FirstMold Half Logo

What is Mold Trial Run?

Share This Article:
mold trial featured image

After an injection mold is completed, does it go directly into production? The answer is, of course, no, because there is a crucial step – the mold trial. Before the injection molding of a new mold or when a machine switches to producing other molds, mold testing is an essential part. The quality of the mold trial results will directly impact the subsequent smoothness of production.

What is a Mold Trial?

Simply put, a mold trial is the test injection molding step conducted in the product development and manufacturing process after the mold is completed but before mass production.

Most defects in molded products are caused during the plasticizing and injection stages, but sometimes they are also related to improper mold design. Possible influencing factors include: the number of cavities, the design of the cold/hot runner system, the type, position, and size of the gate, and the structure of the product itself. Therefore, to avoid product defects caused by mold design, we need to analyze the mold design and process parameters when making the mold.

Improper mold design often causes defects in the final product. Therefore, before modifying the mold, a mold trial and evaluation are usually performed to optimize the mold design and process parameters. This avoids unnecessary errors, achieves twice the result with half the effort, and meets the high-quality requirements of mass production.

Why We Need a Mold Trial?

It’s impossible to judge the quality of plastic mold design and fabrication by eye. Generally, a mold isn’t used as a final product immediately because it may have defects after production. These defects can’t be fully avoided before manufacturing, and issues may arise during manufacturing that weren’t detected earlier. Therefore, a mold trial is needed, followed by analysis, evaluation, and optimization of the trial samples to meet high-quality requirements.

After obtaining the mold trial results, the operator usually needs to evaluate the mold to avoid unnecessary costs and time during modification. In most cases, this evaluation also includes setting machine process parameters. That is to say, to compensate for deficiencies in the mold design, the operator may unknowingly set incorrect parameters. In this case, the equipment’s production operation isn’t normal because the range of parameter settings required for qualified products is very narrow. Any slight deviation in the settings may result in the final product’s quality falling far outside the permissible error range. The actual production cost resulting from this is often much higher than the cost of optimizing the mold beforehand.

The purpose of the mold trial is to find optimized process parameters and mold design. This ensures a stable and continuous mass production environment, even if factors like material, machine settings, or environment change, rather than just producing a good sample. This is crucial.

Basic Steps for a Mold Trial

1. Set the barrel temperature.

The initial barrel temperature setting must follow the material supplier’s recommendation because different suppliers and different grades of the same material may have considerable differences. The supplier often has an in-depth understanding of their material. Users can follow their recommendations for basic settings, then fine-tune based on actual production conditions.

Additionally, a detector should be used to measure the actual melt temperature because the set barrel temperature often differs from the melt temperature due to environmental factors, temperature sensor type, and depth. Sometimes, due to oil contamination or other reasons, the actual melt temperature and set barrel temperature differ greatly.

2. Set the mold temperature.

Similarly, the initial mold temperature setting must follow the material supplier’s recommended value. The mold temperature refers to the temperature of the mold cavity surface, not the temperature displayed on the mold temperature controller. Often, due to environmental factors and incorrect power selection of the mold temperature controller, the temperature displayed on the controller doesn’t match the cavity surface temperature. Therefore, before the mold trial, the cavity surface temperature must be measured and recorded. Additionally, the temperature at different positions inside the mold cavity should be measured to check if they are balanced and record the results for future mold optimization reference

3. Set parameters

Preliminarily set parameters like plasticizing volume, injection pressure limit, injection speed, cooling time, and screw speed based on experience and optimize them appropriately.

4. Filling test in the mold trial

Conduct a filling test to find the switchover point. The switchover point refers to the point where the injection phase switches to the holding phase. It can be based on screw position, fill time, or fill pressure. This is one of the most important and fundamental parameters in the injection molding process. During the filling test, follow these points:

5. Determine the injection pressure limit

During this process, note the relationship between injection pressure and speed. For hydraulic systems, pressure and speed are interrelated, so these parameters can’t be set simultaneously to meet the desired conditions. The injection pressure set on the screen is the limit for the actual injection pressure, so the limit should always be higher than the actual pressure. If the limit is set too low, causing the actual pressure to approach or exceed the limit, the injection speed will automatically drop due to power constraints, affecting injection time and cycle time.

6. Optimize injection speed

The injection speed should minimize fill time and fill pressure simultaneously. During this process, note:

  • Most surface defects, especially near the gate, are due to injection speed.
  • Multistage injection is used only when single injection can’t meet process needs, especially during mold trials.
  • When the mold is intact, switchover point set correctly, and injection speed sufficient, injection speed isn’t directly related to flash formation.

7. Optimize holding time

The holding time is the gate solidification time. Generally, it can be determined by weighing, setting different holding times, and finding the time when the product weight is maximized.

8. Optimize other parameters

Optimize other parameters like holding pressure and clamping force.

Finally, the purpose and focus of a mold trial are to optimize the mold and process to meet mass production requirements, not just to test a good product sample.

