Screw posts, also known as BOSS posts, are common structures in plastic products and are also one of structures that encounter the most problems during molding. Common molding defects of screw posts include cracking, thread slippage, root fracture, shrinkage, whitening, flow marks, etc., which seriously affect appearance and use of plastic products. This article will analyze causes of these defects from perspectives of plastic materials, structural design, mold design and molding process, and propose solutions.

Figure 1 is typical structure of screw column. For many people, the most confusing thing is how to determine size of screw column. Size of screw column is mainly determined by nominal diameter M of screw and type of material. Among them, outer diameter D=M*outer diameter coefficient; inner diameter d=Mxinner diameter coefficient; thread depth H=M*thread depth coefficient. Specific size coefficients of formed screw column are different for different materials. Please refer to Figure 2 for details.

 

A concave platform can be designed at the entrance of inner diameter of screw column. Function of concave platform is to reduce initial stress when tapping screw. Diameter of concave platform D1 = M* about 0.2mm; degree h = (0.3~0.5) x D1. Sometimes, in order to simplify design, concave platform is changed to a chamfer, as shown in Figure 3, which can also play a role in reducing initial stress. Size of chamfer is generally (1~1.5) x 45°.

 

At the root of inner diameter, outer diameter and concave platform, fillet transition should be adopted. Because during injection molding process, sharp corners will generate high internal stress. The larger transition fillet, the smaller stress concentration. As shown in Figure 4, when transition fillet of 90" corner is less than 25% of wall thickness, there will be a higher stress concentration at this point; if allowed, it is recommended that radius of transition fillet is greater than 50% of wall thickness.
Height of screw column should be as short as possible. When height of screw column is greater than twice its outer diameter, it is generally necessary to add reinforcing ribs to increase strength. Screw column cannot be too close to outer wall, otherwise it will cause uneven wall thickness of part, resulting in shrinkage. When close to outer wall, reinforcing ribs can be used to connect to outer wall, as shown in Figure 5.

 

Figure 5: Using reinforcing ribs to connect screw column to outer wall
Sometimes a crater is made on screw column. So-called crater is to extend hole edge and pin on outer cylindrical surface of screw column upward synchronously, as shown in Figure 6 and Figure 7. At this time, plastic at the root of screw column is no longer so concentrated, and it is fully cooled, which has a good effect on preventing shrinkage marks.

 

 

When tapping, screw posts may crack, which can be analyzed and solved from following aspects:
(1) Material is too brittle, material degrades, and toughness is insufficient. Adding too much water to material may make material too brittle; material's elongation at break is too low, which can also easily cause screw post to crack. Generally speaking, it is not recommended to tap screws on screw posts for materials with a glass fiber content of more than 40%. When molding temperature is too high or material is not dried enough, moisture will be contained in material, causing material to degrade and its strength to deteriorate, resulting in cracking of screw post.
(2) Inner diameter is too small and wall thickness is insufficient. If inner diameter of screw column is smaller than inner diameter of screw, possibility of cracking will be very high. If outer diameter of screw column is too small, that is, wall thickness is too small, strength of screw column will be insufficient, causing cracking. Therefore, when designing screw column, it is necessary to select appropriate inner and outer diameters. You can refer to recommendations of Figures 1 and 2 for design, and use reinforcing ribs for reinforcement when necessary. Figure 8 shows cracking of charger plug screw column in Shenzhen DeZe in June 2009. Customer previously used PC/ABS and switched to our PC. From Figure 2, we can see that inner diameter coefficient of PC is 0.5 larger than that of PC/ABS. That is to say, for M2.6 screw used by customer, inner diameter of screw hole corresponding to PC is 2.21mm, while that of PC/ABS is only 2.08mm. After switching to PC, required inner diameter of screw hole increases, and original inner diameter appears to be too small, so it is easy to crack when tapping screw.

 

(3) Excessive internal stress. Excessive residual internal stress on the part will also cause cracking of screw column. In terms of structural design, root of screw column and top of pin should adopt rounded transition to prevent stress concentration. In injection molding process, parameters that have a greater impact on internal stress of part are mainly melt temperature, mold temperature, holding pressure, holding time and injection rate. Generally speaking, to obtain smaller internal stress, a higher melt temperature and mold temperature, a smaller holding pressure and holding time, and a slower injection rate can be used. Among them, mold temperature has the most significant effect on internal stress. When copper threads are to be embedded in screw columns during molding, it is best to use a higher mold temperature so that heat of mold can be transferred to copper nut in a shorter time, or preheat copper thread first to eliminate internal stress caused by low temperature during molding, which causes product to crack. Figure 9 shows that screw column with copper nut cracked because mold temperature was low during molding and copper nut was not preheated.

 

(4) Environmental stress cracking. When screw column contacts with organic solvents such as acid and oil, if material itself is not resistant to such organic solvents, environmental stress cracking may occur. This contact may come from mold surface or from handling and assembly process. In April 2009, screw column of BBK induction cooker cracked 1 to 2 weeks after screwing. As shown in Figure 10, cracked screw column is random and crack direction is not fixed, that is, not all weld line positions. Observing cracked section, inner surface is smooth and has no lines, and outer surface has radial lines. It is inferred that cracking started from inside, which may be caused by chemical solvents that have been treated with rust prevention, decontamination, lubrication, etc. on the surface of screw, resulting in environmental stress cracking of screw column.

