Product shown in figure below is a part of a copier, which requires high dimensional accuracy. Plastic of PPE+PS+40%GF is used for hot runner molding. Product structure and pouring position have been determined. Customer hopes to shorten the entire cycle time by adjusting cooling water channel or cooling conditions, so mold flow analysis is used to verify whether it is feasible.
Because there is no GEPPE+PS+40%GF plastic used by customer in Mold flow material database, PPE+PS+40%GF plastic of AsahiKasei Corporation with similar physical properties is used as a substitute in analysis. Numerical value will be different from actual test mold, but trend is consistent. This report analyzes and compares several solutions, among which original is original design solution of customer, and Revised is our improvement solution based on Moldflow.

 

Introduction to Plastic Materials

 

Product model introduction
Product is about 303*189*58mm in length, width and height. Most of thickness is relatively uniform, with a basic thickness of 2.6mm. However, some areas are thicker, reaching more than 6.0mm (as shown in the left picture), which may cause serious shrinkage problems; some large areas are thinner, only about 0.9mm (as shown in the right picture), which may cause serious stagnation problems.

 

Analysis model introduction
For this thin shell product, you can use Fusion (double-layer mesh) or Midplane (neutral layer mesh) in Moldflow finite element analysis mesh for analysis, and analysis results are consistent. The former takes a double-layer mesh of outer shell, and outer shape is same as 3D model. Pre-processing time is shorter, but number of meshes is more than twice that of the latter, and analysis time is longer: the latter takes a single-layer mesh in the middle, and shape of local area needs to be equivalently processed. Pre-processing time is longer, but analysis time is shorter. This analysis uses the latter.

 

Original gating system design

 

Original 1 is a three-plate mold with one cavity per mold, using an external hot runner system and two-point gating (gate diameter is 3.0mm).
Original cooling system design

 

Original plan designed a total of ten water channels, including six on female mold side and four on male mold side. Blue pipes are ∅10mm straight water channels, and yellow pipes are ∅16mm baffle water channels.
Basic molding conditions of original scheme
Injection machine settings
Machine maximum clamp force: 350 tonne
Maximum pressure:216.00 MPa
Maximum injection speed:422.52 cm^3/s
Screw diameter:58.00 mm
Filling conditions:
Mold temperature :70.00 deg.C
Melt temperature(Hot Runner):280.00deg.C
Injection time :2.0 sec
Part volume to be filled :255.8 cm^3
Part Weight(Solid) :349g
Total projected area :390.4 cm^2
Cooling conditions:
Coolant Temperature(Cavity)60 deg.C
Coolant Temperature(Core)60 deg.C
Pressure holding curve:

 

Original solution analysis adopted molding conditions similar to actual test mold (HP is about 190MPa), and molding cycle is 43s (including 11.5s mold opening time).

Original solution analysis results

 

Following analysis includes the more important results of cooling, filling, pressure holding, and warpage analysis.
Cooling water temperature change

 

As can be seen from figure, water temperature rises slightly (inlet and outlet water temperature difference is within two degrees), and length design of cooling water channel can meet cooling requirements. When molding, do not connect water channels in series for sake of convenience, otherwise water channel will be too long and water temperature will continue to rise, reducing cooling effect.
Surface temperature distribution of male and female molds

 

Left picture shows surface temperature distribution of male mold side of product, and right picture shows surface temperature distribution of female mold side of product. It can be seen from figure that surface temperature distribution is not uniform and cooling effect is not ideal.
Temperature difference between male and female mold sides

 

As can be seen from figure, temperature difference between male and female mold sides is large, which will cause uneven shrinkage of male and female mold sides of product, lead to warping and deformation problems.
Time required for product solidification

 

Above two figures show time required from start of cycle to complete solidification of product. When mold is opened, circled areas have not yet solidified (as shown in the right figure, most areas can solidify within 16 seconds), and the longest solidification time is about 80 seconds (which is also the thickest area on product), so serious shrinkage will inevitably occur.
Filling time

 

Filling time is about 2.2 seconds, and filling flow is not balanced. Arrow indicates last filling area. Thin rib at circled area has serious stagnation, resulting in short shots. This is attributed to the fact that rib is too thin (only about 0.9mm) and gate is too close to the rib. When plastic flows to this area, it encounters great resistance and stagnate, solidifies quickly. In actual test mold, plastic with GEPPE+PS+40%GF may barely fill mold, but molding window is very narrow and short shots may still be possible. This should be taken seriously.
Filling flow process

 

Fiber orientation distribution

 

This figure shows fiber orientation from beginning of cycle to mold opening. From figure, it can be seen that red line distribution area represents a more serious fiber orientation, while blue line distribution area represents a weaker fiber orientation.
Flow front temperature distribution

 

