Plastics Technology: December 2010 Archives
A blog reader asked a follow-up question to a previous blog posting...
If I don't level the machine, what will happen?
Failure to properly level a molding machine can result in premature wear, equipment damage, and even clamp failure. Any twist or non-levelness of the clamp system results in non-linear movement of the platen and uneven forces applied to the tie bars, bushings, and parting lines. The most wear points are premature wear of bushings, wear plates, and feet. When processing, you may notice uneven clamping and uneven flash. If there is twist, excessive wear of leader pins and parting line seals may occur... and the molding machine might even 'walk' or move on the plant floor.
For more on leveling, please feel free to read:
This concludes the three-part post regarding a reader's question about machine maintenance...
What is the best method of measuring tie bar strain?
The most common method of measuring this is to follow the procedures of measuring parallelism as discussed in the previous post on measuring platen parallelism. The difference between the parallelism before and after clamping will give you important information regarding the change under strain. For instance, if the platens are parallel before clamping, but they are out of parallelism after clamping, the change indicates the amount of additional strain in a particular tie bar
When possible, you should try to measure the stretch using a ultrasonic strain gauges. These use sensors at each end of the tie bar which measure the actual stretch of the tie bar during mold clamping. This is much more accurate since it can measure the actual stretch of each tie bar.
To continue on a previous post regarding machine maintenance, I will answer the second question...
What is the best method of measuring platen squareness?
The most common method is to use a set of inside micrometers to measure the distance between the platens. Before you do this, you should first remove the mold, clean the platens, and use a straight edge to determine corresponding areas on the stationary and movable platen which are flat. Next, install a mold which covers at least 2/3 of the distance of the tie bars. Lastly, measure the parallelism of the platens (at each corner) when the mold is opened, closed, and under full tonnage. This will provide you with a great detail of information regarding the parallelism of the platens, the linearity of platen movement, and possible tie bar stretch.
The best method of measuring the parallelism is to use a laser system. These systems use a laser mounted perpendicular to the movable platen and a detector on the stationary platen. These systems are connected to a computer which will collect data and analyse the parallelism and linearity of movement during mold closing, and clamping. This data will give you great specific information regarding parallelism, angles, twist, and movement.
This morning, I received a three part question regarding machine maintenance. The first question will be addressed here...
I am now working at a plant where all the machines are all over ten years old and have not been well maintained. What is the best procedure for machine leveling?
The most common method is to place a level on the tie bars as well as across the tie bars to level each side of the machine using the adjustable feet. Many molders neglect to check both the top an bottom tie bars as well as the levelness across the tie bars. When doing this, it is worth while to purchase or rent a laser level specifically for tie bar leveling.
The most effectiveness methods is to use a rotating laser positioned and leveled on the floor. In this method, two sensors are placed on each of the two bottom tie bars to measure the levelness of each corner. In this case, the whole clamp area can be leveled with a high degree of accuracy. After the bottom tie bars are leveled, the top tie bars should be checked in the same manner. Since these systems tend to be expensive, many molders will rent the levels or hire an expert to come in for a couple days to help level the molding machines properly.
Since the machines are unlikely to be properly leveled, it is important to measure this once every three months for the first year to ensure the machines can maintain levelness and are not 'walking'.
For a related post, please feel free to read: Remedy For A Walking Machine
I received a comment regarding a commonly overlooked skill the other day...
Our company has it's own system of part drawings so I do not believe general blueprint training applies to what we do.
Good technical training teaches not only the specifics, but the fundamental information necessary for true understanding.
In the situation above, having a good understanding of concepts such as how the drawing represents the physical part and how dimensions are calculated can be critical skills for good decision making. When this general understanding is combined with specific instruction on how to use in-house documents, the employee learns applicable skills along with additional information critical to good decision making.
A customer asked me this question the other day...
We are using a screw specifically designed for our grade of PP, we have performed the tact temperature study, and have optimized both injection and packing. All these have helped, yet our cycle time is still limited by the screw recovery time, do you have any suggestions?
Once the process and screw have been optimized, the next step you can undertake is to preheat the polymer. Even though this material does not need drying, a simple hot air hopper dryer can be used to preheat the material to reduce he amount of heat necessary to melt the material. The more heat that is added to the material before it enters the barrel, the less energy that is required by the screw and heater bands.
Semi-crystalline polymers benefit most from pre-heating since the polymer will typically endure more preheating due to their sharp melting point.
Always be cautious of your temperature in both the feedthroat and rear zone of the barrel. Since preheating hastens the melting, you may need to repeat the tact temperature study.
After answering some follow-up questions from the last post, I decided to clarify the difference between Dynamic cavity balancing and Static cavity balancing with repsect to either injeciton molding or injection-blow molding.
Dynamic Cavity Balancing
When the mold cavity is filling, the polymer flow front is considered dynamic, or in motion. The balance of two or more mold cavities at this stage primarily relates to the consistency of pressure loss and shear balancing between the mold cavities. This relates to both hot and cold runner systems. The more the cavities are dynamically balanced, the more consistent the process will be in the long term.
Static Cavity Balancing
Once the flow front reaches the end of the mold cavity, it becomes static. At this stage, part weight differentials are primarily based on the variability in pressures and temperatures. This information is important for final part quality, but can be significantly different than the dynamic balance.
I received this question over the weekend...
What is the right procedure for performing a cavity balance test for a blow molding machine with an injection, pack and hold?
Basically, the procedure is similar to that of an injection molding machine. You would turn off the second and third stage, and then compare the shot weight of the different cavities.
To calculate the % variation is as follows:
100% x (maximum weight - minimum weight) ÷ (maximum weight) = % Variation
During an on-site consultation, I was recently asked this question...
When troubleshooting a Decoupled III process for variations in overall part dimensions, what parameters should we investigate.
note: A DIII process has many specific attributes, but the key aspects include a velocity controlled 1st stage fill (90%), 1st to 2nd stage transfer by position, velocity controlled 2nd stage pack, 2nd to 3rd stage transfer on cavity pressure at the gate, and 2rd stage hold by pressure control.
The parameters and process outputs which affect part size in a DIII process are commonly:
- Melt Tempertaure
- Transfer Setpoint
- Packing Speed
- Hold Pressure
- Mold Temperature
Additional ThoughtsOne of the best ways to approach dimensional issues with a DIII process, would be to compare the pack rate and cooling rate to the documented standard and return them to the standard. These are the upward and downward slopes of the in-cavity pressure curves representing the rate of packing and cooling within the cavity.