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||Rotational Molding: Advantages and Disadvantages
Rotational molding offers a number of benefits, but it’s not the best production process for every part. So how do you decide if it’s a fit for you? Understanding the advantages and disadvantages of the process is the first step towards making a decision with confidence.
Rotational Molding Uses
Rotational molding, also known as rotomolding, is a thermoplastic molding process best suited for large, one-piece hollow parts and double-walled open containers such as tanks, kayaks, and coolers. It’s most cost-effective for production volumes of less than 3,000 annually, making it ideal for inventors, start-ups, and small businesses.
Rotomolding is often used for parts that require high-quality finishes, uniform wall thicknesses, and high stability. Features such as inserts and spin weld attachments can be incorporated directly into the rotomold and foaming can be used to create thermal insulation and stiffness. Unlike competitive processes such as blow molding and thermoforming, rotomolding produces no pinch-off seams or weld lines, resulting in a finished product without the use of secondary processes.
Rotational Molding Process
The rotational molding process is quite simple:
A hollow mold is filled with powdered plastic resin.
The mold begins rotating bi-axially and is transferred into an oven.
The mold continues to rotate as the resin melts and coats the walls of the mold.
The mold is cooled until the resin hardens into the desired shape.
The rotation is stopped, and the mold is opened to remove the finished part.
Given the low-pressure, high-heat nature of the process, rotomold tooling is usually made from a soft metal such as aluminum and the majority of the resin used is polyethylene due to its low chemical degradation when exposed to high heat. Inserts, ribs, kiss-offs, undercuts, and foam reinforcements are often incorporated into the part in-mold or by means of secondary processing.
Advantages and Disadvantages of Rotational Molding
The main difference between rotational molding and competitive molding methods such as blow molding and thermoforming is that the resin melts into the mold walls instead of being forced by pressure. This distinction gives way to a number of advantages over other manufacturing processes, but also carries its share of drawbacks.
Advantages of Rotational Molding
Rotomolding boasts a number of advantages over comparable molding methods:
Low-cost tooling: low operating pressures allow rotomold tooling to be crafted from low-cost metals such as aluminum
Consistent wall thickness: the constant rotation of the mold coats the walls evenly during both the heating and cooling processes
Double-wall construction: complex double-walled open containers can be produced without secondary processing
High durability: parts are molded as one solid piece, eliminating the need for joining techniques such as welding and joint fabrication which creates weak spots
High stability: the molding material isn’t exposed to external pressure, increasing its stability and reducing the risk of defects in the finished part
High strength: rotomolding creates thicker corners, reducing the risk of failure in these stress-concentration points
Appearance: the soft metal used for the rotomold tooling easily accommodates surface finishes such as fine-detail textures, logos, symbols, and lettering
Disadvantages of Rotational Molding
As with any plastic molding process, rotomolding has its disadvantages:
High cycle times: at eight rotations per minute, rotomolding can take up to three hours to complete one part
Limited material options: raw material used in rotomolding must be readily converted from granules to a fine powder and must have high thermal stability, limiting material selection to poly-based resins
High cost of raw material: material costs are high due to high thermal stability requirements, the cost of required additives, and the cost of grinding the material into a powder
Low repeatability: the soft metal used in rotomold tooling must be refurbished or replaced after 3,000 cycles, inducing quality issues due to a lack of repeatability
High labor costs: mechanization and automation have not yet been realized for rotomolding, requiring greater labor intensity than comparable manufacturing processes
What is pulverizer and how does it work
Pulverizers provide material size reduction services for customers with a variety of goals, such as creatine powder for medicine, creating pulp for paper production, grinding grain for food production, tire-shredding and recycling, breaking down building materials, turning soil, crushing vehicles for scrap, grinding rock samples, and more.
Some of the industries that rely on pulverizers include construction, agriculture, industrial manufacturing, power generation, pharmaceutical products development, landscaping, laboratory, printing, recycling, and material processing.
