There are some reasons to cause the lack of material in final injection molded parts. First of all it starts with the mold design. Second we have the plastics. Third we have the injection molding machine. Forth we have the people involved orchestrating the whole concert. Fifth we have the machine and sixth we have the environment. Last but not least we have the process itself.
Starting with the injection mold. The mold has to have a design that allows you to produce good plastic parts. Clear for all of us every injection mold has a process window. That means that whatever your molding machine is doing at a certain viscosity the mold needs a certain pressure within a certain time limit in the cavity. This is unique for each and every injection mold like a finger print.
Variations in the viscosity of the melt can lead to an under packed or over packed cavity and with this to scrap. Basically the viscosity of the melt is influenced by the melt flow index of the plastic in process as well as by the heat applied to the plastic on the path to the cavity. A heavy flow (high viscosity) plastic has a pig loss of the pressure over the travel. That results in a low pressure (may be in a too low pressure) in the cavity although you have a high consistent pressure in the barrel. A light flow (low viscosity) plastic has a smaller loss of pressure over the travel. That results in a higher pressure in the cavity what can be well above the sharp limit between short shot and packed out part.
So first advice: increase the melt and mold temperature (Within the specs.) The approximate leverage Melt/Mold temperature to viscosity change is 1°Melt temp. = 7° Mold temp. Consider your injection molding machine has to stay in thermal balance. Cold air rush will destroy the thermal balance and result in changed viscosity although your molding machine does not show you that. An indication for that is given when you start your injection molding machine you have to start with a higher melt temperature and decrease step by step for about an hour for the same quality result.
Decoupled molding is a good idea. Inject the plastic with the max speed possible and ramp down the speed when the cavities are nearly filled. Switch over by 99% filling (without holding pressure the screw should stay). Apply a holding pressure profile. Consider that the first pressure step has to be in a way that your molding machine is not pulling back the screw. Step down the holding pressure following the natural pressure drop in the cavity. Have in mind that on hydraulic machines when the change between injection phase and holding pressure phase is not well adjusted the hydraulic pressure drops very low for up to 1.5 seconds depending on the injection molding machine. Within that time the melt flows back into the barrel.
Considering this above will help you to setup a stable process within no time. Using cavity pressure sensors will show you all the described correlations immediately and assist you. Not using cavity pressure sensing there are points in the process you will never be able to find or fine tune. Considering the points at the beginning of this contribution and systematically adjust will make the difference between standing in the bright light or standing in the mud. It’s also a fight with the people involved.