Existing product design description
Product thickness distribution is shown in Figure 1. On both sides of each "track", thickness is 1.3mm. In addition, thickness of product at pouring point is about 5.1 mm, thickness of protrusions is 1.9 mm, and diameter of latent gate connecting protrusions is 1.2 mm, as shown in Figure 2.

 

Product problem description
Tail end of product has shrunk too much and cannot be delivered.
Physical context in which problem occurs
Why does tail end shrink too much?
Plastic shrinks in volume as it cools from operating temperature to room temperature. If there is no pressure holding action or pressure holding pressure is insufficient, added plastic cannot enter cavity, and shrinkage of plastic will be very obvious, especially far away from gate. Since added plastic is the least, shrinkage will be particularly large. When solving such problems, gating system design (vertical runner, runner, gate) and product thickness distribution must be considered at the same time. The key is to confirm whether there is a bottleneck in flow of plastic during filling process. In design 1, when cavity still needs plastic replenishment, latent inlet gate (1.2mm) has solidified and plastic cannot be pushed into cavity at all. This is bottleneck of pouring system. In addition, when tail end of cavity needs to be replenished with plastic, plastic entering from gate may not be able to enter tail end due to thick design of product. Even if holding time is longer, purpose of holding pressure cannot be achieved. This is bottleneck of product design. Therefore, two points can be considered in problem diagnosis: size of gate and change of product's thickness (or strengthening flow).
Problems and solutions of design scheme

Inputs used in analysis:
Analysis material GE Plastic PC/ABS CYCOLOY LG9000
Filling time 2Sec
Press holding time 20 Sec
Figure 3 (a)~(d) are filling short shot diagrams of Design 1. From short shot diagram, it can be found that wave fronts on both sides of cavity advance faster at the beginning of filling, and the flow in the middle is slower. After half of cavity is filled, plastic filling speed in "track" closer to gate is faster than other places, and flow presents two inverted V shapes, but flow on outside is still faster. Last filling point is located in the center of tail end of product.

 

Why does flow present two inverted V shapes?
Thickness between tracks (about 1.3mm) is too thin compared to thickness of upper and lower sides of product (4.1mm, 2.5mm), and plastic flow cannot cross thin wall. Therefore, during filling, plastic replenishment is only provided by a single track, and cannot be supplied from adjacent tracks. This phenomenon is more serious at the end far away from gate. Because amount of plastic replenishment is relatively insufficient. Plastic shrinkage at tail end will be greater than that at gate, so size of product tail end will be smaller than that at gate.
Effect of holding pressure from thickness of solidified layer
As plastic is reduced from processing temperature to room temperature, its volume will shrink. Therefore, during injection molding process, there is a holding pressure stage to compensate for shrinkage of plastic. Figure 4 (a) is distribution diagram of solidified layer at 14 seconds, and Figure 4 (b) is distribution diagram of solidified layer near gate. Red part is 1, indicating that plastic at this location has been completely solidified. Color chart is close to blue, indicating that the thinner solidified layer at this location, the easier it is for plastic to pass through. From Figure 4 (b), it can be seen that at 14 seconds, tabs and latent gates have solidified. Even if holding pressure time is extended, plastic cannot enter cavity. Figure 4 (a) shows that plastic can only be replenished by a single track. Only central track has not yet solidified. The rest have solidified, and plastic cannot be replenished to the end of product.

 

How to solve product shrinkage problem?
From Design 1, we know that volume shrinkage of product is mainly due to insufficient pressure holding. Solution is to start from two bottlenecks: gating system and product design.
Enlarge gate: Existing protrusion inlet is only 1.9mm, while product thickness at this point is about 5.1mm. Thickness of protrusion is only 37% of product thickness. Due to space limitations, thickness of protrusion can only be increased to 2.5mm. Therefore, protrusion is changed to 2.5mm thick (thickness ratio is about 49%). Although it is still a distance from recommended ratio of PC/ABS (more than 70%), it can still avoid premature solidification of protrusion. Based on same reason, small end diameter of latent gate must be enlarged to 2.0mm. In short, it is to ensure that gating system can provide enough plastic to fill volume shrinkage of product during pressure holding stage.
Add four equally spaced ribs with a thickness of 2.5mm on male mold side of product (see Figure 5): In addition to increasing rigidity of product, more importantly, plastics between product tracks can complement each other, so that filling pressure farther away from gate is no longer dependent on only one track.