Pulverizers are generally sorted into three main categories: crushers, impactors and grinding mills. Crushers are designed to reduce the size of large, dense materials such as rock and stone to gravel or dust. Primarily, crushers are used for size reduction, easy disposal or recycling and to simplify differentiation of materials.
One of the most common crusher designs is the jaw crusher, which has two jaws, one that is stationary and one that is mobile. Impactors also referred to as impact crushers, are very similar to crushers but differ in the manner of size reduction. Impaction is force that is transmitted through a collision or by striking one body against another, whereas crushing is the use of pressure created by two opposing forces.
Crushing and impacting utilize pressure and collision-wrought force. Grinding mills use friction to break down materials. The friction in grinding mills is brought about as a result of grinding media, which can refer to many different coarse materials such as non-sparking lead, ceramics, brass, bronze and flint.
Two common types of grinding mills are ball mills and hammermills. A ball mill is constructed from a rotating cylinder that is mounted horizontally. They use grinding media such as steel balls or rods, which, as the cylinder turns, are tossed around the cylinder, smashing into the material to be grinded as they do so. Hammermills utilize numerous hammers encased in steel that rapidly revolve in a vertical plane.
How Pulverizer Work
Pulverizers process materials in batches or continuously by accepting incoming material, often on a conveyor, rotating it and pressing a crushing into it, then sending it out. They may also cool, heat, or aerate material as it exits. For the convenience of the user, pulverizers can crush materials to varying levels of fineness, from very fine to coarse.
Typically, pulverizers are sorted into three major groups: grinding mills, crushers, and impactors. Note that within these groups are many specific types of pulverizers, defined by their application, such as coal pulverisers, concrete pulverizers, plastic pulverizer, food pulverizers, and plastic pulverizers.
Grinding mills break down materials using friction, which they generate via grinding media. Any number of coarse materials may serve as grinding media, but some, such as brass, bronze, ceramics, flint, and non-sparking lead, are more common than others. The two most utilized types of grinding mills are hammer mills and ball mills.
Single mill pulverizer machine
Pulverizers are the precision engineered machines that are used for grinding various materials such as plastic, aluminum, glass, concrete, rock, coal, resins, tires, and medical waste. Also, these are quiet useful in the rotational molding process for grinding granules/ powder before they are fed into the molds. Pulverizers are heavy duty machines designed to ensure increased production capacity. Despite their heavy duty designs, the machines can be operated by single person. Pulverizers find applications in plastics pulverizing including rotomoulding, PVC recycling, compounding & master batching industry.
Single mill pulverizer is also one of the types of the pulverizer machines which are used in various industries widely. The single mill pulverizer machines are known for their high capacity, low maintenance and steady performance. These machines produce top quality plastic powders for rotation molding, coatings, flame spraying processes, carpet backside coatings, extrusion and additional processes.
Some of the standard features of Single Mill Pulverizer Machine are as follows:
Direct Drive Mills
Heavy Gauge Quick Disconnect Piping
Heavy-duty Mill Housings
High HP Efficiency Iron Motor
High Efficiency Cyclone
High Output Vibratory Feeder
Single Mill Pulverizer Machine: Salient Features
Single Mill Pulverizer machine
Less power consumption and high production output
Economical production cost
Low maintenance cost
Consistence output quality
Cooling provision for Stationery Mill body, Disk, Duct line & Bearing housing
Powder temperature monitor & control system
Microprocessor based control panel with a digital display
Production of fine micro powder
High quality and low wastage
Easily operable by single person
Compact design for space saving
Take a look at the Constructive & Process-technological characteristics of Pulverizer:
Grinding path made of wear-resistant material
Central grinding gap adjustment
Designed for optimize air cooling
Broad application fields
This article presents all the information you need to know about rotational molding. Read further and learn more about the following:
Overview of rotational molding and its history
Types of rotational molding machines
Rotational molding processes
Materials used in rotational molding
Chapter 1: Rotational Molding
Rotational molding, commonly referred to as "rotomolding", is a plastic casting technique used to produce large hollow, seamless, and double-walled parts. It is a three-stage process that involves a mold on a rotating frame, a heating chamber, and a cooling chamber. Molds for the rotomolding process are specially designed and are capable of producing single and double wall products.
