Mould Trial

What is mould trial?

A mould trial is a process by which a new mould is optimised in order to produce the desired products as efficiently as possible. This makes it one of the most important aspects of injection moulding.

A injection mold trial is a test injection step in the product development and manufacturing process that is performed after the product is finished with the mould and before mass production. After the mold is completed and assembled with all the parts, it is necessary to go through the actual injection moulding and get the injection sample, and then the sample test can determine whether the mold is made in full compliance with the design requirements.

If the sample fully meets the design requirements, the injection mould can be put into mass production; otherwise, the injection mould needs to be re-molded according to the feedback from the sample. Depending on the change, a trial run may be performed several times before mass production until the injection mould is fully corrected.

Most defects in molded products are caused during the plasticizing and injection stages, but sometimes they are also related to improper mould design. Possible influencing factors include the number of cavities, the design of the cold/hot runner system, the type, location, and size of the injection port, and the structure of the product itself.

In order to avoid product defects due to mold design, we need to analyze the mold design and process parameters when making the mold.

After obtaining the mould trial results, the operator usually needs to evaluate the injection mould specifics to avoid adding unnecessary costs and time in making modifications to the injection mould.

In most cases, this evaluation also includes the setting of the machine process parameters. That is, in order to compensate for deficiencies in the mould design, the operator may have unknowingly made incorrect settings.

In such cases, the production operation process of the machine is abnormal because the range of parameter settings required to produce a qualified product is so small that any small deviation in the parameter settings may result in a final product quality far outside the allowed error range, and the resulting actual production costs are often much higher than the costs incurred for prior mould optimization.

The purpose of the mould trial is to find out the optimized process parameters and mold design. This way, even if factors such as material, machine settings, or environment change, it is still possible to ensure a stable and uninterrupted mass production environment, not just to get a good sample. This is very important.

The mould trial steps

Step 1. Set the temperature of the barrel

It is important to note here that the initial barrel temperature setting must be based on the material supplier’s recommendation. This is because the same material from different manufacturers and grades can vary considerably, and material suppliers often have considerable research and knowledge of their own materials. The user can make basic settings based on their recommendations and then fine-tune them as appropriate for specific production situations.

In addition to this, the actual temperature of the melt needs to be measured using a probe. This is because the barrel temperature we set is often not guaranteed to be 100% consistent with the melt temperature due to the environment, the type and location depth of the temperature sensor, etc. Sometimes, due to the presence of oil or other reasons, the actual temperature of the melt differs greatly from the set temperature of the barrel.

Step 2. Setting the injection mould temperature

The initial injection mould temperature setting must be based on the recommended values provided by the material supplier.

It is important to note that by injection mould temperature we are referring to the temperature of the cavity surface and not the temperature displayed on the mold temperature controller. In many cases, the temperature displayed on the controller does not match the temperature of the cavity surface due to the environment and the improper selection of the controller power.

The temperature of the cavity surface must be measured and recorded prior to the official  mould trial. At the same time, measurements should also be taken at different locations within the mould cavity to see if the temperature at each point is balanced, and the corresponding results should be recorded to provide reference data for subsequent mould optimization.

Step 3. Appropriate optimize

Based on the experience, the parameters such as plasticizing volume, injection moulding pressure limit, injection speed, cooling time, and screw speed are initially set and optimized appropriately.

Step 4. Conduct a filling test to find the transition point

The changeover point is the switching point from the injection stage to the holding pressure stage, which can be the screw position, filling time, and filling pressure. This is one of the most important and basic parameters in the injection moulding process. In the actual filling test, the following points need to be followed.

(1) The holding pressure and holding time during the test are usually set to zero.

(2) The product is generally filled to 90% to 98%, depending on the wall thickness and the structural design of the mold.

(3) Since the injection speed affects the location of the turning point, the turning point must be reconfirmed each time the injection speed is changed.

The filling test allows the user to see the flow path of the material in the mould cavity and thus determine where the mold is prone to trapped air, or where it needs improved venting, etc.

Step 5. Find out the injection moulding pressure limit

In this process, attention should be paid to the relationship between injection pressure and injection speed. For hydraulic systems, pressure and speed are interrelated. Therefore, it is not possible to set both parameters so that they meet the required conditions at the same time.

The injection pressure set on the screen is the limit of the actual injection moulding pressure, so the limit of the injection moulding pressure should be set to always be greater than the actual injection pressure.

If the injection moulding pressure limit is too low, so that the actual injection moulding pressure is close to or exceeds the injection pressure limit, the actual injection speed will automatically decrease due to the power limitation, thus affecting the injection moulding time and injection moulding cycle.

Step 6. Find the optimized injection speed

The injection moulding speed referred to here is the injection injection speed that simultaneously satisfies the requirement to keep the filling time as short as possible while keeping the filling pressure as low as possible. In this process, the following points need to be noted.

(1) Most of the product surface defects, especially those near the gate, are caused by the injection speed.

(2) Multi-stage injection is only used when one injection cannot meet the process requirements, especially in the trial mold stage.

(3) If the injection mould is intact, the transfer point is set correctly, and the injection moulding speed is sufficient, the injection speed is not directly related to the generation of flying edges.

Step 7. Optimize the holding time

The holding time is also known as the condensation time of the gate. Generally, the condensation time of the gate can be determined by weighing the product to obtain different holding times, while the most optimized holding time is the time when the product reaches its maximum mould weight.

Step 8. Optimize other parameters, such as holding pressure and clamping force

Finally, it should be emphasized that the purpose and focus of the mould trial are to optimize the injection mould and process to meet the requirements of mass production, not just to test out a good product specimen.