Drop Test theoretical calculations & material selection

[arturbdg]

Hello,

I am currently assigned to perform a drop tests on a unit that we sell. Thing is that I am an Electronic Engineer so I am not best at mechanics.

I am wondering whether you can calculate forces and required material strength to withstand the drop test. The reason I am asking is because one of the units I testes shattered at the corner.

Here are the specs:

Height - 48 inches (122cm)
Unit weight - 5kg
Dropped on 3 chipboard plates carpet and concrete floor.
Every corner, bottom and sides is secured with 7mm thick rubber bumpers. The corners are rounded and bottom one form kind of rectangular with an empty space

-------------------
| |
| | (diagram)
| B |
|\ /|
-------------------

B stands for battery and the corner which shattered was underneath it.

During my test unit fell roughly flat on the chipboard and on one corner where slightly more weight is located above it (the battery) shattered to pieces despite being protected by rubber bumper.

Now my questions are:
1) Injection moulding - can knitlines (non are visible in the shattered area but they are present in some other areas of the case) or any other injection moulding defects contribute to that, if so what to look for?

2) How to calculate the best material for use (current case is made from Minlon 10B140NC010 but the front cover was made from Zytel70G30HSL which seem to be stronger. Front cover do not protrude much but having said that 2 drop tests were performed (2 back covers - one damage to mounting posts inside case, one shattered corner) while only 1 front made from Zytel used without any sign of damage.

Unfortunately I can't provide detailed pictures due to nature of the project and as such any information can be only roughly estimated but I would appreciate any help in that matter (especially calculations).

Kind Regards
Artur

 

[Achy47]

Hello Artur,

As a scientist with a background in mechanics, I would be happy to assist you with your drop test concerns. Based on the information provided, I can offer some suggestions and calculations to help you determine the best material and design for your unit.

Firstly, let's address the issue of the shattered corner. It seems that the weight of the battery may have played a significant role in causing the corner to shatter, despite the rubber bumpers in place. This could be due to the distribution of weight and the impact force on that particular corner. To prevent this from happening in the future, I would recommend redesigning the unit to distribute the weight more evenly and potentially reinforcing the corner with a stronger material or additional supports.

In terms of calculating the forces and required material strength for the drop test, there are a few factors to consider. The primary force acting on the unit during the drop test is gravity, which can be calculated using the equation F=mg, where F is the force, m is the mass of the unit, and g is the acceleration due to gravity (9.8 m/s^2). In this case, the force acting on the unit would be approximately 49 N (5 kg x 9.8 m/s^2).

Next, you will need to consider the impact force on the unit when it hits the ground. This can be calculated using the equation F=ma, where F is the force, m is the mass of the unit, and a is the acceleration due to impact. The acceleration due to impact can vary depending on the surface the unit is dropped on, but for concrete it is typically around 10 times the acceleration due to gravity (98 m/s^2). Using this equation, the impact force on the unit would be approximately 490 N (5 kg x 98 m/s^2).

Now that we have calculated the forces acting on the unit during the drop test, we can use these values to determine the required material strength. This will depend on the design and structure of the unit, but as a general rule, the material should have a strength of at least 2-3 times the expected impact force. In this case, the material should have a strength of at least 980-1470 N.

In terms of injection molding defects, knitlines can certainly contribute to the failure of the unit during the drop test. It is important to carefully inspect the unit for any potential defects and make sure they