network, changes in polymer crystallinity, and the flow path in the mold.
High shear equals higher resistivity
The main enemy of carbon black filled materials is shear. Injection molding process is known to produce high shear forces and, therefore, may cause degrading of the carbon black network. This reduction of the carbon black structure, resulting mainly from carbon black aggregate compression, leads to lower conductivity. Thus, electrically conductive PRE-ELEC® compound are recommended to be processed under low shear conditions.
High shear conditions can be prevented with proper balance of injection rate, temperature and time. Figure 1 describes how barrel temperature, mold temperature and constant injection speed affect the surface resistance of sample plates made of polypropylene based PRE-ELEC® compound. As can be seen, barrel temperature and injection speed as well as their interaction have a significant effect on the molding results of the electrically conductive compound, highlighting the importance of adjusting these variables carefully. The mold temperature has a lesser effect on the electrical properties, but it determines in part quality aspects such as shrinkage, surface appearance and residual stresses.
Figure 1. Impact of processing conditions on surface resistance of a PP-based PRE-ELEC® grade.
Low barrel temperature and high injection speed reduce conductivity in particular. Lowering the barrel temperature decreases the melt temperature and increases viscosity, resulting in higher injection pressure and carbon black aggregate compression. Fast injection, in turn, increases the amount of shear forces that the carbon black filled material is subjected to during the mold filling stage. Conversely, increased conductivity is caused by low shear processing conditions, that is, lower melt viscosity and/or injection rate. It is also important to optimize the switchover point in order to avoid a harmful pressure peak caused by the delay in switchover from injection pressure to the lower holding pressure.
Control crystallization with cooling
In the case of semi-crystalline polymer compounds, the final resistivity is also influenced by the crystallization behavior. Since polymer crystals do not incorporate carbon black, increase in crystallinity produces a higher carbon black concentration in the amorphous phase of the polymer. High local carbon black concentration strengthens the conductive network and thus increases the conductivity of the final material. Therefore, appropriate temperature control of the mold and cooling fluid are essential for consistent cooling of the molded article during every cycle.
Tips for product and mold design
The flow conditions of the molded part have a major impact on the electrical conductivity of the finished product. As the molten compound is pushed through the mold, it bends, turns and distorts. Hard or sharp shapes in the mold exert more stress on the melt and cause more damage to the carbon black structure than gentle shapes do. Furthermore, the melt pressure varies with the distance of the flow. The pressure is at its highest close to gate and decreases when moving away from this point. As a result, the surface resistivity is typically higher around the gate when compared to the surrounding area, as demonstrated in figure 2.
Figure 2. Impact of flow path on the surface resistivity of a planar injection molded article.
Along with electrical properties, there are other important things to consider when molding electrically conductive PRE-ELEC® grades. Carbon black filled materials have a higher melt viscosity compared with virgin polymers. This is especially important in the design of structures with high flow path/wall thickness ratios. Extremely high flow path/wall thickness ratios are to be avoided in order to fill the mold optimally. In addition, sprues, runners, and gates should be large enough to improve material flow. The shrinkage of the PRE-ELEC® grade will be less than that of virgin polymer due to the presence of carbon black filler in the compound. For the same reason the compounds absorb moisture easily, especially under poor storage conditions.
General guidelines for processing
The general guidelines for processing conditions of electrically conductive PRE-ELEC® grades have been listed in table 1. The recommendations for initial processing conditions of specific PRE-ELEC® grades are available on the technical product data sheets. You can access them online at www.premixgroup.com.
Table 1. General guidelines for processing conditions of electrically conductive PRE-ELEC® grades.