February 2011 Archives

In the world of design and fabrication of molds for plastic injection molding, it can be said that standardization spread relatively faster than other fields of machine design and fabrication.
Misumi has had a role to play in this matter , and we would like to discuss here again the merits and demerits of standardization of design and fabrication by making quantitative comparisons.

Merits of Mold Design and Fabrication

1. The design time can be made short

Because of this, the design delivery time becomes short, and even the design cost gets reduced.

2. It is possible to shorten the machining time, the finishing and adjustment times

Because of this, the component preparation time gets shortened, and even the production cost gets reduced.

3. It is not necessary to increase the number of machines and equipment

Since it is possible to depend on purchasing, it is not necessary to increase the number of machines and equipment within the company. Therefore, the fixed expenses (cost of depreciation) need not be increased.

4. The interchangeability during maintenance gets enhanced

By using standard components, since it is possible to unify the specifications of replacement parts, the interchangeability becomes excellent.

5. Maintenance becomes easy in the case of exports to overseas destinations

At the time of maintaining molds exported to overseas destinations, if the same standard component can be procured locally, it becomes possible to carry out maintenance speedily.

6. The cost of order management can be reduced.

The additional work load of ordering and associated accounting is eliminated, and as a result it is possible to reduce the cost.

Demerits of Mold Design and Fabrication

1. It is likely that it becomes difficult to pass on the basic knowledge or mold design to the freshly recruited designers.

2. The basic component production technology within the company may be lost.

In order to counterbalance the demerits of standardization, there are some companies that carry out design seminars within the company or internal production within the company of parts for repairs.
Considering the advantages and disadvantages and the losses and gains, it can be said that further progress in the standardization of design and fabrication of molds is definitely advantageous.

In general, the following are the constituent factors of an injection molding cycle.

1. Mold closing time

This is the time until the mold is closed. The times of opening and closing the mold vary depending on the speeds of the mold opening stroke and the mold closing stroke of the injection molding machine.
Since the mold has considerable mass, if the mold closing speed is made too fast recklessly, it is possible that the mold may break at the time of closing the mold due to the kinetic energy, and hence it is necessary to consider applying brakes.

2. Filling time

The filling time is the time taken until the molten plastic flowing in from the sprue completely fills the inside of the cavity.
The filling time is determined by the injection speed of the injection molding machine (actually, by the injection volume per unit time = injection rate (cm3/sec) which is determined by the movement speed of the injection screw or piston and the diameter of the injection cylinder).
In addition, this is also affected by the viscosity of the molten plastic.

3. Holding time (Packing time)

The holding time is the duration of time during which pressure is being applied after filling the inside of the cavity with molten plastic until the gate part solidifies.

4. Cooling time

The cooling time is the time duration after holding has been completed while the cooling takes place until the molded product, the sprue, and the runners solidify to some extent.
In general, the cooling time tends to be the longest time in a molding cycle.

5. Charging time

The charging time is the time required for measuring the amount of molten plastic for the next injection molding. In general, the charging is carried out simultaneously with the cooling and the charging time is shorter than the cooling time.

6. Mold opening time

This is the time duration until the mold is opened completely. The mold opening and closing times vary depending on the speeds of the mold opening stroke and the mold closing stroke of the injection molding machine.

7. Taking out time

This is the time taken for taking out the molded product from the cavity. This includes the ejection time and the time of operation of the take-out robot.

"Molding cycle" is an index for evaluating the production capacity of injection molding.
A molding cycle is the unit of time from starting one injection molding operation until it is ended.

In more concrete terms, a molding cycle consists of the following.

Molding cycle (seconds / shot)　＝　Mold closing time
　　　　　　　　　　　　　　　　　　＋Filling time
　　　　　　　　　　　　　　　　　　＋Holding (packing) time
　　　　　　　　　　　　　　　　　　＋Cooling time (including the charging time)
　　　　　　　　　　　　　　　　　　＋Mold opening time
　　　　　　　　　　　　　　　　　　＋Product taking out time

As the molding cycle becomes shorter, it becomes possible to increase the quantity of production of the molded product per unit time.

Example:

Consider that there is a mold whose molding cycle at present is 10 seconds. By modifying the cooling circuit of the mold, the cooling efficiency was increased, the cooling time was reduced, and as a result, it was possible to make the molding cycle 9 seconds.
What kinds of factors are involved in this case?

Example Answer:

Since the molding cycle was reduced from 10 seconds to 9 seconds, it became shorter by 1 second.
Although it is merely 1 second, if we think of it as a rate of change, it becomes -

Rate of change＝（10−9）seconds / 10 seconds ×100
　　　　　＝10％

And hence the productivity is improved by 10%.

If a large number of pieces are obtained from a mold such as 12 or 24 pieces per shot, and if there are many similar molds, the productivity of the entire factory will be improved by 10%, and when converted into money in terms of the production cost, it can become a very large amount.
In other words, evaluating the molding cycle directly results in strengthening the profitability of the factory, and not merely reducing by 1 second but leading to an extremely large good effect.
Therefore, in the design of a mold, it is very important to consider at all times by how much the molding cycle can be shortened.
In addition, it is also very important that the mold designer estimates the target molding cycle.
An excellent mold designer always estimates the target molding cycle.