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• Structural reaction injection molding
• Thin-wall molding
• Vibration gas injection molding
• Water assisted injection molding
• Rubber injection
Stress
The main enemy of any injection molded plastic part is stress. When a plastic resin (which contains long strains of molecules) is melted in preparation for molding, the molecular bonds are temporarily broken due to the heat and shear force of the extruder, allowing the molecules to flow into the mold. Using pressure, the resin is forced into the mold filling in every feature, crack and crevice of the mold. As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part. Turning hard or sharp corners exerts more stress on the molecule than taking gentle turns with generous radii. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to.
As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part. Part stresses can cause warpage, sink marks, cracking, premature failure and other problems.
While some stresses in an injection molded part are to be expected, you should design your parts with as much consideration for stress reduction as possible. Some ways to do this are by adding smooth transitions between features and using rounds and fillets in possible high stress areas.
Gates
Each injection mold design must have a gate, or an opening that allows the molten plastic to be injected into the cavity of the mold. Gate type, design and location can have effects on the part such as part packing, gate removal or vestige, cosmetic appearance of the part, and part dimensions & warping.
Gate Types
There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates.
Manually Trimmed Gates:
These type of gates require an operator to separate the aprts from the runners manually after each cycle. Manually trimmed gates are chosen for several reasons:
• The gate is too bulky to be automatically sheared by the machine
• Shear-sensitive materials such as PVC cannot be exposed to high shear rates
• Flow distribution for certain designs that require simultaneous flow distribution across a wide front
Automatically Trimmed Gates
These type of gates incorporate features in the tool to break or shear the gates when the tool opens to eject the part. Automatically trimmed gates are used for several reasons:
• Avoiding gate removal as a secondary operation, reducing cost
• Maintaining consistent cycle times for all parts
• Minimizing gate scars on parts
Common Gate Designs
The largest factor to consider when choosing the proper gate type for your application is the gate design. There are many different gate designs available based on the size and shape of your part. Below are four of the most popular gate designs used by Quickparts customers:
The Edge Gate is the most common gate design. As the name indicates, this gate is located on the edge of the part and is best suited for flat parts. Edge gates are ideal for medium and thick sections and can be used on multicavity two plate tools. This gate will leave a scar at the parting line.
The Sub Gate is the only automatically trimmed gate on the list. Ejector pins will be necessary for automatic trimming of this gate. Sub gates are quite common and have several variations such as banana gate, tunnel gate and smiley gate to name a few. The sub gate allows you to gate away from the parting line, giving more flexibility to place the gate at an optimum location on the part. This gate leaves a pin sized scar on the part.