Above two figures show distribution of flow front temperature during filling process. Most areas are relatively uniform, all around 280 degrees. However, due to severe stagnation of plastic in circled area (i.e., thin rib of about 0.9 mm), flow front temperature drops sharply to 145 degrees, which is close to solidification temperature, preventing subsequent plastic from entering area, resulting in short shots.
Temperature change during cycle

 

Filling pressure

 

Left picture shows injection pressure required for filling/holding pressure switching. Pressure is relatively high, reaching 104MPa, but this pressure is safe for 350t injection molding machine used.
Suture position & wind distribution

 

Red line in the left picture indicates position of suture, and circled suture is more obvious, but it may not be important for this product. Pink circle in the right picture indicates possible wind position. Pay attention to setting up relevant mechanisms to exclude it, especially marked position.
Cooling and solidification process

 

These six figures show cooling and solidification process of product. Red area represents area that solidifies first. Generally, the thinnest area solidifies first and the thickest area solidifies last. It can be seen from figure that thicker area solidifies first and cuts off pressure holding circuit, so that thicker area cannot be effectively pressure-held.

 

Cooling and solidification of product from beginning of cycle to opening of mold. Red area indicates area that solidifies first, please pay attention to circled position. Generally speaking, solidification rate of product needs to reach more than 80% before mold can be opened and ejected, but in this solution, solidification rate of the thickest area when mold is opened is only 50%.
Volume shrinkage and sag index

 

Left picture shows volume shrinkage distribution of product. Most areas shrink evenly, while red area shrinks more. Sag index on the right picture shows severity of shrinkage and sag relative to thickness of product. It can be seen that marked part is severely sag (at least 0.7mm).
Injection pressure and clamping force variation curve

 

Left figure is variation curve of injection pressure over time during the entire molding cycle, and right figure is variation curve of clamping force over time. Maximum pressure is 104MPa, and maximum clamping force is 193ton. 350t molding machine can fully meet requirements.
Warping deformation (magnified 20 times)

 

Total deformation in X&Y&Z directions

 

Warping deformation directions in X&Y&Z directions are shown by arrows in figure, and deformation is not large.
Uneven cooling factors that cause warping

 

Uneven cooling factors have little effect on warping deformation, and deformation direction is shown by arrow in figure.
Uneven shrinkage factors that cause warping

 

Uneven shrinkage factors have a greater impact on warping deformation (uniform deformation in Y direction) and are main factors causing warping deformation. Deformation direction is shown by arrow in figure.
Molecular orientation factors that cause warping

 

Molecular orientation factors have little effect on warping deformation, and deformation direction is shown by arrow in figure.

From analysis results, we know that:
Temperature distribution on the surface of mold cavity is not uniform, and cooling effect is not ideal
Molding requirements of product can be met by using a 350t molding machine.
There is a thin rib with severe stagnation, resulting in short shot of product. This is attributed to the fact that rib is too thin (only about 0.9mm), and gate is too close to rib. When plastic flows to this place, it encounters great resistance and stagnates. Stagnation time is too long, temperature drops sharply and solidifies quickly. In actual test mold, plastic of GEPPE+PS+40%GF may barely fill it, but molding window is very narrow, and short shot may still occur. This should be paid great attention to.
Local area is too thick, surrounding area solidifies first and cuts off pressure holding circuit, resulting in ineffective pressure holding and severe shrinkage and depression.
Warpage deformation is not large, and shrinkage unevenness factor is main factor.

CAE mold flow analysis will first be improved on the basis of Original. Main improvement direction is to overcome stagnation and short shot problems, shorten cooling time. Improvement plan is Revised1.

 

Revised1 Cooling System Design

 

Since gate on one side is too close to thin rib where stagnation and short shot occur, product structure has been determined and cannot be thickened. Therefore, another improvement method is to move gate away from this area to avoid plastic stagnation time for too long. Revised1 move gate close to thin rib down 10mm (due to limitation of mold structure and flow balance, it cannot be moved down too much), and position of the other gate remains unchanged.

 

Water channel design is basically same as Original1, except that water channel shown in frame is shifted in direction of arrow. Left picture shows gate shifted 5mm; right picture shows gate shifted 38mm, away from thin ribs where stagnation occurs.
Revised1 Basic molding conditions
Injection machine settings
Machine maximum clamp force: 350 tonne
Maximum pressure:216.00 MPa
Maximum injection speed:422.52 cm^3/s
Screw diameter:58.00 mm
Filling conditions (increase material temperature and flow rate):
Mold temperature:70.00 deg.C
Melt temperature(Hot Runner):290.00deg.
Injection time:1.8 sec
Part volume to be filled:255.8 cm^3
Part Weight(Solid):349g
Total projected area:390.4 cm^2
Cooling conditions (lower water temperature):
Coolant Temperature(Cavity)25 deg.C
Coolant Temperature(Core)25 deg.C
Pressure holding curve:

 

Use a pressure holding curve in which pressure gradually decreases over time. Pressure holding time is still 4s and molding cycle is 41S.

