Rotomolding Design | Design for Rotational Molding

Rotomolded Jet Ski Dock Assembly

Rotational Molding Design

Designing a product for the rotomolding process requires experience and knowledge of the nuances and parameters of the manufacturing process used to produce incredibly durable hollow parts. Those familiar with the rotational molding process are usually fans of the durability of the final products. Rotational Molding lends itself to producing durable hollow and even double walls parts, all contingent upon the design, expected functionality and performance of the product or component.

Unlike some other plastics manufacturing processes, rotomolding will allow for special features to be incorporated into the design of the product. Special feature options such as molded-in threaded inserts and fittings, molded in aluminum features such as extrusions, large threaded fittings or even molded-in steel tubing, bushings and more.

Rotomolding Inserts

(above)Rotomolded Insert;
(right) Examples of inserts and extrusions that can be molded in;
Inserts for Rotomolding

Because there is little to no stress involved in the rotomolding process products can be subjected to various industrial or abusive environments. A great example of a rotationally molded product that can withstand extreme abuse is the Granger ISS Tornado Shelter door. This door exceeds FEMA 320, FEMA 361 and ICC 500 debris impact standards. Watch in the short clip below how the door safely withstands the impact of a 15 pound 2×4 at 100 miles per hour!

A number of rotationally molded products have been designed to incorporate a urethane foam filling for additional strength and insulating capabilities to some parts and products. As an example, the ever popular growing number of rotationally molded coolers have incorporated various different types of secondary urethane foams to the products for insulating purposes, which allow for the coolers to hold ice and cold products for extremely long durations of time, some coolers can even keep the ice cold or frozen for days on end. Other foam filled rotomolded products include spine boards, jet ski docks, dock floats, buoys, pallets, material handling trays, doors and more!

Foam Rising in Rotomolded Product

Urethane foam rising inside of a double wall rotomolded part

Rotomolding allows for features or shapes in the design to seamlessly blend. Curves and transitions are accomplished in the rotational molding design process, allowing rotomolded products to have great geometric shape to enhance a products beauty and aesthetics while adding strength and durability. Structural ribs and kiss offs may be added to the design to increase strength and rigidity while angles and shapes can be added for further enhancement of the overrall strength.

Additional features such as spin weld fittings, bulk head fittings, mold in graphics and even post mold graphics are all features that can be incorporated into a rotomolded product design. Throughout the design process, take all desired features and functions into consideration as the design is being completed, as this could potentially eliminate the additional cost, time and headache of needing to have the molds modified once the tooling or the molds for rotomolding are completed.

With the design for the products completed and signed off on, the next step is to have a tool or mold built for the rotational molding process. Most molds for rotomoulding are either cast from aluminum, fabricated from aluminum or steel or machined from an aluminum billet.

Some molds for rotomolding can be designed to house multiple chambers or cavities, thus producing multiple parts in a single round. How well the mold is designed and the quality focus of the mold construction will greatly dictate the quality of the part produced from the mold.

New Mold for Rotomolding
New mold for Rotational Molding

A true rotomolding expert can walk a client through the entire process from concept, design, any necessary design changes, mold fabrication, rotational molding process, trim & assembly, packaging and even future inventory scenarios. In most rotomolding scenarios, it is ideal to have as many of these elements as possible in a single location in effort to minimize costs and maximize operations efficiencies.

Rotomolding Design Issues To Avoid

Rotational Molding design experts can expose most all design issues inside of the initial design meeting, going over the features and functionality of the new rotomolding project. Generally with some education or direction, most design issues can be avoided, which may cause manufacturing or quality issues in production. Some of the most commonly experienced rotational molding design issues are listed below in effort to help you avoid these issues when working on your rotational molding project design.


The design process for rotomolding is crucial to ensure that the aesthetics of the final product meets or exceeds the clients expectations. The repeatability of the aesthetics in a rotomold product design should be addressed in the early stages of the design process. It is important that all aesthetic considerations be carefully thought through as all of the rotomolding design stages are carefully inspected, as it is important to ensure that all of the details and features are quality captured to meet or exceed the customers expectations.

Draft Angles

Draft angles are a critical part of the rotomolding design process. Draft requirements for draft angles are crucial as they could potentially be the difference between a part having to be destroyed to be removed from a mold and a part that releases itself from the mold as it cools. Geometry, heavy texture finishes and even engraved features such as logos, warning text plates or gallonage markers could require additional draft angles in comparison to simple geometric shapes such as a block, cube or a cylinder, such as a vertical water tank. A general rule of thumb, engraved features such as logos or text can function successfully with as little as 3 degrees of draft while undercuts and protruding features may require as much as 5 degrees of draft.

