In this guide, you will find all information you are looking for about plastic rotational molding.
From definition, step-by-step process, advantages, choice of material to application – everything you are looking for about Rotomolding is right here. If you want to be an expert in plastic rotational molding, read this guide.
What Is Plastic Rotomolding?
Plastic Rotomolding is a unique technique for producing hollow fixtures without applying pressure. It is an affordable casting technique that supports low volume productions.
Step-by-step Rotational Molding Process
The key steps in rotational molding process include:
Step 1: Pulverizing of Resin
Involves conversion of plastic pellets into fine powder through pulverizing. It may require several pulverizes to achieve the desired particle size.
There are batch, dry and wet pulverization techniques. You will choose any of these techniques depending on the type of molding process.
Step 2: Loading Material
This involves filling the mold with a precise amount of the powdered plastic material. To obtain a good material flow, you should ensure the plastic powder is dry and consistent. Normally, this will prevent formation of bubbles. The mold’s inner walls have a release agent. It is useful in removing molded parts after cooling.
Some of the main types of mold release agents are:
- Sacrificial Coating: When you release a molded feature, this coating comes off alongside it.
- Semi-Permanent Coating: Common in many industries and can support many plastic heating and cooling cycles before reapplication.
- Permanent Coating: Makes a permanent coating that doesn’t need replacing but only through mishandling.
Step 3: Heating Material
Heating occurs as the plastic powder rotates within the mold till you achieve complete melting. The melting plastic coats the mold’s whole internal surface uniformly.
It is important to establish a correct ratio to ensure a suitable thickness. The finished part’s quality is dependent on the plastic’s heating time.
Longer heating times may cause thermal degradation. As a result, you will get parts with poor mechanical qualities. In fact, you may experience reduced wear and impact resistance.
In sufficient heating time may cause partial melting. This may lead to formation of bubbles.
Step 4: Cooling
During the cooling phase, the melted polymer in the mold hardens. You will require sufficient air for effective cooling of the mold’s exterior. There are two cooling options – natural or forced convection.
Using air internally as the mold cools helps to maintain dimensional stability. However, while you can use water sprays to speed up the cooling process, it can hamper the products mechanical characteristics.
Conducting rapid cooling causes shrinking and deformation. Conducting an extended cooling results in molten resin flow leading to uneven wall thickness.
Step 5: Demolding
In this process, you remove the cooled item cautiously from the mold. An ejection system utilizing air can be helpful in the removal of the part.
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Advantages of Rotational Molding
The following are the benefits in employing rotational molding:
Affordable Tooling
The mold is not subject to high pressures allowing use of inexpensive materials like aluminum in its construction. The production processes are brief thus, a minimal tooling investment suffices.
Consistent Wall Thickness
There’s an increase in part’s durability by having a uniform wall thickness across all sides and edges. A consistent thickness by undertaking the right speed of rotation, heating and cooling times is achievable.
Decoration is Easy
It is possible to insert decorative elements when using rotational molding by adding symbols and texts to the tooling’s surface.
Easily Produce Double Walled Pieces
Making parts with a double wall pairing without having to conduct ancillary processing like welding is possible. These parts have enhanced durability due to the smooth edges eliminating stress points.
Fewer Latter Stage Operations
When using rotational molding, it’s only possible to produce individual pieces that require no further processes. As a result, you generate less manufacturing waste such as cut-offs, runners and sprues.
Production Flexibility
Allows production of multiple pieces simultaneously with one machine. Tool management is straightforward where the rotational molding equipment has self-operating arms.
Possible to Produce Larger Parts
It is possible to make large hollow pieces with rotational molding. The only restrictions can be the heating and cooling chambers’ size.
Disadvantages Of Rotational Molding
The following limitations are evident in employing rotational molding:
Complex Productions are Difficult
Where you have unique and intricate design, undertaking the rotational molding process is difficult. For instance, parts with high-tolerance and strident edges are difficult to produce.
