A jungle of methods – Making container-like pieces

There are all kinds of container-like products in the world, but how are they made? Plastic containers are a part of our daily lives now, ranging from soft drink bottles to washer liquid containers sold at service stations. Of course, we are also used to seeing larger containers as well. For example, less space-intensive underground waste collection tanks are deployed more commonly in urban areas now. Their frames are among the product group of container-like products.

In addition to easily noticeable containers, there is also the slightly less-known group of technical containers. These containers are hidden inside machines and systems, working at various vital functions. In some cases, the components are not seen by users at all during the service life of the product.

A container-like product requires more from manufacturing

So, how do you define a container-like plastic product? Even though the field is extensive, certain constraints can narrow it down. Having the shape of a container and a continuous outer surface surrounding the inner space are the basic elements of the definition. Both the inside and outside usually have a functional purpose.

However, a container-like piece is not the same thing as a shell piece. Below is a rough sketch of the differences between these two.

When making a traditional plastic product, the form tool can be taken inside the product. When making a container-like product, however, the form tool cannot be used in the same way, but the inside must be made to form on its own, so to speak. This can be solved also by making two shells and placing them together, thus creating a container-like shape. The weakness of this method is that it creates a seam in the centre of the product and requires a different means of joining. This additionally increases the risk of leaks and other problems.

There are clear differences in manufacturing methods according to needs

If a genuinely continuous and tight wall structure is desired for a container, there remain two alternative methods of manufacturing: blow moulding and rotational moulding. These two are often seen as competitors, but in actuality these are clearly different methods with their own strengths and advantages.

When deciding which method to use, it is worthwhile to go back to the following fundamental questions:
– What is the size of the product?
– How many pieces of the product are needed?
– What is the final use environment and application of the end product?

Answers to the first two questions go a long way in deciding whether to use blow moulding or rotational moulding. When talking size, container-like products are measured by their volume. Blow moulding can be effectively used when making products with a volume of about 2 decilitres and up to 300-400 litres. It is possible to go larger than this, but the tool will be so large that the required investments are a bit too high for the quantities that are usually made in the Nordic countries. When the size is measured in cubic metres or exceeds 500 litres, rotational moulding is the obvious choice.

Serial production or small batches?

When thinking about produced quantities, annual demand is a good measure of volume. The manufactured quantity again highlights the strengths of the two methods. Blow moulding can be considered a method of serial production, because the manufacturing cycle is relatively short (less than ten minutes) and the quality of the products remains consistent. This means that there will be very few quality differences between the individual products. Blow moulding can be considered a method of serial production for the additional reason that it works well with large volumes: the larger the annual volume, the more efficient blow moulding becomes.

Thanks to its fast process cycle, the strength of blow moulding is the low unit cost of products.
The slightly high tooling cost may however make this method undesirable, in particular when lower annual production volumes are considered. The moulding tools used in blow moulding are always machined and water-cooled steel or aluminium moulds. When working with blow moulding, the moulding tool needs to be relatively durable for it to be able to maintain the pressurised air blown into the mould (6 to 8 bar).

Being the heavily built pieces that they are, they last well and can survive the entire service life of the product if maintained properly. Despite the moulding costs, even relatively small series can actually be profitable for blow moulding. It is a good rule of thumb that as soon as the annual need for products reaches about 500, blow moulding should be considered. This method will not reach its limits very quickly either, because it can be used to make up to hundreds of thousands of pieces using only one mould.

Rotational moulding on the other hand is best for small series that do not exceed 500 pieces in annual volume. The moulding tools used with it may be lighter than those used with blow moulding, and some can even be made of sheet metal. This is made possible by the non-pressurised manufacturing process of rotational moulding, which also keeps the mould tooling costs low. Modern product design and demanding functional forms have however forced even rotational moulds to enter the world of machined tools, which brings the prices close to those of blow moulds. It is typical of the rotational moulding process that the manufacturing cycle is slightly longer (some tens of minutes). This is also the reason why the unit prices of rotational moulding products are higher.

Blow moulding means lighter products, rotational moulding can be used to make sharp corners

Environmental considerations also affect the choice of manufacturing method. These include for example operating temperatures, chemical stresses, and mechanical loads. The plastic raw materials used with the processes discussed here are largely the same, so the most differences are introduced by the properties of the processes themselves, such as wall thickness.

