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.
Threads in a plastic component allow for fastening of your plastic part to another plastic part or to a non-plastic part by using common fasteners. Plastic components can have threads on the external side or internally. External threads can be modified so that they can be molded without any special mold actions. Internal threads require a rotating core that unscrews from the part. Unscrewing molds are used for plastic parts that have threads or ridges that cannot be easily injected using the standard knockout method. In these molds, the parts are unscrewed from the mold to avoid thread damage. Unscrewing-molds remain the most complex of all injection molds and need to move at high speeds and clear previously molded parts efficiently in order to begin the next cycle. They require tremendous technical savvy to build, then maintain. They are designed for many years of production, which is why they are considered a long-term investment when producing high-volume parts.
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 mould, 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).
The most common material for the threaded inserts themselves is brass. However, with increasing sustainability requirements, lead-free alternatives such as stainless steel or aluminum are growing more popular. Stainless steel offers better strength and corrosion resistance, and aluminum inserts are about 70% lighter than brass.
There are two options for creating threads in your part. You can either overmould a metal threaded insert or you can use a rotating core to form the thread in the mould. The overmoulded 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. Moulded 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 mould. If you do decide to move the parting line of the product to include the thread,
In addition, designing molded threads or threaded bosses as features for larger molded parts may be something you’ll want to consider. Designing plastic screw bosses is fairly straightforward, but there are a few things to keep in mind to ensure a strong connection between parts and to mitigate cosmetic defects.
A significant number of these thread failures were the result of improper part design. The designs being used at that time were intended for metal parts. These thread design details had evolved over many years of trial and error to be ideal for metal threads. Some of these metal details were unsuitable for plastic threads, but no one knew any better at the time.
The plastics industry has subsequently learned how to design good-quality injection molded threads. Unfortunately, that information is not widely disseminated. Many engineers are continuing to make the mistake of designing plastic components according to the specifications developed for metal threads.
Once the standards for metal threads were established, they remained basically unchanged. One reason for this was that whether the threads were chased on a lathe or individually cut with taps and dies, they required special tools. It was difficult to produce these special tools for a different thread pitch or profile. The important advantages of interchangeability also encouraged the use of standardized threads. Interchangeability was the primary reason why early plastic parts duplicated metal thread designs.
In order to keep tooling costs low and speed delivery time, we use 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, 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.
at least on external threads, is the parting line—we will always place it lengthwise down the exact center of the thread, essentially cutting the mold into two equal halves. As discussed, there will be some undercutting of the thread form, unless you opt for a side-action mold, in which case some slight flashing can occur where the different mold sections come together. Again, it’s not much, but designers should always know what to expect before committing to a manufacturing solution.
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. recommends using the coarsest pitch possible (regardless of size) for your design.
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,