The main raw material for rotomolding is one of the polyethylene resins, which is loaded into the mold to begin the process. In the heating chamber, the mold is rotated as it is heated. The frame of the mold is capable of rotating the mold to every point of its rotational axis. As the mold rotates, the resin is spread over the interior surface of the mold giving the finished product an equal and even thickness across its surface.
After a set period of time, the mold moves from the heating chamber to the cooling chamber, where the liquified melted resin is allowed to cool before the plastic product is ejected from the mold. Rotationally molded products are of the highest quality and are known for their durability and strength.
Rotational molding is a non-pressure molding process, which makes the toolings of the molds less expensive since they do not have to endure the stress of being pressured. The sizes for rotationally molded products are limitless since the molds and equipment are capable of creating very large complex plastic shapes. There are also few restrictions when it comes to part design, giving the designer freedom to add complicated details.
Included in the products produced by rotational molding are kayaks, sports helmets, display mannequins, water storage tanks, baby cribs, and barriers for road construction. All of these large plastic products can be easily produced using rotational molding at less expense and with great efficiency.
The rotational molding process has been traced back to the ancient Egyptians, who created ceramics hundreds of years ago. The first application of a more advanced rotational molding process was the production of artillery shells in 1855 and hollow chocolate eggs by the Swiss in 1910 to create a uniform wall thickness and density. In these periods, there were several patents registered to document the nature of this casting process. However, it was regarded as a slow process, and some challenges were encountered, leading to the process not being popularized.
During the 1940s, this process was used to create doll heads and other small toys from polyvinyl chloride plastisol resin using an electroformed nickel-copper plastic. The set-up consisted of only electric motors and gas burners and the finished part was quenched with cold water. This method has attracted many industries to adopt this in their production process, which led to the manufacturing of road cones, marine buoys, and armrests.
Nowadays, rotational molding is used in a variety of applications, producing larger and more complicated parts. The nature of the process is better understood, and equipment design is significantly enhanced. The long heating and cooling cycles remain a major bottleneck for some manufacturers. The development is focused on the modification of rotational molding equipment by developers to accommodate increasing demand.
Rotational Cast Aluminum Tooling or Molds
Tooling made from aluminum castings are by far the most common type of rotational molds. They are relatively inexpensive and can be cast, machined, and framed quickly. High quaility castings with low pourousity will produce a wide range of parts with fairly intricate detail, elaborate shapes and patterned finishes. Aluminum tools are well suited to produce air or water tight plastic parts as well.
Aluminum molds are lightweight, and transfer and dissipate heat well making them ideal for repeated cycles through rotational molding ovens. Cast tools can be modified after they have been put into production to accommodate slight changes to the parts design.
When your part goes into production, there is no substitute for using a quality mold. A poorly designed or built tool will inevitably lead to slower production runs and mold failure ad lost production time. Sterling Technologies will guide you through the process to ensure you select the right tool for your application.
This is a two cavity rotational mold producing a large, patterned rain barrel. It’s components include:
The cast aluminum cavity is built with three sides. In this particular tool, the sides are formed with one seam and the top forms a seam just below the lip of the barrel. This design produces a seam-free top.
Each mold has a specific texture inside. This ranges from a highly polished, smooth surface to a deep patterned texture which forms the parts surface.
These molds are filled repeatedly throughout the production run. A steel tube frame provides the structure to fasten the sides of the mold together creating a tight clean seal.
The steel frame contains regularly spaced spring-loaded fasteners, normally 6 to 8-inch harden steel bolts, to securely close the cavity during the molding cycle. Torque-limited power tools are used to carefully open and close the mold.
Some mold components require precise alignment to ensure a clean seal and to improve cycle times. This mold used a track and wheel design to slide the top of the mold into exact alignment.
The framework is welded to a steel platform. This allows the rotational mold to be clamped onto to the machine’s arm so they can rotate freely without obstruction.
Each mold will have four even spaced hooks to easily position the mold onto the machine arm.
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