In plastic injection molding, when using side-gate type gating, it is necessary to have a post-processing process such as de-gating to separate the finished product from the runner. There are many ways to de-gate, including rough manual breaking, angled cutter shearing, punching machine, hot cutting, laser cutting, ultrasonic cutting, etc. There is no fixed correct method according to the process capability of each factory or the characteristics of the product. Any method that meets cost-effectiveness and product shipping standards is a good method. Today, we are going to discuss the less commonly used ultrasonic cutting. Let's first understand when it is suitable for use, how to use it, and whether there are design techniques on the runner that can be compatible with ultrasonic cutting. This article will explain it in detail to everyone!
Overview
Ultrasonic cutting involves the use of an ultrasonic welding machine, utilizing different vibration frequency parameters to achieve either cutting or welding effects on plastic components. Similar to welding, it involves pressing the welding head against the product to transmit vibrational energy onto it. However, the cutting effect is achieved by inducing high-speed vibrations in the plastic product, causing it to fracture at its weakest point, which differs from the principle of conventional ultrasonic cutting knives. Therefore, the density of the injection-molded product and the structural characteristics of the runner itself are key factors determining the effectiveness of ultrasonic cutting, while parameters serve to enhance the fracture effect.
The vibrational energy is transmitted through the contact surface to the interior of the product and then propagates outward. When it reaches an unsupported area, the object experiences momentary oscillations, causing it to vibrate. Since the gate area is typically structurally weak, this results in fracture, thus achieving the cutting effect.
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Working Principle
The operation of an ultrasonic welding machine involves three stages: Triggering --> Excitation --> Holding Pressure.
1. Triggering: This refers to the process during which the welding head descends to the excitation stage. There are primarily two control modes: delay triggering and pressure triggering. Delay triggering involves controlling when the vibration frequency is emitted based on time and coordinating it with the speed of the welding head. It can either emit vibration frequency before contacting the plastic product for vibration cutting or press down on the plastic product before emitting vibration frequency. Generally, for welding larger plastic areas, pre-vibration emission is necessary. However, in cutting processes, the welding head is pressed against the plastic before emitting vibration to prevent product displacement, which may result in uneven gate surfaces. Pressure triggering involves emitting vibration frequency only after the welding head has pressed down on the plastic to a specified pressure level. The setting of the pressure level depends on the plastic's structure and fixing method. Excessive pressure can cause inaccurate stress distribution in the plastic, leading to premature fracture, while insufficient pressure can result in inadequate plastic compaction, affecting stability during vibration cutting.
2. Excitation: Modern welding machines typically feature several control modes: fixed energy, fixed welding time, and fixed position. Fixed energy specifies the magnitude of the emitted energy (power), stopping vibration emission once the designated power is reached. Fixed welding time determines how long the vibration lasts before stopping, while fixed position ceases vibration emission after the welding head moves to a specified depth. When testing a new product, it's challenging to determine the appropriate energy level due to the wide range of energy settings. Therefore, initial testing often employs fixed welding time, gradually increasing it until achieving reasonable cutting results, before determining the energy range to be used. The fixed energy control mode ensures consistent power output and stable cutting conditions, making it suitable for mass production. Adjustments to vibration are mainly made through power, amplitude, and welding time. Amplitude and power determine the intensity, with higher values indicating more intense vibrations, though greater intensity doesn't necessarily mean better cutting results. During cutting, the desired fracture point is at the interface between the gate and the finished product. Therefore, the gate's cross-section is typically narrowed to create the narrowest and weakest point. Energy propagates outward from the welding head, and when it reaches a point sufficient to cause fracture, it breaks. Hence, excessive power or amplitude may cause premature fracture before reaching the gate, resulting in excessive gate residue.
The duration of welding time typically correlates with energy levels. Generally, with higher amplitudes, shorter welding times are chosen. If the energy level is high and the welding time is long, the plastic component may fracture or even damage the structural integrity of the product. In cutting situations, it's not advisable to prolong the welding time. Prolonged welding times result in inaccurate power control, leading to unstable cutting quality, and increasing the likelihood of burrs and damage to the product.
3. Holding Pressure: After the welding head stops vibrating, it remains in place for a period before retracting. This period is known as the holding pressure phase. The purpose of holding pressure is to allow the plastic component to undergo stress relief. It has a significant impact on welding, whereas for cutting, insufficient holding pressure time or none at all may result in the runner not receiving sufficient pressure from the welding head during vibration, leading to excessive movement and product ejection. This situation is particularly critical for small components.
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Fixture Design
The items that need to be designed are the vibrator head and the support plate. Due to the special properties of the material and the requirement for ultrasonic simulation software, vibrator heads are typically manufactured by welding machine manufacturers. Support plates, on the other hand, are simple metal components that can be handled internally by our unit.
The limiting features on the plate are modeled according to the shape of the runner, with a peripheral gap of 0.1 on one side. The depth of the runner sinking is taken as half the height of the runner. The top pin feature completely avoids the central top pin feature.
Springs will be installed under the plate for cushioning purposes, mainly to mitigate the pressure exerted by the welding head. As mentioned earlier, excessive pressure can cause inaccurate stress distribution in the plastic, leading to undesirable fracture locations.
Runner Design
Apart from optimizing cutting effects through parameters and fixtures, the structure of the plastic part itself is crucial. Therefore, we can enhance cutting effects by modifying certain features.
The vibration point is a raised platform positioned at the intended fracture area to enhance the discontinuity of the fracture. When ultrasonic energy is transmitted, it's easier to generate concentrated stress at that point. Typically, it's set on the side of the mold that has less impact on the injection molding results.
In addition to the methods mentioned above, the effectiveness of ultrasonic cutting depends greatly on the quality of the plastic product itself. Insufficient plastic drying or inadequate holding pressure during molding can result in insufficient density of the plastic component. In such cases, ultrasonic vibrations may easily crush or prematurely fracture the plastic. Moreover, the runner itself must be smooth enough. If the runner is curved, it can cause the welding head to exert excessive pressure on the plastic component prematurely, equivalent to creating a state of excessive pressure. If pressure mode is adopted, it may cause vibrations to be emitted prematurely. Therefore, ensuring the molding quality of the plastic component is essential for demonstrating the effectiveness of ultrasonic cutting.
Reference
- The equipment in the picture is taken from Taichung- SHUIYE official website https://www.shujye.com.tw/
