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Importance of Cycle time in Injection Moulding

Mr. Sanat Shah, Plastics Expert

The difference between the most efficient injection molders and the least efficient molders is the number of good parts produced and the quantity of plastic used. Not getting into production quickly, interruptions to production and wasted plastic all cost money.Productivity is the name of the game.

1). The very important factor which influences the productivity in injection moulding is “Cycle Time”. Cycle Time is the time required by an injection molding system to mold a part and return to its original position/state and Cycle is complete, repeating sequence of operations for injection molding a part.

2). Cycle time can impact profitability of the molding job’s …

Injection molding cycle time is made up of three elements

  • Fill, pack and hold time
  • Cooling time
  • Mold open time

Among these three factors, cooling time is the most important factor. Cooling the plastic in the mold is typically 70% to 80% of the total cycle time. How quickly the mold can be opened depends on the mold temperature, the plastic melt temperature, the plastic part wall thickness, the part shape and the type of plastic material being used. The mold can be opened and the plastic parts can be ejected when the parts are cool enough to hold their shape (i.e. no deformation occurs).

  • The two other elements of the injection molding cycle that can reduce cycle time are fill time and mold open time. Most injection molds should be filled as quickly as possible. There are some exceptions as in case of thick cross section of the articles & heavy variation in wall thickness
  • The injection molder should try to reduce fill time to the shortest that will make parts quality wise acceptable.

The wall thickness of a part will directly affect the cycle time; thicker walls create longer cycle times while thinner walls create shorter cycles that’s why thin wall molding has cycle times in the 2 to 5 seconds range. The fact is, the part designer needs to make the wall thickness as thin and uniform as possible while still maintaining adequate strength in line with the application. The height of a part also effects cycle time. The higher the part the further the machine platen has to open and the more time required to eject it off the mould core. Minimizing the mold open time is another way to reduce cycle time. If the molding machine is running in manual or semi-automatic mode, the mold open time should be minimized. Some tips are:

  • Reduce the platen opening to the minimum required distance
  • Reduce the ejection stroke to required distance
  • Reduce number of storks, if possible – make use of robots for ejection / picking-up of articles from machine

But enough time must be allowed for the operator to remove the parts and inspect the cavities and the mold surface. If a robot or a part picker is used, the mold open time must be long enough for the functions of that equipment to finish.
So how does longer cycle time affect profits on a particular molding job?
Let’s take the very practical example. The mold setup person got the machine and mold running and producing quality parts at a cycle time of 32 seconds. But, it can be modified to run at 30 seconds for one or more of the reasons cited above. May be it can run at 29 or 28 seconds also.
Set the molding job up to minimum cycle time. But it can be further optimized to gain more 2 seconds. What’s the cost of this extra 2 seconds?
The molding job should have been completed in 15,000 hours of production, but the 2 extra seconds on a 30 second cycle requires an additional 1,000 hours of production. This extra 1,000 hours of production took Rs 100,000 out of the expected profit (assumes Rs 100/hr machine hour rate)
In other words, if the cycle time were reduced to 28 seconds the profit increases by Rs. 100,000.
If the molding personnel on the production floor were aware of the effects of this longer cycle time on the profitability of the job, they probably would have taken the time to further optimize the cycle.
So molding personnel needs both - technical training on how to reduce cycle time and the awareness of cycle time economics.