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3-degree-draft-cut-view-lego-brick

Draft angles are a slanted angle that's applied to injection molded products.

See above video if you dont fancy reading article ūüôā

Draft angles are essential always! To better understand draft, take the lego brick

The left has no draft applied, The right has 8 degrees of draft.

lego-brick-with-no-draft-angles
lego-brick-with-6-degree-draft-angles

In real life, lego bricks only drafted 0.5 degrees, so they look like the non drafted picture. the 8 degrees is exaggerated for effect.

 

Why do we need draft angles?

¬†Their purpose is to help the part eject from the mold easily. Without adequate draft angles the part would scratch along the mold wall, leaving marks or ‘scuffing’. In more extreme circumstances the force of ejection can distort the part (shown in image below).¬†

 

scratch-marks-on-lego-brick-walls-caused-by-inadeuate-draft
deformed-lego-brick-caused-by-inadeuate-draft

How does inadequate draft cause scatches or deformation?

none-drafted-lego-brick-inside-mold-core-cavity-block
drafted-lego-brick-inside-mold-core-cavity-block

In the image above, which shape will be easier to eject from the mold, the left or right? You instinctively know its the right, but why?

 Because of the ejection forces, in the form of friction and the vacuum effect. Images below show the friction between the plastic object and the mold walls. This friction can cause the shape to distort (especially if part is still slightly soft). The less extreme problem is it will leave scratch marks along the surface which would also not pass production standards.

 Image 2 shows where a vacuum effect happens as the trapped air has nowhere to escape from. As the lego ejects the volume of air creates a suction force on the part which can also dis-form the shape

molded-lego-brick-air-vacuum
molded-lego-brick-air-vacuum-escaping

So why do most plastic products I use not have draft angles?

They do, you just cant see it!

  1. Firstly, many plastic products that you know well have about 1 degree of draft. 1 degree, in many cases is too small to recognise with the naked eye, so, its there, you just can’t see it.
  2.  Secondly, most often the outside walls of a product (the visible walls) are not drafted for aesthetic purposes. The inside walls however (the area you cant see) are drafted all the time.
 
Take the lego brick again. in its real form. Its drafted 0.5 degrees on this inside wall, but 0 on the outside (so they can create perfect square structures with no gaps).
 
lego-brick-inside-and-outside-walls-drafted
How and why is it possible to have vertical walls on the outside, but its not on the inside?
lego-brick-shrinking-onto-injection-mold-core

As mentioned in image above, plastic shrinks as it cools. It always shrinks towards the center of its mass (meaning inwards). The bottom block of mold is the ‘core block’ or ‘male mold’. When the part cools, it shrinks inwards onto the core block, thus moving away from the walls of the ‘cavity block’ or ‘female mold’ (one on the top).

lego-brick-shrinking-onto-injection-mold-core-showing-shrunk-form

This microscopic gap reduces the gripping tension between cavity block and molded part, so its more easily removed.  Whereas this is less true of the internal walls. With all that said, the rules of minimum draft is simple, nothing less then 1 degree draft to avoid potential problems.

 

So in what situations would want more then 1 degree of draft? Tall parts require more then the standard 1 degree minimum.

Add 1 degree for every 25mm of height rule
+-1-degree-draft-angle-rule-of-thumb

Going back to lego, lets say we have 3 lego pieces. small (25mm), medium (50mm), tall (75mm). The small needs 1 degree, medium needs 2, tall needs 3. If there was a forth part measuing 100mm tall, the draft would be 4 degrees. Super simple rule of thumb. For every 25mm of hiehgt, add 1 degree of draft.

It makes perfect sense if you think about it,  the taller the wall, the more surface area, the more surface area, the more friction. More friction means that a heavier draft angle is required to releave the higher level of friction. 

What other factors help control allowable draft angle?
lego-mold-tool-polished-surface-annotation

Anything below 1 degree of draft can cause the part to excessively stick to the mold, but are there any factors that can influence allowable draft? Absolutely, 3 in partcular, material behaviour, processing conditions and surface finish.

  1. A fully polished mold surface is called a ‘Class A1’ surface. The smoother the mold surface is, the easier to eject the part.
  2. Plastics with high shrink rates are easier to remove as it shrinks away from the outside walls of cavity. 
  3. Plastics with lubricant additives lower the materials co-efficient of friction which aids ejection.
  4. Mold surfaces can be sprayed with mold release which helps but its not practical to apply every ejection cycle so should only be used as a solution if absolutely necessary.
  5. Processing conditions play a part, for example if a mold was filled with very high pressure, the plastic will make a more intimate contact between part and mold surfaces. Again requiring a greater force to eject from mold.
 
How it effects cycle times?

Draft Angle dont only enable parts to eject easily, they also reduce cycle time. Cycle times dictate a large portion of the production costing. The shorter the cycle time. The cheaper the part.

 

How do they reduce cycle times? Lightly drafted parts must be fully cooled & solid before ejection as the force of ejection can distort the part if its not totally solid. Heavily drafted parts can be ejected sooner, while there still soft, because the ejection forces are much less. Thus reducing the time it takes to mold 1, eject then start the next cycle. Plastic today also did a good article expands on part cost reduction which you can see here 

 

To see other molding disciplines you should be aware of visit our injection molded part design best practices section