Cooling water temperature change

 

Since cooling water circuit does not change much, using normal temperature water, water temperature rise is also small (inlet and outlet water temperature difference is within two degrees).
Temperature distribution on male and female mold sides

 

As can be seen from figure, surface temperature distribution is not uniform and cooling effect is not ideal.
Temperature difference between male and female mold side surfaces

 

As can be seen from figure, temperature difference between male and female mold side surfaces becomes larger, and warping deformation may become larger.
Time required for product to solidify

 

Time required for product to solidify has not been significantly shortened, and the thickest area of product will inevitably shrink severely.
Filling time

 

Filling time is about 1.9 seconds, and there is no obvious improvement in filling flow. Thin ribs at circled position still have serious stagnation, which still leads to short shots. It can be seen that moving gate 10mm away and increasing material temperature have little effect.
Filling flow process

 

Fiber distribution

 

Red line distribution area represents a more serious fiber alignment, while blue line distribution area represents a weaker fiber alignment. Alignment distribution has slightly changed.
Flow front temperature distribution

 

Most areas are relatively uniform, all around 290 degrees. Plastic in circled area still has severe stagnation, and flow front temperature drops sharply to 145 degrees, close to solidification temperature, which prevents subsequent plastics from entering area, resulting in short shots.
Temperature change during cycle

 

Filling pressure

 

Injection pressure required for filling/holding pressure switching is slightly lower, at 101MPa. During holding pressure stage, it is hoped that pressure will be increased to enhance holding pressure effect.
Seam position & bag wind distribution

 

Seam and bag wind distribution are similar to Original1.
Cooling and solidification process

 

Pressure holding effect is similar to that of Original 1. Thicker area solidifies first and cuts off pressure holding circuit, resulting in thicker area not being effectively pressure-held.

 

Volume shrinkage and sag index

 

Volume shrinkage has not improved significantly, and shrinkage and sag of marked parts are still very serious.
Injection pressure and clamping force change curve

 

Maximum pressure is 120MPa, maximum clamping force is 220ton, and a 350t molding machine can also meet requirements.
Warping deformation (magnified 20 times)

 

Total deformation in X&Y&Z directions

 

The total warping deformation is slightly larger than Original1.
Uneven cooling factors that cause warping

 

Uneven cooling factors also have little effect on warping deformation, and deformation direction is shown in figure.
Uneven shrinkage factors that cause warping

 

Uneven shrinkage factors still have a great impact on warping deformation and are still main factor causing warping deformation.
Molecular orientation factors that cause warping

 

Molecular orientation factors also have little effect on warping deformation, and deformation direction is shown in figure.

From analysis results, we know that:
The overall cooling effect has not been significantly improved by lowering water temperature, and temperature difference has increased.
Filling flow has not been significantly improved. Thin rib of about 0.9mm still has serious stagnation and short shot. It can be seen that moving gate 10mm away and increasing material temperature and shooting speed have little effect. Fundamental reason is that rib is too thin, plastic reaches there too quickly, and selected material has a large amount of corrugated fiber, which has greater flow resistance.
Local thicker area still cannot be effectively maintained and has serious shrinkage and depression.
Warpage deformation has increased, and shrinkage unevenness factor is still main factor.

Based on Original1 or Revised1, we still use two-point hot runner for pouring, make partial adjustments to water channel and cooling conditions. After multiple analyses, it is still difficult to overcome stagnant flow and short shot problems, shorten the overall cooling time. Therefore, we try to use one-point hot runner for pouring, and improvement solution is Revised2.

 

Revised2 gating system design

 

Revised2 uses a hot runner for gating, with gate located on the center line of mold, 30mm away from mold center. In this case, mold structure and hot runner form may need to be changed accordingly.
Revised2 Cooling system design

 

There are eleven water channels in total. Local cooling water channels were adjusted based on mold structure and hot runner. A 10mm baffle water channel was added near thicker area (mother mold side) where severe shrinkage occurred (connected straight-through pipe is 8mm), as shown in upper left picture. Male mold water channel under thin rib where severe stagnation occurred was removed, as shown in upper right picture.
Revised2 Basic molding conditions
Injection machine settings
Machine maximum clamp force: 350 tonne
Maximum pressure:216.00 MPa
Maximum injection speed:422.52 cm^3/s
Screw diameter:58.00 mm
Filling conditions:
Mold temperature :70.00 deg.C
Melt temperature(Hot Runner):280.00deg.C
Injection time :2.0 sec
Part volume to be filled :255.8 cm^3
Part Weight(Solid) :349g
Total projected area :390.4 cm^2
Cooling conditions:
Coolant Temperature(Cavity)60 deg.C
Coolant Temperature(Core) 60 deg.C
Pressure holding curve:

 

Still use one section of pressure holding, pressure holding time is still 4s, but pressure holding pressure is 120% of injection pressure, and molding cycle is still 43s.

