What is the draft angle?
The draft angle is the amount of taper for molded or cast parts perpendicular to the parting line. It can be measured in degrees or mm/mm (in/in).
Why Draft angle injection molding?
- Reduces the chance of damage to the part due to friction during release
- Reduces wear and tear and chances of damage to the mold
- Ensures a uniform, smooth, unscratched finish when required
- Ensures the integrity and uniformity of other surface finishes and textures
- Reduces overall cooling time by lessening or eliminating the need for unconventional ejection setups
- Most, if not all, of these benefits offer either direct or indirect overall production cost reductions
Draft angle injection molding
- Draft angles are generally a requirement of injection molding design (though there are a select few soft materials, like nylon, that can get away with draft angles of zero).
- Draft should be a part of design for manufacture, and all stakeholders and contributors should be aware of its need at the outset of the design process. This includes product designers, design engineers, and of course, the customer. This allows considerations for design requirements to be taken into account.
- Any draft is always better than no draft.
How Draft angle injection molding
1) A draft angle of 1½ to 2 degrees is required for most parts
This rule of thumb applies to parts with mold depths of up to 2 inches. At this size, draft as low as 1½ degrees will allow for an easy, non-damaging release from the mold when the expected shrinkage of the thermoplastic material occurs.
2) Draft should average about an additional degree for each inch of depth
For deeper or larger parts, more draft is required to account for the additional surface area (and potential for friction upon mold release).
3) Draft should occur toward the “top” of the mold
In other words, the draft angle should follow the direction of the mold moving up or away as it separates. If you imagine a hollow box with drafted walls, the open top would be slightly wider than the bottom when draft is applied correctly.
4) Textured parts require more draft — sometimes much more
For parts where intricate textures or other raised or recessed surface features are designed,
draft should be added to the initial optimal 1½ to 2 degrees at the rate of another 1½ degrees per thousandth-inch (0.001”) of surface depth.
5) All components of a piece should be drafted
For complex geometries or parts with several tension-easing features like ribs, gussets or louvers (all integral components of good part design), ideally, all of these features — essentially, any area of contact with the mold — should incorporate a draft angle.
6) For metal-on-metal molding functions, more draft is required
This does not refer to injection molding metal, but rather to mold designs where one metal mold component makes direct contact with another.
Typically, 3 degrees of draft will be required to ensure that the mold can separate properly.
7) Draft may have to be incorporated on two sides of a part.
If the parting line for a molded part is in the middle (as with, for instance, a solid cylindrical part), draft should be incorporated on both ends of the part. This is because there are essentially not one but two mold release actions, both of which require draft.
8) At a bare minimum, half a degree of draft should be incorporated into vertical surfaces
This standard should not be adhered to in typical cases, but is more of a guideline when design requirements dictate a nearly uniform angle from top to bottom of a part wall or vertical surface. Half a degree of draft can provide a reasonable compromise between designs requiring straight walls, and the requirements of the injection molding process. In general,
however, performance and part throughput will not be as favorable as going with 1½ to 2 degrees of draft.
9) Again: Any draft is better than no draft
This point is important to remember (and repeat), especially if designers are highly resistant to incorporating draft. If a product design discussion does reach this extreme point, it is still beneficial for the engineer to advocate for some degree of draft, no matter how small. It is unlikely in most design cases that a perfectly straight, zero-degree draft angle is required.
10) Draft is a requirement — not a suggestion — for successful part production
Another point that benefits from mentioning twice: Draft is not a “nice-to-have” feature of design to manufacturing. It’s an important component of a well-designed part and an efficient, high-quality production process. A lack of draft angles can (and likely will) lead to an unacceptable number of rejected parts, and can damage expensive molds. Longer cooling times and complex part ejection requirements can slow production down to an unmanageable and unprofitable speed. In short, just about any other solution is preferable to forgoing draft angles in your injection molded parts.
It’s also important to remember that draft angles are just one component of an effectively designed injection molded piece. Design for manufacture is one of the easiest ways to improve the quality and efficiency of your injection molding process, and it never hurts to stay up to speed on all injection molding best practices. As injection molding specialists here at RevPart, we’re happy to share our expertise with you as we incorporate those best practices into our service.
Draft angle and feature depth in injection molding
Wall thickness, material selection, ejection, shrink rates, finish/texture, wall depth, and manufacturing capabilities will influence the draft angle selection, When designing a part or before mold design, apply as much draft angle as possible at wall side—a general rule of thumb is 1 degree of draft per 1 inch of cavity depth, but that can change with the aforementioned factors. Try following these general guidelines:
- 0.5 degrees on all vertical faces is strongly advised.
- 1 to 2 degrees works very well in most situations.
- 3 degrees is minimum for a shutoff (metal sliding on metal).
- 1~3 degrees is required for light texture.
- 3~5 or more degrees is required for heavy texture.
Please check Texture Mini Drafting Chart for Mold Tech Texture Minimum Drafting Angle For Releasing
|FEATURE DEPTH||MINIMUM THICKNESS/DRAFT|
|0.25 in.||0.040 in./0.5°|
|0.5 in.||0.040 in./1°||0.060 in./0.5°|
|0.75 in.||0.040 in./2°||0.060 in./1°||0.080 in./0.5°|
|1 in.||0.060 in./2°||0.080 in./1°||>0.100 in./0.5°|
|1.5 in.||0.080 in./2°||>0.100 in./1°|
|2 in.||>0.100 in./2°|
No draft angle (Zero Draft mold) in the injection molding
1)Some of Nylon can be released without draft angle on mold but the feature depth must be short that is a reason why all the cable connector chosen PA plastic material.
2)Some of the holes with short depth or through holes at the flat of a part.
3)Some of the short bosses with tight tolerance, if the long spindle without draft angle with injection molding that part must be laid down on the mold.
4)Sometimes, the experienced mold designer will concern to cancel the draft angle at the core side to stick the part on core side for ejection.
5)Reversed draft angle (undercut) at core side design that will guarantee the part releasing in order.
6)Otherwise, when the plastic injection molding without a draft angle, please consider some situation in the mold design or manufacturing as below:
- The core will need to be well polished ONLY IN DIRECTION OF DRAW.
- You will need a tube ejector, conformal ejector or slide.
- You will need a poppet valve to allow air to blow the molding off the core.
- You will need to pull the core completely clear of the cavity before attempting to strip the molding from the core.
When draft angle injection molding
The best way is done on the injection molding plastic part design, the experienced designer will concern the injection molding cost, product appearance and function to design what draft angle is adequate. but the injection molding mold designer and manufacturer must double check the part design on the draft angle item, you can do something for the injection molding mold successfully.
- Double check the draft angle according to Draft Angle Rule on the DFM phase and discuss with the part designer what is the best draft angle for the part design. if it is possible, the mold maker can provide the right part design with a correcting draft angle to confirm by the part designer that will save the time in the mold design phase and good communication with your customer.
- Sometimes, we must concern the part design is not easy to update with the draft angle, the mold maker can do something to guarantee the mold manufacturing successfully, you will consider some other ways to add the draft angle in the injection molding mold, but all the part design changing must be approved by the customer.
- Correcting the draft angle on the DFM phase, mold maker does it yourself with your own mold designer.
- The draft angle adding by taper tools during machining that must be figured out in the manufacturing process.
- Adding a draft angle by polishing that is not a good method and efficient.
However, adding a draft angle in injection molding mold is better than none that is a key to make a set injection mold successfully.
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