Insert molding in injection mold service

Insert molding is a type of mold used to securely fix nuts, metal parts, or hard plastic parts inside cavities during the injection molding process.

For insert nuts injection molding, materials such as stainless steel, copper, bronze, and steel can be used, with copper nuts being commonly preferred. Copper is easily knurled, facilitating better splicing between nuts and plastics. The tolerance of the nut inner bores should be controlled within 0.02mm to prevent flash formation during molding. During mold fitting, nuts need to be assembled onto insert pins for testing. If the fit between nuts and pins is too tight, it can result in difficulties ejecting the part and cause eject marks or sticking issues. Conversely, if the fit is too loose, it can lead to flash formation.

Insert screw-nut injection molding

Insert metal injection molding

Insert metal&hard plastic injection molding

Insert metal parts injection molding:

Metal parts can consist of stainless steel, aluminum, copper, steel, and other materials. It’s crucial to control the tolerance of metal parts within 0.02mm to ensure proper material sealing and prevent flash formation. The area of metal parts should not be designed too large.

If the filling area for metal parts is excessively large, achieving full injection can be challenging due to significant temperature differences between metal parts. Metal part positions are typically designed in the cavity since the cavity remains stationary, minimizing the risk of flash resulting from metal parts dislodging during movement (in severe cases, this may damage the mold). In specific instances, metal part positions may only be designed in the core or side surface of the product.

Insert hard plastic injection molding:

Hard plastics with high melting points, such as PEEK, PA66+30GF, PP+30GF, PA12+30GF, PPS, etc., are typically chosen. Accurate tolerance is crucial for these hard plastics, with defects like shrinkage, dents, and deformations not acceptable in the sealing area.

During mold fitting, the hard plastic inside the mold is used for testing and left with a pre-pressing of 0.05-0.1mm around the sealing area to ensure better sealing. It’s important not to design the hard plastic part with too large an area, as this can lead to temperature differences and difficulties in material filling during injection.

Generally, the hard plastic part is fixed on the side of the cavity to avoid flashing or even damaging the mold during mold movement, as the cavity remains stationary. In specific cases, the hard plastic part may only be designed in the core or side face of the product.

Design key points

  1. Shrinkage should be designed for products with nut inserts, while no shrinkage design is necessary for products with metal parts and hard plastic inserts. In areas requiring strict tolerance, the product size tolerance is adjusted to the median.
  2. Mold design typically incorporates a standard pin-point gate in the mold base, with inserting parts placed in the cavity during secondary injection whenever possible. For inserts fixed in the cavity, consideration is given to how the part remains in the core after injection molding to facilitate ejection. Elastic blocks and glue are added in the cavity to ensure the part remains in the core, with the distance between them usually kept within 2mm. The quantity of elastic blocks and glue may be increased when metal or hard plastic inserts have larger areas.
  3. Material thickness ideally falls within 1.3-1.8mm (around 1.5mm is optimal). A thinner material may pose difficulties in material filling, while thicker material may lead to shrinkage during production. Gating is crucial in mold design, requiring careful consideration of material filling balance. Material filling speed and pressure may slow down when reaching areas with fixed metal or hard plastic parts due to resistance and temperature differences.
  4. For mold ejector systems, balancing ejecting is essential to prevent deformation after ejection. If the part cannot be ejected in balance, collapsing may be considered to improve structural design balance issues.
  5. To assure the qualification of part appearance after ejection in injection molding, the ejector device must be a hard plastic lump inserted with ABS or PMMA. If over mold has slide sealing, design slides in the cavity as much as possible, because slides in the cavity facilitate mold fitting.
  6. To ensure the quality of part appearance post-ejection in injection molding, the ejector device should consist of a hard plastic lump inserted with ABS or PMMA. If overmold involves slide sealing, slides should be designed in the cavity as much as possible, as this facilitates mold fitting.
  7. To guarantee the strength of the sealing S.A. (seam allowance), for products produced via double injection molding, the width of the sealing S.A. should be at least 0.8mm. For secondary injection material being hard plastic, the width of the sealing S.A. should be at least 1.0mm. Otherwise, it is necessary to suggest product modification to the customer.
  8. In mold design, consideration should be given to the production injection molding machines to determine whether to use vertical or horizontal machines. It is advisable not to design too many cavities, especially for molds with a cold runner, as excessive cavities lead to longer runners, resulting in material waste and reduced injection efficiency. To match the injection molding machine, one must consider whether the product arrangement is compact and reasonable. The product must be fixed to ensure it remains in the same position every time it is placed in the mold. Alternatively, a reaction system can be designed to alert if the part is not in the correct position before the mold closes, aiding in preventing mold closure. This ensures that parts are consistently positioned in the mold, thereby improving the pass rate and production efficiency in injection molding.
  9. To prevent air traps in injection molding, thorough consideration of venting is necessary during mold design. Venting holes should be strategically placed on hard plastic parts, especially in blind angles and areas with long-distance water lines, as these areas are more challenging to fill with material.
  10. To ensure complete material filling and proper adhesion, undercuts should be designed at the corners of parts to enhance adhesive results and promote tighter adhesion.
  11. In areas such as the sealing area and parting line, parts should not be demolded directly from the cavity and core, as clamping lines in the mold and demolding drafts can cause flash during mold fitting. Instead, try to demold using a LISS-OFF method.

