Isn’t there a need to keep the molds heated for the plastic to be able to reach the end of the mold before solidifying, we are having problems with that, and we just wanted to make sure if That was the problem
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This can happen if you have thin sections in the mould and the plastic cools too much before it can reach the extremities. Pre-heating is one way to avoid this, but greatly increases the time needed to mould each part as you have to repeatedly heat and then cool the mould to extract the part, so is usually a last resort unless you only need to mould one item at a time.
Other things that might help include: Putting the injection point in a different place on the mould, heating the plastic to a higher temperature before injecting, or using a different type of plastic. If you are using HDPE and having problems you might get better results with polypropylene or polystyrene.