Rotational molding focuses on the sinter-melting, densification and cooling of polymer, beginning with powder. Typically, polymer powder has a particle size range of -35 mesh to + 200 mesh. The powder is usually manually charged to the mold while the mold is in the open configuration in the servicing stage of the process cycle. This Technical Minute reviews some basic information on the way in which powder moves across the metal mold surface. Be aware that studies of polymer powder flow are just beginning.
Putting Powder Into Perspective
The typical poured but untamped powder packing fraction range is 0.35 to 0.50. The bulk density range for typical rotational molding polymers is given in Table 1.
Powder Bulk Density
|Polymer||Compact Density, kg/m3||Bulk Density, kg/m3||Bulk Density, lb/ft3|
|LLDPE||910||345 to 390||22 to 24|
|HDPE||960||375 to 425||23 to 27|
|PS||1050||430 to 585||27 to 37|
|PP||910||345 to 490||22 to 31|
|Nylon||1100||460 to 615||29 to 38|
|FEP||2200||1000 to 1230||62 to 77|
There are several important aspects about powder charging. First, there must be room for the powder in the mold half during charging. For asymmetric molds, the deeper portion should be that which is filled. The powder must be freely poured, and must not be tamped. Then, there needs to be free space for the tumbling powder during the early portion of the heating cycle. Nonuniform wall thickness and severe corner bridging result when the powder cannot freely flow across the mold surface. And powder must be carefully distributed when the mold has both large and small cross-sections. A classic example is a hobby horse, where the leg cross-sections are substantially less than that of the body.
Determination of the required amount of powder in a specific charge is quite straight-forward. The inner mold surface area is determined, either manually or from CAE software. Tool path software yields the most accurate surface area values. The anticipated uniform wall thickness is obtained either from prior experience or from finite element analysis. The product of the area and the wall thickness yields the volume of plastic required in the finished part. The weight of polymer is determined by multiplying the volume by the polymer density. Of course, this is the weight of the powder charge. The volume of the powder charge can be two to three times greater than the required volume of plastic.
Air-borne dust is a major problem with manual powder charging into an open mold half. Dust can be minimized by filling through an accessway in an already-closed mold, or by using a drop box mounted to the accessway. It can also be minimized by using micropellets or prilled powders.
General Process Description
Consider the following summary of the rotational molding process. The heating cycle begins with powder charging at the service station and ends when the mold assembly is removed from the oven to the cooling station. Table 2 details the various phenomenological steps. In this Technical Minute, we consider only the various ways in which powder flows in a mold, prior to tacking, coalescence and densification.
Steps in the Heating Cycle
|Powder charging||Bulk density of the powder, place for powder in narrow molds|
|Initial heating||Characteristics of powder bed|
|Tacking condition||Hot tack temperature of powder|
|Particle coalescence||Three-dimensional structure|
|Densification||Capillary flow, powder structure collapse, air inclusion|
In conventional rotational molding, rotating speeds are quite low, typically about 4 RPM or so. As a result, the powder charge remains as a powder bed near the bottom of the mold throughout the early portion of the heating cycle. Three types of bed motion have been observed, Figure 1.
For steady-state circulation of the powder in the bed, the powder at the mold surface moves with the mold surface until the mass exceeds the dynamic angle of repose. For most polymer powders, this angle is between 25o and 50o from the horizontal. At that point, the mass breaks away from the mold wall, and cascades across the static surface of the bulk of the powder bed. This type of flow is continuous and the flow rate is altered only by the geometry of the mold itself. Powder having this type of flow behavior is usually characterized as spherical or squared-egg in shape and as freely flowing.
This mode of circulation is analogous to snow avalanche. Initially, the powder in the bed is static with respect to the mold surface. The mold raises the powder bed until the entire mass exceeds the dynamic angle of repose. At that point, the top portion of the mass breaks away from the mold wall, and cascades across the rest of the powder bed. The bed again becomes static and is again raised by the rotating mold. It is known that avalanche flow occurs when the powder is slightly tacky or is not free-flowing, and when the powder is acicular or two-dimensional.
