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Rotational Molding: Advantages and Disadvantages

July 10, 2022

What is Rotational Blow Moulding?

Rotational moulding 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 toward making a decision with confidence.

Rotational Molding Uses

Rotational moulding, also known as roto-moulding, is a thermoplastic moulding 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 roto mould and foaming can be used to create thermal insulation and stiffness. Unlike competitive processes such as blow moulding and thermoforming, roto moulding produces no pinch-off seams or weld lines, resulting in a finished product without the use of secondary processes.

Rotational Molding Process

The rotational moulding process is quite simple:

  1. A hollow mould is filled with powdered plastic resin.
  2. The mould begins rotating bi-axially and is transferred into an oven.
  3. The mould continues to rotate as the resin melts and coats the walls of the mould.
  4. The mould is cooled until the resin hardens into the desired shape.
  5. The rotation is stopped, and the mould is opened to remove the finished part.

Given the low-pressure, high-heat nature of the process, rotomolded tooling is usually made from soft metal such as aluminium and the majority of the resin used is polyethene 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 mould or by means of secondary processing.

Advantages and Disadvantages of Rotational Molding

The main difference between the rotational moulding and competitive moulding methods such as blow moulding and thermoforming is that the resin melts into the mould 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 moulding methods:

  • Low-cost tooling: low operating pressures allow rotomolded tooling to be crafted from low-cost metals such as aluminium
  • Consistent wall thickness: the constant rotation of the mould 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 moulded as one solid piece, eliminating the need for joining techniques such as welding and joint fabrication which creates weak spots
  • High stability: the moulding material isn’t exposed to external pressure, increasing its stability and reducing the risk of defects in the finished part
  • High strength: roto-moulding creates thicker corners, reducing the risk of failure in these stress-concentration points
  • Appearance: the soft metal used for the roto mould tooling easily accommodates surface finishes such as fine-detail textures, logos, symbols, and lettering

Disadvantages of Rotational Molding

As with any plastic moulding process, roto moulding has its disadvantages:

  • High cycle times: at eight rotations per minute, roto moulding can take up to three hours to complete one part
  • Limited material options: raw material used in roto moulding 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 roto mould 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 roto-moulding, requiring greater labour intensity than comparable manufacturing processes

Summary:

  • Rotational moulding is a plastic casting technique that produces a hollow, seamless, and double-walled part by heating a powdered resin in a hollow mould tool while being slowly rotated and cooled to solidify. This process started hundreds of years ago, was popularized in the mid-20th century, and is still developing. This process has a variety of applications.
  • The rotational molding equipment consists of a mold, ovens, and a cooling chamber. The type of rotary moulding machine is distinguished by the direction in which the mould travels according to the sequence of the process.
  • The types of rotational moulding equipment are clamshell machines, turret machines, shuttle machines, vertical wheel machines, rock and roll machines, and open-flame machines.
  • Loading is the first step wherein the fine, homogenized, and dried powdered resin is placed in a mould coated with a mold release agent. The resin is heated while being slowly rotated in a bi-axial direction until it is molten and covers the entire tool. The moulded part and the entire mould are gradually cooled by convection to room temperature, wherein they can be safely removed.
  • Critical parameters of the rotational moulding process are heating time, cooling rate, and rotation ratio. These settings determine the mechanical properties and dimensional quality of the parts to be produced.
  • The polymer resin must have high thermal stability, flow easily in its molten state, and have antioxidant groups in the molecular structure. The resin must be easily ground to powdered form. Polyethene is the most commonly used polymer.
  • The advantages of rotational moulding are the creation of parts with uniform wall thickness, inexpensive tooling, the flexibility of production, less downstream process, and less wastage of the resin. Large and double-walled parts are also easily created.
  • The disadvantages of rotational moulding are a high cycle time attributed to long heating and cooling cycles, limited material options, short tool life, and difficulty of moulding some details.

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Mold Support & Venting

On the outside, blow molds are provided with support surfaces to prevent damage to the cutting edges, especially in set-up mode, i.e. when closing the mold without plastic. The mold support also serves to transfer excess clamping force.

DESIGN GUIDELINES FOR BLOW MOULDING MOULD

Guide pins and guide bushings from common standard parts suppliers are usually used as blow mold guides. The guide length should not exceed 1 * D. This is particularly important for mold movement machines to avoid damaging the parallel guidance of the clamping unit and the blow mold guide. The bore for the bushing must be drilled towards the rear of the mold or be cleared laterally to be able to easily remove pressed in material and ensure a clean closing of the blow mold.

HOW TO SELECT TOOLING IN BLOW MOULDING MACHINE :

With a basic understanding of the differences in the types of head tooling
available, we can now look at calculations that will help make our tooling
choices. It is important to keep these points in mind:
Parison diameter is dependent on head tool size, resin characteristics,
regrind content, temperature, and punch-out speed.
Head tooling diameter is determined by part shape and size.