Blow moulded products are typically much lighter because of better wall thickness management. The wall thicknesses of the products can be specified with great precision, enabling the making of light-weight structures. There is however a minor weakness with this method. It can be a bit difficult to form sharp outside corners because the forming process is based on stretching the material preform.

With rotational moulding, the case is almost completely the opposite: sharp forms and corners usually need many times more material than even walls and rounded shapes. This is why the end products of rotational moulding tend to be quite heavy. The products generated using these different techniques can in fact be best differentiated by their weight. Rotational moulding products feel a bit sturdier, which may be of benefit in certain applications.

Summary of the methods:

  • Making container-like products of various sizes:
    • 2 dl–500 l – Blow moulding
    • 500+ l – Rotational moulding
  • Annual demand for container-like products
    • >500 pcs – Blow moulding
    • <500 pcs – Rotational moulding (in some cases blow moulding can be used)
  • Product characteristics
    • Blow moulding
      • Rounded shapes
      • Light weight, wall thickness well controlled (Sharp corners require special care)
      • Good ability to withstand chemical stress
    • Rotational moulding
      • Enables sharper shapes
      • Heavier structure, thick walls (Strong corners)
      • Good ability to withstand chemical stress

5S – An excellent place to work

At an excellent factory, everything is set up in the best possible way. It is clean everywhere and each work station is well organised. Are these things only possible after the factory has reached excellence, or are they preconditions for achieving it?

VMT Plastic is a busy place. Development projects, progress with regard to quality and the environment, training sessions, cooperation with students, investments, recruitment drives, moving production from one hall to another and much more. Much has happened within a short period of time, yet there is much more work to be done.

Our blow moulding production and a section of our finishing department have been successfully relocated to new facilities. This has provided us with an excellent opportunity to organise everything into their rightful places using labels and coloured tape to mark where each item is supposed to go, clean up and think about where the forklift would be best placed so that everyone has access to it.

We are now talking about the 5S method of the LEAN approach. The purpose of this method is to reduce variation that hinders operations, which means cutting back on factors that cause unnecessary expenses. The term 5S comes from the first letters of the original Japanese words Seiri, Seito, Seiso, Seiketsu and Shitsuke, which represent the five stages of the method. In English the stages could be expressed as follows:

  • Sort – Remove unnecessary tools, materials and items from the workstation.
  • Set in order – Organise the required tools appropriately. Each tool’s proper place is clearly marked.
  • Shine – Clean and maintain your devices and machines.
  • Standardize – Establish the organisation and cleaning as a routine part of the work.
  • Sustain – Maintain the established practices.

 

We organised an info session about the stages of the 5S method for all of our employees on 27.1.2020. Everyone got the chance to see how the standardization of the work environment affects results with the help of a playful exercise. The purpose of the training session was to solidify and unify our idea of how to proceed with the method towards a more functional, effective and safe work environment – towards achieving an excellent workplace.

LEAN specialist, Ella Teivainen

A growing plastics factory

VMP Plastic has received reinforcements in the form of Eila Teivainen, a LEAN specialist-in-training. Her expertise in development and quality work is robust and, most important of all, she has a genuine passion for development.

– As a company, VMT has a positive attitude towards development, Teivainen says and praises the company that cooperates with her as part of her studies.

Teivainen began her LEAN training in the Department of Technology of the Häme University of Applied Sciences in November. She takes on the challenge of clarifying everyday operations at VMT and is especially adept at harnessing the knowhow, ideas and problem-solving abilities of our expert staff.

As an academic engineer who majored in material technology, she got right down to business immediately: after the first few weeks she had already held effective and fruitful workshops for staff members and engaged in vigorous conversation regarding development.

– Ella has started to take things forward at an especially brisk pace, says CEO Marja Mäkinen with admiration.

Larger facility for blow moulding and scaling up of injection moulding

There is no shortage of work or development targets for Teivainen. VMT is expanding its blow moulding production and relocating it to a larger facility by the end of the year. VMT’s injection moulding capacity will also be elevated to an entirely new level with the acquisition of a new Engel injection moulding machine.

