Coolers are an important product area and devices that are general household items for most people. They serve across various domestic and recreational uses and typically have product lives measured in years, and potentially decades!
Strength, insulation capability, shock resistance, weight, hygiene, ease of fit and precision of lids and cosmetic quality are all major factors in the selection of the product at purchase.
Coolers generally consist of a double wall box/drum with polyurethane or polystyrene foam filling the cavity between the two skins. The most common materials for manufacture of the skin(s) of the cooler are LDPE (low density polyethylene) or PP (polypropylene). Most have integrated carry handle(s) and a variety of forms of lid latch.
Sizes vary from a few liters to 100+ liters, but as they get larger they can become installation devices rather than portable. Many high value coolers contain Peltier and even compressor based chiller systems, making them portable refrigerators and even freezers.
I. Types of Molding Methods
There are two basic construction methods for coolers, each adapted to a different production method, each resulting in a highly functional and robust device that can serve the owner for years of careful use.
In both cases of manufacturing process, the polymers used for molding parts of a typical cooler are LDPE (low density polyethylene) or PP (polypropylene), with a limited volume of higher quality (and considerably more expensive) devices made in ABS (acrylonitrile butadiene styrene);
- Rotomolding: Many coolers are constructed from a rotomolded shell which forms both the inner and outer skins of the box, with the interior space generally filled with polyurethane foam as insulation. Separate lids are made in the same way.
The rotomolding process involves manufacturing a cavity tool in which the mold will be made. These cavity tools are generally large but of relatively light construction and usually manually closed at a part line with bolts or toggle clamps.
They increasingly have built in induction heaters that heat the tool to the processing temperature. Older systems put the tool in an oven to raise the temperature. As they operate without any applied pressure, they do not require great strength but must be rigid enough not to flex during the tumbling process.
Polymer granules are introduced into the cavity tool and it is tumbled on two rotational axes to distribute these granules, while heat is applied that slowly melts the plastic. The tumbling ensures an even distribution of granules to all surfaces, to which the polymer sticks when melted.
In this way the tool is coated on its internal faces with a film of plastic whose thickness is controlled by the amount of granules used. These tools, although large, are fabricated by low cost processes from low cost materials, so their cost is surprisingly low, compared with the ‘equivalent’ injection mold tools.
Once the tool is coated and all granules are melted and integrated into the coating, teh tool continues to tumble and the heaters are shut off – or the tool is withdrawn from the oven.
Once cooled, the molding will harden to conform to the desired thickness, coating the cavity walls but not adhered. The tool is opened and the finished molding is withdrawn and deflashed.
The inner cavity of the molding is then filled with polyurethane foam insulation through an injection point, completing the cooler box or lid component manufacture process.
What remains is assembly and inspection/packaging processes, to make a finished product ready for market.
- Injection molding: Other cooler boxes and lids are made from two separate skins that can be injection molded. The injection molding process cannot form closed internal cavities as rotomolding can. Therefore each of the inner and outer skins requires an injection molding tool (or the two can be molded simultaneously in a family tool).
Injection molding requires a tool that forms all faces of a component in an openable cavity that is filled with melted polymer, injected at high pressure. The cavity is then cooled and opened and the finished sub-part or parts are ejected, ready for the next stage of manufacturing.
Injection molding tools for such large components are large and heavily constructed, able to withstand the high pressures applied in the plastic injection process. They are mechanically robust and built for many tens of thousands of molding cycles, making them very costly.
These two skins are then joined together to form a single closed cavity for the box or lid of the cooler. This process can be performed with a thermal weld, an ultrasonic weld or an adhesive step. After this jointing, the two part formed cavity is then filled with polyurethane foam
Alternatively a pre-molded expanded polystyrene insulation box is fitted between the two parts as they’re assembled. This allows a snap fit retention of the inner to the outer skins, as a liquid insulation proof stage is not required when polystyrene insulation is used.
