Rotational Molding Overview 270

Additives

A substance that is added to a material during manufacturing in order to improve its properties. Additives perform a range of functions, from changing the color of plastic to improving its strength.

Biaxially

Along two axes. Biaxially rotating molds in rotational molding rotate along a primary and secondary axis.

Blow molding

A plastic shaping process in which a heated tube of resin is clamped in a mold while positive air pressure blows the resin into the desired shape. Blow molding can be done with either an injection molding machine or an extrusion machine.

Bonding agent

An additive that causes the molecules of polyethylene to form cross links. Bonding agents include peroxide, silane, and electron beams.

Bonds

Connections between molecules of polymers. Bonds are cross-linked in cross-linked polyethylene.

Carousel machines

A rotational molding machine with arms that rotate around a central station to transfer molds between multiple stations. Carousel machines are also called turret machines.

Clamshell machines

A rotational molding machine with a lid in which heating and cooling occur at the same station. Clamshell machines are ideal for low production runs of small parts.

CNC machine

Computer numerical control machine. A machine controlled by a computer that runs special programs to manufacture a workpiece. CNC machines are very rigid and are capable of fast cutting speeds.

Cooling stage

The part of the rotational molding process in which the melted plastic is solidified. The cooling stage takes place after the plastic is removed from the oven.

Cooling station

The physical location in a rotational molding process in which the plastic solidifies. The cooling station may include fans or a mist spraying device.

Cross-linked polyethylene

PEX or XLPE. Polyethylene that has been treated with bonding agents. Cross-linked polyethylene is far more chemically resistant than other forms of polyethylene.

Cubic

Shaped with symmetrical, equally-sized flat edges, like a box. Cubic parts can be created with an open flame in rotational molding.

demolding

Removing from a mold. Demolding occurs after the cooling stage of rotational molding.

Fasteners

A device that holds objects together or locates them in relation to one another. Common fasteners include bolts, screws, and rivets.

Finishing

A final machining process that achieves the desired surface finish and tolerance for a part or component. Finishing is often performed on grinders or with cutting tools.

Fittings

A fluid component that connects, redirects, extends, or terminates the run of a fluid conductor. Fittings that are installed improperly can result in external leakage.

Fixed arm

A configuration of turret machines in rotational molding in which the arms are connected around a central turret. Fixed arm machines move at the same rate at the same time.

Flexibility

A material’s ability to bend without breaking. Flexibility is a feature of some forms of polyethylene.

Flow enhancers

An additive that decreases the viscosity of a material. Flow enhancers can be mixed with powdered resin in rotational molding.

hardness

A material’s ability to resist indentation or scratching. An increase in hardness generally leads to a decrease in toughness.

Heating stage

The part of the rotational molding process in which the plastic is melted. The heating stage takes place in an oven or over an open flame.

Heating station

The physical location in a rotational molding process in which the plastic melts. The heating station usually consists of an oven or an open flame.

High-density polyethylene

HDPE. A common thermoplastic known for its high strength-to-density ratio. High-density polyethylene has a higher chemical resistance than low-density polyethylene.

Human-machine interfaces

HMI. A peripheral device connected to a PLC that allows an operator to monitor a program and interact with the PLC. A human machine interface may have a graphical or textual display.

Independent arm

A configuration of turret machines in rotational molding in which the arms all move independently. Independent arm machines allow parts to be heated and cooled at different rates.

Linear low-density polyethylene

LLDPE. A form of polyethylene that has increased tensile strength and flexibility. Linear low-density polyethylene has good chemical resistance and is used to make fuel tanks.

Low-density polyethylene

LDPE. A translucent to opaque thermoplastic with loosely packed molecules. Low-density polyethylene is not as chemically resistant as high-density polyethylene.

Mechanical properties

A characteristic of a material that describes its ability to compress, stretch, bend, scratch, dent, or break. Mechanical properties include tensile strength and yield strength.

Medium-density polyethylene

MDPE. A form of polyethylene that has increased tensile strength and flexibility but is prone to cracking. Medium density polyethylene is the least commonly used polyethylene in rotational molding.

Mold

A hollow cavity used to shape material. The mold in rotational molding moves to allow melted plastic to coat its walls.

No-pressure

Class of manufacturing that does not use gas or injected plastic to create pressure in a mold. No-pressure manufacturing includes rotational molding.

open flame system

A part of some rotational molding machines that uses flames, rather than an oven, to melt resin. Open flame systems are ideal for spherical or cubical parts.

Oven

An enclosed heating device. Ovens are used in rotational molding to melt resin inside the mold at the heating stage of the process.

oxidation

A material's chemical reaction with oxygen. Oxidation can result in polymer degradation.

Pigments

A powder coloring agent mixed with resin. Pigments are an additive used to color the surface of a plastic product.

polyamide

PA. A semicrystalline thermoplastic that gains strength when the fibers are stretched. Polyamides are commonly referred to as nylon.

Polycarbonate

PC. A group of amorphous thermoplastics with high heat resistance. Polycarbonates are used in products such as safety helmets and appliance housings.