– We are growing. Currently we are in the process of relocating our production to new premises that will triple the size of our production facilities. We have also made investments into machinery, says CEO Marja Mäkinen.

The new machines will enable a wider product selection.

– With an even more powerful machine stock we will be able to offer our customers a competitive edge by utilising the latest technology. Our investments also allow us to expand our product selection to cover many new types of products and raw materials, says Mäkinen to sum up the developments.

Thanks to the reorganisation of production, VMT will now have access to large indoor storage spaces that further improve the quality of the products as there is no longer a need to expose the products to the effects of weather during storage.

– A well planned layout ensures cost-effective production. With this change we are developing our workstations and making our material streams more efficient, says Teivainen.

Technical benefits and properties of blow moulding

What is plastic blow moulding?

Blow moulding is based on extrusion. In a blow moulding, a tube of warm plastic is lowered between the halves of a mould. The mould is closed, and air is led into the tube at a suitable pressure, moulding the plastic tube into the contours of the mould. The piece is then cooled down, the tube is cut, the mould is opened, and the piece is ready for finishing.

The process is quick, which makes a blow-moulded piece very competitive in terms of pricing. What makes blow moulding an excellent mass production method is its good repeatability and because it is very controllable to produce identical results between repetitions.

The benefits and properties of blow moulding

Blow moulded pieces are seamless, vessel-shaped or sinusoid, most often round. Blow moulded pieces can be very long or channel shaped. There are no real restrictions to shape. Different kinds of evasions or include several inserts, such as metal threading, hose clamps, support struts or fastener ears.

“Especially in machining and appliance industries, our clients struggle with cramped spaces in ever-shrinking machining spaces. With blow moulded pieces, we can utilize the tight spots, because the pieces can take many shapes and the thickness of the pieces is controllable.”

The blow moulding method enables equal and thin thickness in the pieces. In blow moulding, the thickness of the pieces can be determined in a very detailed manner, because the method ensures good viscosity. Thus, it is possible to optimize the weight of a piece, because the thickness does not need to be increased to reach required stiffness.

The thickness of the piece’s wall affects material expenditure and thus the price per unit of the pieces. Material choices also enable transparency, enabling for example seeing how full of liquid a piece is without opening it.

Production of dimensionally accurate plastic products is possible, because the range in blow moulding is easy to control.

” The range of two fastener ears at the opposite ends of a piece that is over a meter long can be kept under a millimetre.” 

Strong flat and thin pieces can also be blow moulded. The capsular structure ensures a stiff enough piece but is lighter than a solid structure.

The plastic used in blow moulding is chemical resistant and is thus safe in even challenging targets.

Blow moulding mould

The moulds for blow moulded pieces are very resilient and last almost forever. A yearly yield of 100 000 moulded pieces can be easily done with a single mould. Resilience and a long life are visible in the price of the moulds but is compensated quickly, if the volume of the moulded piece is expected to grow.

” Our oldest moulds are from 2003, and they are still in complete working order. We do basic maintenance and slight adjusts to our moulds from time to time. “

Blow moulding is an eco-positive method

Press release for the Subcontracting Trade Fair 2019

Blow moulding allows for the production of a technical product as a single piece, which results in savings in energy consumption and simplifies the customer’s assembly phase.

The use of plastics has positive impacts on the environment via energy and material savings

Life cycle analyses have demonstrated that in the right product groups plastics produce the smallest total impact on the environment when compared to other materials.

As a responsible partner we are ceaselessly developing our operations. We always strive to utilise materials effectively and save energy, which also cuts costs and thus also reflects positively on productivity.

We recycle material in our own production to avoid raw material waste and instead funnel the valuable material for reuse.

We strive to minimise our environmental impact with high-quality product design that optimises the need of materials and maximises the product’s service life and recyclability. We design the most functional and material-efficient solution for each purpose.

The positive environmental impacts are realised as savings in energy and materials as well as reductions in waste volume and energy needs in distribution and transportation. We use reusable packaging materials with many of our customers.

We are a partner with ISO9001 and ISO14001 certifications.

You are welcome to join us to discuss sustainable and eco-positive plastic solutions and forging a long-lasting partnership!
The Subcontracting Trade Fair 2019 is held in the Tampere Exhibition and Sports Centre on 24.–26.9.2019.

 

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