Plastic compound mold processing—also known as vacuum injection molding or soft mold casting—is a cost-effective manufacturing method for producing plastic parts using reusable silicone-based molds. Unlike traditional steel mold processes, it prioritizes speed, flexibility, and detail replication, making it ideal for small-batch production and product development. This article breaks down its core definitions, step-by-step workflow, material selections, advantages, and limitations—with clear comparisons and real-world examples to help you optimize the process for your projects.
1. Core Definition & Key Principles
Before diving into the process, it’s critical to understand what makes plastic compound mold processing unique and how it differs from traditional methods.
| Aspect | Plastic Compound Mold Processing | Traditional Steel Injection Molding |
| Mold Material | Primarily silicone rubber (flexible, elastic, and detail-retaining). | Rigid materials like steel or aluminum (highly durable but costly to machine). |
| Core Principle | Replicates parts by pouring liquid plastic into a silicone mold (created from a prototype), then curing and demolding. | Injects molten plastic into a steel mold under high pressure (10–150 MPa) for mass production. |
| Batch Suitability | Ideal for small batches (10–500 units) and prototyping. | Designed for mass production (10,000+ units) to offset high mold costs. |
| Key Advantage | Low mold cost, fast turnaround, and ability to replicate complex shapes/textures. | High production efficiency, superior part accuracy, and long mold life (100,000+ cycles). |
Key Question: Why choose plastic compound mold processing?
For projects where speed and cost matter more than ultra-high volume—such as new product testing, niche market parts, or custom crafts—silicone-based molds eliminate the need for expensive steel tooling, cutting both time and upfront investment.
2. Step-by-Step Processing Workflow
Plastic compound mold processing follows a linear, repeatable workflow that starts with a prototype and ends with finished plastic parts. Each step directly impacts the final part’s quality and the mold’s durability.
2.1 Stage 1: Prototype Preparation (The “Master Model”)
The prototype serves as the template for the silicone mold—its quality determines the detail and accuracy of all subsequent parts.
| Prototype Manufacturing Method | Key Characteristics | Ideal For |
| 3D Printing (SLA/DLP) | – High precision (±0.05mm) for intricate details (e.g., logos, textures).- Smooth surface finish (Ra 0.8–1.6μm) reduces post-processing.- Fast turnaround (12–24 hours for small parts). | Complex parts: electronic device shells (phone cases, TV remote casings), decorative items with fine patterns. |
| CNC Machining | – Ultra-high accuracy (±0.01mm) for tight tolerances.- Suitable for hard materials (metal, rigid plastic, wood).- Ideal for flat or geometrically precise parts. | High-precision components: automotive brackets, medical device housings, parts requiring strict dimensional consistency. |
| Hand Engraving/Crafting | – Low cost for simple shapes.- Flexible for one-of-a-kind designs.- No specialized equipment needed. | Simple decorative parts: custom keychains, small figurines, low-precision prototypes for visual testing. |
Critical Tip: Ensure the prototype is clean and defect-free (no dust, oil, or scratches). Even tiny flaws will be replicated in the silicone mold—for example, a dusty prototype will create a mold with embedded particles, ruining the finish of plastic parts.
2.2 Stage 2: Silicone Mold Making (The “Negative Template”)
This stage transforms the prototype into a reusable mold. Proper silicone mixing, pouring, and curing are essential to avoid bubbles, tears, or incomplete detail capture.
Step 1: Mold Frame Setup
- Frame Selection: Choose a frame material (wood, plastic, or metal) large enough to fit the prototype with 5–10mm of space on all sides (ensures even silicone coverage).
- Sealing: Use masking tape or acrylic sealant to seal frame edges—prevents silicone leakage, which wastes material and distorts the mold shape.
- Prototype Positioning: Secure the prototype in the frame with clay or double-sided tape—keep it centered to ensure uniform silicone thickness.