These two processes are interchangeable to a degree, in that both produce a good quality and effective outcome. It is common to mix the two processes, for example by manufacturing an injection molded, two part box, to which is fitted a rotomolded lid.
There are generally other components required such as catches and handles. These are almost invariably injection molded.
II. Roto Molding Vs Injection Molding: Which One is the Best for Your Application?
It is by no means a simple decision whether to rotomold or injection mold for such large components. Both rotomolding and injection molding are popular manufacturing processes for creating plastic products, but they have distinct advantages and disadvantages.
The decision to use one process over the other depends on several factors, including the size and complexity of the part, the desired production volume, and the materials being used. There are complex cost-benefit implications that must be considered.
Rotomolding is well-suited for producing large, hollow parts with complex shapes, such as coolers, tanks and playground equipment. It also allows for the use of a moderate range of materials, including polyethylene, polypropylene, and nylon.
However, the process is generally slower and more expensive per part than injection molding, and the final product may have slight variations in wall thickness.
Injection molding is ideal for producing small to medium-sized parts with high precision and consistency, such as gears, housings, and caps – although larger parts such as cooler components are certainly often produced this way.
It can also produce parts with more intricate detail than rotomolding, due to the higher pressure and more controlled shrinkage. Injection molding can be faster and more cost-effective than rotomolding, but it may not be suitable for very large or complex parts and it carries a larger establishment cost, in that the tools are very costly.
In general, if you need to produce large, hollow parts with complex shapes, and the cost and time are not the primary concerns, then rotomolding may be the better choice. On the other hand, if you need to produce more intricate, higher quality parts with high precision and greater consistency then injection molding may be the better option.
Ultimately, the decision between rotomolding and injection molding will depend on the specific requirements of your project such as expected volumes, required finished goods price, tolerance of up-front costs and the overall quality expectations.
Roto Molding: Benefits and Applications
The advantages of rotomolding are clear cut:
- Rotomolding can produce complex shaped hollow bodies with relatively uniform wall thickness that would be impossible to achieve in a single process with other manufacturing methods.
- The process allows for intricate details such as undercuts, logos, and lettering.
- Rotomolded parts are known for their durability and strength, which make them suitable for use in various industries such as automotive, construction, and aerospace and well suited to molding cooler bodies.
- Rotomolding is relatively cost-effective for small to medium-sized production runs. The cost of tooling is much lower than other manufacturing processes such as injection molding.
- A moderate range of materials can be used in rotomolding, including polyethylene, polypropylene, and nylon, which offer differential benefits such as low cost, flexibility, toughness/strength, and UV resistance to suit varied applications.
Rotomolding has a well established group of applications in which it is a highly competitive option:
- Rotomolding is ideal for producing large, hollow products such as water tanks, fuel tanks, and storage containers. The process allows for the production of custom shapes and sizes that meet specific requirements.
- Rotomolding is a great choice for manufacturing playground equipment such as slides, swings, and climbing structures. The process allows for the creation of unique shapes and designs that are durable and safe for children to use.
- Rotomolding is commonly used in the production of automotive parts such as fuel tanks, air ducts, and instrument panels. The process can produce complex shapes and sizes that meet the industry’s strict quality standards.
- Rotomolding is used in the production of medical equipment such as hospital beds, wheelchair components, and prosthetics. The process allows for the creation of durable and sterile products that meet the strict requirements of the medical industry.
- Rotomolding is widely used for producing street and garden furniture such as chairs, tables, and benches. The process allows for the creation of unique designs that are lightweight and resistant to weather and UV damage.
- Rotomolding is used in the production of marine products such as buoys, docks, and boat components – and including whole craft such as dinghies and kayaks. The process allows for the creation of products that are resistant to water, salt, and UV damage.
