Rotational Molding Vs. Injection Molding: An Analysis of Advantages, Disadvantages, and Selection Guide

When it comes to industrial manufacturing, especially that involving plastic and polymer, the manufacturers are always in search of processes that are cost-effective, yield high quality products and are quick. Different production methods have different strengths and are hence appropriate for different industries.

Two of the most commonly employed manufacturing processes today for plastic products are rotational molding aka rotomolding and injection molding. Both of these have their own set of advantages and disadvantages. Let’s have a brief introduction of both processes along with the pros and cons associated with them.

Rotational Molding

Rotational molding is a process whereby a powdered resin in a predetermined quantity is poured into a mold. This mold rotates biaxially in a very hot oven, causing the resin to melt and coat the entire inner surface of the mold. The mold is then allowed to cool and the resin solidifies into a hollow shape. This process helps manufacture one piece parts in all sizes with smooth and uniform surfaces.

Pros of Rotational Molding

Rotational molding finds its applications in a variety of industries due to some unique strengths. Let us have a look at why it is among the most used manufacturing processes.

• Rotational molding offers high flexibility in designs. You can make any sorts of designs, even the complex ones using a mold. It helps create seamless final products with complicated design details without any complex tools.

• Another advantage of using rotational molding is that it is among the most cost effective manufacturing methods. it does not require highly complex and expensive tools and is hence suitable for small to medium-sized productions.
• Rotational molding is specifically preferred in those industries that call for leakproof and one-piece products. This is because the products made with this production method don’t have any joint and hence there are lesser chances of leakage. This is the reason, products like tanks, boxes, and containers are made using rotational molding.

• Larger products can be made more efficiently with rotomolding. Besides tanks, containers, and storage equipment this also includes outdoor equipment and plastic furniture.
• Since the material is spread uniformly across the walls of the mold due to gradual melting of resin and slow rotation of the mold, the final product has great uniformity in structure and thickness. This contributes to aesthetics of the product.

Cons of Rotational Molding

• One of the major drawbacks of rotational molding is that it is a process comparatively longer than injection molding. This means that it is not suitable for those production cycles that demand speed due to the requirement of bulk production.
• Rotational molding can be used with materials like polyethylene, PVC, and polypropylene etc. However, it cannot be used with those materials which are engineering grade or high-temperature materials. Thus, the material flexibility is limited.
• The finished products may often require further finishing and smoothing out of the surface in many cases as there might be some irregularities left due to mold lines.
• As compared to injection molding, the chances of material wastage are higher in rotational molding as the initial material used here is a powdered resin. The extra resin cannot be recycled and is hence totally wasted.

Injection Molding

It is a process that involves injecting molten plastic into a mold. The material is then allowed to cool down and hence it takes the shape of the mold once it is solid and is then removed from the mold.

Pros of Injection Molding

• Accuracy and intricacy of design are some of the most prominent features of injection molding. It is ideal for those parts that require perfection in design without the slightest irregularities.

• Since the process of injection molding is quicker than rotational molding, the mass production and quick yield is possible. Hence larger volumes of a product with injection molding can be made in less time.
• Unlike rotational molding, injection molding supports a greater number of material selections. You can use engineering-grade polymers, elastomers, and thermoplastics etc with injection molding.

• Post-production finishing requirements are minimal when a product is made using injection molding. This is because it produces highly aesthetic and uniformly shaped products with perfect finish.

Cons of Injection Molding

• Injection molding can be comparatively costly as compared to rotational molding because it employs expensive tools and molds for the process. This makes injection molding unsuitable for smaller scale production.
• Using injection molding for the production of larger parts may not be feasible because of the higher cost of tools and equipment. Hence, the usage is limited mostly to the smaller parts production only.  
• Injection molding asks for higher precision when it comes to pressure control, injection speed, temperature setting, and tools handling etc. Hence, it requires more skilled labor as compared to rotational molding which is comparatively simpler.

Selection Guide

Whether one should opt for rotational molding or injection molding depends on a number of considerations. For example, budget, volume of production, material, size and shapa of the parts, and so on. Let us have a look at what would be a wise choice under certain requirements:

• If the product asks for geometric intricacies and achieving a particular wall thickness is a major consideration, relying on rotational molding would be a better option.
• For manufacturers looking to produce high volume lots of their products, injection molding should be preferred. This is because the production cycle is quicker and shorter with this molding method which means quicker and larger turnaround per cycle. However, for small to medium sized lots, rotational molding will be a better option.
• The material to be used for producing different [arts is another important consideration. For engineering-grade materials and the ones that require higher strength, injection molding is preferable. However, the material selection with rotational molding is limited.

Conclusion

In short, both rotational molding and injection molding have their own sets of strengths and weaknesses. The final choice depends on your own preferences and factors like material selection, production cycle, cost, final finishing requirements and so on. In the end, it is all about making a learned choice by understanding the intricacies involved in both production methods.

References

https://en.wikipedia.org/wiki/Rotational_molding#:~:text=The%20rotational%20molding%20process%20is,hollow%2C%20one%2Dpiece%20parts.

https://art-tangent.com/product/rotational-molding-box-with-rotational-molding-process/

https://www.sciencedirect.com/topics/engineering/injection-moulding#:~:text=Injection%20molding%20is%20a%20process,of%20the%20desired%20final%20object.

https://art-tangent.com/product-category/plastic-products