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Holding pressure use when separated from Packing
Definition: To maintain under control a pressure or variable pressures, this in reference to a packing /holding phase of filling a mold within the injection molding cycle function.
Holding pressure is applied to the screw after packing has reached a position on the screw. Understand that on some presses, processes there may only be one total time for pack /hold but there is also possible multiple positions / time functions for this segment in the area of pack/hold.
It is critical that all understand the function of the press operation in regards to timers, pressures and what the screens are telling us. Going from one manufacture to another one must understand the difference in set up and function of the screen. An example is that on injection forward time on one manufacture it is for total forward time of the screw for all functions. On another manufacture the injection fill portion has separate timer, pack has its own timer and hold has its own timer, thus all must be added up for the total injection forward phase.
Applying the hold pressure one is looking at maintaining the position of the screw from where pack stopped. We are not necessary filling or packing material into the cavity but only holding the material under pressure till the gate freezes off. This is best realized by watching the position of the screw when switching to hold. Given that the material is under pressure one may notice that the screw bounces back, or if one has an indicator the position of the screw changes to a larger number. (This is in reference to the fact that when the screw bottoms out the position is zero.)
If we are holding the material into the cavity under pressure while waiting for the gate to freeze off is the screw going to move forward, yes it is possible as the material shrinks and cools the holding pressure may push more material into the cavity to make up for this.
What makes for a good holding pressure set? When used in conjunction with a packing pressure one is processing for:
- shrinkage calculates out to proper allowances for wall/mold temperature
- part weight is consistent, shot to shot to shot
It is important that holding pressure is held long enough for the gate to freeze off. Once the gate is frozen than no material may be packed into or escape from the part itself. It is further critical that minimum holding pressure be applied which can achieve the above mention characteristics above. Though part design may determine how much pressure is used, (thin wall/thick wall) excessive pressure can tend to increase stress in a part, leading to deformities, part function issues and other issues in the use of the part. It is this reason that minimum pressure be sought for the holding pressure, though as stated part design and gate design may predetermine how much pressure needs to be used and for what period of time.
In review of the series of tips on filling the importance can be explained below:
- Fill the part (cavities) as quickly as is cosmetically possible.
- The fill should be as uniform as possible to 95% full minimum.
- Packing pressure should be to pack the part out to achieve a full to spec part.
- Holding pressure should be to maintain at minimum the screw position of what pack achieved.
- Holding time should be to a minimum of a gate freeze with a minimum of pressure to achieve this.
- Packing /holding pressure/time can be clarified by examination of the part for weight, and size in regards to repeatability.
While there are always exceptions to the rules, especially if one is molding thin walls, the above should assist in molding the majority of parts.
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