Gate point types of insert mold

Gate point for insert mold can be designed to direct hot sprue valve gate, hot sprue pin gate, pin-point gate, sub gate, edge gate…etc.

Hot sprue valve gate: With excellent liquidity and flexible positioning, this gating method features a small gate point and is ideal for mass production and products with thick wall thickness. It helps conserve material, eliminates material waste for gating, and ensures short lead times and high quality. However, it may leave slight gating traces as its only defect.

Hot sprue pin gate: Similar to the hot sprue valve gate, this method offers good liquidity and flexible positioning, with a small gate point suitable for mass production and thick-walled products. It also minimizes material waste and ensures short lead times and high quality. However, it leaves behind 0.1mm of material around the gate point and is prone to burrs, requiring grooves to cover the excess material.

Pin-point gate: This gating method offers flexible positioning but weaker liquidity and longer runner distances, making it suitable for small-batch production. It generates more waste material around the gate point and requires mechanical arms to clamp the gate point during production. It has a longer lead time and leaves 0.1-0.2mm of material around the gate point, necessitating grooves to cover the excess material.

Sub gate: This type of gate can be positioned on ribs within the cavity, core, side walls, and ejector pins. It offers flexible gate point selection, with the pouring gate automatically separating from the part and leaving only slight gating traces. However, it is prone to material pulling around the gate point, leading to drying marks that require manual removal. Additionally, there is considerable pressure loss from the gate point within the cavities.

Edge gate: Molten plastic flows through this gate, distributing laterally to reduce stress and minimize the likelihood of air entering the cavity, thus preventing streaks and bubbles. However, the pouring gate does not separate automatically from the part, leaving sprue marks on the part’s edges that require tools to flatten. Despite this, the edge gate facilitates proportional injection and pressure holding, improving airlines and flow marks.

Processing and fitting for insert mold

  1. Before processing, establish the mold processing technology. Utilize high-precision processing machines, high-speed machines, slow-feeding NC wire cut machines, and mirror EDM (electric discharge machining) machines.
  2. Incorporate a 0.05-0.1mm pre-pressing allowance in the design.
  3. Pay attention to precision requirements during mold base processing. Inspect tolerances upon receipt of the mold base and refrain from using it if tolerances are unsatisfactory.
  4. During mold fitting, insert nuts, metal parts, and hard plastic parts into the mold. If issues arise during fitting, carefully examine nuts, metal parts, hard plastic parts, and molds to identify the source of the problem. Strive to adhere to the drawing specifications to facilitate future data tracing.
  5. Avoid using grinders for mold fitting. Instead, use machines for corrections in areas where mold fitting is inadequate.
  6. Conduct action testing before trials to prevent misalignment and incorrect assembly. Mistaken assembly can lead to damage to the mold base.

Mold testing for insert mold

  1. During mold testing, it’s essential to understand the sequence of mold opening, closing, and ejection, as well as the structural features and properties of metal parts and hard plastic parts.
  2. Determine the number of samples required by the customer and ensure an adequate supply of nuts, metal parts, and hard plastics, as mold testing often requires numerous samples.
  3. Take note of whether it’s possible to test the mold without inserts of nuts, metal parts, or hard plastics. Failure to assemble these inserts in the mold may result in defects such as sticking to the mold or short shots.
  4. Adjustments to the waterline plate on the mold are often necessary, but there may be instances where it’s not possible to adjust the waterline plate in certain insert molds due to their structure. In severe cases, the mold may become heavily stuck and require modification or even damage during opening.
  5. Various issues may arise during mold testing, including short shots, air traps, flash, or sticking to the mold. If these problems can be identified on the injection molding machine, it’s preferable to address them there.