Injection Molding: Benefits and Applications
Injection molding is a high value process that brings various benefits:
- The process is a highly efficient manufacturing method that can produce large quantities of parts quickly and consistently, with minimal waste
- Injection molding is cost-effective because it allows for mass production of parts at a relatively low cost per unit, as long as the hightooling cost can be amortized over a large number of parts
- It can be used to produce a wide range of parts, including components with complex shapes and intricate details
- Injection molding produces parts that are very consistent and uniform in appearance and dimensions across long production runs
- The strength and durability of injection-molded parts is typically high, when design and materials selection are effective
- The injection molding process requires minimal labor, which helps to reduce part costs
- Reduced material waste is a process benefit, as excess material can be reground and reused
- Injection molding offers unparalleled design flexibility, allowing designers to create parts of complex and integrated functionality
Injection molding is among the most universal manufacturing processes.
There is no market sector that does not make use of it, and few products that don’t contain injection molded components:
- Automotive parts made by injection molding are commonly used in all vehicles, as interior trim components, dashboard panels, exterior body parts and engine bay components
- Injection molding is used to produce a wide range of consumer goods, including toys, household appliances, kitchenware and consumer electronics products
- Medical Devices make extensive use of injection molding to produce syringes, catheters, implants, surgical tools, stents, diagnostic equipment and much more
- Injection molding is used to produce aerospace components such as aircraft interior parts, ventilation systems, engine ancillaries, seals and even lightweight structural components
- The packaging sector increasingly uses injection molded parts for specialist product display and protection such as containers, caps, and closures
- Electrical wiring and controls use many injection molded parts such as switches, connectors, breakers, junction boxes and housings for electronic devices
- The construction sector uses injection molding for construction materials such as wall panels, insulation, window and door furniture/fixings and roofing materials
- Injection molding is extensively used to produce sporting goods such as golf balls, snowboards, dive gear and bike parts.
III. Differences Between Roto Molding and Injection Molding
When described in basic terminology, rotomolding and injection molding only differ in that the material is introduced to the rotomolding tool before it is closed, and rotomolding tool is moved during the moulding process.
Otherwise, in both cases, a cavity tool has plastic introduced into the cavity and it conforms to the shape of the cavity and cools back to solid, retaining the shape.
Cost differentials between rotomolding and injection molding can be significant:
- Tooling costs differ because:
For parts of similar complexity/size, injection molding tools are more expensive than rotomolding tools. This is because injection molding requires high pressures, usually a higher level of precision and the tool construction is considerably more complex.
The for an injection mold can range from a few thousand dollars to many tens of thousands of dollars. Rotomolding tools, on the other hand, are simpler in design and cost considerably less.
Injection molding tools allow for high volume production runs and higher speed processing. Rotomolding tooling is better suited for low to medium volume production.
Injection molding tools have considerably longer lead time than rotomolding tools, because of complexity and generally higher precision. This is rarely less than 40 days calendar and can be considerably longer. Rotomolding tools are lighter and simpler construction, so they can be produced relatively quickly – often within 10 days.
Injection molding tools require considerable maintenance, unlike rotomolding tools. Injection molding tools operate at higher temperatures and pressures, so they are more susceptible to wear and debris. Rotomolding tools operate at lower temperatures and atmospheric pressure, which takes a much lower toll on them in use.
- Otherwise similar parts costs differ because:
Material use in rotomolding is waste free – every granule of polymer introduced into the tool becomes part of the finished component. Injection molding generally has a material wastage burden in stabilizing the tool operation and in runner/sprue waste attached to every component.
The equipment used for rotomolding is large but low cost, so the machine costs per part are relatively low compared with injection molding, which machines are very expensive.
Energy use in modern rotomolding is low compared to that of injection molding. With induction heaters on the rotomold tool, only the tool is heated. Injection molding machines run hot throughout, and remain heated in the wait times.
Material grades used in rotomolding can be of lower quality than used in injection molding, yet still achieve good quality outcomes.