Step 2: Silicone Mixing
- Material Selection: Use two-component silicone rubber (Part A: base; Part B: curing agent) with a mixing ratio specified by the manufacturer (typically 1:1 by volume/weight for additive silicone, 10:1 for condensation silicone).
- Mixing Rules:
- Stir slowly and thoroughly (2–3 minutes) to avoid air bubbles—uneven mixing causes weak spots in the mold.
- For large batches, use a vacuum degassing machine (1–2 minutes at -0.1MPa) to remove trapped air—critical for parts with tiny details (e.g., 0.5mm-wide slots).
Step 3: Silicone Pouring & Curing
- Pouring Technique: Tilt the frame at a 45° angle and pour silicone slowly along the frame wall (not directly onto the prototype)—reduces bubble formation. For thick molds (>10mm), use layered pouring: pour 1/3 of the silicone, wait 30 minutes for bubbles to rise, then add the next layer.
- Curing Parameters:
| Silicone Type | Curing Temperature | Curing Time (for 5mm Thickness) | Post-Curing Tip |
| Additive (Platinum-Cure) | 20°C–25°C | 2–4 hours | No post-curing needed; ready to demold once firm. |
| Condensation | 20°C–25°C | 4–8 hours | Cure for an extra 1 hour if mold is thicker than 10mm. |
| High-Temperature Resistant | 20°C–25°C (initial); 80°C (post) | 6–10 hours (initial); 2 hours (post) | Post-cure in an oven to activate heat resistance (up to 200°C). |
Step 4: Demolding the Prototype
- Once cured, gently peel the silicone mold from the prototype—silicone’s elasticity ensures easy removal without damaging either the mold or prototype.
- Trim excess silicone (flash) from mold edges with a sharp knife—creates a clean, precise mold for plastic casting.
2.3 Stage 3: Plastic Part Production (The “Replica Process”)
With the silicone mold ready, this stage produces finished plastic parts through pouring, curing, and demolding.
Step 1: Plastic Material Selection
Choose a material based on the part’s end-use (strength, flexibility, heat resistance):
| Plastic Material | Key Properties | Ideal Applications |
| Polyurethane (PU) Resin | – Fast curing (1–2 hours at 20°C).- Good wear resistance and flexibility (Shore A 30–80).- Low cost ($20–40 per kg). | Functional parts: TV remote buttons, toy components, flexible gaskets, small electronic housings. |
| Epoxy Resin | – High strength (tensile strength: 50–80 MPa) and chemical resistance.- Heat-resistant (120°C–180°C after curing).- Low shrinkage (0.5–1%). | Structural parts: automotive interior trim, medical device handles, parts requiring durability under stress. |
| Polyvinyl Chloride (PVC) | – Low cost and good chemical stability.- Rigid or flexible variants available.- Suitable for food-contact parts (when food-grade). | Custom containers, decorative trim, low-stress household items. |
Step 2: Pouring & Curing
- Pouring: Pour liquid plastic into the silicone mold slowly (use a small funnel for narrow openings) to avoid bubbles. For complex parts with internal cavities, use sectional pouring: fill one section, wait 10 minutes for partial curing, then fill the next—ensures full cavity coverage.
- Curing:
- Room-temperature curing: PU resin (1–2 hours), epoxy resin (4–6 hours).
- Accelerated curing: Use a low-temperature oven (50°C–60°C) to reduce time by 50% (e.g., epoxy resin cures in 2–3 hours).
Step 3: Demolding & Post-Processing
- Gently peel the silicone mold from the plastic part—silicone’s flexibility prevents cracking or deformation.
- Trim excess plastic (flash) with scissors or a utility knife. For better finish, sand the part with 400–800 grit sandpaper.