Sharp Corners

One of the design issues that has the ability to plague many plastics manufacturing processes is sharp corners. Sharp corners should be avoided if at all possible during the rotomoulding design phase. Sharp corners tend to have the ability to minimize material flow through the mold during the molding process, which could cause voids in other areas, as the sharp corners tend to cause the material to not fill in properly where the material could not lay down or flow properly though the mold. Sharp corners also tend to create stress risers in the product, developing an area capable of failure or fatigue.

Because of the manner in which sharp corners are created in the design process, sharp corners become a production issue for the rotomolding process. Sharp corners should be be avoided if at all possible. Sharp corners typically will create a heat sink, an area that collects more heat than in other areas of the mold, thus building up thickers areas of wall thickness with thin inside corners.

Material Flow

Material for the rotational molding process is extremely different than for other plastics manufacturing processes such as injection molding, blow molding or compression forming. Material for the rotational molding process requires the polymer pellets to be pulverized to powder, typically a 35 mesh, where as injection molding requires the material to be in pellet form. The additional step in pulverizing the material to a powder is necessary for rotational molding. During the cook cycle of the process, the air that is caught between the polymer particles is being heated in effort to drive out all of the air. Because of the rotation in the process and the powder materials, getting the material to flow succinctly throughout the mold is a crucial step. An experienced rotomolding engineer should be able to see any potential “bridging” or “webbing” issues between wall surfaces, not allowing the material to flow through the mold properly. The restriction of the material flow will cause voids in other areas of the product.

A general rule of thumb to avoid material flow issues is for a minimum of 5 times the wall thickness between any walls, ribs or surfaces in the product in effort to avoid material flow problems.

Flat Walls

In the rotomolding design process, flat walls are possibly the most common of design issues that plague a rotationally molded product design. Long or large flat surfaces are a nuance to the process, as typically the rotomolding process is trying to form a hollow vessel, as typically the parts have little to no internal structure. Rotomoulding experts will know how to work with your design and desired features to avoid the large flat surfaces, by breaking up the large flat areas by adding geometry or structure to the product. This is often done by adding ribs, kiss-offs or even engraved text or logos to the surfaces.

The rotational molding industry has a standard shrinkage tolerance of 1%. Granger Plastics Company is one of the few rotomolding leaders that can hold a tighter tolerance on many applications through a multitude of practices. These shrinkage, warpage, cooling and other factors are all considerations that have influence on the final molded product. Industry standards for a flatness are ± .20″ per inch.

Designing For Rotational Molding

The rotational molding process has far more flexibility to accommodate the design of products and components or tooling that is required to produce products via rotational moulding. While molds or tooling can be modified once it is created, it is always beneficial to have the tooling as close as possible to the end desired result initially in effort to save time and money. Because of rotomouldings ability to change the wall thickness of the final product by altering the shot weight, or the amount of material placed in the mold without having to make modifications to the molds.

Rotational Molding Design Issues to Avoid

Download Rotational Molding Design Issues (PDF) by clicking below

Rotomolding Design Issues
Request Granger Plastics Information
Fill out my online form.
Granger Plastics on Facebook
Comments Box SVG iconsUsed for the like, share, comment, and reaction icons
Cover for Granger Plastics Company
Granger Plastics Company

Granger Plastics Company

The Granger Plastics Company

"Solutions Beyond Expectations"

Sandsaver Beach Erosion Solution helps 5 star resort, Swahili Beach win internationally recognized World Travel Awards for Africa's Leading Beach Resort 2023.... by naturally rebuilding the resorts beach!

Congratulations Swahili Beach!After installing Sandsaver at their resort on the Indian Ocean, Swahili Beach Resort wins the World Travel Awards for Africa's Leading Beach Resort 2023 at the awards show in Dubai, UAE this week.

Congratulations to Swahili Beach!!!
... See MoreSee Less

Sandsaver Beach Erosion Solution helps 5 star resort, Swahili Beach win internationally recognized World Travel Awards for Africas Leading Beach Resort 2023.... by naturally rebuilding the resorts beach!  

Congratulations Swahili Beach!
Load more
Granger Plastics on Twitter
Load More
Granger Plastics On Instagram
May the Fourth Be With You... Protecting you from Mother Nature's FORCE!

#tornado #tornadoshelters #plastics #grangeriss #stormshelters #rotomolding #rotationalmolding #grangerplastics #grangeriss #starwars #darthvader #stormtrooper #staysafeoutthere
In case you needed a "sign" to stay out of Jail... Here it is! 

