A Guide to Plastic Enclosure Manufacturing: Injection Molding Process

Step 1: Clamping

The two halves of the mold are securely closed using hydraulic or mechanical pressure. The clamping force must exceed the injection pressure to prevent mold separation and flashing. Typical clamping forces range from 5 to 4,000 tons depending on part size and material.

Step 2: Injection

Plastic pellets are fed into the barrel, where they are heated to a molten state. The reciprocating screw pushes the molten plastic into the mold cavity under high pressure. Injection pressure typically ranges from 500 to 2,000 psi, with precise control over injection speed and pressure profile.

Step 3: Cooling

The molten plastic inside the mold begins to cool and solidify, taking the shape of the cavity. Cooling time accounts for approximately 80% of the total cycle time. Efficient cooling channel design is critical for minimizing cycle times and ensuring uniform part cooling.

Step 4: Plasticizing

While the current shot is cooling, the screw retracts and plasticizes material for the next cycle. The screw rotation shears and melts the plastic pellets, while the backward movement accumulates the required shot volume in front of the screw.

Step 5: Ejection

After sufficient cooling, the mold opens and ejector pins push the finished part out of the mold. Proper ejection system design prevents part damage and ensures reliable, automated production. Draft angles and appropriate surface finishes facilitate easy part release.

Step 6: Post-Processing

Additional operations may include degating (removing spruce and runners), trimming flash, surface treatment, painting, assembly, or quality inspection. Automated systems often handle these tasks inline with the molding process.

Material Selection Criteria

When selecting materials for plastic enclosures, consider these key factors:

• Mechanical properties (strength, stiffness, impact resistance)
• Thermal performance (operating temperature, heat deflection)
• Chemical resistance (exposure to oils, solvents, cleaners)
• Regulatory requirements (UL94, FDA, RoHS, REACH)
• Aesthetic considerations (color, finish, transparency)
• Cost constraints (material cost, processing efficiency)

Uniform Wall Thickness

Maintain consistent wall thickness to prevent sink marks, warpage, and internal stresses.

Adequate Draft Angles

Include appropriate draft (1-3°) on vertical surfaces for easy part ejection.

Proper Rib Design

Use ribs for stiffness instead of thick walls to prevent sink marks and reduce weight.

Sink Marks

Depressions on the surface caused by uneven cooling or excessive wall thickness. Solutions include reducing wall thickness, optimizing gate location, increasing holding pressure, or improving cooling.

Warpage

Distortion of the part after ejection due to internal stresses or uneven cooling. Prevention methods include uniform wall design, balanced filling, proper gate placement, and optimized cooling.

Short Shots

Incomplete filling of the mold cavity. Causes include insufficient injection pressure, low melt temperature, or inadequate venting. Solutions involve increasing injection speed, raising melt temperature, or adding vents.