Few Material Options
You will convert the plastic materials into powder to ensure successful processing. Unfortunately, very few materials can withstand this process.
Labour Intensive
The process of using rotational moulding is labour intensive, therefore, negatively affecting production costs.
Long Cycle Times
Where you need high volumes, rotational molding is inappropriate since it takes a long time to complete the molding cycle. The rotation process is slow and requires you to heat the material in addition to the prolonged cooling process.
Reduced Repeatability
There’s ultimate need to replace the metal in the tooling of a rotational mold after three thousand cycles. At this point, it is subject to the quality declining influence of wear and tear.
How Blow Molding Compares To Rotomolding
In blow molding, you form hollow parts by blowing the molten plastic material into a mold cavity of the desired shape. The following is a comparison between blow and rotational molding:
- In case of high volume production, blow molding offers you a better choice due to its high production rate than rotomolding.
- Making parts with thicker walls is possible with rotomolding than blow molding.
- The porosity level of parts made from blow molding is low given the presence of loose materials in the melt in rotomolding.
- The tooling costs in blow molding are low since the mold does not need replacing like in rotomolding.
- The cycle times in rotomolding are high taking about half an hour while blow molding is often less than a minute.
Best Material For Rotomolding
Typical plastic materials applicable in rotomolding include:
Polypropylene
Polypropylene is one of the most adaptable plastics in the market featuring capable resistance to heat, stress and chemicals.
Nylon
Nylon is a type of polyamide plastic which offers strong heat and chemical resistance and is in essence robust
Polyethylene
Most plastics employed in rotomolding are polyethylene since it’s affordable and simple to mold. It is also easily accessible in powder with minimal water absorption and high resistance to chemical.
Polyvinyl chloride
Polyvinyl chloride is an unyielding and robust plastic. You can add a wide range of additives to alter its mechanical characteristics.
Applications Of Rotational Molding
Some of the areas of application of the rotational molding process is in the manufacture of the following products:
Marine Equipment
Such equipment need to be strong and lightweight to tolerate marine conditions. Plastic rotomolding allows you to develop equipment such as buoys, pool liners, kayaks and dock floats.
Containers
The rotomolding process offers a wide range of containers for material handling and shipping such as barrels, chilled boxes, and shipping containers.
Medical Supplies
Rotational molding is handy in developing specialty items for use in the medical industry. These include stretchers, paramedic cases and containers for medical specimens and samples.
Transportation
When manufacturing products ideal for transportation use such as fuel tanks, wheel arches and instrument panels, plastic rotational molding is appropriate.
Tanks
Rotational molded tanks can accommodate thousands of litres for storing water, chemicals, sewage and fuel.
Industrial Products
Some industrial products you can make using rotational molding are water filtration systems and housing for blowers.
Toys and Recreational Items
Rotational molding is imminent in creating toys, balls, playground accessories among others. This process can also furnish outdoor pieces including garden pots and rain tubs.
Types Of Machines For Rotational Molding
The following machines are essential to the rotational molding process:
- Clamshell Machines: These are one-station devices that combine molding and cooling in a single chamber.
- Open Flame Machine: Produce open-ended objects and consist the earliest type of rotational molding machinery.
- Rock and roll machines: Contain cradled molds capable 360 degree rotation perpendicularly and 45 degree swings on and below the horizontal axis.
- Shuttle Machines: These devices transfer the mold though loading, cooling, and unloading stations to the heating chamber using bidirectional independent arms.
- Swing Machine: Have up to four bidirectional separate arms two of which conduct mold transfer from the heating to cooling chamber.
- Turret: Features three to six arms spinning around a central pivot throughout different stations in the rotational molding process.
- Vertical Wheel Machine: Share functional similarity to a Ferris wheel with cradled molds that rotate through loading, unloading, heating and cooling processes.
More Resources:
How to Get High Quality Machined Plastic Parts
Conclusion
Rotational molding has brought about significant technological advancements over the past few years. While previously unexplored, rotomolding finds use in producing high-quality items difficult to produce using other moulding techniques.