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Start your plastic product design off in the right direction!

As a material, plastic offers a number of possibilities, but there are at least as many pitfalls too. Optimism and rashness can lead to a situation where you have at your warehouse a couple of thousand assembled and packed items that – for one reason or another – end up being scrapped. Or maybe the defect is spotted by your customer. The more complex the design is, the more difficult and expensive the going back or the change of the course will be. It pays to ensure that the direction of the project is right from the outset.

A plastic product is always used for a certain purpose and in a certain environment, e.g. at a snowmobile safari or in a desert. Some parts are made in thousands, others – in millions. The life span of a product can vary from a couple of minutes to decades. Some products are kept tucked away in a dark cellar for years, the other are used round the clock, kicked and sprayed with water. With skilful design, plastic can take on most challenges.

Below you will find a list of aspects that should be considered at the outset of the project, and a few times along the way, as the requirements and the plans become more specific. It is important that the design-team and the client have the same understanding of the project’s objectives. As a plastic product designer I would ask my customer the following questions.

As a plastic product designer I would ask my customer the following questions.

What is the purpose of the product, what are the benefits it offers?

A new product is often expected to be better, less expensive or more attractive than its competitors. Even though for you the reason for making the product is as clear as day, it still pays to explain the expectations connected with its functions to other involved parties as well. This way the focus will be on what is important from the point of view of the product as a whole.  

What is the expected life span of the product?

A custard jar ends up as energy waste after its contents are eaten. A milk pallet circulates in use for a predetermined time. A fingerboard of an instrument is expected to last from father to son. A product must fulfil its purpose and serve for the expected period of time, but yet its cost has to be as low as possible. In the designing process this balance is achieved by the selection of materials and proper geometry.

What is the expected production volume of the product?

Soft drink bottle caps are produced annually in millions, while control panels for X-ray devices are manufactured in thousands at most. The expected production quantities steer the design process. If the volumes are high, it is worthwhile to invest almost anything into the tool and the level of completion of the item. Even a small additional value per piece pays the investment back. Accordingly, if the production quantities are low, it is often reasonable to keep the tool cost at a minimum. The calculations required are relatively simple, but they help to save substantial amounts of money.

On what level is the target cost?

Sometimes at stores one can see plastic products the price of which does not cover even the material used to make it. The price of a component is dependent, above all, on three factors:

  1. Material: Volume of the piece * density of the material * price by weight
  2. Machine time: The hourly cost of the injection moulding machine divided by the number of pieces produced per hour.
  3. Investment: The amount invested into the tool divided by the number of pieces produced (there are as many ways of calculating this figure as there are customers).

The approximate price range can be calculated even on the basis of a fairly rough draft. The price estimation often surprises the customer – in either direction.

What is the highest temperature at which the product is used?

A product carefully tested at room temperature will bend when used by an Australian sheep farmer. This does not happen because by the farmer is especially strong, but instead because the reduction of the elastic modulus of the material at high temperatures. Elevated temperature affects the mechanical properties of a product substantially. Strength and stiffness of a plastic product must be optimised taking into consideration the maximum operating temperature.

What is the lowest temperature at which the product is used?

An Italian plastic bucket cracks, when used to scoop water from an ice hole at 30 degrees below zero. Below a certain temperature most materials become significantly more brittle. Problems are easy to avoid if one knows the service conditions and the quirks of different types of plastic.

Does the product need to be transparent?

If transparency is needed, the material of choice will most likely be of the amorphous type. They have certain limitations that must be taken into consideration in design. The choice between transparent and opaque must be made in the very early phase of the development process.

Will the product be exposed to chemicals?

A fly-fisher sprays insect repellent onto his or her temple. Soon one of the sunglass arms drops into their wellingtons. Bonds between polymer chains keep a plastic item intact. The performance degrading impact of chemicals cannot necessarily be noticed at once; the long-term exposure is what matters. What kind of substances will the product get in contact with? Detergents, oils, sunscreens, or perhaps nail polish remover?

Will the product be exposed to water or humidity?

When in spring you start pruning apple trees, you could notice that your pruning shears feel much suppler than you remembered. Many plastic products are affected by humidity, even though they are never actually been submersed in water. Polyamide in particular acts a bit like a sponge. The possible impact of humidity on the strength, stiffness and dimensional accuracy of a product can be controlled, if the circumstances are known.

Will the product be exposed to steam or boiling water?

A material with excellent thermal resistance can become brittle if boiled. Steaming is a commonly used disinfection method, e.g. in hospital environments. The requirement has considerable significance in the selection of the material.

