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Words of Wisdom: Confronting current injection machines' worst design elementsWords of Wisdom: Confronting current injection machines' worst design elements

March 3, 2005

5 Min Read
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Robert F. Dray is the founder and owner of R. Dray Mfg. Inc.


What are the four most poorly designed elements of an injection molding machine? The purging setup, the nozzle and sprue bushing hemispheres, the shutoff nozzle, and the decompression or pullback setup?all of which complicate the molding process and cost productivity.

1. The purging arrangement. Regardless of the machine?s size or price, to remove the air and any possible contaminants, we normally place a piece of cardboard in front of the sprue bushing in the stationary platen. Then we close the protective doors before shooting resin to avoid getting splattered with the hot resin. This is never an easy task and requires at best a cumbersome, nasty, and potentially dangerous procedure. Many molders have at one time or another been splattered with hot resin.

If the cardboard is not placed properly, resin can get into the sprue bushing and freeze or flow onto the bottom guard. Cleaning the bottom guard is easy compared to removing frozen resin from the sprue bushing. Assuming all goes well and the resin is projected to the cardboard, this resin becomes scrap.

2. The nozzle and sprue bushing hemispheres. Most, if not all, molders have had radii on the nozzle and sprue bushing that do not match. When this happens the result is resin all over the stationary platen and perhaps a plugged sprue bushing. With an insulated runner this requires removal, which is time consuming. Nicked or damaged hemispheres also produce the same problems or worse?such as a small leak that is not initially detected. This can be nasty to find and can produce shorted parts, as well as accumulated resin if the leak is not found quickly.

3. The shutoff nozzle. Shutoff nozzles are used to prevent drooling and to rotate the screw during clamp movement to increase recovery capabilities. There are a variety of designs that have different problems. The most common problems are leaking, maintenance requirements, excessive pressure drop, dead spots, fire hazard, and high cost. Not all shutoff nozzles suffer from all of these problems but if you are using a shutoff nozzle, you can undoubtedly select a few from this list.

4. Decompression or pullback. As shutoff nozzles have not solved the drooling problem, decompression is generally used. It is accomplished by moving the screw rearward for a distance?velocity tends to pull the resin upstream to eliminate drooling. The normal problem is splay, trapped air, or gas that is introduced into the resin.

Addressing the Problems

R. Dray Mfg. Inc. has developed a device called the Coupler, which is designed to address the above problems. A summary of the technology is presented below. Since the device is patented (U.S. #6413076), more information is available online from the U.S. Patent and Trade Office (USPTO).

The Coupler improves all of the above lacking designs at about the cost of a sprue bushing and nozzle set. As most injection units have sprue-break capability, machine changes are not necessary.

  • The Coupler purging: The machine carriage moves to the sprue break position. When the Coupler nozzle is inserted into the Coupler extended sprue bushing, it remains inserted until mold change. The purge is contained at all times, eliminating the need for protective guards. Additional barrel support is provided, also eliminating the need for barrel supports. Leakage is eliminated by clearance and length; seals are not required. If shutoff is not required, then purge is the only position needed for sprue break or rearward carriage movement. A second position is needed if shutoff is required. In machines without a linear transducer on the carriage, a second limit switch is required for both purge and shutoff.

    The purge configuration shown is only the canister type; other configurations can form the purge into a regrind that is easily fed into the grinder or directly into the hopper. The purge savings alone can easily pay for the unit in a short period of time.

  • The Coupler injecting, fully open: This is the normal inject position. The carriage is in the full forward position. This is the same with any standard injection unit setup. The Coupler eliminates the costly problems associated with present nozzles and tips and operates with normal machine function. The sprue break positions of purge and shutoff are only used when required. There are no hemispheres to cause leakage. Also, there can never be short shots caused by damaged nozzle tips because nozzle tips are eliminated.

    In relatively new injection machines equipped with linear transducers on the carriage, decreasing the entry clearance can effectively control viscosity. This viscosity can also be programmed in the control on newer machines to lower viscosity (or raise melt temperature), initially by partially closing and then moving to the fully open position. This profiling of viscosity can be used to assist in the filling of molds that have a difficult configuration.

  • The Coupler injecting, partially closed (decreased viscosity); and in a closed or shutoff position: This is the normal sprue break. In this setting the injection unit carriage rearward movement, instead of breaking off the sprue, initially decompresses the resin to eliminate drool and close (or shut off) any resin downstream flow, allowing for recovery to start if desired. This movement is not required, only available if needed. This shutoff is essentially an added feature that costs nothing. It does not leak and does not add resistance to flow as the flow channels are designed to eliminate pressure drop and material hang-up.

    If machine cycle time is increased by recovery time, the Coupler shutoff can be easily initiated. One problem is that many machines do not have the hydraulic oil flow capacity to move the clamp and rotate the screw at the same time. The addition of a pump to provide this additional hydraulic oil capacity is normally not difficult. In electric machines, simultaneous movement is not a problem.

    Adding a longer screw rotate time is desirable in any injection machine using maximum rpm. Reducing rpm lowers melt temperature and can decrease cycle time. If this decrease of rpm causes fill problems, it is easy to increase the rpm and decrease rotate time.

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