Overmolding Processing — Maximizing Adhesion

- Sep 27, 2017-

It is a common practice to use two mold temperature controllers while running GLS materials. If sticking in the A-half of the tool is an issue, reduce its temperature as a possible means of improving ejection performance. Cooling Time The required cooling time is dependent on the temperature of the melt, the wall thickness of the part and the quantity of cooling available. Harder grades will set up faster than softer grades and are easier to eject. Overmolded parts will take longer to cool because the TPE is only cooled from one side as the plastic substrate acts as a thermal insulator. The cooling time for overmolded parts will be approximately 15 to 40 seconds for every 0.100" (2.5mm) of overmold wall thickness. Given a final part design and GLS TPE material selection, there are three primary variables that have an impact on cycle time: • Diameter of cooling line. • Geometric orientation (distance) of cooling line with respect to the tool cavity. • Flow rate of the water through the cooling system.

Substrate Preparation and Molding If insert molding is the process of choice, the preparation and molding of the insert can be critical to achieve optimal adhesion between the TPE and the substrate. If an insert surface is not clean, loss of adhesion between the TPE and substrate is a strong possibility. Ideally, it is best if the inserts are molded, then transferred immediately to the second mold to inject the TPE. The insert should be molded in accordance with supplier recommendations (drying, adequate melt and mold temperatures). If the inserts are molded and stored prior to the overmolding phase of the process, they should be well-protected from contamination sources like dirt, dust, etc. Skin oil can also affect the TPE adhesion to the insert, so hand protection should be worn when handling the inserts. Mold release should not be used in either the insert or the overmolding processes, as the lubricants can seriously degrade the TPEs bond to the insert. Studies have demonstrated that preheating the insert prior to overmolding can be very beneficial to the end quality of insert molded parts. The optimum insert temperature will vary, depending on both the insert material and the type of TPE used. Preheating nylon inserts is strongly discouraged. The adhesion of the new Versaflex® OM 6100 series is outstanding, with no modification of the substrate. In fact, preheating the nylon insert can have a negative impact on adhesion. TPE Melt Temperature The TPE melt temperature is one of the most critical injection molding parameters for overmolding applications. A common practice is to equate nozzle temperatures with melt temperature. In many cases, this practice is misleading, since the real temperature of the melt can vary significantly from the nozzle’s temperature. The size of the barrel, shot size and the calibration of thermocouples are all factors that can lead to inaccurate melt temperature readings. The actual temperature of the melt can be best quantified by measuring the temperature of a TPE airshot using a pyrometer. In many cases, the melt temperature of the TPE is lower than the nozzle temperature. Correspondingly, lower melt temperatures can affect adhesion quality. The melt temperature of the TPE must be selected based on the substrate used and target bond level desired in the finished component. To achieve optimal bond strength, higher temperatures using melt temperature guidelines for specific GLS products as a starting point is suggested. In some critical applications, this temperature can be close to the upper processing temperature limits for the TPE. In order to reduce residence time at high temperatures, the rear zone temperatures should be reduced as much as possible, while maintaining the last zone and nozzle at the high processing temperature. For specific GLS overmolding TPEs, start-up temperature settings are included in the GLS Product Technical Data Sheet to assist molders to determine optimal melt temperatures.