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Is SMED Realistic?

Although true SMED (Single Minute Exchange of Dies) is not likely for all molders, significant reductions in mold change time using the same principles are easily attainable.

Question: Is SMED realistic?
My Response: The underlying benefits to the SMED concept is the preparation. Essentially everything necessary to change a mold is already there and ready and easily accessible. This way the mold can be shut down when the run is done, the changeover can take place, and the new mold is up and running in the shortest time possible. Essentially, if you have to waste time looking for paperwork, finding a hose, replacing a connector, locating a thermolator, checking water lines, verifying hot runner systems, etc. you cannot attain an optimal mold change time. All the work is done before the machine is shut down so you attain the maximum efficiency when the machine is not making parts (ie. the company is losing money).
-Andy

Converting Injection Speed…

This is a good starting point, but the most important factor is to know the 1st Stage Fill Time and 1st Stage Short Shot Weight. If these are matched, then the outputs match which is critical.

Question: Say i have a process with .94sec fill time and 36mm
screw. When i change to a 50mm screw, what is the formula to
match the same fill/shear rate/?

My Response: This is based off of a ratio between the effective surface are of the screw. The basic formula would be as follows:

Machine B Speed = (Machine A Speed) * [(Machine A Radius)^2 / (Machine B Radius)^2]
For your example:
Machine B Speed = (Machine A Speed) * [(18)^2 / (25)^2] = (Machine A Speed) * 0.518
Additional Thoughts: It is typically easier to match the fill time and short shot weight from the previous process. This will not be exaclty correct because it will not compensate for aspects such as decompression and check ring wear.
-Andy

Correcting Tiger Striping…

When these are in the mold surface, small changes such as reduced melt temperature, slower injection speed, and reduced packing pressure tend to help. These reduce the polymer’s ability to duplicate the cavity detail will improve the overall appearance, but do not fix the underlying problems. 
Question: Are Tiger Stripes Solvable? All the processing changes such as increasing melt , mold temperature , adjusting velocity did not yield result to eliminate however improved the condition. 
My Response: ‘Tiger Stripes’ is a visual condition on the final part related to poor surface finish. Since the location of material-related defects are not predictable and consistent form shot to shot, this is not likely to be caused by the material since material coloring defects are more random and not consistent in appearance. Some high viscosity and low gloss grades of material may show less cavity detail, and therefor hide the problem, but they will not ‘fix’ the problem. Three of the most common causes are listed below:
1) Poor Sandblasting – The texture was not well applied to the mold. In this case, the striping is likely parallel with the edge of the mold base. This is because the employee would have passed their hand  back and forth when sandblasting, causing lines in the finish if they hold the blaster too close or do not spray evenly. If this is the case, the mold surface needs to be repaired and returned to its original condition
2) Poor Venting (New Problem) – When air cannot escape from the mold it becomes trapped against the mold surface. These tend to occur perpendicular to the direction of flow. When this happens, the polymer does not properly adhere to the mold surface causing it to ‘skip’, giving the cavity a rippled appearance. If this is a new problem, improving the venting, reducing the tonnage, and thoroughly cleaning the mold surface with safe solvents should significantly improve the situation. 
3) Poor Venting (Old Problem) – Unfortunately, if this has been occurring for a long period of time, the mold surface will actually become damaged due to the slipping polymer and buildup of corrosive volatiles on the mold surface. If this is the case, the mold surface needs to be repaired and returned to its original condition and much more venting needs to be added.
-Andy

Weld Line Strength…

Weld line strength is primarily influenced by the amount of interaction that can be caused at the weld line location. Gas traps and low pressure at the weld line location are some of the primary causes for low weld line strength.
Concern: I am having some issues with one of our newer molds. We are using a PPE 20% glass.  We are getting weld lines in around screw wholes. We have added another gate to move the weld line over, which it did not, and still having some issues with the part cracking. So part of the problem we believe is the mold, part processing, and part the material. 
My Response: Although there are many potential problems, I will address the strength of the weld line here.
If you have gas trapped at the weld line location, it displaces the polymer, reducing the interaction. This typically causes a notch in the surface of the part making it a stress concentration and potential fracture point. If you can feel the weld line with your fingertip or a knife edge, you likely have a gas trap issue.
If you have too little pressure at the weld line location, the polymer chains will not interact, resulting in a weaker weld line. There are many ways to lose pressure but the most common reasons are 1) a slow injection speed will result in a high pressure loss during injection 2) Injecting too much material during 1st stage will cause a spike in pressure causing stress in the part, but the screw will bounce back resulting in a less effective packing phase. This is a common problem and may be contributing to the cracking issues you are encountering.
-Andy

High Temp Machinery…

Just because a machine is capable of processing at high temperatures, does not mean it is optimized for use at these temperatures. A machine which spends most of it’s time at high temperatures should be purchased and designed for that intent. A failure to do so will result in potential inefficiencies and premature equipment failure.

Situation: Recently I was working with a company who processes a large number of high-temperature materials. The manufacturer states the machine can operate at high temperatures without significant modifications, but it is not operating optimally. The melt temperature is consistently 30-50 degrees C below set point and the locking check ring is expanding more than the barrel causing a squealing to occur.
My Recommendations: If you occasionally run high temp materials, then a stock machine may be OK. If you are intending to process mostly high temp materials, especially at high speeds, you should consider a machine deigned for that application. Any thermal expansion differentials should be considered with respect to the materials & tolerances used for the screw, check ring, and barrel. The machine should also be fitted with heater bands designed for high temperature use to heat the material quickly and efficiently. There are many other considerations such as proper heat shield design, adequate nozzle heaters, and a pre-heating capable hopper which can improve the overall efficiency of the machine.
-Andy

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