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 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 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 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).
If you need to make plastic parts with strong joints and reusable threads, metal threaded inserts are the way to go.
The best method depends on the part requirements. Some assembled plastic parts need to be taken apart and reassembled, possibly multiple times. In those cases, the best fastening options are metal threaded inserts. What’s the best method for getting those inserts into a part?
Here are a few considerations to keep in mind when selecting the best insert type, and insertion process:
Strength Requirements
The key strength factors are resistance to the insert pulling out of the part (pullout force) and resistance to the insert twisting in the part when the mating fastener is torqued (torque-out force). The longer the insert, the greater the pull-out resistance; the greater the diameter of the insert, the more torque capacity. The knurl pattern is also critical; more on that below.
Materials
Both the plastic material and the insert material matter, depending on the application. Two insertion processes—ultrasonic and heat-staking—involve re-melting previously molded plastic, so they only work with thermoplastics. For thermosets, the options are molding in the insert, or cold pressing the insert in later, where the elasticity of the resin material will become critical. 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.
Cost
Because threaded inserts can be molded into the part at the outset, or pressed into the part later, consider the total cost of the operation. That includes molding time and cost, component handling, and assembly.