ROLE OF AIR IN BLOW MOULDING

In a blow molding machine, air has multiple purposes: pre-blow and inflation air expands the parison until it contacts the mold wall to reproduce the shape of the blow mold. The inflation air presses the material against the mold wall to reproduce surface details such as lettering, surface textures, etc. Furthermore, the inflation pressure promotes good and permanent contact between the mold wall and the polymer, since the polymer starts to shrink during the cooling period. In addition to these functions required directly with the actual blow molding process, air is required for pneumatically activated functions in the machine, such as the insertion of a blow needle to move molded parts (core pulls) or doors in the machine’s safety guard.

PLC in EBM 

Until the late 1970s to early 1980s blow molding machines were primarily delivered with contractor control. This type of control offered the advantage that the electrical components could be checked by appropriately trained maintenance personnel and easily replaced in the event of a fault. Changes in the machine control system could be made with the help of simple auxiliary devices. However, the disadvantages were the susceptibility to faults and short service life due to wear of mechanically moving parts, the large number of components (up to 150 contactors, 12 temperature controllers, etc.), the size of the components, and how these elements were connected. This type of control was called “hard-wired programmed logic controller”.

CLAMPING UNITS IN EXTRUSION BLOW MOULDING

In all extrusion blow molding processes, a blow mold consisting of two halves closes around the parison once it has reached its full length. These two halves of the mold are mounted on the so-called mold mounting platens, which from part of the clamping unit (or closing unit).

RADIAL WALL THICKNESS DISTRIBUTION IN BLOW MOULDING

In blow moulding, the wall thickness distribution of the finished article is essentially determined by the wall thickness distribution of the parison, its position in relation to the blow mould cavities, and the local stretching paths.

SWELLING BEHAVIOUR OF THE PARISON

The visco – elastic behavior of the thermoplastics (here the memory effects) in conjunction with the flow channel geometry in the head causes the diameter of the parison to change after leaving the die. The wall thickness also changes and thus the length of the parison. This phenomenon is called ‘; die swell’’.

MANDREL HEADS IN EBM

In mandrel heads (also referred to as ‘’heart – shaped – curve heads’’, or cardio did heads) the solid core part is massively integrated into the basic structure of the head. These heads are also called side-fed heads.

SPIDER HEADS IN EBM

The main task of parison heads (blow heads or even just heads) is to divert the compact melt flow as supplied by the extruder into a vertical downwards flow movement and to form melt tube with a uniform circumferential wall thickness distribution.

EXTRUDER WITH GROOVED FEED ZONE

Blow moulding machines with grooved barrel extruders are universal machines for all types of polyolefin; the grooved barrel system is,however, indispensable for high molecular weight, high viscosity raw materials. In these extruders, the cylinder wall in the feed section features tapered longitudinal grooves.

SMOOTH BORE EXTRUDER

Blow molding machines with smooth – bore extruders are more or less limited to the processing of low to medium molecular weight polyolefin types. Smooth bore extruders are less expensive than those with a grooved feed zone and show less screw wear.

Is a Blow Molding Machine Used to Make Anything Other than Plastic Bottles?

For manufacturing plastic bottles, experts rely on a state-of-the-art blow molding machine. With this particular process, a manufacturer produces a broad range of finished products in varying sizes and shapes. Because of that, multiple industries depend on a professional company to make a specific type of plastic bottle needed.

What Are the Future Trends of Plastic Molding Machines?

Although the plastic molding industry took a hit this year due to the instability within the automotive industry, it’s still on target for breaking records. After all, the plastic molding machine is something that a broad range of industries rely on for both internal operations and the making of consumer goods. As demand increases, you can expect to see some exciting changes for plastic molding machines.

Can Extrusion Blow Molding Produce Complicated Shapes

When it comes to extrusion blow molding capability, most people think of standard plastic bottles, tight-head containers, jars, and larger water containers. However, the extrusion blow molding process can do much more. In fact, when using a technologically advanced extrusion blow molding machine, it can produce finished products with complicated shapes, and in varying sizes.

HOW TO START A TOY-MAKING BUSINESS WITH EBM IN INDIA?

Before starting the toys making business, it is important to do market research. It is important to collect as much information as possible about this business so that you get the knowledge of exactly what kind of toys are in demand in the market.

WHY INDIAN GOVT. ENCOURAGES TOY MAKING BUSINESSES MAKING BUSINESS

Indian government motivates and urges the Indian startups in the toy sector to maximize manufacturing and increase the reach of Indian made toys to the world. After holding a meeting with senior officials and ministers, the Prime Minister make a statement and urge the start-ups and young blood to innovate in the toy sector. Also, PM has given the suggestion to the educational institutions to arrange competitions in toy technology and design.

Biodegradable vs. Recyclable Plastics

Biodegradable plastics undergo digestion by microbes to form natural bi-products and what is known as humus (a sludge that makes an excellent plant fertiliser). It also produces gas which is officially known as biogas. Biozone’s landfill-biodegradable plastics have proprietary additives that help them biodegrade naturally in landfills. Once the biodegradable packing ends up in a landfill, it attracts the necessary microbes that break down the plastic into its natural components.

The Future of the Extrusion Blow Molding Machine

Everywhere you look, competition among different industries has exploded. Thanks to a thriving market, companies that once struggled now see incredible success. Just as businesses need ways to stay ahead of the competition, so do extrusion blow moulding machine manufacturers. Because of that, they continually push to come up with more innovative solutions.

