Short shot mold diagnostics is a seemingly simple yet very effective method of evaluating a molds performance. The use of short shots can provide a window into how a mold fills. Uses can include diagnostics of filling imbalances in multi-cavity molds, evaluation of filling patterns within a given cavity and evaluation of the molding pressure development within the mold. Despite the simplicity of this method, it is common that the test is done incorrectly and results in false information. Below are instructions on how to properly collect short shot data.
Multi-cavity Short Shot Instructions
This is the most common type of short shot test. The main goal of this test is to determine the cavity-to-cavity filling balance of a mold. Proper procedures for this test are as follows:
- Have your mold running at its optimized process and injection rate.
- Remove hold pressures or set to the machines minimum value. The cavities should be approximately 95%-98% full after removing hold pressure.
- Adjust the shot size or transfer position until the MOST filled cavity is approximately 60-80% filled while still injecting at your optimized injection rate. It is very important that none of the cavities be even close to being filled (** this may not be possible with some molds as the ejection system may not be able to eject short shot cavities).
- Watch the screw position indicator to make sure the screw is not creeping forward during the zero pressure short shot hold phase. If the screw continues to move forward you may need to reduce your cushion to zero so that the screw bottoms out, thereby preventing any uncontrolled movement.
- Collect three to five short shots molded using this process. The multiple shots can be used to confirm shot-to-shot repeatability.
- Make sure each part has the cavity ID visibly marked.
- Keep each shot separate (bag each shot separately).
- Weigh each part, and organize data for analysis. (Beaumont’s 5-Step Process™ is recommended for isolating and quantifying the primary contributors to imbalances)
Runner Progression Short Shot Instructions
This test is generally run with cold runner molds to determine pressure drop through various sections of the runner system and/or cavities. This can be useful for helping to pin point areas of high pressure. You can then make necessary changes to more strategically relieve pressure if it is excessive.
- Have your mold running at its optimized process and injection rate.
- Remove hold pressure (or set to machines minimum value).
- Watch the screw position indicator to make sure the screw is not creeping forward during the zero pressure short shot hold phase. If the screw continues to move forward you may need to reduce your cushion to zero so that the screw bottoms out, thereby preventing any uncontrolled movement.
- Adjust shot size or transfer position to achieve each of the following shots:
• Just after the sprue
• At each runner intersection
• Just before the gate
• Just after the gate
• Most filled cavity 50% filled
• Most filled cavity 80% filled
• Most filled cavity 95% filled - Record actual injection pressure used and fill time for each shot.
- Make sure each runner section and partially molded parts are fully marked with identification.
Cavity Filling Patterns Short Shot Instructions
This test is run to evaluate the cavity filling pattern within your mold. This can help isolate causes of weld lines, gas traps, and other filling related problems.
- Same as Step 1, 2, and 3 above.
- Adjust shot size to provide the desired short shot progression through the cavity. This could be every 10% of the shot or any other increment that may provide the desired information.
