Injection Molding Screw Design For Custom Parts
The screw is one of the most commonly used locking assembly methods in electronic products. Still, many designers (R&D) often neglect the design requirements of screw and screw columns, resulting in some defects in assembly. For example, the screw slides, the screw cracks or breaking, the screw breaks, or the screw head wears out…
Screws are a common component in injection molding design. They are usually used to connect two or more injection molding parts or other parts. The following are some guidelines on the design of injection screw parts:
Injection Molding Screw Design #1: Screw Size
The size and diameter of the screws should match the size and wall thickness of the injection parts. Choosing a smaller screw will result in poor strength, while choosing a larger screw may result in unnecessary cost increases.
Screw size is usually determined by thread diameter, pitch, and length. Thread diameter is a measurement of the outside diameter of the screw, pitch is the distance between two adjacent threads, and length is the total length of the screw.
The following factors should be considered in the selection of screw size:
Wall thickness and size of injection parts: Select the appropriate screw diameter and length so that the screws can properly hold the injection parts and ensure the strength and stability of the injection parts.
Load capacity: Choose screw size according to the load to be carried. Often, the use of larger diameter or longer screws can increase the carrying capacity.
Cost: Choose the smallest screw size possible to save costs. However, it is necessary to ensure that the selected screw size can meet the requirements of injection parts.
When selecting screw size, reference should be made to relevant international standards such as ISO, ASME and JIS. These standards list the size, pitch, and length of screws, as well as related material and marking requirements.
Injection Molding Screw Design #2: Thread Design
The thread design shall conform to the standard of the selected screws. If possible, avoid installing threads near thin walls or holes of injection molded parts, as this may cause cracking or deformation of injection molded parts.
Thread design refers to the shape and size of the thread, usually including the outside diameter of the thread, pitch, thread shape and thread length, etc. The thread design shall conform to the standard of the selected screw and take into account the specific requirements of the injection molded part.
Here are some guidelines for thread design:
Thread standard: Select the screw standard that meets the required application. Some common thread standards include ISO, ASME and JIS. Screw standards usually define thread size, pitch, thread Angle, and other related requirements.
Thread shape: Common thread shapes include V, U and triangle, etc. Choosing the right shape of thread can ensure the strength and tightness of the screw connection.
Thread depth: The thread should be properly deep to ensure that the screw can be securely attached to the injection molding, but not too deep to avoid the thread damaging the injection molding or causing cracking or deformation of the injection molding.
Direction of thread: When designing the thread, it should be ensured that the direction of the thread is the same as that of the injection part to ensure that the screw will not loosen or rotate during use.
Aperture Size: The threaded holes in the injection parts should be large enough to ensure that the screws can easily enter when installed.
Thread distance: When installing multiple screws, the spacing between the threads should be taken into account to ensure that they are sufficiently far apart to avoid cracking or deformation of the injection parts.
Screw threads should be designed to meet screw standards and to meet the specific requirements of injection parts to ensure the strength and stability of the screw connection.
Injection Molding Screw Design #3: Thread Direction
When installing screws, the direction of the thread should be consistent with the direction of the injection part. This helps to ensure that the screws do not come loose or rotate during use.
Thread direction refers to the direction of rotation of the thread, usually divided into clockwise direction and counterclockwise direction. In injection parts, usually use right-handed thread, that is, clockwise rotation of the thread.
The advantage of right-handed thread is that when screws are fixed to injection parts, they are less likely to loosen or rotate by themselves due to vibration or force because of matching clockwise thread rotation. In addition, right-handed screw threads are widely adopted in international standards, making it easy to obtain matching screws.
Of course, in some special cases, counterclockwise rotation of the thread can also be used. For example, in some special applications, it may be necessary to use left-hand threads so that the screws can be tightened during use. However, choosing the right screw can be more difficult because left-hand threads are not common in the standard.