Cooling water temperature change

 

Water temperature rise is small (inlet and outlet water temperature difference is within one degree), and length design of cooling water circuit can also meet cooling requirements.
Surface temperature distribution of male and female molds

 

As can be seen from figure, surface temperature distribution is still not uniform and cooling effect is still not ideal.
Temperature difference between male and female mold sides

 

As can be seen from figure, surface temperature difference in some areas has been reduced, which is beneficial for uniform shrinkage of product and reduction of warpage.
Time required for product solidification

 

Solidification time required for product is only slightly shortened, and the thickest area on product will also shrink severely. It can be seen that added baffle water channel has very limited improvement in cooling effect of this area.
Filling time

 

Filling time is about 2.1 seconds, and filling flow has been significantly improved. Thin ribs in circle still have a slight stagnation phenomenon, but because gate is far away from this area, this area can become area close to last filling, and plastic stagnation time is short, so it can be filled by increasing pressure a little in final filling stage.
Filling flow process

 

Wave fiber distribution

 

Since gate is reduced from two to one, flow pattern has changed greatly, so orientation distribution has changed significantly.
Temperature distribution of flow wave front

 

Most areas are relatively uniform, all around 280 degrees. Plastic in circled area still has a slight stagnation phenomenon, and temperature of flow wave front drops rapidly, but it does not have time to drop to solidification temperature, and subsequent plastic fills area under greater pressure.
Temperature change during cycle

 

Filling pressure

 

Injection pressure required for filling/holding pressure switching is not higher than that of original two-point pouring scheme, and is still around 102MPa. After filling/holding pressure switching, pressure is increased to ensure filling of thin ribs and subsequent holding pressure effect.
Suture line position & wind distribution

 

Circled suture line is more obvious, and the most obvious suture line in the middle has been reduced compared to original plan. Wind at marked location still needs to be eliminated by setting up relevant mechanisms.
Cooling and solidification process

 

Maintaining pressure in thicker areas is still a problem. Although baffle water channels are added nearby, cooling effect is very limited and it is difficult to significantly shorten solidification time.

 

Please pay attention to circled position to see cooling and solidification of product from beginning of cycle to opening of mold.
Volume shrinkage and sink depth

 

Left picture shows that shrinkage is the largest at gate, which can be improved by extending holding time, but it is difficult to improve marked part, and shrinkage sink is still very serious. Right picture shows sink depth at rib root, and sink at marked rib root is the most serious.
Injection pressure and clamping force change curve

 

Maximum pressure is 122MPa, maximum clamping force in filling stage is 220ton, and maximum clamping force in holding stage is 300ton, which has reached 85% of maximum clamping force of 350t molding machine, which may be dangerous for machine. However, clamping force can be reduced by adjusting holding curve.
Warping deformation (magnified 20 times)

 

Total deformation in X&Y&Z directions

 

The total warping deformation is not large, slightly smaller than Original1.
Uneven cooling factors that cause warping

 

Uneven cooling factors also have little effect on warping deformation, and deformation direction is shown in figure.
Uneven shrinkage factors that cause warping

 

Uneven shrinkage factors still have a great impact on warping deformation and are still main factor causing warping deformation.
Molecular orientation factors that cause warping

 

Molecular orientation factors also have little effect on warping deformation, and deformation direction is shown in figure.

From analysis results, it is known that after changing to one-point pouring:
Filling flow has been significantly improved. Although thin rib is thin, there is still a slight stagnation phenomenon. However, because gate is far away from this area, this area can become area close to final filling, and plastic stagnation time is short. In final filling stage, it can be filled by increasing pressure a little. However, molding window is still narrow, and short shots may still occur if control is not good. Therefore, fundamental solution to this problem is to thicken this thin rib as much as possible.
Although baffle water channel is added near local thick area, due to limitation of mold structure, improvement of cooling effect in this area is very limited, and effective pressure holding is still not obtained, resulting in serious shrinkage depression. For this product, shrinkage depression may not be very important, but these thick areas require a long cooling time, making it difficult to shorten the entire molding cycle.
Amount of warping deformation is not large, and shrinkage unevenness factor is still main factor.
Injection pressure is not greater than original solution, and clamping force increases a lot in pressure holding stage, but it should be possible to reduce clamping force by adjusting pressure holding curve, and it can still be molded with a 350t injection molding machine.
Compared with original solution, one less hot runner can be used, which can reduce production costs, and product quality should also meet customer requirements. It is recommended to consider using Revised 2 solution.
Above conclusions and suggestions are for reference only, and customers need to make trade-offs based on actual conditions.