Why CMP Machines are the BEST?

Innovation is at the heart of any successful company. Innovation keeps an organization at the top of their game, allowing them to flex their competitive muscles and take pride in the work they do.

What is Multilayer Extrusion Blow Moulding Machine?

Nowadays literally all humankind use plastic products 24/7 all over the world. It is super easy to get access to all kinds of plastic products. You might have come across bottles that have a shiny layer on the outer side, or different colours on the outer and a different one in the inner. So, what it is? It is a multilayer extrusion product. Well, let me explain to you how what it is exactly.

Exterior Surface (Mold Cavity) Design

The inner and outer walls of the part are formed simultaneously and integrally, but interior and exterior designs are essentially independent so we review them separately. As the design develops, the designer should begin thinking about the interaction of the plastic and the mould that will produce the part. The visual exterior of many products is formed in one half of the mould called a cavity. Following are some of the features of mould cavities the designer will want to consider.

Interior Surface (Mold Core) Design

The inner and outer walls of the part are formed simultaneously and integrally, but interior and exterior designs are essentially independent so we review them separately. As the design develops, the designer should begin thinking about the interaction of the plastic and the mould that will produce the part. The interior surface of double-wall blow moulded parts is normally formed by a mould core. Since the mould core must fit inside the cavity, there should be no question it meets the same core blow ratio = W>2D overall size requirement as the cavity.

About Parison and Parison Control

Parison blow moulding of intricate plastic bottles and other plastic parts requires air pressure control that is precise, repeatable and rapid responding. Proportion-Air offers electronic closed-loop pressure and flow control used to improve processes in the plastics blow moulding industry.

Why CMP Blow Moulding Machine?

Multiple industries and outside manufacturing companies depend on advanced extrusion blow moulding machinery. Without the right machine, there would be no way to produce superior quality products and we, at CMP, provide quality over quantity. Let us go through the advantages you will get for using CMP’s machine.

Information about the series of Extrusion Blow Moulding Machines in CMP

Have you ever seen a person at the fair or the mall during the holidays, blowing glass to create beautiful, artistic designs? It’s mesmerizing how a random piece of coloured glass can be manipulated by precisely placed hot air and become such a masterpiece. This art form inspired the current method of manipulating plastics to form many of the items we use every day, from water bottles to toys and so many things in between. This artful method is called Extrusion Blow Molding and it’s a common and inventive way of mass-producing hollow, lightweight products that are also durable and waterproof

How to select a Blow Moulding Machine?

The reputation of the blow moulding machine manufacturer you do business with has a lot to do with the quality of the equipment it sells. The right company takes great pride in offering its customers top-of-the-line products at a competitive price. Buying a blow moulding machine can be a nerve-racking experience for fresh entrepreneurs.

Types of products manufactured in Extrusion Blow Moulding Machine

Blow moulding is a manufacturing process for forming and joining together hollow plastic parts. In general, there are three main types of blow moulding: extrusion blow moulding, injection blow moulding (one stage), and injection stretch blow moulding (two stages). In the extrusion blow moulding process (EBM), plastic is melted and extruded into a hollow tube (a plastic parison). This plastic parison is then captured by closing it into a cooled metal mould. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container, or part. After the plastic has cooled sufficiently, the mould is opened and the part is ejected. It is designed to manufacture high volume hollow plastic products.

Types of material used in Extrusion Blow Moulding Machine

When choosing a material for blow moulding; cost, density, flexibility, strength, and other factors should be considered into what resin is best for your part. If the elastomeric properties of the material are exceeded, a hole will tear through the material resulting in a defective part. Other relevant properties are largely based on your application and might include mechanical, physical, chemical resistance, heat, electrical, flammability or UV resistance.

Extrusion Blow Moulding Machine VS Injection Blow Moulding Machine

If you operate a business that requires specially formed plastic bottles or containers, such as juice and water bottles or liquid soaps and shampoos, it’s to your benefit to understand the various manufacturing processes for making those bottles and containers.

What are the 2 types of Extrusion Blow Moulding Machines?

Blow moulding is a manufacturing process used for the production of plastic bottles, containers, and custom shapes. At Central Machinery and Plastic Products, we use extrusion blow moulding (EBM). EBM bottles are easily identifiable by their pinch line across the base of the bottle. This line is created as the mould cavity closes on the parison and the tail is trimmed off.

The process of Extrusion Blow Moulding Machine

Extrusion Blow Molding is the simplest type of blow moulding. A hot tube of plastic material, called a parison, is dropped from an extruder and captured in a water-cooled mould. Once the moulds are closed, the air blown through the top of the neck of the container. When the hot plastic material is blown up and touches the walls of the mould, the material freezes, the container now maintains its rigid shape.

What is Extrusion Blow Moulding Machine?

Blow moulding is an umbrella term for forming hollow plastic parts by inflating a molten plastic tube or parison until it fills a mould and forms the desired shape. Think of it as inflating a balloon inside of a water bottle. The blow moulding process begins with melting down the plastic and forming it into a parison. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass. Water channels are carved into the mould to assist in cooling.

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