In injection parts, in order to ensure the correct direction of the thread, the corresponding threaded mandrel and threaded sleeve holes are usually designed in the mold to ensure the correct formation of the thread during injection. These threaded mandrel and threaded sleeve holes shall conform to the standards of selected threads and shall be maintained and cleaned regularly to ensure their correctness and stability.
Injection Molding Screw Design #4: Screw Hole Design
The screw holes on the injection parts should be designed with a suitable depth of hole bottom so that the screws can be tightly fixed to the injection parts. In addition, the screw holes should be large enough so that the screws can easily enter when installed.
Screw hole design refers to the process of designing and manufacturing holes for mounting screws in injection parts.
The screw hole design needs to consider the following aspects:
Hole size: The hole size should match the screw size. The aperture should be slightly larger than the diameter of the screw to ensure that the screw can easily fit into the hole, but not so large that the screw becomes loose in use.
Hole depth: The hole depth should be appropriate to ensure that the screw can be firmly attached to the injection molding, but not too deep to avoid the screw damaging the injection molding or causing the injection molding to crack or deform.
Hole shape: Usually, the shape of the screw hole should match the shape of the thread. Common hole shapes include V, U and triangle.
Number and position of holes: In injection parts, holes should be designed according to the number and position of screws to be installed.
Hole orientation: Generally, the screw holes should be oriented in the same direction as the injection molding part to ensure that the screws do not loosen or rotate during use. In addition, when designing the location of the hole, the distance between the hole and the rest of the molded part should be considered to avoid cracking or deformation of the molded part.
Injection Molding Screw Design #5: Screw Position
As far as possible, install the screws on the thicker wall of the injection parts to ensure that the screws can be firmly fixed on the injection parts. Also, when selecting the screw location, make sure that the screws do not interfere with the rest of the injection molding.
Here are some design suggestions for screw placement:
- The screw position should be within the main stress area of the injection parts, so as to ensure that the injection parts have sufficient strength and stability when used.
- When designing screw placement, you should avoid installing screws in thin wall or arc areas of injection molded parts, as these areas are prone to stress concentration, which can lead to cracking or deformation of injection molded parts.
- If injection molding requires multiple screws for fixing, avoid installing all screws on the same side. Instead, they should be distributed on different sides to ensure balance and stability of injection molding.
- In the design of injection parts, the screws should be avoided on the outside of the injection parts, which is easy to cause the screws to be damaged or loose. Instead, the screws should be installed inside the injection molded parts as much as possible for greater safety and stability in use.
- When installing screws, gaskets or other appropriate mounting accessories should be used as needed to ensure adequate contact area between the screws and the injection molding parts and to reduce loosening and vibration of the screws during use.
In the design of injection parts, the correct determination of screw position is the key factor to ensure the function and safety of injection parts. Need to design according to the specific requirements and characteristics of injection parts, and need to consider the material, installation and use of environment and other factors.
Injection Molding Screw Design Best Practice
Suppose we take an m2.6x10L screw with the following specifications:
- Screw head diameter: 4.1~4.5mm,
- Outer diameter of screw tooth (D) : 2.47-2.55mm, take 2.5mm.
- Screw length: 10+0/-0.8mm,
- Screw pitch: 0.91mm
Generally speaking, the diameter of the screw hole must be slightly smaller than the outer diameter of the Screw (X0.72-0.85); Ordinary people often ignore the outer diameter of the screw column also need to match the outer diameter of the Screw (X1.7 ~2.2), the factor multiple listed here should be the minimum value, too small outer diameter of the screw is easy to cause cracking; The depth factor is also the minimum requirement, which needs to match the diameter of the screw. If the screw’s screw teeth are not locked deep enough, external forces will easily pull them apart.
① Screw hole needs to be designed guide hole (Counterbore) and locked with a straightening screw.
② The depth of the screw hole must be more profound than the length of the screw to accommodate the self-tapping Screw cutting pin.
③ Out of the plastic chips, to avoid the screw hole burst.
