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[Thesis project]: Design and prototype of a cold rotomould machine

Prototipo TFM - Alejandro Sisternes Pla 8
Reading Time: 7 minutes

Hey there! I am sharing with you some parts of my thesis on the master I took in CAD/CAM/CAE, done the year 2016 in the Universitat Politècnica de València, under the supervision of Mr. Manuel Martínez Torán and with the valuable collaboration of a rotomould company.

Because of the extension of the project and the confidentiality of many information, I am only going to show the initial part that talks about the definition and planification of the project and I will share some images of the design process and prototype of the machine done for the thesis evaluation.

What is rotomoulding?

Rotational molding is an industrial process that produces hollow parts, obtained by rotating a mold on two perpendicular axes. During rotation in the conventional process, the mold with raw material is introduced into an oven, so it melts and adheres to the inner surface of the mold. Then, while still rotating, it is cooled to room temperature and finally the part is removed from the mold.

Although the concept of rotational molding is over 150 years old, the production of hollow plastic parts for fields like playgrounds, furniture or transports is arround 50 years old. During the 1990s, the rotational molding industry grew by 10-15% annually. With advances in process control and material research, the US market in 2000 already represented many millions of euros. Today, this technology is spread all over the global market in a wide range of products.

It is a manufacturing process that has strongly attracted the attention of the industrial community in recent years, mainly due to its characteristics. It offers great design features with low tooling investments, being able to make serial parts that could not be manufactured by other molding procedures. By the other side, dimensional tolerancing is not as precise as other manufacturing processes.

In some cases that require creating complex hollow geometries of uniform cut, rotational molding can be considered as a lower cost alternative to the plastic blowing process. The machinery is relatively simple compared to other systems, and also provides a differential benefit compared to injection or blowing. Rotomolding works at very low pressure, so the resulting parts have a structure with low internal stresses, giving good mechanical behavior compared to other processes.


In this process, thermoplastics like LDPE (Low Density Polyethylene), and PVC (Chlorinated Polyvinyl) in the presentation of Plastisol can be used.

Cold rotomoulding

On the other side we find the concept of “cold rotomoulding“, which is the object of this project. In a cold rotomoulding machine, the product is achieved by mixing a resin and catalyst in a closed mold. Then the mould is rotated biaxially at a constant speed as traditional rotomoulding, and the resin solidifies after a certain time. The main difference, apart from the raw material, is that an oven is not needed.

It is a concept experimentally applied today on craft and DIY scale in small workshops and design studios. This manufacturing process gets similar results as conventional rotational molding, with the great advantage of do not need any energy source like ovens and cooling stations. This allows to significantly reduce the carbon footprint and the environmental impact of the company that applies it. In addition, It can be complemented by supporting the constant development and research of bio-polymeric resins manufactured free of fossil derivatives or others with recycled content, to further improve the environmental impact of the final product.

All this drives the industrial economy towards a sustainable growth, and would be responsible for a more environmentally friendly industry.

Main goal

Being this project a stage of a bigger research project that would involve chemicals, mechanicals and environmental professionals, this project’s main goal is to design and develop a functional rotomoulding machine prototype that does not need heat source.

Specific goals

  • To promote a reduction of the environmental impact of the industrial activity.
  • To promote the use of biological-based resins.
  • To study the history and applications of the manufacturing process.
  • Analize the actual rotomoulding machines to understand their functionalities.
  • Determinate the part list for building the prototype.
  • Design the electric scheme of the prototype.
  • Stablish the budget for the machine.
  • Optimize the resources to have the most economical product.
  • 3D model the structure of the machine.
  • Do finite elements analysis of the parts and assembly.
  • To build the functional prototype.

Project scope

  • Context: Identify the requirement of reduction of the environmental impact of plastic industry.
  • To develop a prototype to validate the system so researchs can be done about resins that do not need heat to be cristalized.
  • Easy to be operated.
  • Easy maintenance.
  • Useful and functional, even being a Minimum Viable Product. Further developments will be done if needed for industrialization.

Acceptance criteria

  • Structural resistance to 1000N shearing force. (+40% for CAE analysis).
  • Stablish valid security perimeter of use.
  • Maximum mold dimensions: 500x500x500mm.
  • Budget for materials of 3000€ with a + 15% of possible overrun.

Restrictions

  • Project needs to be ended by the accorded date (described in project planification point).
  • Budget proposed by the company can’t be exceeded unless by strictly needed things like cuality or security.
  • In this stage, the footprint will not be studied since we want to validate the mechanical design of the prototype.

Exclusions

  • Physical study of electro-mechanical parts or chemicals reactions will not be taken.
  • Marketing and commercialization tasks will not be done.
  • The engineering and research time will not be considered into the budget.

Project planning

Start date: may 2, 2016

Finish date: September 11, 2016

Dedication: A total count of 400 hours are required to complete the project. Dedication will be of 2 hours and 45 minutes per day, from Monday to Thursday (11 hours/week) until 5th of August. Then, full time until the finalization.

Taks to do:

Task nameDays
Project definition (objectives, scope, exclusions…)1
Work Breakdown Structure definition1
Research about rotomould history3
Redaction about rotomould history1
Research about how the process works2
Redaction about how the process works2
Research about rotational molding existing machines1
Research about materials that can be used2
Stablishment of design criteria for rotomoulding2
Industrial applications of the productive process 1
Comparison of with other processes2
Study parameters/finish of manufactured parts (traditional rotomould)1
Visit ABC Rotomoldeo’s headquarters3
General measures definition3
Sketches of the machine2
Ergonomic definition2
Definitive approach1
CAD modeling + security space definition 4
CAE analysis2
Electrical design2
Definitive budget
2
Prototype build14
TOTAL54

As told at the beginning of the post, I am not going to develop here all the thesis since it includes confidential information of the company. Following, we are going to take a look into the final design and the construction of the prototype.

Design process

CAD + CAE

The structure modeling and finite element analysis are done using the software SolidWorks. All the components are simulated with loading charges that are superior than reality, to ensure the right dimensions on the design process.

CAD explosion of the assembly of the structure
CAE process
Structure CAE analysis

Structure CAE analysis

As a conclusion, the model passed the studied overloads cases.

Budget

Supplier’s names have been hided in order to avoid any kind of promotion:

We need to keep in mind that all these price includes 21% VAT tapplied in Spain, so the cost before taxes would be 1620,81€. It is on-price within the budget, so it is feasible to be done.

Parts for designed machine (included in budget)

Prototype building

The project and budget are for a bigger machine, but due to my restrictions as an student I developed a smaller functional prototype in order to be able to bring the machine to the evaluation on the university.

The base of the structure is done using 20x20x1mm profiles, and the frames using 12x12x1mm profiles.

Electrical schema of the machine

Here we can see the electrical schema for the machine, going from 230V to the engine of each axis, passing by controllers, reles, a security system of a key and an enemergency stop button. Without the security key the machine will not work.

Hands on! Last step, building the home-made prototype of the machine:

Prototype building

Article written by:

A Product designer who specializes in CAD modeling, 3D printing and other industrial processes.

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