Rotomolded part design complexity:
- Rotomolded parts can be tooled to have limited undercuts in the mold, although this is not widely done
- While small surface features are generally possible, the degree of shrinkage that occurs in the process can limit this
- Parts for rotomolding are generally designed to be feature light
- They are also designed to be free of sharp features, with corners heavily radiused to encourage good distribution of the polymer charge
Injection molded part design complexity:
- Injection Injection molded parts are generally designed with few constraints consequent on the molding process.
- Various undercuts can be built into the tooling without excessive complexity, unreliability or additional process time
- Small features are faithfully reproduced
- Sharp inward and outward features are also well reproduced
- Low (and pressure counteracted) shrinkage means that precision is maintained to a high degree.
Production time for rotomolded parts:
- Tool fill and closure can be a slow process, where the tool is closed by bolts. It is faster to use toggle clamps, but this is considered less reliable and may not apply enough closure pressure to limit flash
- The tumbling process to distribute the fill onto the tools surfaces can be slow, taking several minutes to evenly coat the tool
- The cooling process to allow the tool to be handled in the manual opeining/unloading can take some time, unless tools have the added complexity of quick release waterways
- Flash can be a major issue in some tooling styles, requiring careful and skilled trimming
Production time for injection molded parts:
- Tool setup is performed once. Although it is a complex process, the setup time per part on larger production run is very low
- Processing times for injection molded parts rarely exceed 3 minutes , at the scale of parts for a typical cooler
- If the inner and outer skins are molded simultaneously in a family tool, the proceeding time is barely increased, compared with a single part tool
- Tool opening and ejection take seconds and are generally fully automated
- Tool opening can happen as soon as the part is robust enough to remain integral (rather than cool enough to be handled)
- Part/tool precision, tool closure pressure and operation are precise, so limited post process handling is required
The manufacturing stress on rotomolding tools is quite low, allowing them to be formed from sheet metal cavities supported in a frame.
The manufacturing and operational stress on injection molding tools is considerable and requires the tools to be built with absolute precision and robustness.
While material wastage is low in rotomolding, there is a tendency for larger moldings to be formed with thicker walls, using more plastic, because of the relative weakness of some entrant features at their innermost faces. The tendency of the polymer to flow the outermost faces during rotation drives this.
Therefore equivalent rotomolded parts will tend to use more material to make good quality components.
Injection molding allows thinner walls with better integrated material, resulting in a higher strength to weight ratio than for the equivalent rotomolded part.
However, injection molding does generally involve waste material in runners and sprues. At the scale of parts for a typical cooler, the waste is a very small percentage of the total charge, however.
Overall, for two equivalent products made by the two manufacturing processes, the rotomolded product will be 3-10% heavier for similar strength, because of the necessity to manufacture with thicker walls.
IV. Typical Materials Used in Roto Molding and Injection Molding Operations
A wide variety of materials can be used in rotomolding, these the most common:
- Polyethylene (PE) is by far the most commonly employed material in rotomolding. It is durable, lightweight, and resistant to impact, chemicals, and UV radiation. All grades of polyethylene that can be used in rotomolding; high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and low-density polyethylene (LDPE).
- Polypropylene (PP) is also commonly used in rotomolding. It is a thermoplastic that is lightweight, rigid, and resistant to chemicals and heat. Polypropylene can be more brittle than polyethylene, so it may not be suitable for some applications.
- Polyvinyl chloride (PVC) is used in rotomolding. It is resistant to most chemicals, to weathering and also contains fire retardants. PVC can be rigid or flexible, to suit a variety of applications – flexible PVC is widely rotomolded to make toys
- Nylon is very strong, lightweight and abrasion/chemical resistant. It is used in rotomolding to create fuel tanks or industrial containers.
- Polycarbonate (PC) is clear, lightweight, and impact resistant. It is rotomolded to create parts that require high transparency, such as lighting fixtures
There are many other materials that can be used in rotomolding, including biodegradable plastics, elastomers, and composites.
