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:
#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.
#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.
#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.
#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.
#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.
Injection Molding Screw Design Best Practice
| Materials Type | Hole Factor | Boss Factor | Depth Factor |
| ABS | 0.80 | 2.00 | 2.0 |
| ABS/PC | 0.80 | 2.00 | 2.0 | ASA | 0.78 | 2.00 | 2.0 |
| PA 46 | 0.73 | 1.85 | 1.8 |
| PA 46 GF 30% | 0.78 | 1.85 | 1.8 |
| PA 6 | 0.75 | 1.85 | 1.7 |
| PA 6 GF 30% | 0.82 | 2.00 | 1.8 |
| PA 66 | 0.75 | 1.85 | 1.7 |
| PA 66 GF 30% | 0.82 | 2.00 | 1.8 |
| PBT | 0.75 | 1.85 | 1.7 |
| PBT GF 30% | 0.80 | 1.80 | 1.7 |
| PC | 0.85 | 2.50 | 2.2 |
| PC GF 30% | 0.85 | 2.50 | 2.0 |
| PE-HD | 0.75 | 1.80 | 1.8 |
| PE-LD | 0.75 | 1.80 | 1.8 |
| PET | 0.75 | 1.85 | 1.7 |
| PET GF 30% | 0.80 | 1.80 | 1.7 |
| PMMA | 0.85 | 2.00 | 2.0 |
| POM | 0.75 | 1.95 | 2.0 |
| PP | 0.70 | 2.00 | 2.0 |
| PP TF 30% | 0.72 | 2.00 | 2.0 |
| PP | 0.85 | 2.50 | 2.2 |
| PS | 0.80 | 2.00 | 2.0 |
| PVC-U | 0.80 | 2.00 | 2.0 |
| SAN | 0.77 | 2.00 | 1.9 |
Source: Dupont
The screw holes need to be counterbore to guide the screws into the holes.
The depth of the screw hole must be deeper than the length of the screw, in order to accommodate the plastic debris that is cut out by the tapping screw to avoid the screw hole bursting.
The choice of screws is also very important, if the depth of the screw hole is enough, choose a screw with a larger tooth distance can effectively prevent the screw hole from sliding teeth.
This table is for reference, and the actual operation shall prevail in the end.