Ultrasonic Welding: Set-up Parameters
When planning the set-up of an ultrasonic welding job, many aspects of the process must be taken into consideration to obtain the most desired results. Most importantly, one should document all steps along the way so they can more easily be duplicated or used as reference material when welding similar jobs in the future. When trying to find the best settings, it is important to remember how much they can vary from one weld to the next depending on a whole host of circumstances. The best way to find these answers is to work through the painstaking process of trial and error.
When looking at the process, we must consider that ultrasonic welding is an umbrella term for at least five different methods for affecting an assembly comprised of one or more thermoplastic parts. The most common form of welding involves the assembly of 2 injection-molded parts, but it also includes staking a thermoplastic boss to retain another part, spot welding for bonding flat pieces together, inserting for driving a metal insert into plastic and for forming or swedging to shape an element, usually for retention.
It is vital that the horn and nesting fixtures are properly designed and fabricated and that the ideal booster and power supply are selected. It should be a goal to establish a setup, which produces acceptable and repeatable welded assemblies. With nesting, it is important to ensure proper leveling and that the energy transmitted from the horn is received at the entire part interface rather than beginning at one area then spreading to the remainder of the part as the initial weld area collapses. The use of jack screws in the nesting fixture to raise areas of the nest to a level condition is debatable depending on what you are accustomed to doing. The reduced contact area with the support structure can promote an in-phase condition. Solid shims might be used as an alternative between the nest and the base structure. Multiple thin shims stacked together to achieve level tend to compress and gradually change the level and support conditions over time and should be avoided. Proper support of the pre-assembled parts is literally and figuratively the foundation of any successful welding application. It is essential that the nest is mounted firmly to the welder base structure and that it maintains the pre-assembled parts in a level condition with the face of the horn. Failing to give special attention to these key elements will negatively affect weld integrity and/or cosmetic quality of the weld. No welder, regardless of its features, will overcome improper support or out of level conditions.
The initial setting up of an ultrasonic welding process can be a daunting task considering the number of variables/parameters that are involved in the process. However, the process of dialing in the operation, or fine-tuning an existing application can be simplified if an understanding of the variables involved, and their interaction, is achieved. The combination of weld time (or energy delivered), pressure applied to the part, hold time and other welder features used to produce the weld are generally referred to as weld parameters. Because applications are so varied and materials are more and more exotic, there are no hard and fast rules for welder set-up. However, there are some basics and key things to remember.
First, it is important to only change one parameter at a time. If after an initial weld is made and then both the weld time and pressure are altered, it may be impossible to determine which change was a positive step. As a general rule, pressure and weld time have an inverse relationship. If you increase pressure, a reduction in weld time will be required to net the same result. Certain materials, such as polypropylene, will require a more aggressive weld. A short weld time along with high pressure is likely more effective than a long weld time at a low pressure. Knowledge of plastic characteristics and a good relationship with a welding vendor goes a long way in solving set-up dilemmas. Most welders incorporate features such as welding by energy and force triggering, which are extremely useful if properly utilized.
The following is a look at some of the different parameters to be considered:
Amplitude, the amount of movement at the face of the horn during the weld, is affected by changing booster ratios or generator/power supply settings. It can be established by following the Amplitude Requirement Charts of most equipment manufacturers. The value is used to calculate the booster ratio needed considering tranducer output and horn gain factors. In general, semi-crystalline plastics will require more amplitude than amorphous resins.
Downspeed of the welder head should also be recorded as a set-up parameter. Its effects are most apparent in inserting, staking, and Polycarbonate welding. The value should be varied from an initial setting of 2-4 in/sec. to produce the desired results. Hydraulic speed controls often provide superior control.
Alignment/leveling of the horn, fixture and part is also an important factor. Besides aligning the horn and nest, there must be a way in the tooling/welder system to level the contact of the horn to the parts. Leveling screws on the corners of the fixture, plate, as discussed before, can be a simple, effective means for this- although others warn against ever using leveling screws, or jack screws and recommend solid shims as a better alternative. If jack screws are used, it is important that only two of the four are employed in the leveling. No matter how it is accomplished, leveling ensures that an even weld occurs around the entire weld joint area.