Common Issues in Mold Trials

1. Sticking to the mold in sprue:

Solutions, in order:

  • Polish the spure.
  • Align the nozzle with the mold center.
  • Lower mold temperature.
  • Shorten injection time.
  • Increase cooling time.
  • Check nozzle heater.
  • Polish mold surface.
  • Check for material contamination.

2. Difficult part ejection:

Solutions, in order:

  • Reduce injection pressure.
  • Shorten injection time.
  • Increase cooling time.
  • Lower mold temperature.
  • Polish mold surface.
  • Increase draft angle.
  • Reduce clearance at inserts.

3. Poor dimensional stability:

Solutions, in order:

  • Change barrel temperature.
  • Increase injection time.
  • Increase injection pressure.
  • Adjust screw back pressure.
  • Raise mold temperature.
  • Lower mold temperature.
  • Adjust feeding amount.
  • Reduce regrind ratio.

4. Streaks:

Solutions, in order:

  • Adjust feeding amount.
  • Raise mold temperature.
  • Increase injection time.
  • Increase injection pressure.
  • Increase melt temperature.
  • Increase injection speed.
  • Increase runner and gate size.

5. Warping and deformation:

Solutions, in order:

  • Lower mold temperature.
  • Lower melt temperature.
  • Increase cooling time.
  • Reduce injection speed.
  • Lower injection pressure.
  • Increase screw back pressure.
  • Shorten injection time.

6. Delamination:

Solutions, in order:

  • Check plastic type and grade.
  • Check for material contamination.
  • Raise mold temperature.
  • Dry material.
  • Increase melt temperature.
  • Lower injection speed.
  • Shorten gate length.
  • Reduce injection pressure.
  • Change gate position.
  • Use large-hole nozzle.

7. Silver streaks:

Solutions, in order:

  • Lower melt temperature.
  • Dry material.
  • Increase injection pressure.
  • Increase gate size.
  • Check plastic type and grade.
  • Check for material contamination.

8. Poor surface gloss:

Solutions, in order:

  • Dry material.
  • Check for material contamination.
  • Increase melt temperature.
  • Increase injection pressure.
  • Raise mold temperature.
  • Polish mold surface.
  • Increase runner and gate size.

9. Sink marks:

Solutions, in order:

  • Adjust feeding amount.
  • Increase injection pressure.
  • Increase injection time.
  • Lower melt speed.
  • Lower mold temperature.
  • Increase venting.
  • Increase runner and gate size.
  • Shorten runner length.
  • Change gate position.
  • Lower injection pressure.
  • Increase screw back pressure.

10. Bubbles:

Solutions, in order:

  • Dry material.
  • Lower melt temperature.
  • Increase injection pressure.
  • Increase injection time.
  • Raise mold temperature.
  • Lower injection speed.
  • Increase screw back pressure.

What Consequences Will a Large Number of Mold Trials Bring?

The number of injection mold trials directly affects production efficiency and product quality, so it needs to be flexibly controlled according to specific circumstances. Generally, around 3-5 trial runs are appropriate, but sometimes more trials are needed to ensure product quality.

Moreover, the number of trial runs is also influenced by the following factors:

  1. Mold Quality: High-quality molds with higher precision, strength, and durability require fewer trial runs.
  2. Injection Molding Process: Proper injection molding process parameters can improve trial efficiency and reduce the number of trials.
  3. Product Requirements: Different products have different molding requirements, so trials need to be conducted accordingly.

Although the process parameters and mold design of injection molds can be optimized, the production cost of injection molds will also increase. If it is found after mold trials that the injection mold structure is unreasonable, improvements are needed, which undoubtedly exceed the budget. For example, some injection molds are well-designed, but there are still cases where the process is not in place. When the optimal molding process and injection mold model are not selected during the trial, the mold may perform well, but the resulting products may not be satisfactory. Also, situations where the physical properties of the plastic and shrinkage rates are not fully understood are common. It’s frustrating that many situations are unsatisfactory, and sometimes even the injection molds need to be scrapped and redone.

How to Reduce the Number of Mold Trials?

The number of mold trials for a set of molds depends on several factors:

  1. Whether the structural optimization of customer products is perfect.
  2. Whether the evaluation of mold drawings is comprehensive.
  3. Whether the processing technology arrangement of mold parts is reasonable.
  4. Whether the process control during mold processing is appropriate.
  5. Whether scientific trials are effective.
  6. Whether the improvement plan for trial issues is strictly effective.

Conclusion

In summary, mold trials are an indispensable part of mold manufacturing services and injection molding services. Assuming there are no significant design changes from the customer, the fewer trials required to achieve high precision and perfect injection molds, in some sense, can also be considered as an indirect proof of the mold manufacturer’s manufacturing capabilities.

If you need fast production of high-quality molds or injection-molded products, feel free to contact the FirstMold team.

Table of Contents
Tags
Comments

प्रातिक्रिया दे

आपका ईमेल पता प्रकाशित नहीं किया जाएगा. आवश्यक फ़ील्ड चिह्नित हैं *