1. MFGT 142 Polymer Processing Chapter 15: Rotational Molding
Professor Joe Greene
CSU, CHICO

2. Rotational Molding Overview Process overview
Equipment (machines, molds, plant concepts)
Product considerations (materials, shapes, design)
Operation and control (critical parameters, trouble shooting)

3. Rotational Molding Introduction Process
Rotomolding uses the rotation of a mold in a heated chamber to form part.
Best for large part, e.g., 25,0000 gallon tanks. (Small ping pong balls)
Molds are inexpensive because it uses no pressure.
Temperatures are lower because the plastic doesn’t fully melt.
Process
1. Loading of fine ground thermoplastic powder. Others can be thermoplastics in solvents and thermosets.
2. Heating is done in an oven. Large enough for whole assembly to rotate in a carousel fashion. Long heating cycles.
The plastic becomes hot and sticky and sticks to the mold.
3. Cooling
4. Unloading

4. Forming Process
Rotational Molding Equipment
Figure 14.1

5. Forming Process
Heat cycle
Figure 14.2

6. Forming Process
Biaxial rotation
Figure 14.3

7. Advantages/Disadvantages
Rotational Molding Advantages
Low Pressures
Thicker corners and stress free parts
Very large parts possible
Low mold and equipment costs
Easy color and resin changes
Easy mold changes
Disadvantages
Simple shapes only
Poor dimensional tolerance control
Generally thicker overall walls
Slow molding cycles
Low part mechanical properties

8. Equipment
Machine Types
Figure 14.5

9. Forming Process Molds Plant Consideration
Most Common material is cast aluminum.
Others are machined aluminum, steel, copper, and copper alloy.
Two-part molds with body and a lid. Lid is clamped in place.
Part duplicates inside of mold. Polish if needed.
Draft angles of 4 to 6 degrees.
Multiple molds are common on one shaft.
Plant Consideration
Large floor space to accommodate large parts. (Photo 14.1)

10. Materials
Materials
All thermoplastic resin can be rotationally molded.
HDPE, LDPE, PVC, nylon, and polycarbonate.
Good resin for rotational molding include:
Grindability. Ability for material to ground into fine powder. Low melting plastics may melt during grinding.
Particle distribution. The distribution of the size of particles should be narrow to achieve even melting time between large and smaller particles.
Mesh size. Mesh size is a measure of the size of screen that 95% of the particles will pass. Common mesh size for powders are 16 to 50 (1.19mm to mm openings)
Pourability. The ability of the powder to flow in the mold as it is rotating without any external pressure. Funnel test (ASTM) with minimum 185 g/min.
Bulk density. A measure of the density of the powder before it is heated or compacted. The higher the bulk density the better because the particels are able to naturally pack tightly together.
Fusability. Particles must fuse together easily during the heating cycle. If MW too high (melt index too low) the particles require too mushc heat which could degrade the polymer. A range of melt indexes are needed, e.g., HDPE 3 to 70.

11. Shape and Design Parts Hollow parts. Figure 14.7 Shapes include
Toys, play houses, toy structures.
tanks, trash carts, buoys, and highway safety barriers.
Wall thickness uniformity is better than thermoforming or blow molding, but not as uniform as injection molding.
Design restrictions
Large, flat sections on parts should be avoided.
Large hollow parts should have ribs and sections to improve stiffnes and reduce warping.
Thickness of part is limited by the material’s ability to transmit heat. Typical thickness is between 0.3 in to 1.2 in (0.75 mm to 30mm).
Thicker parts require longer cycle times.

12. Trouble-shooting
Table 14.3