④ The choice of screws is also very important. If the depth of the screw hole is enough, choose the screw tooth distance.
⑤ a tiny screw can effectively prevent screw hole sliding teeth.
|Materials Type||Hole Factor||Boss Factor||Depth Factor|
|PA 46 GF 30%||0.78||1.85||1.8|
|PA 6 GF 30%||0.82||2.00||1.8|
|PA 66 GF 30%||0.82||2.00||1.8|
|PBT GF 30%||0.80||1.80||1.7|
|PC GF 30%||0.85||2.50||2.0|
|PET GF 30%||0.80||1.80||1.7|
|PP TF 30%||0.72||2.00||2.0|
This table is for reference only. In the end, the actual operation shall prevail.
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Advantages and disadvantages of self-tapping screw for plastic products
In the development and design stage of new products, the selection of plastic material is the most important link, but in addition to plastic, in fact, there are a lot of parts that need to be taken seriously, but there is a small part that is often ignored, that is screwed.
From a manufacturing point of view, a poorly designed screw and the stud are often the last straw to kill a design. Screw really such minor parts? This article will try to discuss first what a self-tapping screw is? What are the advantages and disadvantages of tapping screws? We’ll try to talk more about screw torque later in the article.
First of all, let’s look at what is a self-tapping screw? Most of the ordinary screws refer to the machine screws that have to first make the so-called female thread on the parts so that the screw can be locked in. Just like the nut, there are already threads in the nut hole, so the screw can be easily locked in the thread; However, self-tapping screws do not need to make threads on the locked parts in advance because it can lock in the locked parts and tap threads on the locked parts at the same time, and then make itself fixed to the locked parts.
Below is a drill tail cutting self-tapping screw, its most gap in front of a similar shape and function of the milling cutter, convenient on the plastic hole cutting off redundant components, its thread is designed to be more just can put the wave out of plastic squeeze into the trough of the thread, the role of balance, also can let the screws and plastic closer together. Some self-tapping screw threads are designed to rotate to take out the plastic or wood chips that are pushed out so as to avoid excessive waste chips squeezing inside the screw hole of the locked part and causing the danger of local gouging or breakage.
Tapping screws the conception of good, in fact, really can greatly improve the efficiency of the industrialized mass production, because want to advance the creation of a “female” screw holes, need to spend more time and effort, and plastic injection parts must be commonly used embedded value screws (Insert) can produce the effect of the female, both when waste and work, Self-tapping screws do not need to make a “female thread” screw hole first.
However, self-tapping screws also have many limitations and disadvantages, such as:
The locked parts are easy to bear certain stress due to the force of self-tapping screws and lead to the absence of cracking or rupture around the screw hole, thus affecting the reliability of the product. Because of this, the “self-tapping screw” locking place will require a certain area of flesh thickness to withstand the screw locking stress, such as plastic screw column outer hole diameter usually need 2.5 ~ 3.0 times larger than the inner hole diameter, different plastic materials (resin) need different minimum flesh thickness.
Some designers just blindly apply the previous design, regardless of the different plastic materials used, resulting in the thickness of some screw holes less than the safety factor should be.
Sometimes designers know meat thick design should put the screw holes in its size, but do not take into account the ejection Angle (draft Angle) influence on meat thick, and the stronger the screw column, the greater the ejection Angle influence on meat thick, because screw lives the inside diameter of ejection Angle just upside down, the farther, the less the rest of the meat is thick.
If the plastic injection molding factory may use regrind resin, the minimum flesh thickness needs to be increased. Many designers often wonder why the quality problem of screw crack still occurs after the mass production of plastic parts which have already passed rigorous design quality verification. In fact, a large part of this is because manufacturers add secondary recycling materials; Because of the trial production stage, injection molding manufacturers are afraid of problems and generally dare not use secondary materials, but they will secretly add them after mass production. If the safety factor of the original design is too small, as long as a little bit of secondary material will have the risk of rupture.
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Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.