By Dallas D. Cada

Whenever I get a trouble shooting call about TPE parts it usually is a result of poor adhesion. My experience with TPE materials is in automotive weather seals however this tech brief will help optimize adhesion with all TPE related parts. More often than not, the parts I am asked to help with have splits within them. This is a result of poor adhesion between the material (TPE) and the substrate (usually made with TPV or a polyolefin material). In order to optimize adhesion between the substrate and TPE material I have created a list of important steps to help.

 


Design of the substrate (often referred to as an EPDM pillar).

  • Incorporate a step in the substrate profile to create an abrupt transition between the TPE and substrate.
  • Use a proper shut off design.
  • Using mechanical interlocks in the component design will help immensely.
  • Avoid “feathering” or gradual thinning in the design for over molding TPE.
  • Keep the TPE flow-length/part thickness ratio below 150:1. Take care to have appropriate TPE thickness as too thin can lead to delamination. Delamination can result in poor adhesion.
  • Incorporate a suitable texture on the mold for long flow paths.
  • Optimize venting wherever possible especially at the end of polymer flow.
  • Optimize gating on the tool. Most tools will incorporate a sub-gate. It is imperative to use a gate size that will supply adequate polymer to the part.

Important substrate features.

  • It is best if the EPDM pillars are extruded then fresh cut to length. They should then immediately be placed in the mold to bond with the addition of the TPE material. If the pillars are made in advance they need to be stored in an environmentally controlled area (as least moisture as possible). When these pillars are used it is important to fresh cut them.
  • Once again it is imperative to use fresh cut EPDM pillars.
  • Make sure that the pillars are clean; i.e. no oils, dirt or dust. Wiping the pillars with a rag that is dosed in alcohol works the best.
  • Preheat the pillars only if needed. Most mold shops have “curing ovens” or material drying ovens that can be used for this. Set the temperature on low (usually ~100°F).

Use compatible hard & soft materials; i.e. materials with both thermoplastic and elastomeric properties. In other words, ensure that the chosen TPE is compatible with the substrate material. This will result in better bonding.

Insure that the type of color concentrate carrier used is comparable with both the plastic and the TPE.

Drying conditions of TPE materials.

  • A dehumidifying desiccant drying system is preferred when drying any plastic material.
  • Dry the material accordingly to the manufactures recommendations.
  • This also includes TPO materials. While they are non-hygroscopic they may have surface moisture upon them.
  • Most TPE materials will dry for 2 – 4 hours at 140°F – 175°F. It is highly recommended to examine the material moisture within a moisture analyzer. Materials should be dried to 0.05% or less before molding.
  • Keeping the material in a drying hopper that is placed upon the molding machine is best.
  • Properly dried materials will process better, produce parts with good adhesion and good aesthetics. Do not over dry material as they may be difficult to process and may discolor.

Preferred molding machines and process parameters for TPE materials.

  • For best results, molding machines should be selected so the shot weight is approximately 50% of the machine barrels capacity. This minimizes residence time and prevents excessive thermal degradation.
  • If a machine has a capacity of more or less than 50% of the barrel capacity, profile temperatures accordingly to shot size vs machine barrel size. This is necessary to promote a homogeneous melt.
  • Use additional rear zone heat for short residence time (a reverse heat profile).
  • Use less rear zone heat for long residence time (a forward heat profile).
  • A good molding procedure employs barrel heats at the low setting and working your way up as needed.
  • Higher barrel or melt temperatures will usually create a better bond. Take care to prevent thermal degradation of the material.
  • Machine process conditions can be adjusted to compensate for non-ideal conditions.

Additional processing conditions for TPE materials.

  • Follow the processing or molding guide conditions from the material manufacture.
  • It is best to start any processing of materials using a decoupled molding process. This means, filling the part to 98% with 1st stage pressure than adding 2nd stage or pack pressure to complete the part. Weigh the parts to insure that they are filled and packed out.
  • Control melt temperature by injection speed, first stage pressure then barrel temperatures.
  • Again, using higher melt temperatures will result in better adhesion. Take care to prevent thermal degradation.
  • It is best to use a mold temperature controller to maintain heat on the mold. Most TPE materials will adhere best at a mold temperature of 120°F – 160°F.
  • The more polymer on the substrate surface, the better the adhesion. This is referred to as a resin rich surface.
  • Use a pyrometer to check both melt and mold temperatures.

Correct curing stage of parts.

  • Once the part is molded it is best to cure the parts on a rack. The rack must be formed to distribute the weight of the part evenly. Uneven weight distribution can cause parts to “creep” thus causing splits between the polymer and substrate.
  • Once the parts are cured, they should be stored by laying them flat in a shipping container. Placing a piece of cardboard in-between each part will help maintain their post-molded form.

The information given in this tech brief should help with good adhesion of TPE materials and the substrate. The information covers conditions that will help with an over-molding and insert molding operation. Insert molding is most common where a pre-molded insert (such as a substrate of pillar) is placed in the mold. The TPE material is then shot directly over it. The advantage of this procedure is that a conventional single shot mold machine can be use. Tooling costs associated with insert molding are lower than with multi-shot procedures.

In summary, you will note that an optimized design, mold machine, drying and processing conditions will help with good adhesion. Also taking care to cure and store the parts correctly will help. This all helps eliminate splits within the pillar and TPE material.

Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting), his experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies. For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at dallascada@charter.net. Contact Dallas by phone (507) 452-1584.