Weld lines is something you tend to get in an injection molded product with multiple flows. Question is 1) Is the weld line strong 2) Is the appearance acceptable. If there is only one/ two weld lines which can not be accepted, in addition to what is already covered, I would add sequential valve gating. Open the valve gate just as it is about to form a weld. I have done many projects simulating on Moldflow by changing thickness (flow leaders/ deflectors) and converting to a meld line which is less severe. The results can not be guaranteed as the flow pattern can change with the viscosity of material and if product design has thickness variations. We have observed that in Moldflow, the weld lines are seen but in product, they are not noticed. This happens if the inner layer material shear heating is high which heats the surface material.

The higher the melt skin temperature and injection mold surface temperature at time of contact with melt and higher the pressure, the better the appearance and strength retention. Weld-lines are an inevitable consequence of any tool feature that splits flow before it rejoins, just as night follows day. Their visibility and physical properties versus the rest of the molding will vary depending on the many variables mentioned. Avoid mold release spray like the plague, unless you want dreadful loss of strength. High aspect ratio metallic and pearlescent pigments are bound to accentuate appearance. Post-machining a real option, but adds cost, creates material swarf or cut blanks that can (with good control) be re-fed into the material feedstock (at a slight energy penalty, of course). No magic solution, just common-sense application of one or other of the approaches mentioned.
The concept of injection molding is to fill plastic materials to the cavity with the least pressure. Because of the compressible nature of a polymer melt there will be a pressure difference between the last point to fill and the gate with a matching difference in density and therefore shrinkage. If you can fill the cavity with a low pressure then the pressure difference from lptf and gate is smallest. To fill the cavity with the least pressure means that the apparent viscosity of the melt must be as low as possible and this is a function of temperature and shear rate (which for any single injection mold can be related to speed of filling). Ultimate filling speed is frequently compromised by poor venting resulting in gas burns or 'Dieseling'. Often the filling speed is reduced (wrongly) in preference to pulling the injection mold and improving the venting - but it is easier to do!
Shot Wt. 34 lbs.
Cycle Time 65 sec.
Resin Polypro
spec heat .87 Btu / (lb. F) See Chart
spec heat H2O 1.003 Btu / (lb/ F) @ 55 F
Ejected Part Temp. 120 F
Melt Temperature 440 F
Delta T 320 F
Temp. Rise in H2O 5 F (This is max allowable temp rise 2-3 F is suggested)
One Gallon H2O 8.33 lbs.
One Ton Chiller 12,000 Btu

Heat Load (Resin Thru put x Spec Heat (Resin) x Delta T = lbs. / Hr. of coolant required
(Spec. Heat (Coolant) x temp rise of coolant)
Thermal management approach does require time, effort and a good understanding of thermodynamics that is well beyond most tooling engineers. As with mold flow analysis, the new thermal approaches will take time to learn. The injection mold is basically a heat exchanger. However, it is a unique and specialized heat machine that requires attention if optimum performance is to be reached. I have seen a lot of injection molds done over the past three decades, from small medical to automotive body components. The approach is the same but different, that is applying engineering over design. I am always surprised at the low level of engineering done in regards to injection molds, even down to basics like structural strength (gee my tool is flashing). Transient heat flow, a CFD method, is becoming reliable. There are learnings from this technique and others that will go a long way to accomplishing solid thermal management.
The injection mold designing stage is the best time to place water in an injection mold but many of us have a shop full of molds that were designed by others and usually for slower cycles than expected today. Another part of the water flow equation as in the whole injection molding process is to find the water flow, injection mold temperature or BTU removal that works for your part. When part quality changes that should be one of the points to check before making adjustments to the process. The dynamics of water flow always wanting to follow the path of least resistance causes different water flow in the plant as injection molds are shut down, started up or changed. If the capacity of your cooling system changes so does the ability to remove heat or in some cases remove too much heat from the injection mold.

One of the things I find most frustrating is that when building multi cavity molds with independent circuits to each core/cavity, I find that the injection mold techs have jumped all the circuits together with a single in and out. When questioned they'll say... but I only have 2 in and 2 out circuits on the machine.
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