 

Figure 5 (a)~(d) are filling short shot diagrams of Design 2. The biggest difference from Design 1 is shape of wavefront advancement. Wavefront advancement of Design 1 presents two inverted V shapes, but wavefront advancement of Design 2 presents an inverted V shape. Main reason is that four additional ribs promote plastic filling between product tracks, so that filling is no longer based on a few tracks, but is filled in a nearly straight wavefront advancement mode. Although filling end is still located in the center of product tail end, since plastic is no longer supplemented by one track, holding pressure can be smoothly transferred from gate to the end of product during pressure holding, avoiding excessive shrinkage there. Moreover, new four ribs also have a reinforcing effect on rigidity of product.

 

Effect of holding pressure during filling process from perspective of solidification layer thickness
Figure 6 (a) and Figure 6 (b) are solidification layer distribution diagrams of Design 2. At the same time of 14 seconds, gate part of Design 2 is still not solidified, indicating that plastic can still be filled smoothly. In addition, at the end of product, it is no longer only center that is not solidified, and most of tracks are still not solidified. During holding pressure process, plastic is no longer transmitted by a single track, but can be replenished by all tracks at the same time.

 

In addition to strengthening rigidity of product, rib design of Design 2 can also confirm that plastic can be transferred to the end of product during holding pressure stage. However, from another perspective, holding pressure of this product cannot be effective, mainly because plastic filling distance is long, and thick design of product cannot allow plastic to be filled smoothly to the end of product. If product's gating point is changed to side gating point, flow length can be shortened, and plastic can be pushed to various parts of cavity in a shorter path during pressure holding stage.
Figure 7 shows gating method of Design 3. Gating position is placed at junction with rib of Design 2. Since there is no need to consider thickness limit when adding a tab here, a more appropriate tab thickness can be selected. Thickness of product connected to tab here is 5mm, so tab thickness is 5X0.7=3.5mm.Diameter of latent gate connected to tab is 2mm.

 

Figure 7, Design 3 (ribbed and poured from side)
Choice of pouring position
Generally, pouring position of a product is selected based on principle of equal flow length from gate to product to ensure that plastic can fill all parts of cavity at the same time, so that plastic can have a uniform volume shrinkage and product will not deform. However, due to different thickness designs of products, it is difficult to find an appropriate pouring position. In the past, CAE analysis also required a lot of time to find gate position.

MOLD Design Optimization module provides gate position optimization design, which can determine the most balanced pouring position within one minute. First, set side as pourable area and set number of gates to two. Program can find the best position within 30 seconds, then move found position to side wall connected by two nearest ribs (four ribs added in Design 2). From filling short shot diagram (Figure 8), it can be seen that due to thicker thickness of product at the bottom, speed of plastic filling at the bottom will be faster, so gate position on lower side cannot be moved down any further.

 

Advantages of Design 3
Since thickness of protrusion at inlet does not need to be reduced due to space limitations, thickness can be enlarged to correct designed size. From distribution diagram of solidification layer (Figure 9), it can be seen that at the same time of 14 seconds, plastic entering from gate can still be smoothly filled into various parts of product.

 

Table 1. Comparison of three designs’ shrinkage
How to build your own Know How? (About excessive shrinkage of product volume)
Step 1
Check gating system to confirm that there is no premature solidification of plastic during process of passing through runner and gate to entering product.
Step 2
Check product thickness distribution and confirm from filling short shot diagram and solidification layer distribution diagram that product can transmit pressure to each end during pressure holding stage.