Design Considerations - Injection Molding Is Tricky, Part 3

It's all in the Tool

To understand the design considerations for injection molded parts, really you need to know about injection molding tools and machines.  At school we are shown an incredibly basic line drawing of an injection molding machine.  This image shows what the average non-design punter needs to know.  However a designer/engineer needs to to know, what the machine looks like (so you can spot one and not look like a fool), what consideration are needed when designing a part for one and therefore what an injection molding tool (the mold) looks like on the inside and how it works.  Combine this and the basic knowledge that hot plastic is pushed into a mold and cools to the shape and you can design the part to be able to be made.

http://www.sinotech.com/images/injectionMolded3.gif

Real life injection molding machine

So to know what an injection molding tool looks like, below are some images of injection molding machines.

http://www.plasticimpex.com/images/Pricing/IMSBM1500mlPETBottlesProduction/PlasticInjectionMoldingMachineFTN280F2.jpg

http://image.made-in-china.com/4f0j00LvAEihWGhVql/Injection-Molding-Machine.jpg

http://www.exapro.com/product/2011/05/P10519213/dc1185eceabdbf529ca63dd2e917b2b4/used-windsor-w350-1930-injection-moulding-machine-p10519213_2.JPG

To clarify, the mold is called a tool.  These tools can be big or small and simple (simple open, shut design) or very complex, depending on the design of the part.  This however is in the hands of the designer.  The more simple the tool, the cheaper it is.  Therefore knowing how to design a part that can be made in a simple tool is very valuable.

A Simple tool: http://axisengineering.webs.com/plastic%20injection%20moldss.jpg

The insides

Above shows the insides (both core and cavity respectively) of a simple tool.  With this tool the part is completely drafted and there are no undercuts on the part.  When you add undercuts or areas that cannot be drafted then extra parts must be added to the the tool.  These are called movements.  There are different type of movements, which will be covered more in depth in a later series focusing on specifically on tools, but these movements add to the tools cost.  The more movements you have, the more expensive.  The tool below has two movements called sliders.  The green rods, on the left hand mold go into the holes on the light pink and green blocks on the right hand mold.  As the tool closes the blocks are forced to move into position by the angle of the rods, then when the tool opens the blocks slide back out of the way so the part can be ejected. (The grey rods are so that the tool aligns correctly).

http://www.dwgservices.com/images/mold-base-large.jpg

So how to have less movements you need the part to have draft angles.  Draft angles are needed so the part can come out of the mold when ejected.  If you cannot have draft angles on the part (as little as 0.5 degrees can be fine) then you have to have sections that move away to let the part be ejected after it has been molded.  Hence being called a movement. 

Undercuts also make getting the part off the mold difficult/impossible in most materials.  In some, that give some flex you can use the bump-off technique (shown below) but otherwise you have to have sections in the mold that again move away once the part has been molded to allow the part to be ejected.

This is the last part in the Injection Molding Is Tricky series.  There will be more on injection molding tools in a later series coming very soon.