Polyethylene

PE. A polymer that has good ductility and impact strength. Polyethylene is the most commonly used plastic in rotational molding.

polypropylene

PP. A semicrystalline thermoplastic that is resistant to fatigue. Polypropylene is used in a wide range of products, from laboratory equipment to microwavable soup cans.

Polyvinyl chloride

PVC. A thermoplastic that can be manufactured to produce both rigid and flexible materials. Polyvinyl chloride is a high-viscosity plastic that is often manufactured with lubricants that improve flow.

Primary axis rotation

The vertical rotation of a rotational mold. Primary axis rotation is listed, in RPMs, first in the rotation ratio of an operation.

Programmable logic controllers

PLC. A processor-driven device that uses computer software to provide electrical control to machines. A programmable logic controller delivers part program information to independent arm machines.

Pulverized

To be ground down to fine particles. Pulverized resin is used in rotational molding.

Release agent

A substance that is applied to a mold surface to make it easier to remove the molded part. Release agents in rotational molding can be sacrificial, semi-permanent, or permanent.

Resin

A raw polymer, usually in the form of beads or pellets, that is not yet molded into its final shape. Resin is melted to form plastic parts.

Revolutions per minute

RPM. A measurement that indicates how many times a machine component rotates in one minute. Revolutions per minute is used to measure mold rotation speed in rotational molding.

Rock and roll machines

A rotational molding machine that tilts back and forth as it rotates. Rock and roll machines can be placed in ovens, or they sometimes produce their own open flames.

Rotation ratio

The relationship between the primary and secondary axes rotations. Rotation ratio in rotational molding is expressed by the RPM of the primary rotation and the RPM of the secondary rotation, divided by a colon.

Rotational molding

A plastic shaping process in which resin is heated, rather than melted, inside a rotating mold. Rotational molding, also known as rotomolding, is a no-pressure process that creates hollow parts.

rotational molding machine

A machine used to shape plastic by rotating it inside of a mold. Rotational molding machines rotate the mold biaxially while exposing the mold to a heating element.

Rotomolding

A plastic shaping process in which resin is heated, rather than melted, inside a rotating mold. Rotomolding, also known as rotational molding, is a no-pressure process that creates hollow parts.

Sacrificial

Being used up in the process of creating something. Sacrificial release agents are sometimes used in rotational molding.

Sanding

A form of grinding that uses a fine-grain abrasive to remove small amounts of material. Sanding improves the finish of a surface.

Seams

A line along which two materials or components are joined together. Seams can be weaker and more prone to failure than solid areas.

Secondary axis rotation

The horizontal rotation of a rotational mold. Secondary axis rotation is listed, in RPMs, second in the rotation ratio of an operation.

Secondary operations

An additional manufacturing process required to bring a workpiece into tolerance and create a final part. Secondary operations include removing pre-machined features like dovetails and painting parts.

Shuttle machines

A rotational molding machine that features two mold units on a track with an oven between them. Shuttle machines allow for continuous operation when space is limited.

Spherical

Shaped with a symmetrical domed surface, like a ball. Spherical parts can be created with an open flame in rotational molding.

Spin welding

A process of joining parts together by rapidly spinning them against each other to create friction and heat. Spin welding is used to connect fittings in rotational molded containers.

Swing machines

A rotational molding machine that features two-to-four arms located at the corners of the oven. Swing machines allow multiple parts to be heated and/or cooled at the same time.

tensile strength

A material’s ability to resist forces that attempt to pull it apart or stretch it. Materials with high tensile strength tend to deform, bend, or stretch before breaking.

Thermal degradation

Deterioration of a material due to overexposure to heat or sun. Thermal degradation can diminish a material’s mechanical properties.

Thermoforming

A secondary plastic shaping process that forces heated plastic sheets around a mold. Thermoforming processes use mechanical, air, or vacuum pressure.

Thermoplastics

A grouping of plastics that can be repeatedly reheated and re-shaped. Thermoplastics are the most commonly used plastics.

Threaded

Having a long, spiraling groove. Threaded objects can be securely connected to other compatible threaded objects.

Toughness

A material’s ability to absorb energy without breaking or fracturing. Toughness is a key property that determines a material’s ability to withstand sudden stress.

Turret machines

A rotational molding machine with arms that rotate around a central station to transfer molds between multiple stations. Turret machines are also called carousel machines.

UV absorbers

An additive that protects a plastic against ultraviolet (UV) radiation. UV absorbers can be added to powdered resin in rotational molding.

UV protective agents

Additives that protect materials from ultraviolet (UV) radiation. UV protective agents include black oxide.

Vertical wheel machines

A rotational molding machine in which molds travel up and around to an oven, then down and behind to cool before moving to the front to be released. Vertical wheel machines have a “Ferris wheel” design.

Warp

To physically bend, twist, or deform. Warpage is commonly caused by poor heat exchange and uneven cooling.

Welds

The area where two pieces of material are joined together by heating the surfaces until they melt. Welds in plastic can be weaker and more prone to failure than solid areas.