Is the product exposed to UV light (or other radiation)?

A plastic toy tractor that has spent a couple of summers in a playground sandbox can be a rather sad sight. Sunlight or old-style fluorescent tubes have a certain effect on most of plastics such that over time they turn yellowish, develop cracks and lose a great deal of their durability. Not many plastics are completely immune to UV light, but usually the compound meeting the requirements can be found.

What is the magnitude of the loads to which the product is subjected?

Will the product have to carry loads weighing some kilograms, tens of kilograms, or hundreds of kilograms? It is useful for a designer if the information about the approximate loads is available from the start. This is something that is perhaps difficult to estimate at the outset, as the form of the product is not yet clear. However, you could describe which of the following represents the best fit for the use of the product, for an example:

  1. Smoke detector shell: the product has only its own weight to carry.
  2. Printer cover: during use the product is subjected to moderate forces, but there are no actual loads to speak of.
  3. Heavy duty storage box: the strength of the product is put to the test, but minor bending does not affect its performance.
  4. Brake pedal of a car: the purpose of the product is to transmit larger forces without flexing. The stronger and the stiffer the better.

Injection moulding allows designing the product so that its geometry and the loads affecting it match. Weak spots can be eliminated from areas subjected to loads.

What types of loads are expected?

A shelf fastened to the wall with plastic supports will carry the weight of a toolbox without any problems. After a couple of months, however, the ring spanners are lying across the garage floor. In addition to the magnitude of the forces, the way how loads are applied has a considerable influence on the dimensioning of the product. Of what type are the loads to which your products are expected to be subjected?
The load is …

  1. temporary: an air conditioner’s adjustment lever
  2. static (long-term): a support element of a storage shelf
  3. cyclic (leading to fatigue): a bicycle pedal
  4. impact: a pallet or a toolbox

What is the impression of quality that the product is expected to impart?

The shell of a consumer electronics device has to be extremely neat, while some products in the construction industry are covered in concrete. The feel and gloss of the surface, as well as the overall appearance of the item depend on the selected material and the quality of the tool. Certain types of plastic hardly ever provide the quality impression expected of consumer products. One should also consider the visibility of glass reinforcement on the product surface.

How sensitive is the product in terms of dimensional accuracy?

Small electrical connectors must have an extreme degree of precision, while tolerances of a concrete bucket can be much larger. The majority of injection-moulded products are somewhere in between. The precision of dimensions achieved in the process varies, depending on the material and the geometry. A material showing a high degree of shrinkage challenges the straightness of the item. Moreover, the dimensions of the completed item can change due to temperature variations. A small clearance can be lost when a product is used in freezing temperatures.

Are there any standards or other requirements established for the industry or the application that must be complied with?

The product has been tested and production line is ready, when you are informed that this product needs to comply with the UL-94 V-0 flammability standard, in order to be sold in the US market. Replacement of the material will increase the price, influence other properties and the entire geometry of the product needs to be reconsidered. Usually it is easier to comply with the standard rather than start a process for changing them. The easiest way is to address them early in the process.

Does the product require painting, welding, gluing or printing?

Do you need to glue, print or weld the component as a secondary operation? The post-treatment of the product has to be taken into consideration when selecting the material. Design can also facilitate and speed up secondary operations and assembly stages.

Are there any similar solutions existing already?

Although you might think that you are developing a completely novel and unique product for the market, there is certainly something similar already being used in another area. It is advisable to study and prefer materials and structures that have already been found appropriate in another product used in a similar manner.

Are there any patents restricting the design?

Old and new patents are easy to examine with the help of Google, although in order to be certain it is advisable to seek assistance of a professional. If your competitor’s patents set restrictions to your product, you should investigate this as early as possible. Unless your objective is to merely copy the solution of the competitor, patents rarely create obstacles to an improved product.

This blog focuses on the hands-on design of injection molded plastic products. If it is something close
to your heart, please share your own experiences with all of us. You can also follow plasticprop.com on
LinkedIn or Facebook.

Thank you for reading,

Markus Paloheimo, Managing Editor, plasticprop.com

  • Markus Paloheimo (Ms.Sc) is a Product Designer with two decades of experience in the field of functional consumer and industrial products. His specific area of expertise and source of enthusiasm is injection molded plastic products. Markus Paloheimo is in LinkedIn.

Comments

Serge
Markus - I am currently developing a plastic product which will be molded. I found this site while surfing the web for information on that matter; let me thank you for the information you shared with the world. For a neophyte like me, this is gold and I know I will reach my objective faster with less issues. Merci beaucoup (thanks a lot).
Kumar
thanks for the article

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