History and Current Status of the Plastics Industry

MFGT 142
Polymer Processing
Chapter 15: Rotational Molding
Professor Joe Greene
Rotational Molding
• Overview
Process overview
Equipment (machines, molds, plant concepts)
Product considerations (materials, shapes, design)
Operation and control (critical parameters, trouble
Rotational Molding
• Introduction
– Rotomolding uses the rotation of a mold in a heated chamber to form
– 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
Forming Process
• Rotational Molding Equipment
– Figure 14.1
Forming Process
• Heat cycle
– Figure 14.2
• Biaxial rotation
Forming Process
– Figure 14.3
• 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
• Machine Types
– Figure 14.5
• Molds
Forming Process
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)
• 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 0.297 mm
• 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.
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.
• Table 14.3