Machining Engineering Plastics: Hydro Massage Component Example

Challenges in Milling Engineering Plastics

Machining engineering plastics has always been challenging due to their variable and inconsistent nature. Processing commonly used plastics such as PA, PP, POM, PC, ABS, and PMMA remains a persistent issue in the industry.

One major challenge is deformation caused by the material’s softness. During milling, internal stress can be released, leading to shape changes.

Plasticizers are also a factor. Plastic materials often contain it, which can complicate their mechanical properties and temperature control, however making it challenging to achieve precise tolerances when milling plastics.

Additionally, high temperatures during machining can alter plastic shapes. Like ice melting under heat, plastics can soften or deform due to excessive machining heat.

About Project and Product

Recently, a multinational healthcare company based in the USA approached WayKen for assistance in machining plastic components. The final product was a massaging device that uses high-pressure water jets to provide deep muscle relaxation.

The project involved four parts originally designed for injection molding. However, during the development phase, the customer wanted to find an alternative manufacturing method since only a few pieces were needed, making mold production an impractical and costly option. Meanwhile, the material was also specified to be used, which was PA6 with glass fiber. After evaluating all possibilities, we discussed with the customer and decided to proceed with CNC machining.

As mentioned earlier, plastics are prone to deformation during CNC machining. In the following sections, we will detail the material preparation and milling process of the two parts from this project to share how we address these challenges.

Part Structure Analysis

The first component, the “Leg,” is designed to support and stabilize the massage bed. It has a long structure with multiple hollow sections, added with supporting ribs. However, due to its length (46 cm) and plastic composition, it remains highly susceptible to deformation. Since this part plays a support role in the overall structure, excessive bending could compromise the bed’s stability.

Another component at risk of deformation has an irregular shape with two large openings—one in the center and another at the top. Below are two images showing the openings.

Even if machined from metal, a design with large openings is prone to significant deformation. A careful review of the part drawing also reveals that some tolerances are quite tight, bringing additional challenges for plastic machining.

Engineering Plastic Machining Solutions

To minimize deformation and ensure precision in machining the plastic parts of hydro-massage equipment, a series of processing steps was carefully implemented.

Pre-Annealing the Material

To mitigate deformation during machining, before machining, we annealed the material by ramping up the temperature, which helped to reduce the inherent stress in the material.

The first step was to source a nylon sheet (480 × 70 × 110 mm) and place it in an oven to ensure uniform heating. The oven was preheated to 80–120°C, and the material was heated for approximately 20 minutes. Next, we raised the temperature to 80–110°C and maintained it for about four hours. Once the heat treatment was complete, the sheet was gradually cooled to room temperature. Finally, we measured and inspected the material to confirm it met processing requirements.

Machine Selection

Machine selection is also an important part of this project. For the long plastic component, we used a 3-axis CNC machine, as it required lower precision but had significant length constraints. For the circular plastic part, a five-axis CNC machine was used to prevent tolerance accumulation from multiple clamping operations. Additionally, the higher spindle speed of the 5-axis machine reduced cutting stress on the material, improving overall machining accuracy.

CNC Milling Process

For the long supporting component, we adopted an anti-deformation CNC machining method. We used a 3-axis CNC machine to work from both ends simultaneously. The machine milled a small section on each end.

This machining method helped to minimize internal stress due to smaller amounts of material being cut each time. This reduced the risk of excessive deformation and made the component more balanced as both ends had been machined equally. It avoided any over-deformation of a single section, however, it made the total machining time much longer. So machining expense was relatively higher.

For the circular plastic part, careful programming was essential to balance stress release and minimize deformation. Given the semi-open design in the middle and top of the part, the top opening was machined first while the central section remained solid, maintaining structural stability. Next, we machined the internal features.

The final step was to cut the semi-open section in the middle. At this stage, the uneven shape made the part highly susceptible to deformation. To counteract this, engineers used filler materials to support high-variability areas and stabilize the part, reducing stress-induced deformation.

Temperature Control

Temperature control was a critical factor in preventing machining defects. Since plastics melt at high temperatures, excessive heat could cause the material to adhere to the milling tools. To counteract this, we applied a large amount of coolant to regulate the temperature, effectively preventing part damage and ensuring machining accuracy.

Feedback and Plan

The finished product we machined finally met the tolerances required by the drawing. After sending the product to our customer, they reported that the testing went smoothly and the functionality was fully guaranteed, due to very minor deformations across all parts. As such, the product development and improvement were able to continue as planned. Following this success, we have continued to provide prototype samples and small-batch production services for multiple plastic components for this project.