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.
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.
orbital-tapping
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.
Threaded plastic parts can be created either internally or externally. Typically, the external thread is defined by locating the center of the thread as the parting line.
When this is not possible, or when the machine operates in the direction of the thread, cam-operated side cores or unscrewing devices need to be used.
On the other hand, threads running through the center of the core will require an unscrewing device or a collapsing core.
Threads
They can be removed from the mold if they are well-rounded, with minimal undercuts and depth.
If one chooses the unscrewing approach, the individual part in the mold must be index, so that while the core is being removed, it does not rotate inside the mold cavity.
If threaded components are to be stripped, threads should be shaped in a manner that is sufficient in radius in order to prevent splitting or tangling on stripping. If the thread is so shaped as to provide sufficient radii, it will not break upon stripping.
When parts need to be stripped (such as internal threaded bosses), then first the steel outside of the external diameter needs to be removed.
Consequently, when the core is stripped, the boss can expand.
The boss and parts must be appropriately supported during the stripping process so that the part will not warp. The boss must be firm enough so that it does not collapse or crack while stripping.
Threads injection molding design
Threads with a diameter-to-wall thick ratio greater than or equal to 20 must have an ejection pin that is capable of disengaging the resin.
Glass-reinforced plastics can also be stripped from a mold if the part temperature is sufficiently high and the strain rate is not exceeded.
Hotter parts are able to tolerate a greater amount of material strain during the ejection process.
For example, 33% glass-reinforced nylon can be stripped from a mold kept at 100 degrees Fahrenheit if the strain is less than 1% or a mold kept at 200 degrees Fahrenheit if the strain is less than 2%.
Ensure the ideal design is selected for the optimal ejection by consulting the material supplier.
Molded-in threads need to be terminated when they reach a minimum of 0.1 inches from their end.
Injection Molding Threads Types:
1. Injection mold external threads
Injection Molding Threads
Making half of threads each on cavity and core side on the mold tool. In this way, the thread can be formed completely after cooling, it solves undercut problem for tooling, and raises the production efficiency. The only concern is the tiny parting lines on thread. These lines are fine for thread’s attachment, but not kindly for precised transmission especially for gear components. Depending on the part’s size and geometries, you always need to make the parting lines on injection mold external threads as small as we can!
2. Injection mold internal threads
Injection mold internal threads, or thread which not allows to have parting lines on, for these moldings, the thread needed to made on mold tool completely without division. We need a third component named thread core/thread cavity, it has the completed geometries of threads. The tooling structure for these moldings is more complicated than injection mold external threads, the production cycle time can be varied depending on the thread released methods.
Mold structure for external thread is not complicated, the simple open-and-close mold is good to release undercuts. But for injection mold internal threads and those external threads that can’t have parting lines, undercuts need to be released before opening the mold. Typically, we have the following three injection molding threads released methods:
1. Force Release Injection Molding Threads
– Thread size, pitch and depth should be small enough to force out.
– Injection material should be flexible enough to avoid crack issue.
– Wall thickness is consistent, it can’t be too thick and without sharp corners.
– Draft and radii on thread are necessary.
2. Manual Insert Release Injection Molding Threads
Thread core/cavity is placed into the mold, and ejected out together with the molded part, then released threads by hand. The method of manual insert is suitable for low volume production. In order to raise the production efficiency, we usually makes 2 or even more thread core/cavity for interchanging.
3. Fully Automatic Release Injection Molding Threads
Fully automatic unscrew molding, it is a unique injection molding process includes movement and rotation to produce injection molding threaded parts. First of all, we need to build automatic unscrew molds. Unscrewing action of these injection molds are accomplished with hydraulic or electric motor, making the thread core/cavity precisely have threads ground on them. The cycle time for automatic unscrew molding can be much shorter than manually unscrewing, but the cost of automatic unscrew mold is expensive. Depending on your production amount, such as large volume production, the automatic unscrew molding cuts down the unit part price, even if by several cents per part, the total cost can be cut down much!
Threads are critical in Injection Moulding. 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 I’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 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, be extra cautious about moulding 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 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).
Mouldings with thread on the outside
Mouldings with thread on the outside can generally be made by making a mould tool with half of the thread in the core and half of the thread in the cavity. That way, the moulding can be removed from the tool without problem. A very thin joining line will be visible on the thread where the two halves of the tool met.
Mouldings with thread on the insidethreaded plastic moulding
However, to obtain a plastic moulding with the thread on the inside, or a moulding with the thread on the outside where no join lines may be visible, a different process is needed. For these mouldings, a removable threaded core (or cavity, if the thread is on the outside of the moulding) is placed inside the mould tool. The polymer is injected into the mould tool and sets to create the thread. The core and the moulded part are then removed from the mould tool.
Threads are very useful mechanical design features, but they do complicate the molding process. The inside thread shown in Figure 3 represents undercuts. In relatively soft materials these undercuts can sometimes be stripped from the mold. In the majority of instances, the threaded core pin must be rotated and withdrawn from the molded part before the part can be ejected from the mold.
Moldmakers have evolved many different ways of rotating threaded cores. A hydraulic-cylinder-activated gear and rack is the most frequently used mechanism. Inside threads require a more costly and maintenance-prone mold. These threads may also increase the cost of molding a part. With more than one full turn of a thread, additional cycle time will be required to unscrew the threaded core and return it to its original starting position.
Outside threads may or may not require an unscrewing mechanism. If the thread can be located on the mold’s parting line, the part can be molded without an unscrewing mechanism. Locating the thread on the parting line eliminates the added cost of an unscrewing mechanism in the mold and the longer molding cycle required to unscrew the threaded core.
Every effort must be made to locate outside threads on the mold’s parting line. In some cases this may require a complete redesign of the part. In most instances the redesign is a good investment.
success with plastic injection molding requires designers to follow the “best practice” rules covered in design threaded parts:
1) Draft angles of 1 degree or greater should be used to assure easy part ejection.
2) Wall thicknesses should be fairly consistent, and sharp corners avoided.
3）Avoid locking or counter-rotating threads, and ask for an A2 or better surface finish to make insert removal easier.
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.
orbital-tapping
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.
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.