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plastic injection mold design errors


As a sales for Custom plastic injection molding industry since 2009, below I want to share a few common tool design errors to avoid in plastic injection molding with you:

1, Incorrect material selection – The coefficient of thermal expansion for all steel, hardened steels, copper & aluminum, must be taken into consideration when designing and building tools. Large differences can cause damage and uneven wear to all components.

2, Incorrect mold shrinkage of different raw material - Different raw material plastics has different shrinkage rate. If mold shrinkage is put incorrectly, the final part dimensions will be incorrect, too.

3, Incorrect runner design – A runner system that is not balanced will cause filling issues and inefficiency in running the tool. Some parts may be over-packed and other under-filled; both scenarios could cause quality issues with the part.

4, Incorrect gate design – Selecting the incorrect gate has a similar effect with filling issues and inefficiency in running the tool. The gate is the area where the polymer transits from the runner to the part, and if this is wrong the gate could cause restrictions, incorrect polymer flow, turbulence, or other defective filling issues.

5, Part designed with incorrect wall thickness – The part should be designed with the thinnest wall section the part can tolerate (taking into account strength, functionality and other critical aspects of the design). The two risks as:

Too thin – if the wall section is too thin it may not fill correctly or it may break off in the tool. Too thick – wall sections that are too thick take longer to cool and solidify before ejecting and may result in sink marks, warping and even cracking. A good suggestion: If you are not sure about the thickness, it is better to make it thinner, and then easier to increase the thickness: just need to cut more steel. But if too thick, you need to increase more steel, then maybe the whole cavity or core should be replaced, much extra labor and cost. Our company, if to add thickness, we usually do it for free, but if to reduce thickness, it depends on how much extra cost. If too much, we have to charge it if the mistake is made by customer.

6, Sharp internal corners – Having a sharp corner at an intersection will cause stress within the part and could result in part failure. Best practice is to design corners with a radius.

7, Thick solid sections – Designing a part with thick solid section because you need the strength or rigidity is not the answer. Having a thick solid section will only result in sink marks and defective molding due to the cooling effect of the polymer after injection. Best practice is to core out the section and add ribs to strengthen that area (this will allow the molten polymer to fill the part and for cooling to be even).

8, Deep pocket with parallel sides without draft – In order for a part to be ejected off of the core without sticking, there has to be draft on the walls. A parallel-sided part will inevitably cause molding issues and high reject rates. Adding draft will overcome these problems.

Author: Vicky Liu --- Sales Manager of Respon Moulding Co., Ltd.

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