This type of flow occurs when the mold surface is very smooth. There are two types of slip flow. The more common slip flow is really a cyclical slip-stick flow. Initially, the powder in the bed is static with respect to the mold surface, as with the avalanche flow. As the mold raises the powder bed, the entire mass reaches a point where the friction between the powder and the mold wall is no longer sufficient to prevent the mass from sliding against the mold surface. At that point, the entire static bed simply slides to the bottom of the mold, without any measurable type of powder circulation. The bed then becomes static and is again raised by the rotating mold. The less common slip flow is a steady state slip. For this type of slip, the bed essentially remains fixed relative to the horizontal axis of the mold and the mold simply slides beneath it. Powders that pack well and that have very low coefficients of friction with the mold material, such as olefins and FEP, will exhibit slip flow, particularly if the mold is also plated or highly polished. Early permanent teflon mold releases also promoted slip flow.
Table 3 summarizes these major types of powder flow.
Types of Powder Flow – Rotational Molding
|Steady-state circulation||Ideal flow
Best heat transfer
Spherical or squared egg particle shape
Cohesive-free or freely flowing powders
Smooth powder surfaces
Relatively high friction between mold surface and powder bed
|Avalanche||Adequate powder flow
Relatively good powder mixing
Relatively good heat transfer
Squared egg, acicular or disk-like particles
High friction between mold surface and powder bed
|Slip flow||Poor powder flow
No powder mixing
Poor heat transfer
Disk-like, acicular particles
Powders with high adhesion or cohesion
Agglomerating or sticky powders
Very low friction between mold surface and powder bed
Polymer powder particles fluidize during avalanche and steady-state bed flows. From in-mold cameras and from dimunition of light through rotating beds [see comments about Exxon Canada's experimental analyzer, below], particle size segregation and decrease in overall powder bulk density are observed, particularly in the layers farthest from the mold surface. There is substantial debate as to the best way to treat the mechanics of powder flow. In reality, flowing powders are discrete particles that are temporarily suspended in air. There have been many studies on the rheological or flow characteristics of powders. Single powder particles falling in quiescent air or another fluid are characterized by Stokes flow. That is, the drag force on the particle is directly proportional to its relative velocity, with gravity being the only body force. As the particle density increases, Stokes flow is compromised by interparticle collisions, where kinetic energy interchange occurs. Fluidization is the lifting of a stationary bed of particles by upward flow of air or another fluid.
Unfortunately, throughout most of the rotational molding process, there are so many particles interacting with one another, in swarms or as streams, that most discrete particle theories cannot be used. The possible exception is in the latter stages of powder flow, when most of the polymer is adhered to the mold surface or to other pieces of powder. The concept of viscosity of a flowing powder stream, proposed many years ago, has not received wide acceptance. This concept was based on the decrease in velocity of a falling powder layer owing to shear with a solid inclined plane. This decrease implies a shear layer or region and a resistance to flow. Additional work indicates that the velocity of a flowing powder stream is not necessarily maximum at the free surface, and that a viscosity of sorts is defined only when the shear surface is static. When the shear surface exchanges particles with the flowing surface, the flowing fluid can either increase or decrease in mass during flow across the shear surface. The change in mass is dependent on the effect of external factors such as gravity, fluid velocity, the relative size and shape of the particles, and the relative boundary conditions.
Since the nature of powder bed motion is so critical to the early fusion state of the powder against the mold surface, it is recommended that a simple lab-scale rotating unit be employed to evaluate the flow behavior of new polymers and new grinding techniques. The unit shown in Figure 2 yields rotation in a radial direction only, as seen in Figure 3.
Nevertheless the unit is useful for determining the effect of mold fill level on bed motion and the nature of the powder flow characteristics during dry flow and melting. In fact, at the 1997 ARM conference, Blair Graham at Exxon Canada demonstrated a correlation between measured avalanche characteristics and the processability of molding powders. The experimental device is shown in schematic in Figure 4 and the nature of the experimental data is shown in Figure 5.
It should be noted that the bed flow mechanism can change during heating. For example, as powder becomes sticky or begins to stick to the mold surface, the bed flow can change from slip flow to avalanche, or from steady-state circulation to avalanche flow. As a result, the particle-to-particle temperature uniformity can change dramatically. This aspect of rotational molding will be the subject of another Technical Minute.