Carbon paper placed between the horn and the part or fixture can be used to indicate the contact area initially and welded parts can be broken to examine the degree of weld. Leveling can then be used to raise (or apply more force to) those areas lacking weld. Sometimes the horn must not be perfectly level to the fixture to ensure the best weld. Using a known good part from the last run to align the horn and nest is a real time saver.
Welding/hold force is the amount of force exerted on the horn, and in turn, the parts, by the press system during the weld and hold cycles. Initial approximations of good values to use are based upon overall joint length in linear inches and are shown in the accompanying graphs. The process is fine-tuned from these general guidelines. Generally, larger parts require higher forces. Some modern computer controlled welders allow the changing of forces during the weld cycle to allow more complete control over the heating rates of the joint material and collapse of the materials.
Trigger force is a pre-load force exerted against the parts before ultrasonics are turned on. Most applications require this pre-load to seat the pieces together before welding. Values are based upon overall joint length. Warped or distorted parts weld better with higher forces or delay of the ultrasonics for a certain time of distance. Applications such as inserting, staking, and swaging may require pre-triggering activation of ultrasonics before the horn contacts the workpiece, which enables very small, or no initial forces to be applied to start the process.
If a particular application has no obvious starting point for set-up, it is often useful to start at the extremes and work towards the middle. In most cases, a window for successful weld parameters will begin to reveal itself as you go through this exercise. Patience and sometimes dozens of parts will be required. A successful window for welding implies that a given range of parameter variation will weld any and all part combinations. If this cannot be achieved, then look for part combinations that seem to be more difficult to weld than others. Parts will differ subtly from each other, especially when multi-cavity molds have been used to produce them.
The use of computer-controlled welders has made set-up much easier and allows much finer tuning of the process. These closed loop types also add immensely to the repeatability of the weld process. Common weld parameters include the collapse (or relative) weld distances, as measured from the trigger point, and are approximately equal to the joint welding distance such as the energy director height, shear joint depth, etc. These will not be exactly equal to the heights of the joints due to part flexure under welding forces. The absolute distance allows precise welding to the desired overall height of the completed assembly. Welding by peak power output is usually done on thin films or fabric materials, while welding by energy is sometimes used to good affect on the welding of rigid plastics as well as films and fabrics.
Other more elusive problems that affect welding repeatability and quality may be encountered. Mold release agents are typically sprayed onto the mold cavity to facilitate a quick release of the part from the mold. Release agents can reduce heat generation at the joint during the weld process. The resin grade must be considered in all ultrasonic welding. For best welding results, same grade resins should be maintained. In addition, moisture content from the atmosphere can cause bubbling at the weld surface. Because of this, hygroscopic (moisture absorbing) resins will typically yield weak strength and poor cosmetic appearance. Fillers such as glass or talc can increase the weldability of thermoplastics, but only to a point, as too much of a filler can reduce the weld strength. Lubricants, such as waxes, stearates, and fatty testers can also reduce weld strength by reducing the intermolecular friction within the resin. Plasticizers, when added to acquire softness and flexibility, can interfere with the resins ability to transmit vibratory energy as well. The last variable is pigment. Some oil-based colorants may also affect weldability. With all these variables controlled, the weld or bond obtained with ultrasonics is typically 90-95 percent as strong as the parts themselves.
Learning the specifics of each welding job before beginning any process is the most vital component to successful ultrasonic welding. By merely knowing which button to push and learning how each parameter affects the welding process is the surest way to prevent disasters.
Remember that any worthy ultrasonic company will take time, often at no charge, to work through your applications with you. Many prefer to be involved in the conceptual stages to make sure joint design and material will be properly specified. Most have laboratories filled with equipment for application testing and proper equipment selection. Keep in mind that most welder set-ups are easily solved with patience and attention to the fundamentals.
Plastics Decorating would like to provide a special thanks to the following individuals for their contribution to this article: Kenneth Holt of Dukane Corporation (630-584-2300), Gary Sullivan of Specialized Assembly Systems Northern California Representative of Forward Technology (209-839-9678), and Rick Lombardo of Tooltex, Inc. (614-539-3222). Dukane Corporation also supplied the photographs and graphs shown.