A much wider group of materials can be used in injection molding, because of tighter control of molding conditions:
- Polypropylene (PP) is resistant to chemicals and heat. It is one of the most commonly used materials in injection molding, and is used to make a wide variety of parts, from bottle caps to automotive components.
- Acrylonitrile butadiene styrene (ABS) is tough, rigid, and impact resistant. It is the most commonly used material in injection molding, creating parts that require high strength and durability, such as for automotive, toys and consumer electronics.
- Polystyrene (PS) is low cost, lightweight, rigid, and easy to process but has poor impact resistance. It is commonly used in injection molding to create parts that require high clarity, such as lighting fixtures and packaging materials
- Polyethylene (PE) is tough, lightweight, wear resistant and resistant to most chemicals/solvents. It is commonly used in injection molding to create parts such as caps, closures, and household items.
- Polyoxymethylene (POM) is strong, stiff, and resistant to wear and tear. It is commonly used in injection molding to create engineering parts such as gears or bearings.
- Polycarbonate (PC) is a thermoplastic that is clear and impact resistant, although easily scratched and UV sensitive. It is commonly used in injection molding to create parts that require high transparency, such as ‘unbreakable glass’, lighting fixtures, particularly for automotive.
There are many other materials that can be used in injection molding, including elastomers, composites, and bio-based materials.
V. Roto Molding Vs Injection Molding: an Overview of Each Molding Process
The two processes are closely related, but differ in every detail.
Roto Molding: Principle of Process
The fundamental principle of the rotomolding process is to coat the surfaces of a closed cavity with a slayer of polymer. The plastic granules are introduced as powder into the tool which is then tumbled to distribute the powder and heated to melt it. This forms a uniform coating on all faces.
The tumbling continues as the tool is cooled, so the polymer solidifies in the cavity forming a hollow molding that faithfully represents the cavity, from which it can be removed when cooled sufficiently.
Injection Molding: Principle of Process
Injection molding also uses a cavity tool, but the polymer is injected already melted and fills the cavity (rather than forming a hollow skin part).
Pressure of molten plastic is maintained as the filled cavity cools, to counteract shrinkage – and when sufficiently cooled to maintain the shape of the part, the tool is opened and the part ejected.
VI. Making the Choice Between Roto-molding and Injection Molding for Coolers
There are a number of simple and complex factors that can drive the selection of manufacturing processes for a cooler’s components. They make this a complex decision that requires balancing various considerations.
Why Choose Roto-molding?
Rotomolding offers these advantages:
- Low cost of tooling, if the volume expectations are low – additional tools increase productivity
- Fast to establish moderate volume production
- Simple one step manufacture of the main components
- Hollow parts allow liquid foam fill for insulation, increasing rigidity and immobilizing parts by adding bonded rigidity
- Lower grade materials and more variation in material type can be tolerated
- Wide market acceptance of the limitations imposed by rotomolding make for a relatively simple design process
- Reduced manufacturing stages results in generally fewer potential quality issues in assembly
Why Choose Injection Molding?
Injection molding offers different advantages:
- Higher precision parts allows more complexity and feature integration, potentially reducing part count
- Higher quality outcomes can lift product cosmetics
- Fewer design constraints can result in better functionality and greater user appreciation
- High volume production is more easily achieved from single tools
- Pre-molded expanded polystyrene insulation can be used, easing some production issues
- Color changes can be easily integrated, meaning white box linings (for apparent hygiene) don’t impose outside color choice
Choose Kemal for Your Injection Molding Needs
Kemal is a market leader in its areas of operation. We are highly experienced and ready to fulfill your needs in injection molding.
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Kemal has built a highly skilled team, and large scale leading edge facilities. The range of services we offer is huge and we always strive to be the best partner for your engineering and production needs. Our quality standards are high and excellent project management and communications will smooth your path to production.
We will be pleased to receive your call, to allow us to show you our team and capabilities. We can make your transfer to production the smoothest possible experience.