 

Figure 10 Screw column of induction cooker is cracked
(5) Reduce initial stress during tapping. As mentioned above, adding a concave platform or chamfer at the entrance of inner diameter of screw column can reduce initial stress during tapping and reduce cracking of screw column.

Corresponding problem of screw column cracking is slippage, which can be analyzed from following aspects:

 

Figure 11 Screw thread of power tool screw

(1) Material is too tough and not rigid enough, which can easily lead to slippage;

(2) Inner diameter of screw column is too large, and thickness of screw biting is thin, which is also easy to cause thread slipping. In July 2009, screw column of power tool of Bosch's outsourced factory Shenzhen Xiangxing was thread slipped. Screw used was M3.5. According to reference coefficient in Figure 2, theoretical inner diameter is 2.625mm, whilethe designed inner diameter of customer's screw column is 3.0±0.05mm, as shown in Figure 11. Inner diameter is too large and causes thread slipping.
(3) Torque is too large. Figure 12 is reference torque of screws of different specifications. When torque is too large, inner side of screw column cannot withstand torque, while outer part is strong enough without damage, which leads to thread slipping.

 

Figure 12 Reference torque of screws

Figure 13 shows situation of root of screw column being broken. Stress concentration can cause root of screw column to break. Due to sharp corners at the root of screw column and top of pin, stress concentration is easy to cause during molding. In the case of no shrinkage, these places should adopt a larger fillet transition.
If molding temperature is too high or there is too much water in material, material will degrade during molding, resulting in a decrease in strength and possibly root fracture. In addition, proportion of water inlet should not be too large.
When pin of screw column is too long and strength is insufficient, deformation and deviation may occur due to impact of plastic melt. As shown in figure, after filling is completed, pin needs to be reset and rebounded, so that root of screw column will be repeatedly impacted and root is easy to break. In this case, shrinkage mark behind screw column will move downstream (when pin rebounds, downstream pressure is small), and there is also possibility of forming a weld line.

 

Shrinkage problems are often encountered behind screw column. Common solution is to add a crater, as shown in Figures 6 and 7. Reason why crater can prevent shrinkage is that it can reduce equivalent wall thickness (inscribed circle diameter 2R) of root of screw column. As shown in Figure 15, when equivalent wall thickness of root is not much different from wall thickness T of part, part is not easy to shrink.
When screw column is too close to side wall, shrinkage is likely to occur due to increase in local wall thickness, as shown in Figure 16. Referring to Figure 5, a reinforcing rib may be used to connect screw column to side wall to avoid shrinkage.

 

Figure 15 Principle of preventing shrinkage in the crater

 

Figure 16 Screw column close to side wall causes shrinkage
When designing mold, setting a gate near screw column can also prevent shrinkage. The closer to gate, the more sufficient pressure holding, the stronger shrinkage compensation effect, and the less obvious shrinkage.

Figure 17 shows whitening of root of screw column. Reason is that in the case of insufficient cooling, plastic is stretched to a micro-crack state near glass transition temperature (pin is pulled out when mold is opened, a vacuum is formed at the top of pin, and plastic is subjected to force when it is sucked in). Solution is to extend cooling time so that root of screw column is fully cooled before demolding. Design of adding a crater makes wall thickness of root of screw column thinner, easier to cool, and can also eliminate whitening.

 

Figure 17 Base of screw column turns white
Figure 18 shows that after tapping, main body of screw column is white. Reason may be that torque used is too large and wall thickness of screw column is too thin. Refer to Figure 12 to select standard torque and increase outer diameter to increase strength of screw column.

 

Figure 18 Screw column turns white

When pin is extended too long, it is easy to be eccentric and cause root to break. If pin is stiff enough, it is not easy to be eccentric, and other defects may occur on the surface of stud. In January 2009, at Dongguan Fuwei, upper drum surface of Flextronics electronic drum had flow marks when testing, as shown in Figure 19. Adjusting material temperature, mold temperature, injection speed, and holding pressure did not improve it. Screw column was cut open and it was found that pin extended into surface by about 1.2mm, accounting for 40% of average wall thickness of part (3.0mm). When melt passed through this place, channel suddenly narrowed, material flow became chaotic, and flow marks appeared downstream.

 

Figure 19 Flow marks on the back of screw column
A similar case is whitening phenomenon on the back of screw column. In February 2009, Xiamen Bofeng's high-gloss ABS parts had whitening behind some screw columns. Adjusting molding process had no effect. Screw columns were destroyed and it was found that pins of screw columns corresponding to whitening areas extended into surface of parts, causing melt to be sheared too strongly when passing through area, resulting in whitening of surface; while in the areas without whitening, holes were flush with surface, as shown in Figure 20.

 

Figure 20 Whitening on the back of screw column

For common defects of screw columns, it is necessary to analyze their rationality from perspective of structure of screw columns, prevent their occurrence from perspective of plastic materials and mold design, and overcome them from perspective of molding process. Many defects can be easily solved.