For those of you in injection molding industry, many of you know an old saying in our industry that we don't buy injection molded parts, we sell machine time. This is not entirely true. In fact, what we sell is estimated machine time. And accuracy of machine time estimates will determine whether your project makes money or loses money.

As with any project in injection molding industry, an injection molding plant must quote a machined part before mold is made, or before an existing mold is received from customer. This requires injection molding plant to accurately estimate cycle time of project product.
Cycle times are estimated even if a mold flow analysis is performed. While mold flow analysis may be able to predict required fill and cool times fairly accurately, it cannot predict how mold, machine, and in many cases operator will work. It also cannot predict craftsmanship on any given machine, especially older ones. Therefore, estimation of cycle time is critical when quoting injection molded parts.

 

If project's annual output is low, underestimating cycle time will not cost factory much. But for projects with very large annual volumes, accuracy of cycle time estimates is critical. In real estate industry, industry slogan is "Location, Location, Location." In injection molding industry, industry slogan should be "cycle time, cycle time, cycle time".

The total cycle time of mold is combined time of 11 steps listed below. When trying to reduce cycle time, these steps need to be considered separately for optimization. Interactions between steps also need to be considered.
1. Mold clamping and locking time.
2. Fill time.
3. Packing and holding time.
4. Screw delay time.
5. Screw plasticizing time.
6. Screw back suction time after rotation.
7. Cooling time.
8. Cooling delay or idle time before mold opening.
9. Mould opening time.
10. Part ejection (and robot capture) time.
11. Return time of ejector mechanism (and robot).

Start discussion by evaluating time it takes to cool part. Cooling time typically accounts for the largest percentage of any mold cycle time. In fact, it can be 80% or more of cycle time. However, cooling is often not adequately considered during part design, mold design and machining stages, especially machining stage. But now that parts are designed and molds are made, how can some optimizations be made to cooling time?
The first thing to look at is demold temperature of part.
While visiting some injection molding plants, I am sometimes surprised to find that demolding temperature of some project products is even lower than temperature of customer's cooling tower. Reason client gave me was that if demolding temperature was too high, accumulated heat from part could cause some internal stress to cause part to deform. Some customers will say that part of runner is still molten and they will stick to part. I've seen cycle times on nylon products get significantly extended because operators complained that product was hot to touch when demolding. It seems that every customer has an excuse to extend their cycle time, but few of those excuses are justified.
For products with different precision requirements, RJG has recommended demolding temperatures for corresponding products for reference.
When it comes to jetting temperature, we're talking about part, not runner. Injection molding plants are not selling runners but products. The best way to check temperature of a molded part is to use an infrared (IR) thermal imaging camera. Laser aiming (pinpointing) infrared thermometers are not as precise, but can check temperature of part immediately after it is ejected.
Image below shows a thermal image of a center-gated part with uniform wall thickness. Gate or origin of material flow is the hottest area on part. Hot spots on part often determine cooling time of process. Anything that can be done to mold to reduce hot spots, such as beryllium copper inserts or 3D printed conformal cooling loops, can save cycle time.

 

Immediately after mold is opened, it is very helpful to take a thermal image of mold (pictured below) with a thermal imaging camera. But die steel is highly reflective, and even adjusting parameters often produces erroneous temperature readings. An aerosol, such as Magnaflux SKD-S2, can be sprayed before measurement, which reduces reflectivity of steel and provides better measurement results. You can also apply some dull-colored tape to specific areas.

 

It is not uncommon for gates and runners to affect cycle time. Problem is usually at intersection of gate, runner, and cold well. Usually this area is the thickest part of material in mold, as shown in picture below, it is easy to cause heat accumulation and form a hot spot.

 

If cooling time is too short, cold well can be separated from runner, as shown in picture below, or gate can be separated from runner. When gate separates from runner, it can remain in cavity when mold is opened, damaging mold.

 

 

So, how can this situation be improved and cycle times reduced? Here are some suggestions:
• Make sure gate bushing is polished (most materials) or sandblasted (polyolefins and elastomers).
• Use gate bushings with less internal taper.
• Recess sprue bushing to make it shorter.
• Replace cold gate bushing with hot gate bushing.
• Add gussets where runner meets gate and where flow meets cold well (pictured below).
• Add stiffeners to runners, especially on three-plate molds, to prevent curling and ejection problems.