Monday, May 7, 2018

Variance with Injection Molded Parts

I have a habit of inspecting items in the store before purchasing them, and more than once I have had people with me that have questioned why I bother with this since "they are all the same". Since I have found it can be difficult to convince these people that there is indeed variance in mass produced parts I have decided to create this post to show the differences between some injection molded boxes that I recently purchased from the same store about one month apart from each other.

At first glance, they may appear to be identical, however upon inspection it is clear that the quality of the one that I purchased later was of noticeably lower quality. The most noticeable instance of this is the difference in how much the box lids close:


In the photo above the boxes are shown without the lid clamp used. Notice that the lid on the left box does not close all of the way and has a gap, where the one on the right does not.

The next image shows the difference between the lids when the lid latches are closed. Notice again how the box on the left doesn't fully close compared to the box on the right.

This problem is caused by variance in the gasket used in the lid, as can be seen below. Note that in this image the left and right boxes have been swapped from the images above in order to show the variance more clearly:


In addition to this, there are other differences between the two that suggest that a different mold design was used. There are a number of small variations that can be observed, but I will only include the following two examples since they are the clearest to see:
Note how the handle on the left has extra plastic in the middle, while the one on the right is clear all of the way through.

Comparing the bottom of the two boxes also shows that the difference between the gate design (the opening in the mold where the hot plastic comes in):

Conclusion
Although it would be great if mass produced items were all identical, in reality manufacturers will ship anything that fits within a certain amount of tolerance from the ideal design. The amount of variance that is allowed depends on the customer (that is, the company that is paying to have the part manufactured), and typically cheaper products tend to have more allowance on the amount of tolerance that is acceptable. A number of factors can cause variance between parts that "should be" the same, from the temperature of the mold and plastic, the amount of time allowed for cooling, and even if there has been a change to the tooling used to make the part. This variance exists for all products, not just injection molded parts. Depending on what you want/need from the product, you may want to do your own quality check before purchasing it.

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