Our mold designers often see threads on a plastic part， they are called internal threads and external threads，and mold designer absolutely know cores for internal threads and sleeves for external threads，in order to get high quality threads and to be molded economically and in large quantities，unscrewing device will be used inside a mold. Threads are critical in Injection Molding. Not only do they require more complexity in the tool design, the resulting plastic part must work perfectly, with next to no warpage and align perfectly when assembled. It’s important that you get it right the first time, so in this article we’ll explore what you need to know about threads in Injection Moulding.
There are two options for creating threads in your part. You can either overmold a metal threaded insert or you can use a rotating core to form the thread in the mould. The overmolded insert provides slightly better strength and better alignment, however it costs more as it is overmolded and requires an Injection Moulding operator at all times. Molded Threads are cheaper and do not require supervision, but the still need a rotating core to properly form the thread. This means an increase in cycle time and. Most moulded threads produce undercuts, though this can be avoided if the thread is located on the parting line of the mold. If you do decide to move the parting line of the product to include the thread, be extra cautious about molding it as it may create flashing.
In general, try use threads that have radiused crests and roots, a thicker cross section and wider spacing between threads. This gives good engagement to the plastic part, which is more likely to break or fail rather than the screw or bolt you intend to use. The Unified Thread Standard is commonly used in the design of plastic threads, however acme or buttress thread profiles also produce good results. Extremely fine pitched threads are difficult to mold, if you need a finely pitched thread, the overmoulded thread insert is your best option. It’s best practice to leave 0.8mm clearance at the very end and beginning of threads. Also if you are connecting plastic to metal, it’s best to have the plastic as a male thread (external), and metal as the female thread (internal). When required, external and internal threads can be automatically molded into the part, eliminating the need for mechanical thread-forming operations.
External Threads
Parts with external threads can be molded in two ways. The least expensive way is to locate the parting line on the centreline of the thread. It should be considered however that it is generally not possible to avoid an undercut in the parting line. This should lead to deformation of the thread on ejection. If this is not acceptable, or the axis of the thread is in the direction of mold-opening, the alternative is to equip the mold with an external, thread-unscrewing device. Moulding external threads without side core
Internal Threads
Internal threads are molded in parts by using automatic unscrewing devices or collapsible cores to produce partial threads. A third method is to use hand-loaded threaded inserts that are removed from the mold with the part.
Stripped Threads
When threaded parts are to be stripped from the mould,
the thread must be of the roll or round type. The normal
configuration is R = 0,33 pitch. Requirements for thread stripping are similar to those for undercuts. Threaded parts with a ratio of diameter to wall thickness greater than 20 to 1 should be able to be stripped from a mold.
don’t forget that there’s more than one way to skin the proverbial cat. If finding the perfect threaded-part design eludes you, remember that Protolabs might be able to machine your threads in a secondary operation more cost-effectively than trying to mold them, at least on smaller quantities. You could also consider using self-tapping screws or an overmolded thread insert as an alternative.
Internal threads are another matter. In order to keep tooling costs low and speed delivery time, Protolabs uses hand-loaded inserts to create internal threads. These must be unscrewed from the workpiece at the end of each molding cycle using a special hex key machined into the end of each insert. Yes, this does take a little time, which, in turn, drives up part costs slightly. It’s for this reason that the number of internal threads per part are limited to two, and their size is limited as well—at Protolabs, internal threads can be no larger than 2.75 in. (70mm) in diameter, and no smaller than the 0.3 in. (7.6mm) minimum already mentioned. The threads should also be located at or near the end of the insert, lest the insert become irretrievably stuck in the workpiece after molding.
The use of hand-loaded inserts presents another small limitation: material choice. Many engineering-grade polymers, especially those that are glass or fiber-filled, are a little “sticky,” making it difficult for a human to remove the insert just described. To keep production rolling along smoothly, any molded parts with internal threads should be made of ABS, POM (Delrin), or nylon. Avoid glass-filled and high-temp materials. No such limitation exists for external molded threads.
Aside from the issues discussed so far, success with plastic injection molding requires designers to follow the “best practice” rules covered in other design tips, and threaded parts are no exception:
Draft angles of 1 degree or greater should be used to assure easy part ejection.
Wall thicknesses should be fairly consistent, and sharp corners avoided.
Avoid locking or counter-rotating threads, and ask for an A2 or better surface finish to make insert removal easier.
Let’s face it: Screw threads were originally designed for metal parts, and were standardized long before plastic injection molding became mainstream. This has created a few challenges for plastic part designers. The first is that the classic, 60-degree V-style thread seen in the fastener aisle of your local hardware store is relatively shallow. This isn’t a problem when parts are made of strong, tough metal. But those made of plastic tend to strip more easily, especially on fine pitch threads or those the size of a pencil or smaller.
This might sound like a lot, but it’s hardly noticeable except on very large or coarse threads. For these, it might be necessary to remove the undercuts using a “half thread” design, although this will reduce thread strength slightly and may make the threads a bit less free-running, depending on what they screw into. One way around this is to design the mold with cam-activated side-actions, which eliminate undercuts and provide a fully formed thread. Be aware that this approach drives up the initial tooling cost
Screw threads were originally designed for metal parts, and were standardized long before plastic injection molding became mainstream. This has created a few challenges for plastic part designers. The first is that the classic, 60-degree V-style thread seen in the fastener aisle of your local hardware store is relatively shallow. This isn’t a problem when parts are made of strong, tough metal. But those made of plastic tend to strip more easily, especially on fine pitch threads or those the size of a pencil or smaller. That’s why Protolabs recommends that internal threads should be no smaller than 0.3 in. (7.6mm) in diameter, and recommends using the coarsest pitch possible (regardless of size) for your design.
Getting that perfect thread in a moulded plastic part takes some skills and know how.
Unfortunately many toolmakers still do not quite know how to achieve this.
The secret is in most cases to use a process called Orbital tapping.
Orbital tapping is accomplished by utilizing a vastly
undersized electrode, whose O.D. is
small enough to drop down vertically
full depth into the tap drill sized hole.
A planetary motion is then introduced feeding radially outward
until the finished thread size is reached.
Orbit tapping engages the electrode over the entire
depth of the hole, allowing a much larger “frontal burn” area, while spreading
the electrode wear over the entire surface of the electrode.
This reduces the problem with uneven wear and gives a very nice result.
This is something you should make sure any toolmaker you work with is aware of, and have the equipment for.
A good CNC EDM machine is crucial.
thank you for sharing your experience
thank you for sharing your experience