3. Advantages of Plastic Compound Mold Processing
This method solves key pain points for small-batch and prototyping projects:
| Advantage Category | Key Benefits | Real-World Example |
| Low Cost | – Silicone mold cost is 50–70% lower than steel molds (e.g., \(200 vs. \)5,000 for a small part mold).- No expensive machining equipment needed for mold production. | A startup producing 50 test samples of a new smartwatch casing saves $4,800 by using a silicone mold instead of steel. |
| Fast Turnaround | – Entire process (prototype → mold → parts) takes 3–7 days, vs. 2–4 weeks for steel molds. | A consumer electronics company needs 20 TV remote prototypes for user testing—plastic compound processing delivers them in 4 days, enabling faster design iterations. |
| Complex Detail Replication | – Captures tiny textures (0.1mm–0.5mm), curved surfaces, and internal structures that steel molds may struggle to replicate. | A jewelry brand replicates a hand-sculpted pendant with fine engravings—each plastic part retains the pendant’s intricate patterns, which would require costly steel mold machining. |
| Flexibility | – Easy to adjust mold design (e.g., modify a prototype and make a new silicone mold in 1–2 days).- Switch plastic materials or colors without changing the mold. | A phone case manufacturer tests 3 colors (black, white, blue) using the same silicone mold—no need for multiple molds, cutting color-testing costs by 60%. |
4. Limitations to Consider
While highly effective for specific use cases, plastic compound mold processing has constraints:
- Limited Mold Life: Silicone molds last 20–500 cycles (vs. 100,000+ for steel molds). Parts with sharp edges or high friction (e.g., plastic clips) wear down molds faster—after 50 cycles, a mold may start to lose detail.
- Lower Part Accuracy: Dimensional accuracy is ±0.1mm–±0.3mm (vs. ±0.01mm for steel injection molding). This is sufficient for most consumer parts but not for precision components like aerospace parts.
- Lower Production Efficiency: Each part requires manual pouring and demolding—production speed is 1–10 parts per hour (vs. 100+ parts per hour for steel injection molding). For batches over 500 units, it becomes slower and more costly than traditional methods.
5. Yigu Technology’s Perspective on Plastic Compound Mold Processing
At Yigu Technology, we see plastic compound mold processing as a “development enabler”—it lets clients test designs fast and avoid overinvesting in unproven products. A common mistake we address is clients overusing it for large batches: one client tried to produce 2,000 phone cases with a silicone mold, only to face mold wear and inconsistent parts after 300 cycles. We advised switching to steel molds for mass production, saving them 40% in rework costs. For prototyping, we recommend additive silicone (1:1 ratio) for detail retention and PU resin for fast functional testing. Our key insight: This process shines when paired with a clear transition plan—use it for 10–500 units, then scale to steel molds if demand grows.
6. FAQ: Common Questions About Plastic Compound Mold Processing
Q1: Can I use plastic compound mold processing for high-temperature parts (e.g., parts exposed to 150°C)?
A1: Yes, but use heat-resistant materials. Choose high-temperature silicone (service temp: 200°C–300°C) for the mold and heat-resistant epoxy resin (cured temp: 120°C–180°C) for the part. Test a sample first—expose it to 150°C for 24 hours to ensure no deformation. Avoid standard silicone (max temp: 150°C) or PU resin (max temp: 80°C) for high-heat applications.
Q2: How can I extend the life of my silicone mold?
A2: – Clean the mold with mild soap and water after each use (avoid harsh solvents like acetone, which break down silicone).- Apply a thin layer of silicone oil to the mold before pouring plastic—reduces friction and wear.- Store the mold in a cool, dry place (humidity <60%) and avoid folding or stretching it—prevents tears.
Q3: Are parts made via plastic compound mold processing suitable for food contact?
A3: Only if you use food-grade materials. Choose food-safe silicone (certified by FDA or EU standards) for the mold and food-grade plastic resins (e.g., food-grade PU, PVC, or epoxy). Regular materials may leach chemicals into food—always test the final part for compliance (e.g., FDA 21 CFR 177.2600 for resin) before use.