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During the installation of a mold, is it a good practice to build tonnage prior to applying torque to mold bolts, or energizing a magnetic platen?

Is there anything wrong with torquing the bolts or energizing the magnetic platen before torquing the bolts?

Regarding the first question: You should always secure the mold by torquing or energizing before applying full clamp tonnage. The mold should be fully closed without clamp tonnage when the bolts are torqued or magnetic platen energized.

Regarding the second question: The purpose of the clamps or magnetic platen is used to support the full weight of the mold when it is open or without clamp pressure as well as align the two mold halves. Since both the mold base and platen flex during full tonnage, securing the mold while it is under full clamp tonnage may result in an improperly aligned mold base or inadequate torque.

-Andy

We were discussing the different types of screw tips: sliding ring (check ring), poppet valve, ball valve and smear tip. I was asked, under what circumstances would you use each type and I feel I could not give a good answer. I was wondering if you maybe able clarify this for me?

When discussing non-return screw tips in general, there are two competing aspects: backflow vs. material stagnation.

In general, tips with high backflow are not very well suited for high precision molding or materials with very low viscosity. Adversely, tips which cause material stagnation have dead-spots where material can become caught and remain indefinitely. Material stagnation is avoided when processing heat sensitive materials or products which require frequent material or color changes.

Smear Tips – High Backflow & Low Stagnation – These screws are ideal for high viscosity, heat sensitive materials such as PVC.

Ball/Poppet Valves – Low Backflow & High Stagnation – These tips are ideal for low viscosity materials which are not heat sensitive such as polyolefins.

Check Ring – Moderate Backflow & Moderate Stagnation – These tips are the most widely used non-return valve because the versatility of applications which can process using check rings.

Keep in mind, the discussion above focuses on the general uses of these screw tips. There are a wide array of screw tip designs for each of the general categories above. I have seen some check ring designs which have very little backflow, and some ball valves which have very little stagnation.

-Andy

If you are using a good stable molding process with a slight short shot during first stage fill, then you will not see a significant change with the use of a new machine due to two factors: 1) A specific amount of pressure is required to force the material to the end of fill, regardless of the machine manufacturer, and 2) a short shot during fill allows or variation in the degree of overshoot that occurs during the transition from first stag fill to second stage pack. As a result, the amount of pressure necessary to pack thew mold will not change, thus requiring the same amount of clamping force.

Anyone who prefers to mold using a completely full part (thus packing the mold during first stage) might see a drop in necessary clamping force because the machine controls will provide better control over the overshoot of the injection unit, resulting in a more consistent packing during first stage.

-Andy

I received an interesting question from recent webinar attendee…

There have been some large advances in hybrid molding machines over the years. These machines gain many of the benefits of electric molding machines, but have a slightly smaller cost. Initially, these machines were believed to be a great compromise, but the smaller initial cost is quickly offset by the increased costs to run the machine. Aside from using more electricity to operate the machine, these machines require 2-3 times more water to keep the hydraulics cool, and require more routine maintenance for items such as filters and hydraulic fluid.

I have seen many molders replace hydraulic mold actions with comparable electric components. If this is not an option, there are many hydraulic units which can be easily added to the molding machine for powering a hydraulic core.

As for nozzle contact force, I recommend you discuss this with the manufacturer and determine how they overcome this potential issue. Also, check with the manufacturer to determine whether the contact force motor can be upgraded if necessary.

Some molders I know have purchased both hybrid and all-electric molding machines from the same manufacturer. In each case, the all-electric machines became the preferred machines.

-Andy

 

Although this seems strange, it is not uncommon. Since changing the injection rate causes a difference in the amount of material entering the mold during fill, this might occur when molding a part where the highest pressure losses occur near the end of fill. You might see a drop in pressure if the part thickness increases, and a rise in pressure when the thickness decreases at the end of fill.

In most cases, you will see a smooth in-mold rheology curve.

-Andy