In-mold decorating (IMD) gives manufacturers across industries an alternative process for creating more durable plastic parts. With IMD, graphics are printed on a film which is then inserted into a mold cavity. That cavity is then filled with resin to create a single finished part. But it’s how the resin actually gets into the mold — also known as “gating” — that can have a major impact on the success of IMD projects. Here are eight gating considerations for second-surface IMD that can help molders and manufacturers achieve target outcomes for IMD applications.

Eight gating considerations

Use one gate whenever possible to reduce knit line effects.

If two gates are used to inject resin into multiple places on a single part, where that molten plastic meets in the middle will create a knit line. This can ruin the clean aesthetics of an IMD product. So, when creating an overmold product (i.e., molding over the top of the applique, not molding behind the applique), it’s best to use only one gate, if possible, to eliminate that knit line and preserve the intended look and feel of the product.

It should be noted that this is only true for overmolded products. When creating an undermold product (i.e., doing a second-surface molding from the back), using two gates is not problematic because users won’t see the knit line because the top plastic layer hides it.

Use short land lengths to ensure good flow.

As resin enters a mold, it immediately starts to cool. So, using short land lengths will help ensure sufficient flow of the resin throughout the part. Land lengths are largely dictated by gate placement and tool design, so it’s best to consider these factors early in the IMD development process.

Use impinging gates whenever possible to lock the film into place during fill.

The flow of injection molding resin can cause the IMD graphic film to fold, stretch, crumple or get washed away entirely. Using impinging gates will help secure the film in place to avoid these issues. In some instances, a tab gate can be used to gate straight down into a tab on the edge of the part and pin the applique against the tool before the molding resin flows across it. The molder would then cut that tab off after the part was molded.

Direct flow toward vents to prevent gas entrapment.

As resin fills the mold, gas can get trapped within the part and create unwanted issues — potentially catastrophic issues — if not properly vented. This should be taken into consideration when designing gate placement because directing flow toward vents will help eliminate this issue and ensure a smoother production process.

Gate from thick to thin areas to avoid venting issues.

If a molded part design includes thicker and thinner areas, it’s best to place gates so the resin flows from the thicker areas to the thinner parts. This will help with venting and ensure the resin completely fills the mold. Sometimes molders will add an extra gate in thinner areas to fill that area. To provide venting in these areas, molders will often include an ejector pin which doubles as a vent to push the piece of plastic out of the mold once it has cooled.

Gate away from impact areas to minimize stress in those areas.

If a gate is placed too close to the IMD film or label, the flow of the resin can potentially blow away the printed inks and ruin the aesthetics of the part. If it’s blowing into a tight corner or the gate is close to the IMD label, it can even blow away the entire applique or melt through the film if the applique is not tight against the tool. This is why it’s important to design gate placement away from high-impact areas to minimize stress in those areas and ultimately improve production yields.

Use indirect runner systems to prevent jetting and potential gate wash.

Using an indirect runner system can help minimize stress like that discussed above. With this process, molders add sprues which provide a place for the plastic to cool slightly and become more stable before moving into the rest of the part. The sprue creates an indirect runner that shoots into the part and is then discarded. The disadvantage to this approach is that it wastes plastic material and therefore increases costs.

Minimize flow restrictions near gates.

Molding is great for creating products with complex geometries. But if there is an indent in the part or the part gets particularly thin in certain areas, this can restrict the flow of resin throughout the part. If these restrictions are too close to the gate — where the resin is hottest and flowing with the most force — it can blow away the ink or applique. This is why molders and tool designers should seek to minimize flow restrictions near gates or find other areas to place gates.

Ready to start an in-mold decorating (IMD) project? Contact DuraTech today to learn more about our IMD services and how we can help you improve manufacturing efficiency and create better products.