How to diagnose & cure short shots.

* Short shots during injection molding are a common filling problem in thick and thin parts and multi-cavity tools or family molds. The root cause is typically attributed to some problem related to injection pressure. However, non-filling of the mold cavity can be traced to many other issues, such as the machine's settings and its ability to maintain them, the molding process, the balancing of the runner system, the machine's plasticating system, the mold, or the melt itself. Injection speed, shot size, melt and mold temperatures, venting, melt contamination, and tool surface quality are all factors that can lead to the mold not filling correctly.

Find pressure problems

Lack of sufficient melt pressure may be the most common cause for short shots. Pressure drop can occur at any point along the path of melt delivery to the mold--in the nozzle, sprue, runner, gate, or mold. However, increasing first-stage (injection or filling) pressure or second-stage (pack and hold) pressure should not be the first response to correcting the problem.

Try examining what has occurred after first-stage filling is complete.

Use one velocity setting to fill the mold and reduce the second-stage (pack and hold) pressure to nearly zero, but keep the mold closed for the normal second-stage packing time. The mold cavity should be 95% to 99% full, and the degree of fill should be consistent from one shot to the next. If the parts are not consistent, take a close look at components of your molding system. The problem may be due to a problem with shot-size control, barrel wear, or a leaking non-return valve. In this case, adjusting injection pressure will not eliminate the occurrence of short shots. You will need to fix the root causes.

Check the non-return valve and machine barrel to be sure they are in specification. The non-return valve should not have a mating angle between the seat and sliding ring--it should be a stepped angle for positive shut-off. Repair or replace it, if necessary. Also check the barrel for wear and ovality.

If insufficient pressure is the problem, find out where pressure losses are occurring. Compare the set first-stage pressure value and the maximum actual pressure exerted during first-stage injection. The set pressure must be higher than the actual maximum pressure observed. The machine hydraulic pressure should be 200 to 400 psi higher, while for an electric machine the set pressure on the resin should be 1000 to 2000 psi higher. If the set pressure is not higher than the actual pressure by about those amounts, then adjust the first-stage pressure setting to improve velocity control over the melt front in the mold. If you cannot establish this pressure differential, you need an injection unit that develops higher melt pressure.

As the test shots are being made, compare the actual stroke transfer cutoff position from the first to the second (pack/hold) stage. The transfer position of the screw should be consistent within 1 mm from shot to shot. If the transfer position is inconsistent or vastly different from the set position, then the machine needs repair.

If the part is not 95% to 99% filled after first-stage injection, and the set pressure is higher than the actual maximum pressure, then try moving the stroke-transfer position or increasing the shot size to get more plastic into the cavity during this stage. Sometimes molders set the transfer position too early in an effort to prevent flash. If you are setting the transfer from first to second stage before the cavity is even 90% filled, then it is a good bet that the tool has a parting-line problem. You will have little chance of properly packing the mold if the parting line cannot withstand 2000 to 6000 psi hydraulic pressure--or 20,000 to 30,000 psi of melt pressure in the nozzle.

If the pressure needed to fill the part is near the machine's limit, perform a "Pressure Loss" or short-shot analysis to determine if there is a large pressure drop in your system. Start with one velocity setting and no second-stage pressure. Make four test parts: a 99%-full part, a sample where the resin just gets through the gate, one that fills just the runner system, and a purge shot into the air. Record the hydraulic pressure at transfer for each test shot. Use the machine's intensification ratio to calculate the actual plastic pressure and make a bar chart of the plastic pressure required for each of the sample shots. The chart can help you spot where pressure loss is occurring. For example, the sprue and runner may be too restrictive, consuming too much of the applied pressure. Acceptable ranges are typically up to 4000 psi for the pressure drop through the nozzle, up to 6000 psi for the sprue/runner, up to 5000 psi at the gate, and up to 30,000 psi in the cavity.

If you discover no first-stage pressure problems, examine next the second-stage pack and hold pressure. First, be sure there is an adequate cushion and that the screw is not bottoming out at the end of the shot. If the cushion repeatability varies by more than 0.200 in., check the non-return valve for leakage, or try a larger decompression stroke to help set up the check valve, but be careful not to suck air into the nozzle, which will cause splay. Increase the second-stage pressure in increments, but be certain you know how long it takes the gate to freeze off. If the second-stage time is less than it takes to freeze the gate, you may have plastic back-flowing out of the cavity into the nozzle.

Applying a higher injection speed can 'help compensate for short shots by lowering the resin's viscosity through added shear. But this requires higher initial injection pressures and ability to halt the resin flow-front momentum as it approaches the end of the cavity in order to prevent over-packing and flash.

Other places to look

Several melt-related factors can also cause short shots. Check gates and parts for foreign material or other contamination. Check the quality of the regrind if any is being used. Look for unmelted granules in the part, as well as color streaks. Unmelts may be traced to screw design or the material. Check that melt temperature is within the resin supplier's specified range. Also check for cold slug formation at the gate. A cold slug from the nozzle can get past the sucker pin and plug the gate. Nozzle drool can be a source of cold slugs.

You might consider changing the resin to one with a higher flow rate. But resin properties may be affected by a lower molecular weight. You would have to test parts made with a higher melt flow to confirm the part properties.

Improper balancing of multi-cavity runner systems can be a major cause of short shots and uneven packing. One solution is the "5-Step Process" software from Beaumont Technologies Inc., Erie, Pa., which helps diagnose filling imbalances.

Trapped gas or air can also inhibit proper cavity filling. Re-evaluate the tool venting and consider porous steel inserts to eliminate gas traps. Also, check for moisture in the tool or system that can generate steam, or for volatiles coming from the plastic. Other solutions may be to change the gate location to prevent gas traps or to pull a vacuum on the cavity during fill.

Last, you might try raising the mold temperature to facilitate melt flow into the cavity, though this may involve a tradeoff with cooling time.

John Bozzelli has taught seminars on plastics design and processing for more than 15years. He has extensive experience in polymer development and processing from more than 20 years with Dow Plastics. He holds 10 patents and has written more than 60papers covering plastics, processing, and machine specifications. He is the founder of Injection Molding Solutions in Midland, Mich., a provider of in-plant training and consulting services to injection molders. Tel: [phone] * PT Direct: 369MF