Rotational Molding - Polymer Processing - Lecture Slides, Slides of Software Engineering

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MFGT 142

Polymer Processing

Chapter 15: Rotational Molding

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Rotational Molding

• Overview

• Process overview
• Equipment (machines, molds, plant concepts)
• Product considerations (materials, shapes, design)
• Operation and control (critical parameters, trouble

shooting)

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Forming Process

• Rotational Molding Equipment

• Figure 14.

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Forming Process

• Heat cycle

• Figure 14.

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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

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Equipment

• Machine Types

• Figure 14.

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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 0.297 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.Docsity.com

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Shape and Design

• Parts

• Hollow parts.
• Figure 14.
• 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. Docsity.com