Factor of safety
Making the housing wall for the main body of the spinner thickness strong enough to not fail and be able to hold the weight of the upper assembly of the spinner whilst aiming to use as little material as possible whilst still maintaining a factor safety of 2. (Meaning it should be able to withstand 2x the amount of load and stress that it should be reasonable expected to withstand without failure.)
The calculation for the factor of safety is : Factor of safety = failure load/design load
To calculate the amount of force that could be reasonably expected to be exerted on the bingo spinner housing when it is being held by the user, I set up weighing scales at the same hight as the handle of the product and rested my hand on them as if I was holding the product. The weight exerted by my forearm was just over 500grams as seen in the photos below
Forearm force test
I am a 5”11 male with a weight of 75kg. I took the 75th percentile of weight for males as a reasonable heaviest user (as most users will be well below this as average weight drops off rapidly after 60 years of age and the target age group for the product is 75years+) , which is highest at 50 years of age at 82kg
82kg (reasonable heaviest user) / 75kg (my weight) = 1.0933
1.0933 x 500g (force exerted by my resting forearm) = 547g reasonable expected upper force exerted on body of bingo spinner from normal use.
As I am aiming for the body of the bingo spinner body to have a safety factor of 2, the material and material thickness of the part should be able to withstand a vertical downwards force of 1.1kg.
Manufacturing method
3D printing was chosen to make the parts of the prototype but is obviously not a viable method of mass production for the parts of this product as it is vastly more expensive and time consuming than other polymer part production methods.
Injection moulding was chosen as the manufacturing method for a number of reasons including; it’s ability to produce detailed features and complex geometry (this was one of the drawbacks in the prototype production method of 3d printing as highlighted in ‘technical prototyping report’ section of the report), high efficiency, enhanced strength, ability to use multiple plastic types and it’s automation to save manufacturing costs. It is totally uneconomically viable to 3D print these parts as the price of materials to print just the dibber tip was £10.83 as can be seen in the screenshot below.
Materials
The material I chose for the injection moulding process was Polypropylene due to it’s high resistance to stress and cracking, excellent impact strength, and does not break down easily from reactions with water, acids, and detergents. These are all very relevant properties that the product must have as it will be used in an environment where it is not unreasonable to predict it being dropped or knocked off a table or having something spilt on it if there are drinks on the same table it is resting on when people are playing the game.
Stress test simulation
I ran a Solidworks simulation where 10.787315 Newtons (1.1kg) [this is the force of the aforementioned desired factor of safety] of force was applied to downwards on the upper edge of the female part of the body of the bingo spinner. The material was selected as Polypropylene and the recess for the stand arm was locked in place in the simulation (marked by the green arrows in the screenshot) as it would be in the finished product in reality.
Results of the simulation showed no stress or strain to the point of failure so the body of the bingo spinner would not have to be re-engineered to meet the pre-established factor of safety of 2 when made out of the selected material of PP.
