TROUBLESHOOTING

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Short shots are caused when there is not enough material entering the mold cavity during the fill process. The problem may be under-packing (also see “Part Dimensions Too Small") of the mold. The conditions for correcting dimensional shortages are similar to remedying short shot problems. Simply, there needs to be enough melt flow with enough pressure and proper fill and hold time to allow for filling the cavity. An incomplete part will be produced with a short shot, which distinguishes it from dimensional problems.

Available Melt Volume Is Insufficient
Increasing shot size is the obvious solution to increasing the amount of the material in the mold cavity. However, we suggest a cushion of 6.35 to 3.2 mm (0.250 to 0.125") to adjust for variations in different screws and nozzles. If a short shot still exists, increasing first stage injection speed or velocity will increase shear in the melt flow and lower viscosity, requiring less pressure to fill the cavity. Increasing nozzle and front barrel temperature will also decrease viscosity by adding more heat to the melt flow prior to it entering the mold. If these suggestions do not solve the problem, there may be an issue with the physical equipment restricting the melt flow. Ensure that nozzle and runners are clear of obstruction and that the non-return check valve on the screw is operating properly.

Mold Has High Pressure Drop
A reduction of pressure in the mold cavity can be caused by the gate freezing off prematurely, poor mold design, or low pack/hold pressures. We always recommend gating a part in the thickest section with flow towards thinner areas. This helps ensure that pressures in the mold are balanced and maintained. If a large cavity is receiving melt flow from a narrower channel, there will be a substantial drop in the pressure pushing the melt flow. It can cause the narrow channel to freeze off, denying the large cavity an opportunity to fill. Moving or adding a gate to the thicker area may alleviate this problem. However, care must be taken when adding a gate as it will create another flow front in the mold and may require additional venting where the flow fronts meet. If this is not an option, maximizing the pressure in the mold and reducing the viscosity of the material may be the only options. Increasing the back pressure and injection speed will increase shear (lowering viscosity) of the TPV material and ensure a more homogeneous mix of the melt. Increasing pack/hold pressures will increase the pressure in the mold, helping the melt flow pack out. If these suggestions do not solve the problem, it may be necessary to use a higher flow grade of our TPVs.

Mold Function Is Incorrect
Restrictions in a mold cavity or an unbalanced multi-cavity tool can also cause short shots. Be sure that all gates in a multi-cavity tool are the same size. For balanced melt flow and cavity filling, design runners for the shortest, straightest flow path consistent with the best gate location(s). In order to reduce pressure drop and facilitate heat retention, the ratio of surface area to volume should be kept to a minimum. Runners also function to transfer pressure and regulate the pressure drop between an injection molding machine and a mold cavity. Also, ensure that the mold is vented correctly and that vents are clear, allowing air to escape.

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