#rotomolding #laundrycarts #materialhandling #laundry #commerciallaundry #uniforms #towels #jail #CountyJail #Prison #plastics #inmates #getoutofjailfreecard
One of the video clips from the Granger ISS debris impact test at Texas Tech University's Wind Lab performing the FEMA 320 and FEMA 361 debris impact test.... If you're considering purchasing ANY tornado shelter or storm shelter... Ask to see the debris impact test video... If they don't or won't show you, run away and find a reputable supplier who will!!! #tornado #tornadoshelters #StormShelters #GrangerISS #plastics #rotomolding #rotationalmolding #grangerplastics #plastics #weather #environment #emergency #FEMA #disaster #prepared #survival #Ohio #Texas #texastech
The Sandsaver continues to produce amazing results after only 6 weeks- over 20,000 cubic yards of sand accumulated and over 100' of increased width profile in some areas already!! #beacherosionSolution #beacherosion #beaches #plastics #rotomoulding #rotationalmolding #environment #beachconservation #stopbeacherosion #reversebeacherosion #savethebeaches #Sandsaver #grangerplastics #Ohio #Africa #Kenya #UnitedStates
See the Sandsaver Beach Erosion solution get installed and produce significant positive results at the beautiful @swahilibeach resort in Diani Beach Kenya in just 2 weeks! Video link in bio!

#Beach #beacherosionSolution #beaches #beacherosion #stopbeacherosion #dredgingalternatives #reversebeacherosion #Kenya #Africa #environment #tourism #travel #seetheworld #luxuryresort #resorts #dianibeachkenya #dianibeach #Sandsaver #grangerplastics #rotomoulding
Granger ISS Tornado Shelter installed at an incredibly beautiful horse farm in Southern Pines, North Carolina.

One of the rotomolding industry's best innovations, ever. 
#GrangerISS #tornadoshelters #StormShelter #tornado #tornadowarning #weather #plastics #rotomolding #rotationalmolding #grangerplastics #rotationalmoldingdotcom #Granger #NorthCarolina #Ohio
A photo of the tide coming in at the Sandsaver installation location at Diani Beach in Kenya... So incredibly beautiful... #Beach #BeachRenourishment #beacherosionSolution #Sandsaver #rotomolding #rotationalmolding #Granger Plastics #Africa #Kenya #dianibeach
The Sandsaver, a natural solution to beach erosion gains it's second install, this time on private property located on the Gulf of Mexico 🇲🇽! In less than 10 weeks, in an extremely low tide environment, The Sandsavers are over 50% buried in Sand and have a generous amount of accretion between the modules and the surf.  Behind the modules is starting to fill in creating a nice gradual slope.  The property owner is ecstatic with the current progress and cant wait to see the beach completely rebuilt naturally by Sandsaver, another #rotomoulding innovation from Granger Plastics Company! 
#Sandsaver #naturalSolutionToBeachErosion
#Beach #beaches #dredgingalternatives #BeachRenourishment #Florida #Texas #NorthCarolina #SouthCarolina #California #Alabama #Georgia #Virginia #Maryland #delaware #NewJersey #newyork #rhodeisland #massachusetts #Maine #California #Oregon #washingtonstate #savethebeaches #savetheoceans #rotomolding #rotationalmoldingproducts #rotationalmolding #plastics
Congratulations and a very special thanks to Hall of Fame Announcer for the @reds, Marty Brennaman who is calling his last home game after 46 incredible years for the Cincinnati Reds!  You will be missed Mr. Brennaman!!! Pictured is EJ, the daughter of Granger Plastics' GM.  EJ also doubles as our in-house baseball expert, who was lucky enough to get this photo with Marty earlier in the season.  Now shes trying to figure out how to get it signed!! #MLB #Baseball #CincinnatiReds  #Cincinnati #Ohio #MartyBrennaman #GrangerPlastics #Rotomolding #Plastics #HallOfFame #Retirement
America's leading underground tornado shelter, The Granger ISS in full production... The only Tornado Shelter with a 500+ year lifespan... #Tornado #tornadoshelters #StormShelters #GrangerISS #HowToSurviveATornado #RotationalMoulding #Rotomolding #GrangerPlastics #RotationalMolding #Survival #preparedness #savealife #ohio
This error message is only visible to WordPress admins
Error: There is no connected account for the user 4036753673082371.

Contact Us

Give us a call or fill in the form below and we will contact you. We endeavor to answer all inquiries within 24 hours on business days.